Deuterium Isotope Effect in the Radiative Triplet Decay of Heavy Atom Substituted Aromatic Molecules
J. Friedrich, J. Vogel, W. Windhager, and F. Dörr
Institut für Physikalische und Theoretische Chemie der Technischen Universität München, D-8000 München 2, Germany
(Z. Naturforsch. 31a, 61-70 [1976] ; received December 6, 1975)
We studied the effect of deuteration on the radiative decay of the triplet sublevels of naph- thalene and some halogenated derivatives. We found that the influence of deuteration is much more pronounced in the heavy atom substituted than in the parent hydrocarbons. The strongest change upon deuteration is in the radiative decay of the out-of-plane polarized spin state Tx. These findings are consistently related to a second order Herzberg-Teller (HT) spin-orbit coupling. Though we found only a small influence of deuteration on the total radiative rate in naphthalene, a signi- ficantly larger effect is observed in the rate of the 00-transition of the phosphorescence. This result is discussed in terms of a change of the overlap integral of the vibrational groundstates of Tx and S0 upon deuteration.
I. Introduction presence of a halogene tends to destroy the selective spin-orbit coupling of the individual triplet sub- Deuterium (d) substitution has proved to be a levels, which is originally present in the parent powerful tool in the investigation of the decay hydrocarbon. If the interpretation via higher order mechanisms of excited states The change in life- HT-coupling is correct, then a heavy atom must time on deuteration provides information on the have a great influence on the radiative d-isotope electronic relaxation processes in large molecules. effect. Besides this, deuteration must strongly affect Much work has been done concerning the nonradia- the 00-rate of the phosphorescence. Moreover, the tive decay. In 1972 Johnson and Ziegler2 found polarized phosphorescence excitation and emission that in the case of benzene, deuteration likewise spectra are expected to be sensitive to d-substitution. affects the radiative decay rate. This radiative d-iso- It is the aim of this paper to investigate the in- tope effect was interpreted in terms of vibronic fluence of d-substitution on the radiative rates of the interaction in the orbitally forbidden T1-S0-transition triplet sublevels of naphthalene in the presence of 3 4 of benzene. Later on, Lim and coworkers ' repre- heavy atoms in order to obtain further insight into sented a quantitative study of the luminescence of the various decay mechanisms of the lowest triplet aromatic hydrocarbons in an organic glass matrix. state in aromatic molecules. They found that the radiative d-isotope effect is a common feature in aromatic molecules, and that it may arise even in cases where the phosphorescence II. Experimental transition is not orbitally forbidden. Fischer and The experimental setup for measuring the degree Lim interpreted those observations in terms of a of polarization was described elsewhere 6. The band- non-adiabatic coupling 5. passes used were about 50 cm-1 in the emission In a recent study of the influence of a heavy atom region (in the case of naphthalene 100 cm"1) and on the various triplet decay mechanisms, Friedrich, about 100 cm-1 (naphthalene 200 cm-1) in the ab- Metz and Dörr proposed a connection between the sorption region. heavy atom effect and the d-isotope effect via a To determine the quantum yield, the spectral re- sponse of the detector side of the spectrometer was second order HT-coupling mechanism6'7. By ob- corrected by use of a tungsten band lamp (Osram). serving the polarization of the phosphorescence 00- The spectral emissivity of tungsten was taken from transition of haloaromatics, they found that the reference8. To produce corrected emission spectra automatically, a procedure was used originally em- ployed by Grüneis et al.9: The output of the lock-in Reprint requests to Dr. J. Friedrich, Institut für Physika- lische und Theoretische Chemie der Technischen Univer- amplifier (PAR mod 129) at a particular wave sität München, D-8000 München 2, Arcisstraße 21. number was multiplied by the corresponding cor- 62 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay reclion factor by use of PROM's (Programmable equations Read Only Memories). The fluorescence quantum kr = vl (1 - T ; v> =
