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 = <I>vl (1 - T ; v> = <V/ (1 - * • yield, was obtained from the corrected emission The triplet lifetime r was measured by using a phase spectra using well established methods10. As a modulation technique. The exciting beam was reference standard we used diphenylanthracene in chopped at a suitable frequency and the phase shift EPA solution, the quantum yield of which is very of the emitted light was determined with the lock-in near to unity n'lla. amplifier. For those molecules which had a lifetime The phosphorescence quantum yield, &y>, was longer than the reciprocal cut-off frequency of the determined assuming that internal conversion from lock-in amplifier (5 cps) we used a shutter and a the first excited singlet state is negligible. Because of ary-recorder (HP 7045 A). the high energy gap, this approximation should All results were obtained in an elhanol matrix at hold very well in the case of the molecules con- 77 K. The dihalonaphthalenes investigated were syn- sidered. thesized from the corresponding diaminonaphtha- The quantum yield of the OO-transition, was lenes 12'12a. The substances were carefully purified determined by taking the ratio of the area of the by means of column chromatography and repeated 00-band and the area of the total phosphorescence sublimation. The Br- and I-derivatives were deu- spectrum. To be sure that the results were not in- terated by an exchange reaction with perdeutero- fluenced by reabsorption (we used concentrations benzene as a deuterium source13. In the case of of about 10~3m) we measured the quantum yield 1,4-dichloronaphthalene, deuteration was achieved of a very dilute solution (10-5m) of diphenyl- by starting the synthesis with perdeuteronaphthalene anthracene versus a 10~3m solution. The results (Merck). Thus, in this case, the isotopic purity agree within the error limit of the experiment. should exceed 98%. In the case of the I- and Br- The total radiative rate k and the 00-rate k° from derivatives, the degree of deuteration was deter- the lowest triplet stale were obtained from the mined from the NMR spectra to be at least 90%. 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 - e in the in-plane and in the out-of-plane polarized 2 = 3 V © <U 13 rates. II libromo ithalen 11 b) In the halonaphthalenes, especially in the case & & 1—1 — IM C <N C of the Br- and I-compounds, however, there is in- deed a significant change in the degree of polari- 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- o kT° (H) i- 14%) 1.5 1.75 1.6 f l phorescence 00-transitions of the protonated and - kr° (D) T deuterated species respectively.