Elucidation of Organohalogenochromism (OHC) of D-A Pyridinium and D-π-A Pyridinium Dyes: Effect of Halogen Bond.

Bathochromic shift-type OHC (b-OHC) was found in donor-acceptor (D-A) type pyridinium dye bearing halide ion as a counter anion, as well as donor-π-acceptor (D-π-A) type pyridinium dye; the intramolecular charge transfer-based photoabsorption maxima (λmax abs) in halogenated solvents show a large bathochromic shift, in comparison with those in non-halogenated solvents. It was revealed that there is a good relationship between the most positive surface electrostatic potential (VS,max) values associated with the most positive σ-hole on halogen atoms in organohalogen molecule and the λmax abs, indicating that the b-OHC of the D-A and D-π-A pyridinium dyes is attributed to the formation of halogen bond (XB) or complex such as [R─X·Y]- between the halogen atom (X) of organohalogen molecule and the counter anion (Y) of dye molecule. Moreover, the plots of the λmax abs against the VS,max values demonstrated that the b-OHC characteristic of D-A pyridinium dye is lower than that of the D-π-A pyridinium dye. It was suggested that the formation of XB induces a decrease in the ring current of the pyridinium ring, that the LUMO is mainly localized, resulting in the expression of b-OHC. Consequently, this work offers a deeper insight into the mechanism for the expression and the origin of OHC.