Table 1. Quantum yields and rate constants of naphthalene and its derivatives. PG(O.)'La) and PG(0,i'Lb) represent the degree of polarization in the La and Lb-transition respectively, taken with respect to the 00-band of the phosphorescence. fcr°>'P and A"r0'0P denote the in-plane (ip) and the out-of-plane (op) polarized radiative rate constants of the lowest triplet to the vibrationless ground state of S0 . The errors in the various rates of naphthalene are somewhat larger due to the uncertainty in the determination of 0isC •
1,4-dichloronaphthalene 2,7-dibromonaphthalene 2,7-diiodonaphthalene naphthalene
h6 d8 K d« h6 dB h8 d8
0F (±10%) 0.04 0.04 0.0009 0.0008 io-4 10~4 0.45 0.41 0ISO = 1-0F 0.96 0.96 1 1 1 1 0.55 0.59 (±0.1%) (±5%) (±4%) 0p (10%) 0.25 0.85 0.51 1 0.64 1 0.039 0.34 (±11%) (±11%) 0,,o 0.05 0.05 0.04 0.04 0.045 0.05 0.08 0.07 0p- <±10%> (±11%) (±11%)
PG(O.VLa) -0.2 -0.2 -0.31 -0.315 -0.29 -0.29 -0.32 -0.32 (±2%) PG (0, J'Lb) -0.175 -0.205 -0.17 -0.2 -0.13 -0.16 -0.29 -0.29 (±2%) [sec"1] 9.1 1.8 200 59 1560 555 0.43 0.049 (±3%) 2 2 kr [sec-1] 2.4 1.6 102 59 1000 555 3.05X10- 2.9X10- (±10%) (±12%) (±12%) Ay [sec-1] 0.12 0.08 4.1 2.36 45 28 2.54X10"3 2.14X 10~3 (±10%) (±12%) (±12%)
fcr°>°P [sec"1] 1.8 X 10 2 0.96 X 10 ~ 2 8.1X10 -1 3.4X10 1 10.6 5.75 1.3X10-4 l.ixio-4 (±11%) (±12%) (±12%) 3 A-ro.°P [sec"1] 0.1 0.07 3.3 2.0 34,4 23.3 2.4X10"3 2.03 X 10~ (±11%) (±12%) (±12%) 63 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay
III. Results of polarization the influence of deuteration on the individual radiative rates from Tz and T!/ or T* In this section we summarize the principle find- (see Fig. 6). A few facts are obvious: ings from our results (see Tables 1 and 2). a) In naphthalene-d8 and -h8 there is no differ- ence in the degree of polarization when excited Table 2. Ratios of rate constants of the protonated and either in the Li,- or L;l-state. From this observation deuterated compounds. it follows that there is no different deuterium effect in the in-plane and in the out-of-plane polarized 2 = 3 V ©
II libromo - ithalen e 11 b) In the halonaphthalenes, especially in the case & & 1—1 — IM C 5 3.4 2.8 8.8 zation upon deuteration with respect to Li,-excitation *(D) * ' (see Figure 5). MH) + 1.05 14%) 1.5 1.7 1.8 On the other hand, La-excitation produces no sig- (±17%) MD) nificant difference in the polarization of the phos- kT° (H) 14%) 1.5 1.75 1.6 phorescence 00-transitions of the protonated and - l kr° (D) T f i- o deuterated species respectively. /cr°.'P(H) 1.2 (±15%) 1.88 2.44 1.9 (±17%) /cr°.'P(D) From this behaviour we conclude that there is a A-r°>°P (H) 1.2 much more pronounced d-effect in the in-plane than (±,15%) 1.45 1.63 1.48 (±17%) /cr°'°P (D) in the out-of-plane rate. How can we explain the influence of deuteration on the radiative decay of the triplet substates in a) The total decay rate of the lowest triplet state heavy atom substituted aromatic molecules? is much less influenced by deuteration in the case of the haloaromatics than in naphthalene itself. IV. Discussion b) The radiative rate, however, is much more affected in the heavy atom containing molecules 1. Sources of the radiative d-isotope effect than in the parent hydrocarbon. In naphthalene the There are several interactions which may in- ratio A:r(H)/A;r(D) deviates little from unity (about fluence the decay of the lowest triplet state and 5%), whereas, for example, in 2,7-I-naphthalene, we which depend on d-isotope substitution. To arrive have a change in this ratio upon deuteration by at an explanation of the experimental facts, we con- about 80%. That is, in the presence of a heavy atom, sider these interactions and discuss their relative the d-isotope effect in the total radiative rate in- magnitudes. creases. c) The 00-rate changes in the same way upon The hyperfine interaction (HFI) deuteration as does the total radiative rate in the case of the halogen substituted molecules. However, As early as the influence of deuteration on triplet even in naphthalene itself we observe a remarkable lifetimes was discovered, the HFI was considered to influence of deuteration on the 