Fast excited state dynamics in the isolated 7-azaindole-phenol H-bonded complex

Abstract

The excited state dynamics of the H-bonded 7-azaindole-phenol complex (7AI-PhOH) has been studied by combination of picosecond pump and probe experiments, LIF measurements on the nanosecond time scale and ab initio calculations. A very short S(1) excited state lifetime (30 ps) has been measured for the complex upon excitation of the 0(0)(0) transition and the lifetime remains unchanged when the ν(6) vibrational mode (0(0)(0) + 127 cm(-1)) is excited. In addition, no UV-visible fluorescence was observed by exciting the complex with nanosecond pulses. Two possible deactivation channels have been investigated by ab initio calculations: first an excited state tautomerization assisted by a concerted double proton transfer (CDPT) and second an excited state concerted proton electron transfer (CPET) that leads to the formation of a radical pair (hydrogenated 7AIH(●) radical and phenoxy PhO(●) radical). Both channels, CDPT and CPET, seem to be opened according to the ab initio calculations. However, the analysis of the ensemble of experimental and theoretical evidence indicates that the excited state tautomerization assisted by CDPT is quite unlikely to be responsible for the fast S(1) state deactivation. In contrast, the CPET mechanism is suggested to be the non-radiative process deactivating the S(1) state of the complex. In this mechanism, the lengthening of the OH distance of the PhOH molecule induces an electron transfer from PhOH to 7AI that is followed by a proton transfer in the same kinetic step. This process leads to the formation of the radical pair (7AIH(●)···PhO(●)) in the electronically excited state through a very low barrier or to the ion pair (7AIH(+)···PhO(-)) in the ground state. Moreover, it should be noted that, according to the calculations the πσ* state, which is responsible for the H loss in the free PhOH molecule, does not seem to be involved at all in the quenching process of the 7AI-PhOH complex.