Investigations on the nature of non-radiative transitions from excited singlet and triplet states of dimethyl substituted phenols in the presence of acceptor 2-nitrofluorene at 77 K

Abstract

Steady state emission measurements are combined with time-resolved experiments to examine the nature of the different non-radiative transitions from the photoexcited donors (D) (3,5-dimethylphenol, 35DMP; 2,6-dimethylphenol, 26DMP; 3,5-dimethylanisole, 35DMA; 2,5-dimethylanisole, 25DMA) in the presence of the acceptor (A) 2-nitrofluorene (2NF) in ethanol (EtOH) rigid glassy matrix at 77 K. At such low temperature observations of large negative driving energy (Δ G 0) values (−1.42 eV to −2.03 eV) for the systems of present excited singlet (S 1) donors and ground state 2NF are indicative of the occurrence of highly exothermic electron transfer (ET) reactions in the singlet state S 1 whereas relatively lower values of Δ G 0 (−0.17 eV to −0.70 eV) for the excited triplet (T 1) donors and the acceptor 2NF in the ground state suggest the involvement of moderately exothermic ET reactions within these D–A pairs. Following the Weller equation, the destabilization energy (Δ G 0 d) has been computed (to estimate the driving energy for ET reactions in solid solution) for the ion pair states of the present donor–acceptor molecules. However, the computed value of Δ G 0 d (∼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 glass. It seems in EtOH solid solution at 77 K dipole rotations of the solvent may not be fully eliminated unlike the situation observed by Wasielewski et al. in the case of less polar methyltetrahydrofuran glass environment. From the energy gap dependence of ET rates and the observed relationships between λ, nuclear re-organization energy parameter and Δ G 0, it is apparent that singlet (S 1) ET reactions might be occurring in the Marcus inverted region (MIR) while such reactions between triplet donors and ground state acceptor 2NF seem to proceed through the normal/intermediate region. From the observations of the large spectral overlapping between the donor emission and electronic absorption spectra of the acceptor coupled with the high values of T (99%), the theoretical transfer efficiency of non-radiative energy transfer of Förster's type, and R 0 (∼27 Å), Förster's critical energy transfer distance, it seemingly indicates that the combined effect of the concurrent processes of photoinduced ET and Förster's type energy transfer is primarily responsible for the observed fluorescence quenching of the present donors in the presence of the acceptor 2NF at 77 K. The same trend was observed at the ambient temperature as reported earlier. Moreover, steady state and time-resolved data reveal that triplet–triplet (T D 1→T A 1) energy transfer along with triplet state ET reactions might be responsible for the observed donor phosphorescence quenching phenomena. Reaction schemes describing the various possible pathways for the non-radiative depletion of the excited (singlet and triplet) states of the donors effected through the interactions with the quencher (electron acceptor) 2NF have been proposed from the observed experimental results.