00-rate (nearly 20%). be a possible explanation of the observed facts 15. This is a large enhancement as compared with a Though it is nowadays well understood that the change of 5% in the total radiative rate. main influence of d-substitution is via the Franck- Looking now to Fig. 5 we see that deuteration in- Condon (FC)-factor which governs the radiationless deed causes a significant change in the polarization rate, it is nevertheless true, that the coupling between of the phosphorescence. From the change in the the electronic and nuclear spin may in principle degree of polarization we can determine the in- dränge the rates from the triplet sublevels. There is fluence of d-substitution on the in-plane- and on the a twofold influence of the HFI on the triplet state: out-of-plane polarized rate using the photoselection On the one hand it mixes the zero field wavefunc- equation 14. Since the different directions of polari- tions, thus changing the properties of an individual zation originate from the radiative decay of different substate; on the other hand, it can mix singlets and triplet sublevels we can determine from the degree triplets. 64 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay However, there are several reasons which allow lation and the decay of the lowest triplet state in us to rule out the HFI as a possible source of the aromatic molecules. Though this mechanism is in- radiative isotope effect: From ODMR-spectroscopy deed strongly dependent on isotope substitution, we know the HFI splitting to be in the MHz-region there is in the meantime much theoretical18,19 and while the spin orbit splitting is in the GHz region. experimental20 evidence available which indicates Thus HFI can not successfully compete with SOC. that this mechanism is of minor importance in triplet This result is also confirmed by calculations of decay. Another argument against non-adiabatic Colpa et al. and Stehlik et al.16-17. spin-orbit coupling arises from our own experimen- tal results: From the polarized excitation spectra Moreover HFI fails to explain the overall trend in (Figs. 1—5) and from Table 2 we see that there is our experiment: since it does not depend on the a correlation between heavy atom and deuterium nuclear charge this interaction can not account for isotope-effect in the 00-rate of the phosphorescence. the observed influence of the heavy atoms on the As already stated in Sect. Ill it is the T^-rate which d-effect. is predominantly influenced by deuteration. From symmetry arguments, however, it follows that the T^-rate in the 00-transition (in-plane polarized) can- The non-adiabatic interaction not be affected by non-adiabatic spin-orbit inter- The non-adiabatic interaction was in the past action via one center integrals at the heavy center. often considered to be responsible for vibronic spin- Thus we are left with the HT-interaction to explain orbit coupling and therefore to govern the popu- our results. u> 3 103cm"1 Fig. 1. Phosphorescence P and absorption spectra A with the corresponding degree of polarization of naphthalene-h8 and -d8 in an EtOH-glass matrix at 77 K. The band passes used are shown in the figure. 65 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay 15 20 30 40 15 20 30 LO 103cm_1 Fig. 2. Phosphorescence P and absorption spectra A with the corresponding degree of polarization of 1,4-dichloronaph- thalene-h6 and -d6 in an EtOH-glass matrix at 77 K. The band passes used are shown in the figure. The H T-1 n t er ac t i o n The contribution due to the totally symmetric modes are of minor importance compared with the The a-component (a = x,y,z) of the transition zero order term (ca. 10%). We therefore neglect moment of a sublevel TXT is given by their contribution to the isotope effect. T 1 T (S0 | #so I Tj X Tj 1 Ta | TX ) MJ&X ({operator for spin-orbit coupling. The influence affect only the overall radiative rate, the influence of the normal vibrations on the transition moment on the 00-rate being very weak. The question is, are taken into account by an expansion around the whether or not the promoting modes in the first equilibrium position {()} = 0 of the molecule: order mechanism are CH-vibrations, which would lead to a d-isotope effect. We try to answer this Ml?, Ml&AQ}) =MI?Ti({ 0})+2 3 SoT, 'QP question by looking at the polarized phosphores- p Qr = 0 cence emission spectra. There is a large difference 32 in the polarization of the vibronic bands in the case + 2 MIfTl QPQP+... (5) 2 dQp3Q, QP,QP-=0 p.p' of naphthalene and naphthalene-d8, but there is In this expansion we only took into account the only a very small difference in the case of the cor- variation with the out-of-plane promoting modes responding halogenated derivatives (see Figs. 1 —4). {(?