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1965 Studies in Molecular Spectroscopy; I. Excimer Fluorescence, II. Heavy-Atom Spin-Orbital Coupling Effect and IIi. The lecE tronic Spectra of Ferrocene. Fred Jewel Smith Louisiana State University and Agricultural & Mechanical College
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Recommended Citation Smith, Fred Jewel, "Studies in Molecular Spectroscopy; I. Excimer Fluorescence, II. Heavy-Atom Spin-Orbital Coupling Effect and IIi. The Electronic Spectra of Ferrocene." (1965). LSU Historical Dissertations and Theses. 1092. https://digitalcommons.lsu.edu/gradschool_disstheses/1092
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SMITH, Fred Jewel, 1939- STUDIES IN MOLECULAR SPECTROSCOPY} I. EXCIMER FLUORESCENCE, II. HEAVY- ATOM SPIN-ORBITAL COUPLING EFFECT AND HI. THE ELECTRONIC SPECTRA OF FERROCENE.
Louisiana State University, Ph.D., 1965 Chemistry, physical University Microfilms, Inc., Ann Arbor, Michigan STUDIES IN MOLECULAR SPECTROSCOPY; I. EXCIMER FLUORESCENCE, II. HEAVY-ATOM SPIN-ORBITAL COUPLING EFFECT AND III . THE ELECTRONIC SPECTRA OF FERROCENE
A Dissertation
Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy
in
The Department of Chemistry
by Fred Jewel Smith B.A., University of Southern Mississippi, 1960 August, 1965 ACKNOWLEDGMENT
The author wishes to express his sincere appreciation to Dr. S. P. McGlynn, under whose direction this work was performed, for his helpful guidance throughout the course of this investigation. The author is also Indebted to the rest of the faculty and graduate * students at Louisiana State University for many helpful suggestions and comments, and especially Andrew T. Armstrong for his assistance in computer programming the calculations of Part I. Considerable gratitude is also due his wife, Phyllis, for her patience and encouragement throughout this investigation, and for aid in the preparation of this manuscript. The author gratefully acknowledges financial support received from the Petroleum Research Fund of the American Chemical Society, The United States Atomic Energy Coimnission-Biology Branch, and the Dr. Charles E. Coates Memorial Fund of the L.S.U. Foundation donated by George H. Coates.
i i TABLE OF CONTENTS
PAGE ACKNOWLEDGMENTS...... 11 LIST OF TABLES...... v LIST OF FIGURES...... v l l l ABSTRACT...... Ix I. EXCIMER FLUORESCENCE...... 1 A. Exclmer Fluorescence of Naphthalene and Its derivatives ...... 1 1. Introduction ...... 1 2. Experimental andResults * ...... 4 3. Simple Molecular Exclton and Charge Resonance Concepts ...... 5 4. Configuration Interaction between Molecular Exclton State and Charge Resonance States ...... 17 5. Energy of Exclmer Fluorescence ...... 21 6. C onclusions ...... 28
B. Delayed Exclmer Fluorescence of P y re n e ...... 30 1. I n tr o d u c t i o n ...... 30 2. Results and Discussion ...... 30
C. The Mechanism of Delayed Exclmer Fluorescence ...... 36
II. HEAVY-ATOM SPIN-ORBITAL COUPLING EFFECT ...... 41 1. I n tr o d u c t i o n ...... 41 a) A Primitive Model of Spin- Orbital Coupling ...... 41 b) Heavy-Atom Effects of M olecules ...... 48 2. Experimental ...... 50 3. Results and Discussion ...... 53 4. Phosphorescence Spectra ...... 63 5. Conclusions ...... 6 7 '
i l l PAGE I I I . THE ELECTRONICSPECTRA OF FERROCENE...... 69 1. Introduction ...... 69 2. Experimental ...... 69 3. R e s u lts ...... 70 4* Discussion ...... 76 5. C onclusions...... 87
APPENDIX...... 89
SELECTED BIBLIOGRAPHY ...... 100
VITA ...... 109
iv LIST OF TABLES
TABLE PAGE
I. Energy of Exclmer Fluorescence ...... 10 II. Exclmer Properties ...... 11 III . Comparison of Observed^L Stabilisation and *L Exclton Splitting .* ...... 13 IV. Simple Charge Resonance Energies ...... 14 V* Calculated Exclmer Energies for the Internuclear Distances at which the Calculated Energy of the B 3»~ State Equals the Experimentally Observed Exclmer Energy, Z=3.18 ...... 29 VI. Proportionality of the Intensity of Various Luminescence of Pyrene to Incident Excitation Intensity, I ...... 34 VII. Decay Lifetimes of Long-Lived Luminescences of Pyrene ...... 35 VIII. Quantum Yields and Mean Phosphorescence Lifetimes of a-Halonaphthalene and Naphthalene in Heavy-Atom Solvents ...... 34 IX. The Limits of Variation of the Intersystem Crossing Rate C o n sta n t ...... 60 X. Comparison of External and InternalHeavy- Atom E ffect ...... 61 XI. Comparison of the Prediction of Case A and Case B ...... 62 XII. Effect of Medium of the Frequency (0,0) of Phosphorescence ...... 66
v TABLE PAGE
XV. The Vibrational Frequencies and Vibrational Analysis of System V ...... 82 XVI. The Vibrational Frequencies and Vibrational Analysis of System VI ...... 83 XVII. The Vibrational Frequencies and Vibrational Analysis of System VII ...... 84 XVIII. The Vibrational Frequencies and Vibrational Analysis of System V I I I ...... 85 XIX. The Vibrational Frequencies and Vibrational Analysis of System I X ...... 86 XX. Some Possible Cooling Systems for use with the Low Temperature C e ll ...... 96 XXI. Low Temperature G lasses ...... 99
v i LIST OF FIGURES
FIGURE PAGE
1. Potential Energy Diagram for Excimer Formation ...... 3
2. Fluorescence Spectra of 1-methyl- naphthalene ...... 7
3. Fluorescence Spectra of 1,2-Dimethyl- naphthalene ...... 9
4. Plot of I-A Versus the Experimental Excimer Energies ...... 16
5. Definition of Molecular Axes and the Symmetries of the Molecular O rbitals of Naphthalene ...... 19
6. Energies of Excimers Calculated at D=3.5 as a Function of the Effective Nuclear Charge, Z ...... 23
7. Energies of Excimers at Z=3.18 as a Function of Interplanar Distance ...... 25
8. Energies of Excimers at Z=3.18 as a Function of Interplanar Distance ...... 27
9. The Intensity of Delayed Fluorescence of Pyrene in Liquid Solution and De layed Excimer Fluorescence of Pyrene in both Liquid Solution and in the Crystalline State as a Function of the Relative Number of Quanta Absorbed ...... 33
10. Temperature Dependence of the Quantum Yield Ratio of Delayed Excimer Fluores cence to Delayed Monomer and the Quantum Yield Ratio of Non-delayed Excimer Fluorescence to Non-delayed Monomer Fluorescence ...... 40
v i i FIGURE PAGE
II* Illustrating the Transformation from a Nuclear Fixed Coordinate Center to an Electron Fixed Coordinate Center ...... 43
12. Illustrating the Effect of Weak Spln- Orbltal Coupling on Transitions Between States of Different Multiplicities ...... 46
13. Absorption Spectra of Halide Solvents at Room Temperature ...... 52
14* Total Emission Spectra of Naphthalene In Propyl Halide-Alcoholic Solvents at 7 7 ° K ...... 56
15. Illustrating the First-Order Processes which Connect the Ground State, the Lowest Singlet State, and the Lowest Triplet State ...... 58
16. The Phosphorescence Spectra of Naphtha lene in EP (Diethyl Ether and Isopentane In a 1:1 v/v Mixture), Propyl Chloride Cracked Glass, Propyl Bromide Cracked Glass, and Propyl Iodide Cracked Glass ...... 65
17. Absorption Spectra of Ferrocene Vapor at 35°C...... 72
18. Absorption Spectra of Ferrocene in Isopentane at 24°C ...... 74
19. The Temperature Dependence of the Spectrum of Ferrocene ...... 78
20. Schematic of the Low Temperature C e l l ...... 92
21. The Inner Window Assembly of the C e l l ...... 95
v i i i ABSTRACT
The interpretation of the energies of excimer luminescence of naphthalene and twelve of its alkyl derivatives is considered, and it is shown that there is considerable configuration interaction between molecular exclton and charge resonance states and that the energy of excimer fluorescence may not be interpreted without invoking this configuration interaction. The energies of excimer fluorescence are calculated for the substituted naphthalenes using a four-electron MO treatment of the interaction as a function of Z, the effective nuclear charge to be used in a Slater orbital exponent, and of the interplanar distance D. Agreement with excimer luminescence energies is obtained for values of Z=3.18 and values of D between 3.45 and 3.7&, with the largest intermolecular distance D being obtained for those compounds in which the steric hindrance is expected to be largest. The delayed fluorescence and delayed excimer fluorescence of pyrene in liquid solution are shown to originate in triplet-triplet annihilation, and the triplet population is shown to be kinetically limited by first order decay mechanisms. It is shown that the delayed excimer fluorescence of pyrene crystal at 77°K also originates in triplet-triplet annihilation, but that the triplet population is kinetically governed by second order annihilative processes. It is also shown that at lower temperatures and higher viscosities triplet-triplet annihilation results predominantly in the initial production of excited monomers, whereas at higher temperatures the initial production of excimers also becomes an important process. The relative phosphorescence to fluorescence quantum yields and phosphorescence lifetimes of naphthalene have been measured in a number of glassy media, some of which contain perturbing atoms of large atomic number. It is shown that the intersystem crossing
ix process is Che most sensitive to environmental spin-orbital coupling effects. The phosphorescence quenching process is less sensitive than the intersystem crossing process, but more sensitive than the phosphorescence emission process. It is strongly suggested by the data contained herein that heavy atom quenching of fluorescence can indeed be attributed primarily, if not entirely, to Increase in the probability of the intersystem crossing process. A number of possible uses for the heavy atom technique are suggested. A total of 8 singlet-singlet transitions are observed in the Ferrocene Vapor spectra. Transitions at 42,181, 46,933 and 53,078 cm are reported for the first time. Vibrational analyses of the vapor spectrum are given and assignments of the electronic transitions are made. I. EXCIMER FLUORESCENCE
A. EXCIMER FLUORESCENCE OF NAPHTHALENE AND ITS DERIVATIVES
1. Introduction The fluorescence spectrum of a dilute solution of naphthalene or ot any methyl substituted naphthalene consists solely of the normal structured fluorescence band which originates from singlet excited unassociated monomer molecules S^.
