Structure of donor–acceptor biphenyl derivatives in the excited state

This paper presents a study on structural changes of donor–acceptor derivatives of biphenyl: 4-hydroxy-4′-cyanobiphenyl, 4-hydroxy-2,6-dimethyl-4′-cyanobiphenyl and 4-methoxy-2,6-dimethyl-4′-cyanobiphenyl in the excited state. An analysis of spectral and photophysical parameters of those compounds was completed and quantum-chemical calculations were conducted in order to explain if the excited state of these molecules is a mesomerically stabilized less polar intramolecular charge transfer (MICT) state of near planar geometry, or a more polar one, with strongly decoupled intramolecular CT resembling a TICT state. A strong solvatochromism of fluorescence and values of dipole moment in the excited state suggest the TICT character of the emitting state, especially in compounds with a sterical hindrance. It was demonstrated that structural changes of excited derivatives containing an OH group, resulting from strong charge transfer from the donor subunit onto acceptor part of those molecules, are associated with so strong increase of acidity of studied compounds, that they can be included in the group of, so-called, “super” photoacids.