INVESTIGATIONS ON THE NATURE OF NONRADIATIVE-TRANSITIONS FROM EXCITEDSINGLET AND TRIPLET-STATES OF DIMETHYL SUBSTITUTED PHENOLS IN THE PRESENCE OF ACCEPTOR 2-NITROFLUORENE AT 77 K

Steady state emission measurements are combined with time-resolved exp eriments to examine the nature of the different non-radiative transiti ons from the photoexcited donors (D) (3,5-dimethylphenol, 35DMP; 2,6-d imethylphenol, 26DMP; 3,5-dimethylanisole, 35DMA; 2,5-dimethylanisole, 25DMA) in the presence of the acceptor (A) 2-nitrofluorene (2NF) in e thanol (EtOH) rigid glassy matrix at 77 K. At such low temperature obs ervations of large negative driving energy (Delta G(0)) values (-1.42 eV to -2.03 eV) for the systems of present excited singlet (S-1) donor s and ground state 2NF are indicative of the occurrence of highly exot hermic electron transfer (ET) reactions in the singlet state S-1 where as relatively lower values of Delta G(0) (-0.17 eV to -0.70 eV) for th e excited triplet (T-1) donors and the acceptor 2NF in the ground stat e suggest the involvement of moderately exothermic ET reactions within these D-A pairs. Following the Weller equation, the destabilization e nergy (Delta G(d)(0)) has been computed (to estimate the driving energ y for ET reactions in solid solution) for the ion pair states of the p resent donor-acceptor molecules. However, the computed value of Delta G(d)(0) (similar to 0.26 eV) for the present D-A systems indicate that though, the driving energy decreases in magnitude does not reduce to a large extent on going from polar acetonitrile (ACN) Liquid to EtOH g lass. It seems in EtOH solid solution at 77 K dipole rotations of the solvent may not be fully eliminated unlike the situation observed by W asielewski et al. in the case of less polar methyltetrahydrofuran glas s environment. From the energy gap dependence of ET rates and the obse rved relationships between lambda, nuclear re-organization energy para meter and Delta G(0), it is apparent that singlet (S-1) ET reactions m ight be occurring in the Marcus inverted region (MIR) while such react ions between triplet donors and ground state acceptor 2NF seem to proc eed through the normal/intermediate region. From the observations of t he large spectral overlapping between the donor emission and electroni c absorption spectra of the acceptor coupled with the high values of T (99%), the theoretical transfer efficiency of non-radiative energy tr ansfer of Forster's type, and R-0 (similar to 27 Angstrom), Forster's critical energy transfer distance, it seemingly indicates that the com bined effect of the concurrent processes of photoinduced ET and Forste r's type energy transfer is primarily responsible for the observed flu orescence quenching of the present donors in the presence of the accep tor 2NF at 77 K. The same trend was observed at the ambient temperatur e as reported earlier. Moreover, steady state and time-resolved data r eveal that triplet-triplet (T-1(D) --> T-1(A)) energy transfer along w ith triplet state ET reactions might be responsible for the observed d onor phosphorescence quenching phenomena. Reaction schemes describing the various possible pathways for the non-radiative depletion of the e xcited (singlet and triplet) states of the donors effected through the interactions with the quencher (electron acceptor) 2NF have been prop osed from the observed experimental results. (C) 1998 Elsevier Science S.A. All rights reserved.