S.^S-.+hv- normal monomer fluorescence 1 U r However, as the concentration is increased a second d istin c t band, which is broad and structureless, appears about 6,000 cm ^ to the red of the normal monomer fluorescence. At high concentrations and In the pure liquid the fluorescence spectrum consists almost exclusively of 1 2 this Latter broad structureless emission. ’ This broad structureless band has been attributed to a singlet excited molecule which inter acts with an unexcited neighbor molecule to produce an excited dimer, or excimer, S^Sq which can then break up into de-excited monomer mole cules with the simultaneous emission of a characteristic excimer fluorescence quantum.^ excimer formation S +Sf.-*S S -*2S +hv„ ...... followed by excimer 1 w X U w u * « fluorescence There is no corresponding change in the absorption spectrum as the concentration is increased. The term "excimer" has been introduced in order to differentiate the transiently stable excited dimeric species from the excited states of ordinary dimers which are also 4 stable in th eir ground states. These observations are illustrated in Fig. 1. As two molecules in the ground state approach each other only repulsion R results; however, as a singlet excited molecule approaches a molecule in the
1 Figure 1. Potential energy diagram for exclmer formation. t o t r w
NTERPLANAR DISTANCE- % + %
u> ground state stabilization results, referred to here as binding energy B, yielding an excimer. Emission can occur from both the monomer and excimer yielding the normal structured monomer fluorescence hv^ and broad structureless exclmer fluorescence hv^, respectively. Kinetic considerations of excimer formation have been given by 5 Birks; it seems to have been established, at least for liquid systems, that the broad structureless band is indeed due to excimers. Recently the types of intermolecular forces which are responsible for excimer 6 7 8 formation have been the subject of several investigations. * * It has been shown for non-substituted aromatics that neither simple molecular exclton considerations involving the structures
Sl W l nor charge resonance considerations involving the structures
so so ^so so are sufficent to explain the observed excimer stabilization energy B. It is necessary to consider configuration Interaction between the molecular exclton states and the charge resonance states in order to obtain agreement with the observed energies of excimer fluorescence of non-substituted polyacenes. It Is the purpose of this section to correlate the experimental data for naphthalene and thirteen of its alkyl derivatives with the predictions of simple molecular exclton and charge resonance formalisms, and to point out the inadequacies of both in predicting the changes in excimer energies which occur on substitution. It is also our purpose to point out the necessity of considering configuration interaction of the zeroth-order charge resonance and molecular exclton states, and to show that the same theoretical treatment of excimer formation which has been used successfully for non-substituted aromatics is also adequate to explain the changes which occur on substitution. 2. Experimental and Results The spectra of naphthalene, its two monomethyl derivatives, all of its ten dimethyl derivaties, and 2,3,6-trimethylnaphthalene were recorded by an Aminco-Keirs spectrophosphorimeter. The excitation source was a 150-W xenon lamp and the detector was an RCA 1P28 photomultiplier. In each case the spectrum was observed at room 5 temperature, about 25°C, in the pure liquid or in concentrated solutions, W.1H, in n-heptane. All of the compounds studied were first purified by vacuum distillation; with final purification step being preparatative gas chromatography. An Aerograph Autoprep Model A-700 gas chromatograph was employed in conjunction with a 20 f t. long by 3/8 in. diameter 20% Aplezon column. The average column temperature used was approximately 280°C. Only the top 507. of each band was collected. Fluorlmetric grade n-heptane obtained from Hartman-Leddon Company (Philadelphia, Pennsylvania) was used without further purification. A sampling of the spectra obtained are given in Figs. 2 and 3. All the other compounds have fluorescence spectra similar to these. The resulting excimer fluorescence energies are tabulated in Table I 2 along with the recently reported results of Birks for comparison. It is noted that although the absolute values are in general slightly lower than those obtained by Birks, the relative change which results on substitution agrees within about .02 ev. This Is considered to be within the experimental error of determination of the position of the broad excimer emission band. The one main exception is 1,5- dlmethylnaphthalene for which a substantially larger shift is observed 2 here than was reported by Birks. % In Table II we have summarized the other exclmer properties defined in Fig. 1. The energy of the lowest excited singlet state of the excimer relative to the energy of two unassociated unexcited monomers at infinite seperation is (hv +R). The relative change of £ excimer energy which occurs on substitution is A(hVg+R). The energy of excimer fluorescence and the excimer binding energy B, as well as the rate constants for formation and dissociation of the rate excimer, have also been determined for naphthalene and four of 9 its methyl derivatives by Selinger. Although the values given by 9 Selinger differ quantitatively from those given in Table II, the conclusions to be derived through the use of Table II later in this 9 section are not changed by the use of values given by Selinger . 3. Simple Molecular Exclton and Charge Resonance Concepts The lowering of energy AE due to exclton splitting is given by^
AE=M2/D 3 6
Figure 2. Fluorescence spectra of 1-methylnaphthalene. o . PURE UQUD ^ ------,5M M n-HEPTANE
20 22 24 26 28 30 32 WAVENUMBER (xIO-3) 8
Figure 3. Fluorescence spectra of 1,2-Dimethylnaphthalene. ,4 .. » , i . i, i — i i — * 00 S- (S> lO to C \J — A 1 IS N 3 1 N I TABLE I
ENERGY OF EXCIMER FLUORESCENCE
This Work Birks® Compound hv£b -A(hv£) u b -Mhv ) E (ev) (ev) (ev) (ev)
Naphthalene 3.129 — 3.168 I-Methylnaphthalene 3.067 .06 3.124 .04 2-Methylnaphthalene 3.060 .07 3.099 .07 1,2-Dime thylnaphthalene 3.030 .10 3.044 .12 1,3-Dimethylnaphthalene 3.052 .08 3.043 .07 1,4-Dimethylnaphthalene 3.016 .11 3.081 .09 1 ,S-Dimethylnaphthalene 3.023 .11 3.124 .04 1,6-Dimethylnaphthalene 3.038 .09 3.081 .08 1,7-Dime thylnaphthalene 3.030 .10 — 1,8-Dimethylnaphthalene 3.009 .12 3.050 .12 2}3-Dimethylnaphthalene 3.053 .08 3.087 .08 2,6-Dimethylnaphthalene 3.045 .08 3.068 .10 2,7-Dimethylnaphthalene 3.038 .09 3.050 .12 2,3,5-Trimethylnaphthalene 3.016 .11 3.093 .07 a) Aladekomo, J.B. and Birks, J.B ., Proc. Roy. Soc. (London), A284. 551 (1965). b) Franck-Condon maximum TABLE II
EXCIMER PROPERTIES
1T (a) HVE<» B(c) R
COMPARISON OF OBSERVED lL STABILIZATION AND *L EXCITON SPLITTING a a
*La Stabilization Observed -A(Observec$ D Predicted •A(Predicte<9 i (a) 2(b) Compound L M For D=3.5 For Dt=3.5 a V (hY R)
(ev) (Debye)2 (ev) (ev) <«> (ev) (ev) Naphthalene 4.3339 U .64 2.2 .16 1-Methylnaphthalene 4.2426 12 .63 .01 2.3 .17 -.01 2-Methylnaphthalene 4.3319 9.2 .74 -.10 2.0 .13 .03 1,2-Dimethylnaphthalene 4.2314 13 .69 -.05 2.3 .19 -.03 1,3-Dimethylnaphthalene 4.2525 14 .70 -.06 2.3 .20 -.04 1,4-Dimethylnaphthalene 4.1285 16 .54 .10 2.6 .23 -.07 I , 5-Dimethylnaphthalene 4.1533 16 .59 .05 2.6 .23 -.07 1,6-Dimethylnaphthalene 4.2599 14 .68 -.04 2.3 .20 -.04 I ,7-Dimethylnaphthalene 4.1285 12 .55 .09 2.4 .17 -.01 I , 8-Dimethylnaphthalene 4.2190 15 .51 .13 2.6 .22 -.06 2,3-Dimethylnaphthalene 4.2971 9.8 .66 -.02 2.1 .14 .02 2,6-Dimethylnaphthalene 4.3517 10 .79 -.15 2.0 .14 .02 2,7-Dimethylnaphthalene 4.3331 9.8 .72 -.08 2.0 .14 .02 2,3,5-Trimethylnaphthalene 4.1285 17 .71 .07 2.5 .14 .02 a) 0-0 Transition; Bailey, A. S., Bryant, K. C., Hancock, R. A., Morrell, S. H., and Smith, J. C., J* Inst. P etr.. 33, 503 (1945). b) Calculated from spectra given in reference a. TABLE IV
SIMPLE CHARGE RESONANCE ENERGIES
i<.) A I-A -A(I-A) Compound (ev) (ev) (ev) (ev)
Naphthalene 8.07 .65b/• 7.42 1-Me thylnaphthalene 7.88 .84 A 7.04 .38 2-Methylnaphthalene 7.90 . 82 7.08 .34 1 ,2-Dimethylnaphthalene 7.74 .98 /> 6.76 .66 1,3-Dimethylnaphthalene 7.77 .95r* 6.82 .60 1,4-Dimethylnaphthalene 7.72 1.00 A 6.72 .70 1,5-Dimethylnaphthalene 7.74 .98 A 6.76 .66 1,6-Dimethylnaphthalene 7.77 .95 A 6.82 .60 1,7-Dimethylnaphthalene 7.75 . 97 A 6.78 .64 1,8-Dimethylnaphthalene 7.67 1.05A 6.72 .80 2,3-Dimethylnaphthalene 7.85 .87 A 6.98 .43 2,6-Dimethylnaphthalene 7.74 .98 A* 6.76 .66 2,7-Dimethylnaphthalene 7.77 6.82 .60 *95c 2,3,5-Trimethylnaphthalene 7.66 1.06 6.60 .82 a) Aladekomo, J. B. and Birks, J. B., Proc. Roy.Soc. (London), A284. 551 (1965). b) Pople, J. A., J. Phys.Chem.. 61. 6 (1957). c) Calculated on basis of I+A=constant using the I and A values for naphthalene todetermine the constant. For discussion and justification of this rule see Hush, N.S. and Pople, J.A,, Trans. Faraday Soc.. 51. 600 (1954). 15
Figure 4. Plot of I-A versus the experimental excimer energies, hVR 4.0
3.9
1 3 .8
+ 3.7
JZ 3.6
3.5
31 . 17
It appears then that the charge resonance states must be seriously considered as being Involved In some way in the generation of exclmer states. However, the I - A values are higher than the observed exclmer energies by 2 or 3 ev. We therefore conclude that while there Is some evidence Implicating charge resonance states in the exclmer fluorescence phenomenon, there remains an energy disparity that cannot be accounted for by simple charge resonance theory. It has been shown that the exclmer energies predicted by molecular exciton theory are too small and that no correlation exist between the observed and predicted changes which occur on substitution. Charge resonance theory predicts exclmer energies which are too high but there is some evidence of correlation between the predicted and observed changes. It therefore seems reasonable that configurational mixing of molecular exciton states and charge resonance states might yield results which are in agreement with experiment. 4. Configuration Interaction Between Molecular Exciton States and Charge Resonance States The symmetry of naphthalene and the "relevant" symmetry of its alkyl substituted derivatives is (For definition of molecular axes and symmetries of molecular orbitals see Fig. 5). An exclmer composed of two identical molecules of symmetry in which the two molecules are parallel to each other with their principle axis coinciding, is itself of species. ^ ^ The symmetries of the two 1L exciton states are B„ and B- , respectively, and the symme- a , 2u 3g’ tries of the two L, exciton states are B_ and B_ , respectively, b ju zg the g state being lower in both cases. These states are designated as |b2 u (E x c )> , |B3g(Exc)>, |B3u(Exc)> and |B2g(Exc)> , respectively. The lowest energy charge resonance states are of B2u and B3g species; these states are designated |B2u(CR)) and |B3g(CR)). It is immediately seen that configuration interaction between |B3g(Exc)) and |B3g(CR)> and between |B2u(Exc)) and |b2u(CR)> is nonvanishing and can become important. Recently several theoretical calculations have taken consideration of such interactions for non-substituted aromatics of 6 7 8 symmetry. * ’ Here we adopt the attitu d es of one of these, that of Azumi, Armstrong, and McGlynn^ (hereafter referred to as AAM), and extend it to the case of alkyl substituted naphthalenes. 18
Figure 5. Definition of molecular axes and symmetries of the molecular orbitals of naphthalene. The group tables used were from Eyring, H., Walter, J. and Kimball, G., Quantum Chemistry. John Wiley and Sons, Inc., New York, 1957, p .189. DEFINITION OF MOLECULAR AXES
CO
Y
SYMMETRIES OF MOLECULAR ORBITALS
biu ------lowest vacant MO “sg— o-o— highest occupied MO 20
The energies of the excited exclmer states are given by secular 6 equations of the form
H -E H . -S .E as ab ab = 0 E H.b - s.bE “bb" where
H = < (Exc) | 5C |
^ <(CR) \X\ (CR) >
Hflb= < (Exc) | 3C| (CR) )
Sflb= <(Exc) | (CR) >.
