Electronic spectra and fluorescence of dithiinodiquinoline compounds. An experimental and theoretical study

Absorption and emission properties of six dithiinodiquinolines 1-6 (composed of 2,3- and/or 3,4-quinolinediyl units fused with 1,4-dithiin moiety) in methanol and cyclohexane solutions were studied. Differences in fluorescence properties of the compounds 1-6 are observed, depending on the type of condensed pattern of quinoline units. Emision bands of 1 and 2 occur at wavelengths over 500 nm are shifted systematically towards shorter waves when coming from 1,2 to 3,4 and 5,6. The compounds 1-6 posses long fluorescence lifetimes of several microseconds, which can be explained by an effective electronic conjugation in the excited state. In particular, the emission spectra of 3-6 in cyclohexane show two peaks characteristic for dual luminescence. The absorption and emission properties of compounds 1-6 were discussed in terms of DFT and TDDFT calculations. The geometry optimization of the excited nstates results in a planar structure as compared to a folded structure in the ground state of ditthiinodiquinolines 1-6.