Steady state and time resolved studies on photophysical properties of carbazole and 9-phenyl carbazole molecules and their quenching reactions with suitable electron accepters in the excited electronic states both at the ambient temperature and at 77 K

Studies at the ambient temperature by employing both steady state and time resolved techniques reveal that the observed fluorescence quenching phenomena of the present electron donor molecules, carbazole (C) and 9-phenyl carbazole (9PC) in the presence of the well-known electron accepters 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) in acetonitrile (ACN) fluid solution are due to the combined effect of the static and dynamic processes involved. To dissect the donor fluorescence quenching data into its dynamic and static components, a model in the form of a modified Stern-Volmer (SV) relation has been proposed. By treating the data, obtained from the present investigation when the donor chromophores are excited, by nonlinear least squares curve fitting procedure, static (V) and dynamic (K-SV) contributions in overall quenching mechanisms were evaluated separately. The contribution of the static component (V) is observed to be so large that it overwhelms the dynamic process and plays major role in overall quenching mechanisms. In dynamic quenching, photoinduced electron transfer (PET) process, whose occurrence being confirmed by measuring redox potentials of the reacting systems (C and 9PC as electron donors and 9FL and 2N9FL as electron accepters) in ACN solvent, is found to be operative concurrently with the Forster energy transfer process. However, when the electron acceptor molecules are excited in presence of a ground state donor, the quenching of the acceptor fluorescence appears to be mainly of dynamic nature. In this dynamic process, electron transfer (ET) seems to play the major role in nonradiative deactivation of the lowest excited singlet state (S-1) of the acceptor species. From the observed results at 77K, it is inferred that donor fluorescence quenching is primarily due to the combined effect of the concurrent occurrences of ET in the excited singlet state and the Forster long range energy transfer (S-1(D) --> S-1(A)). Low temperature studies further demonstrate that the triplet donors are not involved in energy transfer as well as in ET reactions with the accepters. From the observed room temperature oxidation potential values of the donors, it is apparent that the electron donating capability decreases when substitution is made along the short molecular axis of symmetry of C, i.e., by replacing > NH hydrogen atom of C molecule by a phenyl ring (in case of 9PC). (C) 2001 Elsevier Science B.V. All rights reserved.