It is convenient to calculate the energy with respect to the ground state energy of the exclmer. When this is done, H is the fld energy of the zero-order molecular exciton state, and is the energy of the zero-order charge resonance state. As in the preceding 1 2 3 section, these energies are roughly expressed as L ±M /D and I-A-C±A, fil respectively. The above equalities are rough approximations from a theoretical viewpoint; however, the use of the fu ll theoretical expres sions introduce only minor changes in the calculated exclmer energies. Hence, the above equalities are satisfactory for introduction into the secular equation. This computation procedure is referred to as g "Method A" by AAM. To be more specific we now set
H (B- )=LL -M2/D3 aa' 3g7 a H (B. )=l L +M2/D3. aa 2u a 10 The contribution of A is normally small and may be neglected. Therefore, we write
Hbb(B3B>=Hbb(B2u>=1 ' A ' C' 1 2 The values for Lfl, M , I, and A have been given in Tables III and IV. If simple Huckel molecular orbitals are used as basis molecular orbitals, differential overlap neglected within any one molecule, and overlap introduced only between those orbitals on adjacent atoms of different molecules, we find that S . is identical for all excimers 10 ab of D species. In addition, we assume that the charge distribution is not greatly effected on substitution and take C and Hflb to be the 21 same for a ll compounds. Therefore, the values of S . , H . , and C g AD AD calculated for naphthalene by AAM are used throughout. The Integrals S^, and C are functions of both Df the inter- molecular distance, and Z, the effective nuclear charge tc be used as an exponent in the Slater orbitals which are used as basis functions for atomic o rb itals. The value Z=3.18 has been found to yield the "best calculated values" for intramolecular quantities and hence Z=3.18 is used consistently in our evaluation of intramolecular 6 Integrals. We vary Z values only for those integrals which are over two molecules and it is this intemolecular variation of Z to which we refer when we speak of variation in Z values. The significance of the effect of variation in Z on the calculated excimer energies has been pointed out by AAM.^ Therefore, the excimer energies are calculated as a function of both Z and D. If we assume that the interplanar distances in exclmers should be the order of the separa tions in unexcited crystals (3 to 4$) of molecules which form excimers in the crystalline state (for example pyrene and perylene); then Z-3.18 appears to be the most appropriate Z for calculation of inter- molecular quantities. This is illustrated in Fig. 6 in which the calculated energies of several compounds at D=3.5 are plotted as a function of Z. In all cases at Z less than about 2.4 negative energies are obtained for B_ and only when Z^3.0 do we find what 3g might be considered agreement with experiment. These conclusions are identical to those of AAM.^ We shall therefore present the rest of our data for only Z=3.18 for both intemolecular and intramolecular quantities. 5. Energy of Excimer Fluorescence In Figs. 7 and 8 the calculated energies E for each compound is plotted against intermolecular distance D for Z fixed at 3.18. The higher and lower of the resultant of the B„ states are designated as + + and , respectively, and those of B^ are designated as B^ and B^u , respectively. The lowest excited singlet state of the excimer is B~ and hence i t is the state from which excimer fluo- 3g rescence must occur. The experimentally observed excimer energies (hv_+R) are shown by vertical arrows. The calculated results for the interplanar distances at which the calculated results give the observed 22
F igure 6 Energies of excimers calculated at D=3.5 as a function of effective nuclear charge, Z. 23
5 0 - 30
20 20
10
Z z
30
UJ
Z Z 24
Figure 7 Energies of excimers at Z=3.18 as a function of inter planar distance. The vertical arrow represents the excimer energy. 25
9 8 CO 05 M 0 0 "* 7 \ 6 « t , . v ■s. 5 4 f " 3 4 I % 2 I / 0- 9 o 5 h , 8’ b; a>» M 7 6 5- •s-O... 4 i m / 3 a* 2 b ; O I $ S‘4 & 6 ‘ f $ S*4 6 fe' I S'S *4 $"6' I ? S*4 a - r Q a 26
Figure 8 Energy of exclmers at Z=3.18 as a function of In te r planar distance. The vertical arrow represents the exclmer energy. P $ a ? ; e ' 1
I: > - • • • • * ■pr.i~.tr ™.3 irrni
Hi. y ' - ' ' k
*/ } J * * 1
1 \ \ 0 0 S5 PO* CO "5 * r
^ \ \ >--'•■■■' ^ V . l - 'prnttiiM s '------:; 111 , 4 A ! ' % / / 2 ^ / [ : 1 8 i , j : f ■ i */ '/ 01 i > - y M ft” ft 1 » V 4 ft f t1 i > ** 4 ft f t 1 *4 ft ft1 28 excimer energies are given In Table V. The rest of the calculated data is omitted for reasons of brevity. The coefficient a is the weighting factor of a molecular exciton state and the coefficient b is the weighting factor of a charge resonance state in the final configurationally mixed state function defined as K*'> - lv Exc)> +b lv c*» It is seen that the interplanar distances required in order to obtain absolute agreement between the calculated results and the experimental results are slightly greater for the excimers of substituted naph thalenes than for the naphthalene excimer. Furthermore, it is noted that the interplanar distances are largest in excimers of compounds in which there is one or more methyl group in the a positions of naphthalene. This behavior is to be anticipated from the previously mentioned effects observed for similar methyl substituted compounds (i.e. methyl 12 13 benzenes, 9-methylanthracene, and 9,10-dimethyl 1,2-benzantracenes 14). In these cases the inhibition of excimer fluorescence has been attributed to steric hindrance by substituents at ends of the inter- 13 acting dipoles which are responsible for dimerization. Since the interacting dipole lies along the 9,10 bond (i.e., short axis) in naphthalene, steric hindrance for excimer formation is expected to be
greater when there is a substituent in the a position which is adjacent to the end of the interacting dipole than when there is a substituent in the 8 position which is further away. In light of the order of steric hindrance predicted above, it therefore seems reasonable to expect the interplanar distances to be largest for excimers of a substituted naphthalenes, next to largest for excimers of 8 sub stituted naphthalenes and smallest for naphthalene. 6. Conclusions It is found that the energy of excimer fluorescence for naph thalene and its alkyl derivatives may not be interpreted using either simple molecular exciton or charge resonance concepts. There is considerable configuration interaction between molecular exciton and charge resonance states and the energy of excimers fluorescence may not be interpreted without invoking this configuration interaction. Ilf Iff fit f
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) l « f I l I i I I I i I I iiliiiiiiiilil
62 30
These conclusions are identical to those obtained for non-substituted aromatics.^*^*®’^ In addition, one of the same computational procedure used by Azuml, Armstrong, and McGlynn for non-substituted aromatics^ is found to give agreement with the experimental excimer energies for naphthalene and 13 of its alkyl derivatives using Z=3.18 and D between 3.45 and 3.7. The largest interplanar distances are predicted for the cases for which steric hindrance for excimer fluorescence would be expected to be largest.
B. DELAYED EXCIMER FLUORESCENCE OF PYRENE*
1« Introduction Delayed fluorescence and delayed excimer fluorescence are defined as emissions spectrally identical (both wavelength and intensity) to ordinary fluorescence and excimer fluorescence respectively, but possessing a much longer decay half-life. It has been proven experimentally that delayed fluorescence originates in triplet-triplet annihilation processes. However, there is conflicting evidence in 16 the case of delayed excimer fluorescence. Parker and Hatchard have shown that the delayed excimer fluorescence of pyrene observed in ethanol solution at room temperature originates in triplet-triplet annihilation processes. On the other hand, Azumi and McGlynn^ observed that the delayed excimer fluorescence intensity and the in tensity of the phosphorescence of pyrene in perturbed lsopentane matrices at 77°K were linearly related; this indicated an origin other than triplet-triplet annihilation for the delayed excimer fluorescence of pyrene in this latter medium. It is the purpose here to present further experimental data, and to resolve the apparent dilemma. 2. Results and Discussion We have observed simultaneously the fluorescence, excimer fluorescence, delayed fluorescence, delayed excimer fluorescence and phosphorescence of the same out-gassed concentrated solution of pyrene in liquid paraffin (Nujol) in the temperature range 0°C to -35°C.
♦Published in part in Chem. Phys.. 42. 4308 (1965). 31
At various temperatures throughout this range, and despite consider able variation in the intensities of the different luminescences, it was found that the intensity of delayed fluorescence emission, as well as the Intensity of delayed excimer fluorescence emission, was proportional to the square of the intensity of exciting light and to the square of the intensity of phosphorescence emission. A sampling of the relevent data are presented in Fig. 9 and Table VI. These data demonstrate conclusively the biexcitonic nature of both delayed exclmer fluorescence and delayed fluorescence in fluid media. In addition, it is observed that the lifetimes of the delayed fluorescence and delayed excimer fluorescence increase considerably with decrease of temperature, but that at any one temperature they are equal; it is also observed that they equal % the phosphorescence lifetime in those cases in which the phosphorescence may be observed. The lifetim e data are given in Table VII. These observations con- 18 clusively show that (1) the triplet state population is governed by f ir s t order decay mechanisms, and (2) the delayed fluorescence and delayed excimer fluorescences initiate in triplet-triplet annihilation (second order) processes. It is also noted that the lifetime and intensity of delayed excimer fluorescence are sen- 19 s itiv e ly dependent on the oxygen content of the solution; in view of the well-known oxygen quenching of triplet states, this might be adduced as further evidence for the triplet-triplet annihilative origin of the delayed excimer fluorescence. The lifetimes reported 19 In Table VII are not in disagreement with a value of 60 msec, for the delayed excimer fluorescence of pyrene in liquid parafin at -10°C, 19 or a value of 1.8 msec, for pyrene in ethanol at room temperature; these values can be rationalized with those of Table VII by invoking more efficient quenching of triplet states at lower viscosities. He have observed the excimer fluorescence and delayed excimer fluorescence of pyrene crystal at 77°K; we have not been able to detect fluorescence, delayed fluorescence, or phosphorescence of pyrene crystal at 77°K at the highest sensitivities available to us. The Intensity of delayed excimer fluorescence is shown to have an ^1.1±0.1 ,]epen(jence on the intensity of exciting light. This 32
Figure 9. The intensity of delayed fluorescence of pyrene in liquid solution and delayed exclmer fluorescence of pyrene in both liquid solution and in the crystalline state as a function of the relative number of quanta absorbed. The intensity of both delayed fluorescence and delayed excimer fluorescence in liquid solution show a square dependence on the relative number of qunata absorbed while the intensity of delayed excimer fluorescence of crystalline pyrene shows a linear dependence on the relative number of quanta absorbed. QUANTA EMTTTED (RELATIVE UNITS) 1000 100 UNA BOBD(EAIE UNITS) (RELATIVE ABSORBED QUANTA 10 EXCIMER - DELAYED ~
(SOLUTION) EXCIMER DELATED (CRYSTAL)
20
FLUORESCENCE DELAYED (SOLUTION) 40
60
80
100 33 TABLE VI
PROPORTIONALITY OF THE INTENSITY OF THE VARIOUS LUMINESCENCES OF PYRENE TO INCIDENT EXCITATION INTENSITY, I
Type of Solution Crystal Luminescence in Nujol
1 Fluorescence none observed 1 Excimer Fluorescence I 1 _1 Phosphorescence none observed _2 Delayed Fluorescence none observed ..2 jl.lisO.l Delayed Excimer Fluorescence TABLE VII
DECAY LIFETIMES OF LONG-LIVED LUMINESCENCES OF PYRENE
ueiayea Delayed Excimer Temperature Medium Fluorescence Fluorescence Phosphorescence
(msec.)______(msec.) (msec)
23 Nujol solution 3±0.2 3±0.2 none observed - 15 II 20±3 20±3 40±3 - 30 II 30 30 60 - 35 IT 50 50 100±10 -196 Crystal none observed 10 none observed
u> Ul 36
20 dependence approximates closely the linearity which is expected if the triplet-triplet annihilation is so efficient that it controls kinetically the triplet state population. The lifetime of the delayed excimer fluorescence is ~10 msec., which Is actually longer than the lowest value observed in solution under conditions where the triplet population is kinetically limited by first order pro cesses. It is thus necessary to suppose that (1) triplet-triplet annihilation efficiency increases upon crystallization, (2) first- order trip le t depopulative mechanisms decrease upon cooling and as a result (3) triplet populations in the pyrene crystal at 77°K are annihilation limited. The mechanism of delayed excimer fluorescence despite different intensity dependencies thus remains the same in the crystal as in solution. The previous observations^ of Azumi and McGlynn are now readily understood if it be assumed that their "solution" of pyrene in o isopentane at 77 K contained pyrene crystals. The lifetime of delated excimer fluorescence observed by them was 10 msec., identical to that of the crystal observed here. Consequently, their plot of delayed excimer fluorescence intensity versus phosphorescence Intensity was that of the Intensity of delayed excimer fluorescence of pyrene crystallites (more efficient that that of solution) versus the intensity of phosphorescence of pyrene in solution (since the crystal does not phosphoresce). As a consequence, the objections^ to the triplet-triplet annihilative origin of delayed exclmer fluo rescence are no longer valid, and the results obtained support those of the present work, as well as the present conclusions.
