EFFECT OF A HEAVY ATOM IN THE NONRADIATIVE TRANSITION

V. G. Klimenko
Central New Mexico Community College 525 Buena Vista Dr SE, Albuquerque

EFFECT OF A HEAVY ATOM IN THE NONRADIATIVE TRANSITION

Results of numerous experimental studies of deactivation of singlet (S) and triplet (T) electronic ππ* states of organic compounds show that the probability of intercombination transitions, both the T1 S0 transitions and the S1T1 nonradiative transitions, usually increases upon the introduction of heavy atoms into a molecule (the heavy-atom effect) [1]. This result is explained by a significant increase in the spin–orbit (SO) coupling in an atom with its atomic number and, consequently, by an enhancement of the SO interaction in the molecule [2]. At the same time, there are experimental data [1] according to which the introduction of a heavy atom does not lead to an increase in the probability for the T1 S0 and S1T1 transitions (anomalous heavy-atom effect). The theoretical studies of T1(ππ*)S0 radiative transitions resulted in the description of experimental findings and revealed several causes for the anomalous heavy-atom effect [3,4 ,5]. In these interactions, the virtual electronic states of type πσ*/σπ* are considered and the influence of a heavy atom on the T1(ππ*)S0 transition is controlled by one-center integrals of SO coupling in atoms A whose magnitudes depend on an effective nuclear charge ZA* and are characterized by the SO coupling parameter ςA [3]. For the S1(ππ*)T1(ππ*) nonradiative transition, the theoretical description of the experimental data on the heavy-atom effect encounters considerable difficulties if, in the adiabatic approximation, the vibronic interactions are ignored and only the SO interactions are considered. The reason is that the purely SO interactions that are important in the S1(ππ*)T1(ππ*) transition are determined by three-center integrals that are almost independent of Z* [6]. At the same time, it was previously assumed [7] based on approximate theoretical estimates for several molecules that the next theoretical approximation—the model of vibronicinduced spin–orbit (VISO) coupling, which takes the SO + VIB interactions into account and allows such nonradiative transitions—generally makes a negligible contribution to KST. An interesting effect of a heavy atom on the fluorescence quantum yield φfl whose value is changed due to nonradiative intersystem crossing was observed experimentally, in particular, with different halogen (Cl, Br, I) substituted anthracenes (AC) [1]. Both ordinary and anomalous heavy-atom effects was observed. For example, in solutions of 9,10-dichloroanthracene (DClA), as compared to AC, the value of φfl appeared to be larger rather than smaller, as one might expect. In cryosolutions (77 K) and in argon jet (about 1.4 K), the value of φfl equals 0.60± .5 and 0.67, respectively, for AC [8], while, for DClA, φfl amounts to 0.78 (solution) and 1.00 (jet) [9]. On passage to room temperature, φfl decreases but the difference is retained, i.e., φfl = 0.48 and 0.22 in DClA and AC, respectively [1].
Table 1. Estimates of the position of the energy levels Ecal of the S1 singlet state relative to the calculated values of energy levels of the Tm triplet states
The rate constant KST for the nonradiative intersystem crossing transition S1(1B1u)T1(3B1u)(I) in 9,10-dichloroanthracene (DClA) is calculated in terms of the model of vibronic-induced spin–orbit (VISO) interactions. The magnitude fluorescence quantum yield φfl is estimated. Comparison of KST(I) and φfl for DClA with the corresponding values obtained earlier for anthracene (AC), where KST is governed by the conversion channels (I) and S1(1B1u)T2(3B3g) (II), shows that the theoretical estimates reflect the anomalous heavy atom effect in these molecules in accordance with the experimental (literature) data. The cause for this effect is revealed. The influence of different factors on the KST(I) constant and on the ratio of its components (where s denotes the z and y spin-sublevels) is established for DClA. These factors are the magnitude of the spin–orbit coupling parameter in a chlorine atom, the change, as compared to AC (in the same conversion channel (I)) of the distribution of electrons in the carbon core of the DClA molecule, and the change in the form of out-of-plane vibrational modes involved in VISO interactions.

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