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 phenome na of the present electron donor molecules, carbazole (C) and 9-phenyl carb azole (9PC) in the presence of the well-known electron accepters 9-fluoreno ne (9FL) and 2-nitro-9-fluorenone (2N9FL) in acetonitrile (ACN) fluid solut ion are due to the combined effect of the static and dynamic processes invo lved. To dissect the donor fluorescence quenching data into its dynamic and static components, a model in the form of a modified Stern-Volmer (SV) rel ation has been proposed. By treating the data, obtained from the present in vestigation when the donor chromophores are excited, by nonlinear least squ ares curve fitting procedure, static (V) and dynamic (K-SV) contributions i n overall quenching mechanisms were evaluated separately. The contribution of the static component (V) is observed to be so large that it overwhelms t he dynamic process and plays major role in overall quenching mechanisms. In dynamic quenching, photoinduced electron transfer (PET) process, whose occ urrence being confirmed by measuring redox potentials of the reacting syste ms (C and 9PC as electron donors and 9FL and 2N9FL as electron accepters) i n ACN solvent, is found to be operative concurrently with the Forster energ y transfer process. However, when the electron acceptor molecules are excit ed in presence of a ground state donor, the quenching of the acceptor fluor escence appears to be mainly of dynamic nature. In this dynamic process, el ectron transfer (ET) seems to play the major role in nonradiative deactivat ion of the lowest excited singlet state (S-1) of the acceptor species. From the observed results at 77K, it is inferred that donor fluorescence quench ing is primarily due to the combined effect of the concurrent occurrences o f ET in the excited singlet state and the Forster long range energy transfe r (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 reacti ons with the accepters. From the observed room temperature oxidation potent ial values of the donors, it is apparent that the electron donating capabil ity decreases when substitution is made along the short molecular axis of s ymmetry of C, i.e., by replacing > NH hydrogen atom of C molecule by a phen yl ring (in case of 9PC). (C) 2001 Elsevier Science B.V. All rights reserve d.