C. THE MECHANISM OF DELAYED EXCIMER FLUORESCENCE In the preceding section, I.B., we have shown that both delayed excimer fluorescence and delayed fluorescence originates in triplet- triplet annihilation. In this section we discuss the different pos sible mechanisms for delayed fluorescence and delayed excimer fluo rescence. 21 The mechanism of non-delayed excimer fluorescence is 37
2S_+hv A 0 fluorescence
(A) VS0
E-»2S_+hv . \L 0 exclmer
yEF = kEFkAC 1 'P F kF(kEF+kQE+kD> where is an excited singlet molecule, Sq is a ground state molecule, E is the excimer and cp /cp_ (Eqn. 1) is the Intensity of EF r excimer fluorescence divided by the intensity of fluorescence; where k__ is the rate constant for excimer fluorescence, k. is the EF A rate constant for association of an excited singlet molecule and a ground state molecule, k is the rate constant for fluorescence, kn_ F is the rate constant for internal quenching of excimer fluorescence, kp is the rate constant for dissociation of the excimer into an excited state singlet molecule and a ground state molecule, and C is the concentration. The mechanism of delayed fluorescence may b e ^
2Sn+hv . / * 0 excimer T^T^Evi%! | (B) \s'S, +s+S_-*2S_+hv-1 1 0 0 fluorescence
'PpEF kEF(1y kOF*kIS> kEFkAC k_k_ ^DF F D kFkD where T^ is a trip le t state molecule, and cppgp/cpjjj,(Equation 2) is the predicted intensity of delayed excimer fluorescence divided by 22 the intensity of delayed fluorescence. It might also be
2S +hvf ~ 0 fluorescence
Tj+T - S.+S' (C) 1 ° ^ E -•. 2SQ+hvexcimer 38
!DEF EF A 3 'Pdf kF^kEF+kQE+kD^
23 fiirks has shown that both the mechanisms B and C participate in the production of delayed fluorescence, and to an extent which in room temperature ethanolic solutions of pyrene is given by the ratio 2/1. T rip le t-trip le t annihilations which produce an excimer directly (mechanism B) are diffusion controlled and will decrease in im portance with increasing solvent viscosity. However, triplet- triplet annihilations which initially produce an excited monomer (mechanism C) have been observed to occur at large distances both 24 18 25 in crystals and in rigid matrices. * Therefore, at lower temperatures and higher viscosities mechanism C should increase in relative importance and ^Dpp/^Dp should approach a limiting value CpEF/cpF (coinPare equations 1 and 3). On the other hand, if mechanism B dominated, we should expect ^DEp/'PDp^pp/'Pp* ^ata on relative intensities graphed in Fig. 10 indicate that at lower temperatures mechanism C is dominant; mechanism B is shown to become operative at higher temperatures (lower viscosities), and to produce the positive deviation of cp^^/cp^ - Figure 10. Temperature dependence of the quantum yield ratio of delayed excimer fluorescence to delayed monomer fluorescence and the quantum yield ratio of non-delayed excimer fluorescence to non-delayed monomer fluorescence. QUANTUM YELP OF EXCIMER QUANTUM YIELD OF MONOMER II. HEAVY-AT CM SPIN-ORBITAL COUPLING EFFECT* 1. Introduction a) A primitive model of spin-orbital coupling. Let us, for simplicity, consider a one-electron atom. In the usual representation, the electron, while spinning on its own axis orbits the nucleus. Let us now affect a transformation to an electron-fixed coordinate system such as is shown in Fig. 11; relative to this electron-fixed system it is the nucleus which is orbiting the electron, and by virtue of the nuclear charge and acceleration, a (relativistic) magnetic field is produced and envelops the electron. The direction of this field is perpendicular to the plane of the orbit. Electron spin motion still occurs and, by virtue of some assumed asymmetry of charge d istrib u tio n on the electron, results in the production of a magnetic field along the electron spin-axis. It is the interaction of these two magnetic fields, the one due to relative nuclear motion, the other to electron spin motion, which is described as 'spin-orbital interaction or coupling1. The energy of the system will be determined by the relative orientations of spin axes and orbital angular momentum axes of the electron, and if several relative orientations are possible or allowed then a corresponding number of states of different energies will result. It is this effect which causes the multiplet states of atoms and molecules to divide into closely spaced sets of energy levels, giving rise to the so-called "fine structure" of atomic line spectra, and which is also causative of the multiplet splitting of some organic unsaturated molecules. *Published in part in Photochem. and Photoblolo. . _3, 269 (1964) and J^. Chem. Phys. . 39, 675 (1963). 41 42 Figure 11. Illustrating the transformation from a nuclear fixed coordinate center (left) to an electron fixed coordinate center (right). The interaction of the magnetic field produced by seeming nuclear orb ital motion (right) and that due to electron spin motion causes spin-orbital coupling. 43 44 A 'pure spin1 state is one in which the spin angular momentum is time independent. A transition between two such states of differ ent spin m u ltiplicity is accompanied by a change of spin momentum, which by reason of momentum conservation must appear or disappear elsewhere. But electric or magnetic n-pole moment operators are not functions of spin-coordinates, consequently the excess spin momentum may not appear in or disappear from the radiation field; since the original 'sharpness' of the spin quantum numbers are pre served in the radiation field, a transfer of the excess spin angular momentum to other types of angular momentum may not be induced. Such transitions may therefore not occur; they are forbidden. The perturbing effects of spin-orbital coupling, however, are such that it is no longer proper to consider the spin angular momentum to be time independent. Indeed the simple model presented above implies that angular momentum is being continually shuttled back and forth between the spin and orbital degrees of freedom. It is the total angular momentum, spin-pk's-orbital, which is now conserved, and the concept of a spin state with 'sharp' properties is not valid. If, however, the coupling of spin and orbital angular moments is weak the spin description may be retained and one may talk of 'nominal spin s ta te s ' where it is understood that any given spin state contains a small admixture of states of all other possible multiplicities. The situation for two states of two different multiplicities is illustrated in Fig. 12, whence it becomes evident that the spin allowedness of a transition between nominal singlet and trip le t states is actually due to the spin allowedness of transitions between two singlet states and between two trip le t states. The Intensity of the singlet triplet transition is then said to be 'stolen' from a triplet *—* triplet transition (or transitions) and a singlet *—> singlet transition (or transitions). This description is probably reasonably satisfactory for molecules containing no atoms heavier than nitrogen. The above model is undoubtedly oversimplified, and yet a number of statements and/or conclusions are warranted by it: (1) Spin-orbital coupling is a relativistic phenomenon, and the photochemistry and chemistry, biophysics and 45 Figure 12. Illustrating the effect of weak spin-orbital coupling on transitions between states of different multiplicities. The meaning of the symbols is as follows: F-spin for bidden; A-spin allowed; S-pure spin singlet state; T-pure spin triplet states; t-small amount of pure spin triplet states in a nominal singlet state; s-small amount of pure spin singlet state in a nominal triplet state. PURE SPM NOMNAL SPN STATES STATES T ! STRICTLY A7 f ! WEAK f SPN ORBITAL F ■FORBDCEN C0UPLING I S s t 47 biology which it occasions are relativistic also. (2) The larger the nuclear charge, the larger is the magnetic field produced by its seeming o rb ital motion and hence also the larger is the spin-orbital coupling. The spin- orbital coupling should therefore tend to be large in atoms towards the end of the periodic table and smaller In atoms towards the beginning of the periodic table. It is this trend in spin-orbital coupling which is largely but not wholly responsible for the occurrence of the electron isomers of helium: orthohelium and parahelium. (3) The closer the electron is to the nucleus, the larger must the seeming angular momentum of the nucleus be In order to counteract the larger coulombic forces. Hence, the larger is the spin-orbital coupling. It is to be expected then that spin-orbital coupling will be especially Important in atoms having fille d or nearly filled outer shells (rare gases and halogens, respectively) provided that they are reasonably heavy ( i.e . have reasonably large atomic number Z). (4) Electrons which have penetrating orbits, and which therefore 'see* more nuclear charge will be more strongly spin-orbitally coupled. (3) Attention is now directed to molecules. Observation of a singlet-triplet absorption requires some spin- orbital coupling. Significant singlet-triplet absorption intensity will then be facilitated by chemically affixing a heavy atom ( i.e . atom of large Z) to that region of the molecule within which the transition is localized. Transitory perturbation provided by collisiat with another molecule which contains a heavy atom, such as a solvent molecule, might be sufficient to enhance singlet triplet absorbtivity, as might any other perturbation which induces even a slight amount of charge-transfer from any of the optically combining states of the heavy atom solvent or matrix. 48 In our description it is seen that the spin-orbital interaction is due to the spin magnetic dipole Interacting with the relativistic orbital magnetic dipole. Since dipole-dlpole Interactions decrease 3 with distance r as 1/r , so also will spin-orbital coupling inter- 26 action energy, c, is given by e = (Ze2/2m2c2) ( l / r 3)*L»"? where Z is the nuclear charge, m and e are the electronic mass and charge respectively, c is the velocity of light and~£ and if are the orbital and spin angular momenta, respectively. The average value 3 26 of 1/r for a hydrogenic atom is found to be r ‘ 3=Z3/a 0n3(je+l)(jt+*)je where a^ is the Bohr radius and n and & are the principal and o rb ital angular momentum quantum numbers. Therefore, we find e = [z 4 /n 3 (£+l)(je+*)£] (e2 /m2 c2a03) T**S* which we rewrite as • * where § ^ is the spin o rb ital coupling constant of an electron with principal quantum number n and angular momentum quantum number i . The important points to note are: (1) The Z dependence (heavy-atom effect). 3 (2) Inverse dependence on n and on a cubic in & (screening and penetration effect). b) Heavy-atom effects in molecules If a heavy-atom is chemically affixed to a molecular skeleton there is reason to expect increased spin-orbital coupling. This increase should manifest itself by augmenting the probability of T^*— transitions as well as all other T*—'S processes, including intersystem crossing. The limiting parameters for the extent of spin-orbital coupling which occurs are (a) 5 t^ie heavy atom, and (b) the extent of penetration of the electrons of the molecular skelton into the field gradient of the heavy atom. All of these 27 associated effects are called "Internal Heavy-Atom Effects" 49 It has recently been shown that an environmental molecule which contains a heavy-atom such as ethyl bromide, propyl Iodide, etc. will cause spln-orbltal coupling In another molecule (e.g. benzene, 27 naphthalene, etc. ). This environmental phenomenon Is referred to 27 as an "External Heavy-Atom Effect," In contrast to the "Internal Heavy-Atom Effect" observed when the heavy-atom Is chemically affixed to the molecular skeleton whose intercombinational transitions It perturbs. The external heavy-atom spin-orbital coupling effect depends on the same factors as the internal heavy-atom effect al though factor b is now intermolecular. The external heavy-atom effect is of a magnitude comparable to that of the internal effect, and both 26 effects operating simultaneously are linear in the perturbations. 