*>} which yield one center spin-orbit terms. That is, in naphthalene itself there is a considerable 66 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay i V/Vv i IV Hk 1 IP n n r / , ifi 1t II' p / II V\ I « 1 II • 1 1 1 1 j / 1 / 1 1 \ / i 1 i y ! 1 J I j ll'jl 1 ri V 1 j in i l- pjyJ 1 I / ¥ y / 1 / 4 i fr p A P 1 A / i / \ / i \ 03- 03- 02- 02- PP(L b>\' PP (Lb)/"' \ * \PGt0J- \PG 0.1 - \1 \ s 1 ! 1 i 1 J \ 'ca •oa h6 V > 1 d6 \ \ {! * 0- l -0.1- -02- PP(La) A/ l -Q3- l -03- 15 20 30 40 15 20 30 40 » 103cm-1 Fig. 3. Phosphorescence P and absorption spectra A with the corresponding degree of polarization of 2,7-dibromonaphtha- lene-h6 and -d6 in an EtOH-glass matrix at 77 K. The band passes used are shown in the figure. amount of CH-modes in the lst-order HT-mechanism, Since in the spectra we observe a large activity of but in the heavy atom containing molecules, there is the in-plane-polarized component with increasing only a very small influence of CH-vibrations in the nuclear charge at the heavy center, we only take into lst-order HT-coupling. This observation is consis- account those terms which lead to a one center spin- tent with theoretical results6: It is an out-of-plane orbit contribution at the halogen. The only term vibration of that C-atom which is the nearest neigh- with this property is of the type * bour to the heavy substituent, which accounts for the most important contribution. < r Vs|#so| äTT V*rXv*r I ra | t/V); oQp d(Jp nd Thus the 2 -order HT-interaction remains as the The change of the vibrational ground only explanation for the radiative d-isotope effect in state overlap integral on deuteration the case of heavy atom substituted aromatic mole- Though there is no influence of deuteration via cules. the vibrational overlap integrals in the total radia- There are many terms such as * Very recently there appeared a paper by Kanamaru and Lim 21 on the d-isotope effect in pyrazine in which they considered the derivative of the matrix elements of the < VS Itfso I VTXYT | ra | ^T ) , dipole operator to be the dominant source. Though this mechanism may be effective in the singlet-singlet absorp- 3 i 3 tion and in the radiative triplet decay in the case of < q^-VSI^SOIvt) (v*rk«|yT > » nearby mr* and ^r7r*-states in N-heterocyclics, it seems to fail in explaining our results. The reason ist that a term like (Hso)d2/dQp2 (ra) cannot provide one center 2 9 spin-orbit-terms if the intermediate states are of JZJI*- 1 I / I / Hb / I 1 I'll a ' \ 1 I -ft Ii \\ 1 1 1 VI i * I / 1 /i . 1 \Ll 1 1 1 I i > 1 1/ ! i ' 1W ! Ü 1 1 1 1 I. fi / r l'\' i / 1 / / 1 / p i A < P A / i I / / i a, 1 Q3- 0,3- k r \ r^ 1 \ i rS -s /I , s 1 i 1 1 \ Q2- II ' j 02- 1 1 | PG t 0,1 - J J \Jl l \ 1 JpGtai- 1 j j y OO 00 d6 he V 0- 1 oj 11 1 -0,1 - PP(Lb)' vvv PPlLfal J v-y 1 -ai ^ -03- / ^ -03- A/ 15 20 30 LO 15 20 30 LO 103cm-1 PPlLa) PP(La) Fig. 4. Phosphorescence P and absorption spectra A with the corresponding degree of polarization of 2,7-diiodonaphtha- lene-h6 and -d6 in an EtOH-glass matrix at 77 K. The polarization spectra are taken with respect to the marked (|) tran- sitions. The band passes used are shown in the figure. tive rate, there may arise sudi an influence in the given by 22 00-rate. The rate in the 00-transition of the phosphores- cence is given by where gt is a parameter depending on the equilib- rium shift of the modes t: ^-SlUsjMI&J^)!