Investigations on the photophysical properties of 2-methylindole and 2-methylindoline in various environments. Studies on the nature of non-radiative transitions in presence of the electron acceptor 2-nitrofluorene

Abstract

Electronic absorption, steady state and time-resolved emission, in the domain of nanosecond, spectroscopic methods were employed to compare the spectroscopic and photophysical properties of the electron donors (D) 2-methylindole (2MI) and 2-methylindoline (2MIN) in the presence of the well-known acceptor (A) 2-nitrofluorene (2NF) in fluid solutions at ambient temperature (296 K). Steady-state excitation polarization and emission polarization spectra were also measured. From all the studies presently carried out it was apparent that the two lowest lying electronic excited states 1L b (S 1) and 1L a (S 2), which seem to be very closely spaced in the case of the donor 2MI, become well-separated in the case of the other donor 2MIN. Both the donors were found to undergo large fluorescence quenching in the presence of the acceptor 2NF in solvents of different polarity. The experimental evidence suggest that numerous processes might be responsible for the observed large fluorescence quenching effect of the present donors (2MIN and 2MI) in the presence of the acceptor 2NF. These processes seem to be mainly (1) photoinduced electron transfer, as evidenced from large negative values of driving energy (Δ G 0) obtained from electrochemical measurements, (2) energy transfer of Förster's type from the observed large overlapping of donor emission and acceptor absorption spectra, (3) transient-type static quenching phenomena, etc. Further investigations are now underway to reveal the exact mechanisms of the observed large quenching effect within the present D–A systems.