26 It has also been shown that the interaction prerequisite to spin mixing is probably donor-acceptor in character. The manifestations of the external spin-orbital coupling effect thus far investigated have been the enhancement of singlet-*triplet 28 29 30 (T^S-) absorption probability, * ’ and the lifetime of phos- 31 phorescent decay. The present work will concern itself with the effect of environmental molecules which contain heavy-atoms on the relative phosphorescence (cpp) and fluorescence ((pj) yields and phosphorescence decay (t ) of the solute naphthalene. It is thought that the data obtained are of some importance to studies of fluorescence quenching and intersystem crossing, that the magnitude of the variations displayed by the ratio cpp/(p£ an^ Tp will lead itself to utilization in analysis and, with optimization of the effects recorded here, that the chances of stimulated emission from the triplet state of an organic molecule may be increased somewhat. It must be noted that external heavy-atom effects on cp /cp, 32 ? have been observed by Robinson £ t a_l, the matrix being xenon, and 33 by Graham-Bryce and Corkhill, who used a glass containing ethyl iodide as the perturbing medium. The present experiments differ 32 33 from those of the authors mentioned * in that they are quan titative, but they are otherwise very similar to those of Graham- 33 Bryce and Corkhill. 50 2. Experimental All quantum yield and lifetim e measurements were performed on an Aminco-Keirs Spectrophosphorlmeter (SPM). The excitation source was a Xenon XB0-150 W Lamp (Osram) and the detector was a RCA 1P21 photomultiplier tube connected to an Electro-Instruments X-Y recorder. The wavelength error was approximately 2mp,. All the lifetimes reported are "first observable lifetimes", a term defined 31 elsewhere. The most probable error in any group of half-life measurements was 0.11 secs, while the average error was 0.009 secs. The better-resolved phosphorescence spectra were obtained using a Becquerel phosphoroscope and a Steinheil spectrograph; the dispersion of th is instrument was 80 X/mm at 5800& and 2oX/ mm at 4250&. A Beckmann DK spectrophotometer was used to monitor all solvent and solute pu rificatio n s. In the present work, the heavy- atom solvent composition was 3.2 EtOH: .8 MeOH: _1 propyl halide by volume. Absorption spectra of these solvents were run at 22°C, and are compared in Fig. 13;only the iodide solvent absorbs appreciably at 330mp,, the wavelength of maximum fluorescence intensity of naphthalene. It is indicated then that the fluorescence will be selectively absorbed (compared to phosphorescence) by the medium in the specific case of propyl iodide and that an increase of the ratio cpp/cpf will result for this reason alone. Correction for this spurious increase is not readily effected; however, it is simple to over-correct: one merely presumes that the volume of the medium has contracted at 77°K to % of its volume at 295°K, the extinction remaining the same as in Fig. 13, and that all of the luminesced light must traverse the to ta l internal width of the sample tube (2mm), and therefore made to experience absorption along a 2mm path. It is known from low temperature absorption measurements that the first statement above is an exaggeration, while the fact that emission and excitation beams are at 90° to each other renders the assumed path of 2mm much too large. Over-correction of a ratio cp /cp^ for selective absorption of fluorescence is then readily carried out and such grossly reduced t p ^ / t p ^ values are henceforth used* 51 Figure 13. Absorption Spectra of Halide Solvents at Room Temperature; 4-(4-Ethanol: 1-Methanol) : 1-Propyl Halide, by volume. 1. Propyl chloride alcoholic solvent 2. Propyl bromide alcoholic solvent 3. Propyl iodide alcoholic solvent 4. Alcoholic solvent 0 4 0 2 ABSORPTION (%) 300 WAVELENGTH (mMmierons) 0 0 4 52 It is for the same reason (i.e.,simplicity in making over-corrections that and cp^ values are calculated from phosphorescence and fluorescence intensities at their moat intense peaks (465 and 330m^, respectively). No correction is necessary for the wavelength dependence of detector sensitivity and instrument, since both cp^ and (p^ were determined by comparison of the intensity of phospho rescence and fluorescence in each case to the intensity of phos phorescence and fluorescence respectively of naphthalene in EPA. The absolute values are then calculated by comparing these relative values to the absolute quantum yield values for naphthalene of Gilmore et a l.^ * ^ In this work excitation was carried out at 300m|j.. Inspection of Fig. 13 immediately makes Important the competition between solvent and solute for excitation light; because of this the fluorescence yields (or phosphorescence yields) obtained in different media may not be rig id ly compared, even though otherwise they are s tr ic tly proportional to absolute quantum yield values. It is for this reason that some reluctance is evident in quoting absolute yields in different media of either phosphorescence or fluorescence values alone. 27 All solvents were purified as previously described. The naphthalene was an Eastman-Kodak product which had been extensively zone-refined and recrystallized. 3. Results and Discussion A sampling of the results obtained with naphthalene in various media is presented in Table VIII* Data on the internal heavy-atom effect ispresented for comparison. A graphic illustration of the effect is given in Fig. 14. It is possible to be more explicit. If we admit to a kinetic scheme which includes the first order rate constants as shown in Fig. 15 we may w rite: TABU V III QUANTUM YIELDS AND MEAN PHOSPHORESCENCE LIFETIMES OF ar-HALONAPHTHA- LENES AND NAPHTHALENE IN HEAVY-ATOH SOLVENTS External Heavy-Atom Effect Internal Heavy-Atom Effect Naphthalene t (sec) Compound T (sec) P *f ^P P *f YP in EM3 2.5 0.55 0.055 Naphthalene 2.5 0.55 0.055 PCI3 2.27 0.44 0.080 a-Fluoronaphthalene 1.5b 0.84b 0.056b PBr3 1.73 0.13 0.24 a-Chloronaphthalene °.29b 0.058b 0.30b -2 PI3 1.33 0.026 0.35 a -Bromonaphthalene 2x10 0.0016b 0.27b -3 a - Iodonaphthalene 2x10 J 0.0005b 0.38b a) EM=(4:I) EtOH, MeOH; PC1=(3.2:.8:1) EtOH, MeOH, Propyl Chloride; PBr=(3.2:.8:i) EtOB, MeOH, Propyl Bromide; PI (3.2:.8:1) EtOH, MeOH, Propyl Iodide. b) According to data of V. L. Ermolaev and K. K. Svitashev, Optics and Spec.. _7» 399 (1959). u 55 Figure. 14. Total Emission Spectra of Naphthalene in Propyl Halide- Alcoholic Solvents at 77°K; 4-(4-Ethanol : 1-Methanol) : 1-Propyl Halide. All Spectra are uncorrected Aminco Curves. Curve 3 should be reduced by factor of three (3). Excitation is at 300mp,. EM = Alcoholic Solvent PCI = Propyl chloride alcoholic solvent PBr = Propyl bromide alcoholic solvent PI - Propyl iodide alcoholic solvent pa PBr PI 400 500 600 W/VELEN6TH (mft) 57 Figure 15. Illustrating the first-order processes which connect the ground state, the lowest singlet state, and the lowest triplet state, is the fluorescence rate constant, is the rate constant for internal quenching of fluorescence, k is the rate constant for phosphorescence, k is the P * qp rate constant for internal quenching of phosphorescence, and k^g is the rate constant for intersystem crossing. 58 I------6 • I 37 If we now set k^sl.06 x 10 sec” as found by Kasha and Nauman, and If we presume, as seems reasonable, that k^ changes but little from one environment to another, we note that Eqs. 1 and 2 contain two observables and three unknowns. We further note that T for P the ethanol methanol glass is much the same as for EPA, and that we may accordingly adopt the values k =0.06 and k =0.34 sec. * 32 p qP as found by Robinson for EPA, for both glasses. We may now adopt two extreme attitudes: we may fixate k qp =0.34 sec. in all media, and thus maximize the variation in k, (cf. Eq. 2), or we may fixate -1 k = 0.06 sec. and thus maximize the increase of both k and k, ; p qp is we call these two extremes case A and B, respectively, and it seems that by this expedient we may at least obtain some idea of the relative effects of heavy-atoms on the rate constants k , k , J qp p’ and k^s which connect states of differing multiplicities. The necessary abstractions are carried out in Table IX, whence it will be observed from the bracketed numbers that the maximum increase of kp on going from the ethanol: methanol glass (EM) to the ethanol:methanol:propyl iodide glass (PI) is by a factor of 7, of k by a factor of 2, and of k, by a factor of 140; the minimal qp J * is J increases of k and k are by factors of unity (no change) and of p qp ^is a factor 21. It is necessary to conclude therefore that the process most sensitively affected by external heavy-atom perturbation is the intersystem crossing process. It is further of note that the values of cp^/cp^ used in construction of Table IX are "over corrected" and that as a consequence Table IX understates the evidence in favor of the high sensitivity of intersystem crossing. Table X is presented for a comparison of the internal and external heavy-atom effect. One may now utilize Eq. 3 and the data of Table IX to obtain the predictions for case A and case B. These are shown in Table XI. TABLE IX THE LIMITS OF VARIATION OF THE INTERSYSTEM CROSSING RATE CONSTANT CASE A3 CASE Bb -6 K K. X10 K KlsXl EM .06 (I)° .71 (1)° .34 (DC .71 (1)C PCI .10 (1.6) .85 (1.2) .38 (1.1) 1.4 (2) PBr .24 (4.0) 4.6 (6.5) .52 (1.5) 18. (25) PI .41 (6.8 14.6 (21) .69 (2) 100. (140) a) K =0.34 sec * in a ll four media qp -1 b) K =0.06 sec in a ll four media P c) Relative values are quoted in brackets TABLE X COMPARISON OF EXTERNAL AND INTERANL HEAVY-ATOM EFFECT INTERNAL HEAVY-ATOM EFFECT3 EXTERNAL HEAVY-ATOM EFFECT Compound k k . Naphthalene in k^g P is Naphthalene 1 1 EM 1 1 or-Fluoronaphthalene 1.6 2 a-Chloronaphthalene 8.6 1.5xl02 PCI 1.6 2 a -Bromonapht ha1ene 2.5xl02 5xl03 PBr 4.0 25 a-Iodonaphthalene 2.5xl03 3xl04 PI 6.8 1.4xl02 a) Data of V. L. Ermolaev and K. K. Svitashev, Optics and Spec.. 399 (1959); all values are relative. TABLE XI COMPARISON OF THE PREDICTIONS OF CASE A AND CASE B SOLVENT *f EM PCI PBr PI Exp. .55 .44 .13 .026 Case A .59 .55 .20 .068 Case B .59 .43 .06 .011 63 Despite the fact that some uncertainly attaches to these last experimental values the author feel that the actual situation approaches much more closely that of case (B) than of case (A). This predicts that the order of case (B) than of case (A). This predicts that the order of decreasing sensitivity to this per turbation is: intersystem crossing process (Sj- quenching process (T^-»Sq)> phosphorescence emission process (T^-*SQ) . It is strongly suggested by the data contained herein that heavy atom quenching of fluorescence can indeed be attributed primarily, if not entirely, to Increase in the probability of the Intersystem crossing process. These conclusions are contrary to those reached 41 by Siegel and Judelkis who concluded that is more sensitive to the nature of the solvent than are k and k. . It is difficult qp is to compare the relative validity of these conflicting results; however, Siegel and Judelkis used the total output of a PEK 500 high-pressure, compact mercury arc lamp for excitation; this high intensity source might have caused photoionization and it may very well be the source of disagreement. We have used the dispersed output of a 150 W Xenon at 300 mp, for excitation. 