The intramolecular external heavy atom effect in bromo-, benzo-, and naphthonorbornenes

Abstract

A theoretical analysis of the external heavy atom effect of a halogen atom on the radiative rate constant of phospho- rescence is examined as a function of position of a bromine atom or atoms relative to a naphthalene or a benzene chromophore for a series of mono- and dibromo-, naphtho-, and benzonorbornenes. The theoretical results are then compared to experimen- tal data and lead to the conclusion that the enhancement of the phosphorescence process takes place through the second-order mixing of the triplet states of the chromophore with the singlet charge transfer states arising primarily from an electron trans- fer from the orbitals of the heavy atom perturber to the unfilled x* orbitals of the chromophore. the spin-orbit coupling is generally very small, but an intro- duction of heavy atoms into such molecules and/or into the surrounding medium has been found to create a dramatic in- crease in the rates of the various multiplicity-forbidden pho- tophysical processes. The effect of a heavy atom bonded di- rectly to the chromophore (internal heavy atom effect)2b-d or of a heavy atom not affixed but close to the chromophores (external heavy atom effe~t)~ has been the focus of much theoretical4 and experimental work.5 The exact mechanism(s) of the heavy atom effect has not been clarified. The heavy atom could either enhance the singlet-triplet mixing already present in the molecule or introduce new perturbing states. In the case of the internal heavy atom effect, both mechanisms have been proposed.5 MurrelF and Tsubomura and Mulliken7 presented evidence for the formation of a charge-transfer complex of aromatic hydrocarbon with oxygen as an external perturber and pro- posed that the enhancement of TI - SO transition in aromatics is due to the intensity borrowing from the charge-transfer transition in the complex. Later, McGlynn and his co-workers3 extensively studied the role of external heavy atom on the phosphorescence process and offered evidence for charge transfer complexes with alkyl halides as donors. However, the exact mechanism by which the external heavy atom increases the spin-orbit coupling has not been advanced. There are a number of experimental observations, notably those of Kearns et aL8 and also of McGlynn et al.,3 which indicate that an ex- ternal heavy atom perturbation is purely electronic in nature and involves no vibronic coupling. Siege1 and Judeikis9 have shown, using the combined ESR and optical studies, that the phosphorescence process is more sensitive to the external heavy atom than either the intersystem crossing or the TI - SO ra- diationless process. The object of the present investigation is to understand, more quantitatively, the role of an external heavy atom or atoms in the enhancement of phosphorescence. For this purpose, we have chosen to examine the spectroscopic data of a series of mono- and dibromonaphthonorborneneslo~'i and a few