*. T,A Kt 2 _ 2 Since we expand Ml'„x, only in terms of the promot- 9i A -01 frequency change in the excited state, and taking the OO 2 24 PG parameters gt from , we find a ratio CI £r°(D)/&r°(H)^0.6. -02 Though this result does not agree with our experi- ment very well, it shows nevertheless that the change in the overlap integral may indeed provide a large a/. contribution to the radiative isotope effect in the I i i \ i , , I 1 1 1 1 1 i -03 00-transition. Let us now consider the polarized excitation spectra. There is no difference in the degree of po- larization between the protonated and the deuterated -01 species in the 00-transitions when excited into either txf \ „ PG the Lh or the La-band. The wavefunction of the La- state transforms like Z?iu and thus the corresponding v / \ \ \ A v * \ transition is z-polarized. From Fig. 6 we see that \\ -02 \\ 1 / there is no z-polarized component in the phospho- 1 / / rescence which could be changed upon deuteration. V\\ / This situation prevails in both the protonated and \\ !/ / 1iV/ / ' the deuterated species. 1 1 1 1 ,// 1 1 1 i i i r -03 On the other hand the Lb-transition (Bou) has a parallel polarized component in the phosphorescence emission (from T*). Though this is small, it may in principle be influenced by deuteration via 2nd-order Fig. 5. The polarized excitations spectra of the protonated HT-coupling. Because, however, the polarization ( ) compared to the deuterated ( ) compounds. does not change, we conclude that the radiative Tx- decay in the phosphorescence 00-band is mainly 2. The radiative isotope effect in naphthalene governed by three center spin-orbit integrals which are not sensitive to deuteration, and (or) by vibronic 23 From experimental work we know that about spin-orbit interaction due to C-C-vibrations. There- 90% of the total radiative rate comes from the T^- fore we relate the d-isotope-effect in the 00-band of substate (see Fig. 6) due to one-center spin-orbit the T1-S0-emission to a change in the overlap factor. coupling between the n n* and o JI* states. That is, In this case it is clear that there is no different iso- in the parent hydrocarbon, the matrix elements at tope influence in the in-plane and out-of-plane rate. the equilibrium position of the molecule [Eq. (5) ] are large compared with the vibronic spin-orbit coupling mechanisms. The weak in-plane polarized 3. Radiative deuterium effect and heavy atom effect component is partly due to the decay of the Testate via three-center spin-orbit integrals, and partly due The situation is quite different in the case of the to the vibrational influence on the transition matrix halonaphthalenes. element. Because the total in-plane component is In the first place, there is a great difference of the small, we expect upon deuteration only a small d-isotope-effect in the triplet lifetimes. From experi- change ( < 10%) in the total radiative rate. Looking mental and theoretical work (for a review see 1) we at Table 2, we see that this is indeed the case; the know that the main influence of deuteration in aro- rate decreases by 5% upon deuteration. Discussing matic hydrocarbons is upon the radiationless rate the 00-rate, however, we find a significantly larger via the FC-factor. Since there is no reason to assume d-effect than in the total radiative rate. Since, as that the electronic matrix elements in the radiation- argued, vibronic interaction (i. e. the 2nd-order HT- less decay are changed less efficiently by the heavy spin-orbit coupling) is very small, the d-effect must atom than the matrix elements in the radiative arise from the difference in the overlap integral be- decay, we argue that the FC-factor decreases upon tween the vibrational ground-states. Assuming no halogen substitution. Thus in the halonaphthalenes 69 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay the non-radiative processes are diminished compared cies is always less than that of the protonated spe- with the radiative processes and the quantum yield cies. This is because the 2nd-order HT-spin-orbit becomes larger. As a consequence the d-effect on coupling is reduced by deuteration, thus the relative the triplet lifetime is much smaller than in naphtha- extent of the out-of-plane polarization which is less lene itself. The situation is quite different, if we sensitive to deuteration, increases. consider the radiative rates. On the other hand, there is no difference in the There is a very large influence of deuteration in 00-transition of the La-band of the protonated and the presence