4. Phosphorescence Spectra The phosphorescence spectra of naphthalene in various media are given in Fig. 16. It seems evident that there is no significant self-absorption of the phosphorescence. The triplet state ex periences a red shift relative to the ground state as the spin- orbital coupling nature of the matrix is increased, which is en tirely to be expected if the perturbing singlet which mixes with the triplet of the aromatic lies higher in energy than the triplet 39 state. These results are in agreement with Tsubomura and Mulliken who presume that this singlet is the charge-transfer singlet state of a donor-acceptor complex; as far as the present resu lts are concerned, however, i t could be any higher energy sin g let. The 0,0 frequencies of the phosphorescence in different media are abstracted in Table XII. 1 40 It is noted that the L transition of naphthalene experiences d 64 Figure 16. The phosphorescence spectra of naphthalene, in descending order, in EP (diethyl ether and isopentane in a 1 : 1 v/v mixture), propyl chloride cracked glass, propyl bromide cracked glass, and propyl iodide cracked glass. Exposure times were in a ll cases about 20 min. Con clusions regarding quantum yields should not be deduced from this graph since a phosphorescope was interposed between spectrograph and sample, and the decay times of the samples were not iden tical. The dashed lines belong to traces of longer exposures, and are shown merely to validate some weak emission bands. All "cracked" glasses consisted of a 2:5 mole ratio solution of naphthalene and propyl halide. Kodak spectroscopic plates 103a-F(3) were used throughout. PLATE 90% 79% 90% • 0 % 19000 P000 19000 19000 tOOOO 81000 81000 tOOOO 19000 19000 P000 E UBR CM NUMBER, VE W 00 81000 10000 1 " 79% 90% • 90% 0 % 65 TABLE XII EFFECT OF MEDIUM OF THE FREQUENCY (0,0) OF PHOSPHORESCENCE Solvent (cm-1) Red shift (cm *) Ether-isopentane glass 21,335 ----- Propyl chloride cracked glass 21,280 55 Propyl bromide cracked glass 21,180 155 Propyl iodide cracked glass 21,010 325 67 a red shl£t of 101 cm * upon variation of aolvent from l-bromobutane to 1-lodobutane, and that this shift la of the same order of magnitude and in the same direction as that observed for naphthalene between the two cracked glasses containing propylbromide and propyliodide, respectively. It would appear then that the major portion of the red shifts observed In the various glassy media is due to simple electrostatic interaction with the solvent, and is attributable to the polarlzabllity of the triplet states. In other words, the red shifts are due at least in part to triplet-trlplet mixing, and it seems unlikely that any other than a small part of the observed shifts is due to solvent-chromophore interaction via spin-orbital coupling, i.e., through solvent Induced singlet-triplet mixing. 5. Conclusion In view of the limitations imposed by solvent absorbtlvity (see Fig. 13), naphthalene is hardly an ideal case for investigation. It is concluded that a solute species capable of significant excitation in the 4000-X region with fluorescence above th is wave length should be utilized, since thereby re-absorption of fluorescence by the solvent and competition by the solvent for the excitation light is minimized. It is suggested that use of heavy-atom solvents may facilitate the following: (1) A decrease in the lower concentration limit of phos- phorimetric detection, in certain cases, by a factor of 10 or greater would not be unexpected. (2) Stimulated emission from triplet states of suitable organics might be more feasible. The increasing rate of population of the triplet state, and the increasing emissive rate constant for phosphorescence which are associated with heavy-atom perturbations are both conducive to the onset of laser oscillations. However, the difficulties associated with linewidth (cf. Fig. 16) and the non-emissive phosphorescence rate constant (cf. Table IX) would, if anything, Increase; quality factor difficulties and the wasteful loss of pumping energy and 68 inversion capacity in absorption processes would still persist. (3) This last observation might however prove utilitarian for the study of l^T^ absorption processes of molecules which normally possess low triplet populations. (4) It is also suggested that study of trlplet-triplet energy transfer between like species in noncrystalline matrices or solutions would be more readily studied. (5) The blmolecular recombination of two excited triplets 20 to yield an excited singlet might be facilitated by heavy-atom techniques. The present work is semiquantitative in nature, but is sufficiently accurate to bear the deductive burdens imposed on it. I I I . THE ELECTRONIC SPECTRA OF FERROCENE 1. Introduction The nature of bonding in the ferrocene molecule has been of much concern to theoretical investigators, but progress has been considerably hampered by a lack of experimental information. The primary purpose of this chapter is to present a detailed absorption spectrum of ferrocene vapor in the region 17,000 to 55,000 cm The vapor spectrum of ferrocene discussed herein differs markedly from that of ferrocene in solution and should be of great interest to those concerned with energy levels and bonding in ferrocene as well as in other rr-bonded sandwich compounds. Ultraviolet solution spectra of ferrocene were also obtained at low temperatures by use of a specially constructed low temperature absorption cell which was mated to the Beckman DK Spectrophotometer. Solution absorption spectra of ferrocene were determined at 23°C, -78°C, -100°C, and - 196°C. 2. Experimental Ferrocene was obtained from Or. J. G. Traynham of this Laboratory and Arapahoe Chemicals, Inc. (Boulder, Colorado). It was purified by repeated vacuum sublimation; the final purification step in all cases was zone refining. Fluorimetric grade solvents obtained from Hartmann-Leddon Company (Philadelphia, Pennsylvania) were used without further purification. Ferrocene was sublimed into the vapor phase in a 10 cm. long high-temperature cell or in a 1 meter Cary gas cell and the vapor spectrum was recorded by the Cary Model 14 spectrophotometer. The temperature used in these determinations ranged from 23°C to 100°C. Samples of ferrocene in KBr gave the same infrared spectrum both 69 70 before and after heating to 150°C, thereby verifying that the bands present in the vapor spectrum were not those of a decomposition product. It has also been established in the literature that o 42 ferrocene vapor obeys the ideal gas law up to 400 C, which would indicate that a moderate temperature of 100°C should not cause it to decompose. Low temperature absorption spectra were obtained on a Beckman DK spectrophotometer using the special low temperature double path absorption cell described in the appendix. The solvent used for the low temperature work was EPA (5 parts diethyl ether, 5 parts isopentane and 2 parts ethyl alcohol) which forms a solid glass upon being cooled to liquid nitrogen temperatures. Baselines for the solvent were obtained on the spectrophotometer at all temperatures and the spectra of the samples were corrected accordingly. Absorption spectra of ferrocene were measured at -78°C, -110°C, and -196°C using the low temperature cell and the following cooling agents: dry ice-acetone mixture, methyl eyelohexane-liquid nitrogen mixture and liquid nitrogen, respectively. Spectra were not recorded beyond 42,(.30 cm ^ because of the reduced transmittance of EPA in this region at low temperature. 3. Results The vapor spectrum of ferrocene is shown in Fig. 17 and is to 43 be compared with the solution spectra of Scott and Becker shown in Fig. 18. The bands observed in the solution and vapor spectra are classified into nine systems as shown in Figs. 17 and 18. Table XIII contains a comparison of the solution and vapor spectra. The first long wavelength band, system I, observed in the solution spectrum is not detectable in the vapor spectrum. Systems II and III are not shown in the vapor spectrum of Fig. 17; they are indeed observable in the vapor spectrum, but only with considerable diff iculty because of their low extinction coefficient and no extra resolution relative to the solution spectrum has been obtained. The maxima of these two bands in the vapor are approximately at the same wavelength as they are in the solution spectra. 71 o Figure 17. Absorption spectra of ferrocene vapor at 35 C. The Roman numerals represent a classification of the bands observed In the vapor spectrum and those observed In the solution spectrum (Fig. 18) Into systems. Additional broad structureless bands are observed at 22,700 and 30,800 cm ^ with extinction coefficients of approximately 90 and 50 respectively. 30- 25- 2 0 - h X I A \ J •* V. 10- m/ENUMBER, cnr* x ICT 73 Figure 18. Absorption spectrum of ferrocene In lsopentane at 24°C. The Roman numerals represent a classification of the bands observed In the above spectrum and those observed the vapor spectrum of ferrocene (Fig. 17) Into systems. The spectrum Is taken from Scott, D.S. and Becker, R.S., J. Chem. Phys.. 35, 516 (1961). T— I I I 11 HI ' 1— I I I IMF Kf ■ « 1 ‘ 1 1 ■ I t i i l-Lll TABLE XIII COMPARISON OF THE ABSORPTION BANDS OF FERROCENE IN SOLUTION AND IN THE VAPOR PHASE ( a \ Solution Vapor Band Max. Band System (cm'1) e (cm"1) e I 18,900 7.8 not observed — II 22,700 102 22,700 (Max., no structure) '*'100 III 30,800 57.9 30,800 (Max., no structure) <100 IV 37,700 1,600 37,700 (Max., no structure) 2,000 V 41,600 3,500 40,506 (0,0 transition) 4,520 VI 42,181 (0,0 transition) 2,800 VII 46,933 (0,0 transition) 9,070 VIII 49,400 51,400 50,890 (0,0 transition) 32,700 IX 53,078 (0,0 transition) 26,000 a) Franck-Condon maximum, D.R. Scott and R. S. Becker, of Chem. Phys. . 35. 1516 (1961). 76 The solution spectra of ferrocene in EPA in the temperature range 23°C to -196°C are shown in Fig. 19 and the reaults are tab ulated in Table XIV. 4. Discussion The first system (system I, Table XIII and Fig. 18) of low intensity which occurs at 18,400 cm * and which is not observed in our work, has been assigned as a T «-Sn transition by Scott and 43 Becker. Their assignment is based on its low extinction co efficient) c=7.8, a slight external and Internal heavy-atom enhancement effect, and an apparent emission from this state with a mean lifetim e of approximately 2 sec. We have not been able to confirm this system since efforts to observe this band in hydro carbon and heavy-atom solvents have been unsuccessful. In addition, we have not observed the reported emission by Scott and Becker. System II (max. at 22,700 cm S , as well as Systems I II (max. at 30,800 cm *") and IV (max. at 37,700 cm ^), occur in the vapor spectrum little changed in appearance from that in the solution spectrum. However, the remainder of the absorption systems in the vapor spectrum show considerable vibrational structure. The reason for this is not immediately clear; however, there is consider able evidence that there is either a free or slightly hindered 44 internal rotation of the rings about the principal axis. The energy of the 3d ligand field bands of iron are extremely sensitive to the position of the rings; hence, rotation of the rings relative to one another should tend to induce a diffusiveness of the 3d levels of the iron atom of ferrocene and any electronic transitions involving these levels should also be diffuse with no vibrational structure expected. However, the energies of the ring orbitals are relatively insensitive to position of the metal and therefore transitions involving primarily ring orbitals should be effected much less by this hindered internal rotation of the rings and might be expected to show considerable vibrational structuring. It is therfore concluded that System II, as well as Systems III and IV, must in some way involve the 3d levels of iron while System V, 77 Figure 19. The temperature dependence of the spectrum of ferrocene. All spectra were measured in the same solution of ferrocene in EPA. X,A 5000 4000 3000 23 *C ■73 •110 103 right tcolo loft 801- 60h M/ 40- 20 20 40 44 WA/ENUMBER, crrr* x KT3 TABLE XIV EFFECT OF TEMPERATURE ON SYSTEMS II AND III IN FERROCENE System Temperature Band Maximum e C°c) (cm-1) II 23 22,700 90.1 II -78 23,200 80.7 II -110 23,400 80.0 II -196 23,700 78.8 III 23 30,700 49.4 III -78 30,700 48.5 III -110 30,800 47.9 III -196 30,900 46.8 - 4 \ 0 80 VI, VII, VIII, and IX probably involve transitions between ring molecular o rb itals. System II shows a sh ift from 22,700 to 23,700 cm**, a narrowing of the band, and a slight decrease of intensity on going