Fluorescence loss mechanism due to large-amplitude motions in derivatives of 2,2'-bipyridyl exhibiting excited-state intramolecular proton transfer and perspectives of luminescence solar concentrators

Fluorescence quantum yields and lifetimes as functions of temperature and solvent polarity are compared for two different hydroxy derivatives of 2,2'-bipyridyl. Both dyes show strongly red-shifted excited-state intramolecular proton transfer (ESIPT) fluorescence at all temperatures investigated. In protic solvents, the ESIPT emission is of a more allowed nature and somewhat blue shifted, owing to specific solvatation. A strong difference in behaviour can, however, be observed for the non-radiative losses. The symmetric dye 1 possesses large fluorescence quantum yields, whereas sizeable fluorescence quantum yields from 2 could only be obtained under high viscosity conditions. Increased solvent polarity leads to enhanced fluorescence losses. These facts and the comparison with quantum chemical calculations for planar and twisted structures of 1 and 2 are interpreted as evidence for a photochemically reached low-lying state with charge separation (twisted intramolecular charge transfer (TICT)),responsible for these fluorescence losses and especially active for 2. For 1 in aprotic solvents, this reaction is discussed within the quantum-chemical extension of electron transfer theory and shown to involve nuclear tunnelling. In addition to this viscosity-dependent decay channel, 2 also decays via a viscosity-independent (presumably n-pi*) channel. The identification of the TICT and n-pi* non-radiative channels allows a new approach to the development of highly fluorescent ESIPT dyes with very large Stokes shift for use in fluorescence solar collectors or other devices utilizing the principle of fluorescence-based light-pipes.