of a heavy atom. While there is only a the corresponding deuterated molecules. This is due 5% change in the radiative rate of naphthalene, we to the fact that the La-transition obviously has no get, for example in 2,7-iodonaphthalene a change parallel component in the phosphorescence (the of 80%. Nearly the same situation holds for the corresponding transition is forbidden, as is shown 00-rate. From this fact we conclude that the isotope in Fig. 6). Thus no vibronic coupling mechanism effect in the 00-transition is mainly due to a vibronic can influence the relative direction between the La coupling mechanism and not to a change in the over- band and the 00-transition of the phosphorescence. lap integral. Moreover we see that even in the total radiative rate this mechanism must be the 2nd-order T 2 HT-coupling since deuteration has no different effect y on the 00-rate and on the total radiative rate. This T statement is consistent with the argument we used in Sect. IV 1, concerning the magnitude of the first order HT-coupling. Let us now compare the deuterium effect in the in-plane (Tz) and in the out-of-plane rates (Ty or forbidden T2). We see that in the Br- and I-derivative the ratio V,ip(H)/A;r0'ip(D) is considerably larger than &r°'op (H) /£r°'op (D). To explain this fact, we re- member that the out-of-plane polarized decay from the T^- or T2-substate is governed by large matrix elements at the equilibrium position of the mole- cule. Higher order HT-coupling surely is of minor importance, though possible. Thus the isotope effect in the T^ (or T2) decay should be due to both a relatively small influence of second order HT-spin- orbit coupling and to a change in the zero point overlap integrals. However, the in-plane polarized decay at {()} = 0 is governed only by very small three-center spin- orbit integrals in which there is no influence of the Fig. 6. Schematic level diagram and choice of axes. heavy atom because the nuclear charge is shielded to a high degree. Therefore these equilibrium terms V. Summary are dominated to a high degree by vibronic spin- orbit mechanisms which are sensitive to heavy atom In this paper we investigated the influence of deu- substitution6. An immediate experimental conse- teration upon the radiative decay of the Tj-substates quence of these conclusions is that there should be in naphthalene and some halogenated derivatives. a change in the degree of polarization upon deute- The relation between deuterium isotope and heavy ration, if the promoting modes are CH-vibrations. atom effects we found is consistently related to a Looking at the polarized excitation spectra (all second order HT-spin-orbit coupling. The isotope are taken with respect to the phosphorescence 00- effect in the phosphorescence 00-transition of naph- transition), we see that the degree of polarization thalene, in which vibronic spin-orbit coupling is with respect to Lb-excitation of the deuterated spe- small, is interpreted in terms of a change of the 70 J. Friedrich et al. • Deuterium Isotope Effect in a Radiative Triplet Decay overlap integral of the zero point vibrations upon 3. A clear dependence of the polarization of the deuteration. There are a few outstanding points con- phosphorescence upon deuteration. cerning the d-isotope-effect in heavy atom substi- tuted aromatic molecules: We find 1. A large deuterium effect in the 00-transition of Acknowledgement the phosphorescence. The authors wish to thank Dr. F. Metz and Dr. 2. A very strong influence of deuteration on the S. Schneider for many stimulating discussions. — radiative decay of the out-of-plane polarized spin The financial support by the Deutsche Forschungs- substate T*. gemeinschaft is gratefully acknowledged. 1 E. W. Schlag, S. Schneider, and S. F. Fischer, Ann. Rev. 12a B. Bossenbroek, D. C. Sanders, H. M. Curry, and H. Phys. Chem. 22, 465 [1971]. Schechter, J. Amer. Chem. Soc. 91 (2), 371 [1969]. 2 P. M. Johnson and L. Ziegler, J. Chem. Phys. 56, 2169 13 J. L. Garnett, M. A. Long, and R. F. W. Vining, J. Amer. [1972]. Chem. Soc. 94(16), 5913 [1972]. 3 R. Li and E. C. Lim, J. Chem. Phys. 57, 605 [1972]. 14 J. Czekalla, W. Liptay, and E. Döllefeld, Ber. 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