from room temperature to liquid nitrogen temperature (see Fig. 19 and Table XIV). This is exactly what is expected if this transition be a relatively pure symmetry forbidden d-d transtlon which obtains Intensity because of some vibrational distortion of 43 45 the molecular geometry. * In addition, System II is insensitive to solvent effects and relatively insensitive to substitution on the ring. Therefore System II is assigned as a relatively pure 3d-3d transition which is highly localized on the iron atom. This 43 assignment Is in agreement with that of Scott and Becker. System III (30,800 cm *), as previously mentioned, is similar to System II In that it also shows no vibrational structure in the vapor spectrum; hence, it must involve the 3d orbitals of iron in some way. However, unlike System II, System I I I shows l i t t l e change in either intensity or wavelength when the temperature is lowered (Fig. 19 and Table XIV). Therefore, this band cannot be a 3d-3d 46 43 tran sitio n . Data of Weinmayr, and of Scott and Becker on the ultraviolet spectra of substituted ferrocenes indicate that System III is more sensitive to substitution on the cyclopentadienyl rings than is System II. System III is therefore probably a forbidden transition of the type 3d-ring MO. System IV (37,700 cm *) was f ir s t reported as a shoulder in 43 the solution spectra by Scott and Becker. This band is confirmed by the spectra of ferrocene at -196°C in EPA (Fig. 19). At this temperature this band is completely resolved. This band is also confirmed by the vapor spectrum in which System IV appears as a broad structureless band which has a structured System V super- 43 imposed on top of it. It is reported by Scott and Becker that System IV, as well as System V, undergo a red sh ift and intensi fication upon substitution of various groups on the rings. However, most of the derivatives referred to contain substituted groups such as phenyl, p-nitrophenyl, etc. which themselves show absorption in this region. Therefore, little significance is placed on the 81 position of the band maximum which occur In this region of most of the derivatives studied. In light of the structureless form of System IV in the vapor spectrum, an apparently greater, but similar, sensitivity to substitution on the rings than System III and its medium intensity; System IV is probably also a transition of the type 3d-ring MO but which involves more ring character than System III, and which may be allowed. The remainder of the electronic absorption bands present in the ferrocene vapor spectrum have a considerable amount of vi brational fine structure associated with them. Vibrational analysis of these bands are given in Tables XV, XVI, XVII, XVIII and XIX. These analysis should be viewed with caution since in general they are not unique. The primary reason for this is the error Involved in determining band maxima. In addition observed intervals are compared with the frequencies found in the infrared and Raman spectra 47 of ferrocene by Lippincott and Nelson. These are ground state frequencies and it is possible that they may be changed somewhat In the excited states. With this word of caution we discuss the analysis of each system below. Since a band has been found in the Infrared spectrum of 170 cm } many hot bands arising from this vibrational level might be expected in the absorption spectrum. A brief calculation of KT energy shows that about four-tenths of the molecules might be found in this upper vibrational level at room temperature. This point was considered in assigning the 0-0 transition of band System V since the intensities of the first two vibrational bands are very weak compared to the rest of the system. However, neither was considered temperature dependent enough to have arisen from hot bands. System V can be analyzed, for the most part, in terms of frequencies (see Table XV). The intensity of System V a fte r the broad structureless System IV is subtracted out is small relative to Systems VIII and IX, and hence it may be a forbidden electronic transition which has become allowed through vibrational coupling. If this be the case, then in 48 order to be allowed in the X,Y direction TABLE XV THE VIBRATIONAL FREQUENCIES AND VIBRATIONAL ANALYSIS OF SYSTEM V (a) v Av Intensity' 7 Assignment Species Identification (cm"1) (cm"1) 40,506 0 VW (o,o) ----- 40,687 181 VW 170 Ring metal ring bending Elu 41,005 499 S 492 Antisym. ring t i l t Elu 41,123 617 M Unassigned 41,178 668 M Unassigned 41,276 770 M 303+492=795 Sym. ring metal stretch + Antisym. ring t i l t V Elu-Elu 41,361 855 M 38&f478=866 Sym. ring t i l t + Antisym. ring metal stretch Elg+A2u=Elu 41,515 1009 CH bending1* U 1002 Elu 41,846 1340 VW 1411 Antisym. CC stretch Elu a) VW=Very Weak, W=Weak, M=Medium, S=Strong b) Of doubtful assignment since CH vibrations are not expected to couple. N)CD TABLE XVI THE VIBRATIONAL FREQUENCIES AND VIBRATIONAL ANALYSIS OF SYSTEM VI t & Assignment Species Identification (cm-1) (cm-1) _ 42,181±36 0 (0,0) ------42,522±18 333 303 Sym. ring metal stretch Ailg 42,942±38 753 804 Ai Sym. CH bending lg 43,352±60 1163 1108 Ai Sym. ring bending lg a) Of doubtful assignment since CH vibrations are not expected to couple. uoo TABLE XVII THE VIBRATIONAL FREQUENCIES AND VIBRATIONAL ANALYSIS OF SYSTEM VII V Av Assignment Species Identification (cm'1) (cm ) _ 46,933±50 0 (0,0) ----- 47,199 266 170 Rind metal ring bending Elu 490 492 Antisym ring t i l t 47,423±25 Elu 47,740 807 388+478=866 Sym. ring tilt + Antisym. ring metal stretch Elg+A2u=Elu 47,969 1,031 1002 CH bending8 Elu 48,296 1,360 1411 Antisym. CC stretch Elu 48,598 1,665 1411+303=1714 A, +E =E Antisym. CC stretch + Sym. ring metal stretch lg lu lu 49,391 2,458 1408+1108=2516 Sym. CC stretch + Antisym. ring breathing Elg+A2u-Elu 49,661±50 2,728 Unassigned 50,025 3,092 3075 CH stretching 8 Elu a) Of doubtful assignment since CH vibrations are not expected to couple. 00 ■p- TABLE XVIII THE VIBRATIONAL FREQUENCIES AND VIBRATIOHAL ANALYSIS OF SYSTEM V III V Av Assignment Species Identification (cm"1) (cm ) (cm'1) 50,890±25 — (0,0) 51,177 287 303 A Sym. ring metal stretch lg 51,440 550 303x2-606 Sym. ring metal stretch Ailg* O 51,733 843 303x3=909 Sym. ring metal stretch lgO 51,948 1,058 1105 A, Sym. ring breathing lg 52,164 1,274 303x4=1,212 A, Sym. ring metal stretch lgO 52,410 1,520 303x5=1,515 An Sym. ring metal stretch lgO 52,742 1,852 303x6=1,818 A, Sym. ring metal stretch lg TABLE XIX THE VIBRATIONAL FREQUENCIES AND VIBRATIONAL ANALYSIS OF SYSTEM IX V Av Assignment Species Identification (cm-1) (cm ) (cm"1) 53,078±25 — (0 ,0 ) 53,390 312 303 Sym* Ring Metal Stretch Ailg 53,669 591 2x303=606 A, Sym. Ring Metal Stretch lg 87 ESrvib ^ Alg A, x E. x E, = A, + A_ + E„ ; lg lulu lg 2g 2g or in order Co be allowed in the Z direction A, x A. x E. ^ E, . lg 2u lu lg Therefore if the analysis given be correct the excited state must belong to one of the following representations: ^2g’ ^2g* or E, . System VI is also of low intensity and is probably also for bidden. It can be analyzed in terms of A^ (Table XVI) frequencies and thus the excited state is predicted to be of either the E^^ or A^u representation. After the t a il of System VIII is subtracted from System VII, System VII is of comparable in ten sity to System V (Fig. 17) and hence may also be forbidden. System VII can also be analyzed (Table XVII), for the most part, in terms of E^u frequencies. The predicted representations of the excited state are thus the same as for System V, i .e . , A, , A„ , E. , or E. . lg 2g 2g lg Systems VIII and IX are of sufficient intensity to be con sidered allowed, (Fig. 17). System VIII can be analyzed in terms of an A^ 303 cm * (see Table XVIII) vibrational progression and an A- 1105 cm"* vibrational frequency. System IX can also be analyzed -1 using a progression of the A^ 303 cm vibration (Table XIX). These systems should therefore involve transition of a similar type, and the predicted possible representation for the excited states of Systems VIII and IX are the same as System VI, i .e ., A. or E. . 2u lu 5. Conclusions The vapor spectrum of ferrocene shows a total of 8 slnglet- singlet (S^*-Sn) transition. The transitions at 42,181 cm *, -1 -1 47,288 cm , and 53,078 cm are reported for the first time. The 88 band at 22,700 cm-1 la assigned as a highly localized ligand field 3d-3d tran sitio n and the bands at 30,800 cm 1 and 37,700 cm 1 are shown to be transitions probably of the type 3d-ring M0. Vibrational analysis of the rest of the bands in the vapor spectrum are made and it is predicted that the excited state of the bands at 40,506 and 46,933 cm"1 belongs to one of the following representations: A. , E_ , or E. , and that the excited state of the bands at 42,181, *8 *8 „ i ^8 . 50,890 cm and 53,078 cm belongs to either E^u or A^u representa tion. APPENDIX. A LOW TEMPERATURE DOUBLE PATH ABSORPTION CELL* 1. Introduction The Importance of low temperature absorption spectroscopy has been recognized for years, but relatively little work has been reported In the field because of the experimental difficulties 49 - 55 which have existed. Recently, several authors have attempted to remove some of these experimental problems by designing low temperature absorption cells for various spectrographs. Described here is a low temperature absorption cell designed especially for Beckman DK, DK-1, and DK-2 recording spectrophotometers and which has been in use in these laboratories for approximately four years. It has been found that th is cell removes some of the problems associated with this field and offers some distinct advantages over previous designs. Visible and ultraviolet absorption spectra at room temperature are in general broad band spectra with little, if any, structure. This is primarily due to: (1) thermal motion of the molecules and associated perturbation effects which include collisional broadening, the Doppler effect, and random variations of internal Stark and Zeeman effects; (2) thermal population of the higher vibrational levels of the ground state and (3) natural band widths. By lowering the temperature of the sanple, one should observe several important effects. Among these are: additional resolution, 56* 58 59 a greater possibility of locating the 0,0 transition, a 58 . 59 greater ease of classification of bands, changes in intensity with temperature, specific thermochromic effects (hot bands, etc.) 60-61 photochromic e ffe c ts .*** Some of these effects have been * Published in part in Rev. Scientific Inst.. 33. 1367 (1962). 89 90 studied in this laboratory. 2. Description of the Apparatus The detailed construction of the apparatus is shovm in Figs. 20 and 21. The cell consists of a detachable inner and outer chamber separated by an evaculated region for insulation. Both compartments are machined from solid brass in order to eliminate as many soldered joints as possible, which is an advantage over previous designs. Brass was chosen for the construction material because of ease of machining and the relatively high thermal conductivity which it possesses. The optical axis of the inner compartment is located with respect to that of the outer compartment by the mating surfaces (see a of Fig. 20) of the flanges of the two chambers, and by two pins (@, Fig. 20) which are attached to the top flange and fit into corresponding holes in the lower flange. A vacuum seal is secured between the two flanges by means of a rubber or teflon 0-rlng gasket (Y, Fig. 20). With a vacuum of .1 to .4 mm pressure in the outer chamber (6, Fig. 20) the cell may be used for several hours with little, if any, condensation of water on the outside of the cell. However, for short experiments of ~20 minutes, 3 to 5 mm of pressure provides adequate insulation. To preserve the vacuum seal it was found necessary to circulate water within the hollow flanges (e, Fig. 20) in order to prevent ice formation between them. For short experiments 0—20 minute duration) or temperatures above -80°C no circulation was found necessary. In practice the water circulation through the flanges may be halted for short periods of time if necessary; however, if this is done it is advisable to purge the hollow flanges with air to remove all water in them and thereby prevent ice formation which would prohibit restarting of water flow. To ensure that no vapor condenses on the outside windows (|j,, Fig. 20) while spectra are being obtained, these windows are swept with a stream of dry air (T|, Fig. 20). The inner window assembly (\, Fig. 20) is shown enlarged in Figure 20. Schematic of the low temperature cell. C* Ttemfh • LMktafOnM Cm) TIirwfhA W#*SM»Vtow C«t Tlrw^C1 tf rrewi 93 In Fig. 21. The quartz windows (D, Fig. 21) are contained between an Indium gasket (E, Fig. 21) and Belleville washer (B, Fig. 21) which not only makes a good seal over a wide range of temperatures 62 but greatly reduces breakage due to expansion and contraction. This window arrangement Is a distinct advantage over other cells. The quartz windows may be used almost Indefinitely once a good seal is obtained. A range of coolants may be utilized since the coolant is not in the light path. A partial list of slush baths or coolants is given in Table XX and were chosen to give a range of temperature between room temperature and that of liquid nitrogen. It is also possible to use the cell up to about 100°C with the use of water, or oil, and a heater which may be Immersed directly in the water, or o il, in the heating chamber. A path length of about 2 cm is employed in both the sample and reference cell. However, most solvents show reduced trans mission at liquid nitrogen temperatures due to dissolved gas which forms "micro" bubbles when the solution becomes rigid. This causes an increase in light scattering which becomes increasingly important as one goes to shorter wavelengths. Therefore, the cell path length could possibly be reduced to some advantage. 3. Experimental Technique When spectra of liquid solutions are to be run, the experi mental techniques required to use the apparatus are very simple. The two parts of the cell are assembled and water is started cir culating aroung the flanges. A vacuum is then obtained between the two compartments by means of a high speed vacuum pump. It is ad visable to have a cold trap in the vacuum line to prevent any vapor from the pump from diffusing into the cell and condensing on the inner windows. After the desired pressure is obtained for insulation the vacuum is secured by means of a stopcock and the pump disconnected. In this arrangement the cell is completely portable. The sample and reference solutions are then introduced into their respective chambexs and the coolant is introduced into the inner ccmpartment. After 94 Figure 21. The inner window assembly of the cell. The quartz window (D) is contained between the Indium washer (£) and a Belleville washer (B). The aluminum washer (C) aids in distributing the force applied by the Belleville washer over the surface of the window. (F) is the cell and (A) is a screw cap to hold the window in place. 93 A B D * * * □ M n o 1 _ □ /L UQ r t t C E 96 TABLE XX SOME POSSIBLE COOLING SYSTEMS FOR USE WITH THE LOW TEMPERATURE CELL* Coolant Temp. °C. Liquid Nitrogen -196 Liquid Air -190 Liquid Methane -161 Solid Isopentane - Liquid Isopentane -160 Liquid Carbon Tetrafluoride (Freon 14) -128 Liquid Ethylene -103 Solid Methylcyclohexane-Liquid Methylcyclohexane -100 Liquid Ethane - 88 Liquid Nitrous Oxide - 88 Solid Carbon Dioxide - Acetone - 78 Liquid Propylene - 47 Liquid Propane - 42 Liquid Methyl Chloride - 24 a) A method of obtaining a continuous range of temperatures has been proposed by W.C. Neely, Chemstrand Research Center, Inc., private communication. The coolant reservoir is made pressure tight and connected to a vacuum and pressure system in such a manner that the absolute pressure may be varied from ~1 mm to ~100.00 psi. By using only two components as coolants, dry ice and liquid N_, and the above limits of pressure variation, a temperature range of -216 C to -48°C, with a 39 deg. missing gap between -135 C and -174 C is possible. 97 allowing adequate time for attainment of thermal equilibrium the apparatus is placed in the spectrograph and the spectra are re corded. This same procedure may be used for the formation of rigid glass solutions down to about -100°C, and also for solutions which become rigid 10° or less above the desired experimental temperature, whatever that temperature be. For work at liquid nitrogen a (5:1) mixture of isopentane, methylcyclohexane and a (3:1) mixture of propylether, Isopentane have been found to form a glass within 10°C of liquid nitrogen temperature, and hence may be used with the above procedure. The technique involving the formation of most other rigid solutions at liquid nitrogen temperatures in the cell is considerably more difficult than that given above; however, reproducible glasses may be obtained by u tiliz in g the following method. Water circulation is begun in the flange of the inner compartment and this chamber is then partially immersed in liquid nitrogen. While this chamber is cooling, the sample and reference compartment contained therein are flushed with dry nitrogen gas to prevent any condensation of vapor in them. After thermal equilibrium has been obtained the sample and reference solutions are introduced into their respective compartments. Rigid glasses are allowed to form while the inner compartment is s t i l l p a rtia lly immersed in liquid nitrogen. After these glasses are obtained, liquid nitrogen is then slowly added to the inside of the inner compartment. The apparatus may then be assembled and the same procedure used as previously described. For temperature measurements a thermocouple probe may be inserted into the sample solution ju st above the light path. To use the apparatus with the Beckman DK, DK-1, or DK-2 spectrophotometer the cover of the sample compartment and the sample cell holder of the instrument is removed. The low temperature cell is then placed on the floor of the sample compartment and lined up with the optical axis of the spectrophotometer. An auxiliary sample compartment cover is used to eliminate stray lig h t. 98 4. Glasses Since the advent of low temperature emission and absorption spectroscopy a large number of solvents which form rigid solutions or glasses when cooled have been reported. A list of a large number of these is given in Table XXI. This table is Intended to be as complete as possible a reference to the large variety of glasses which one may use at various temperatures. If no temperature is listed beside the glass it may be presumed to be good to liquid nitrogen temperatures. As mentioned before a (5:1) mixture of isopentane and methyleyelohexane has been found to form a rigid solution at -192°C, only about 4°C above liquid nitrogen temperature. In addition, it Is relatively strain free. Therefore, it should be of special value for any study which requires a glass with little or no strain, such as studies of the polarization of emission and absorption spectra. 99 TABLE XXI LOU TEMPERATURE GLASSES Refer- Refer Type T>1* roc* Com|mdtkm 1. Bode odd (wad at room ii mo ii iiid 12. Bade • (2:4:4) Methyl hydmdne, methyfemine, 2. Add b SoModc add (fnod Im p at b a d -4 0 * la trimcthybmine -«0*C) U . Bode d (12:10:6:1) Diethybthcr, forpcnlane, S. Add Phodihndc add (naml (mat at b a d —Ml* ethanol, pyrhHne M -B 0*C ) 14. Rtber r (1:5:12) n-Nulyl ether, dantienteyl ether, 4. Add (4:11:1) TdBonmacttfc add, methyl- methyl ether amine, Iriethibmln* (aond between 15. Rtber n (1:1:1) Diphenyl ether, 1.1-dlpheayle- -110* to-196*C) tbane Iriphenybncihane d K t b e n l 16. Rtber d J-Mrthyltetrahydnduran 4. AfechaBc d fnemi)-! at 17. Rtber b ,d (1:1) (1:2) Diethybther. (Mpcntane T. AfcohoKc d lnbgpanal* JO. Ether t Di-n-pni|tyl ether •. AbohoKc d l-Balanol* 19. Rtber t (1:1) tmtpyl ether, (s*|icnlanc 9. A k d d k l,|.h (4:1), (5:1). (1:2). and (1:9) Ethanol, 40. El her 1 (1:1) propyl ether, ntclhylcyclnhetane met banal 41. Riher d (2:1) Dielnyt ether, prntenr-2 (rill- 10. (1:7) fmnu|iyl alcohol, (repoaleno |ieatene-2 (f n u ) II. (2:1:1) r.ihawii, bomHuw. ethyl ether 42. Ilalide (1:2:2) Rlhyl habile, faipenlane, ethyl (UFA)* ether 12. Alcnhntte “A)|ihanol 79“-commerriaBy avaibbh 4J. Halide r (1:4:7:7) Rlhyl bromide, methyfeyclo- mixture id |*(mary abohob heaone, iiiotrnlane methybyclnpcntane 11. AboboBc (7:1) /jujienlaae, n-batyl abohol 44. Halide n (16:4:1) Riband, methand ethyBodide 14. Alcoholic (6:2) fn m n ian r, o- or iropropyi abohol 45. Halide u (16:4:1) Rthood, methand, pmpyMndbfe 15. AknknUc (1:1:1) Ether, it* octane, w « n a d 44. Halide a (16:4:1) Riband, methand, abolod iim)iybhlntbb 14. A k d d k (1:1:1) TUber, fre-octane, rtbyi abnhd 47. Halide a (16:4:1) Rthand, methand, IT. A battoir (1:1) Fiber, fea-propyi abohol pmtivllirnmide IB. AbohoNc (1:1) Rtber, ethanol* 4B. Halide d (6:6:2:2) Diethybthcr, bapentane, 19. (1:1:1) lie-octane, meihyi cycbh»aaae. fthamd, 1 -chhtmna|>lilhalcne 49. Hydrocarbon IMnbyhientane 20. AfeohoHc (2:l:lT1l)felhy1 ether, tnb enr, ethanol 10. Hydrocarbon V (1:2) 1 Methvlpenlane, bepentane 21. Aleohnlie (2:1:1) fm pmpyl ...... abohol, la^iaalano, SI. Hydrocarbon w Paraffin d) (Nujtd) (food (mm at bad ether -76* to -tO TI 22. AbohoBc (2:1) Profionnl, ether 12, Hydrocarbon e (1:4 to 6:4) /Japenlane, methylryclo- 21. AbohoNc (2:1) Balanol, ether heiane (MPII) 24. Akwhobe (ityreml (aood (mm at bad -7B* 11. Hydrocarbon a (1:1) /npentane, methyteydnhcaane -90*C) 14. Hydrocarbon r (M ) Melhvlcyclnpetilnn*, methydcyeb- 21. Alcoholic Sunar (oord at room temperature) 24. Alcoholic d (12:10:4:1) Diethiidher, (Mpentaae, 51. Hydrocarbon d Penlene-l (chd-Prnlene-l (Iran*) dimethyHnrmamtife, ft bond (Mixture) 27. At) nano* 0,0 (1:1) Water, pmpybne jrtycnt (aood 16. Hydrocarbon, between -» * to -40T) ■ (1:1) Pmpane, pm|icne HftnM* 24. A q w o o 1H«1 (2:1) Rthybnettlycnl, orator (anod between -BO* to -IS0*C) 57. Hydrocarbon, aa (2:9:9) /r^tmpylhenicne, |irn|tane, 29. Oodc r (2:1:1) Trimcthybmine, bapewtane, nqtridr |>m(>ene ethyl ether IB. Alcohol. hb (2:9:9) Propyl alcohd, pmpane, pm|icne 10. Bode a (1:1:1)'j Trbthybndne, inpeQlan*. ethyl Hqttidr ether 19. Badc.Bqoidr cc (2:9:9) TWrupmpylamlne, propane, 11. b Trbthaool amine (aood (ram —Ml* to pmpcne —BD*C) 40. Ether, Mrptidr dd (2:4:4) IMprapyl ether, pmpane, propene ■ O. P. Kvaaa, N om e l b . «T ( | M » > ._____ Ham. )r, p ndaip roWrmaf aa Aaabntral rtamMry aid •O. N. lent* w iD . UeSo j . fheai. **r. M. 1*0(1*41). • 1.1. Grahaat-Bryct awt J . M. t'wkMI Ketai* 144 *M (IM*. theel*. U alw rdlr nf WaaMaefoa. VatlW. WaeMaelaa • t>. a. 6c*tt aid J. aVAIniia. I. Pfcya. Cheat.44. Ml neat). » vdf i n n aal i m*Im i dew m llnQ. • M. Kadi*, daarheiter Laeteiaa. H aarhM rr. KaalaaO (Iff II. • a. Pliw dal m S l. c TEm . I .tr l. Im i. 44114 (l*U>—fmalraHiM latalata • 6. tlid, j. H. t inOaC id W. Datmre, NatarelVl, trll (IM*). • J. CmhaUa. G. brieetrO. W. Herre. a a l It. GUrr, Z. Ktmractwm. 41. 1171 ir. O. L. Haaiadch. O. M. ItaaOe. awO A. C. K. loaOetn | . Cl •G . 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VITA Fred Jewel Smith was born March 15, 1939 in Brookhaven, M ississippi. Following graduation from Forrest County Agriculture High School, Brooklyn, Mississippi in June, 1957, he attended the University of Southern Mississippi receiving his B. A. in chemistry in June, 1960. In September, 1960, he entered the Graduate School of Louisiana State University where he is presently a candidate for the degree of Doctor of Philosophy. He is married to the former Phyllis Love Soard and has one child, T e rrill Lee. 104 EXAMINATION AND THESIS REPOBT Candidate: Fred Jewel Smith Major Field: chemistry Title of Theda: Studies In Molecular Spectroscopy; I. Excimer Fluorescence, II. Heavy-Atom Spin-Orbital Coupling Effect and III. The Electronic Spectra of Ferrocene Approved: M ajor P ro! Chairman Dean of die Graduate School EXAMINING COMMITTEE: e > . ______ Date of Examination: j uiy 28 1965