Investigation of the effect of solvent polarity and viscosity on the Stokes shift of chromeno[2,3-b]indoles

Steady-state absorption and fluorescence measurements were performed on a series of three chromeno[2,3-b]indoles to investigate the origins of their ultra-high Stokes shifts. The effects of solvent polarity and viscosity on these properties were examined through steady state experiments, along with solvent-polarity-dependent fluorescence lifetimes. The chromenoindole exhibit intense emission in the visible region that strongly depends on solvent polarity, accompanied by a weak ultraviolet emission band that remains largely independent of solvent polarity. Addition of a non-polar co-solvent to the medium induces a continuous hypsochromic shift in the visible emission band. Viscosity-dependent fluorescence studies, conducted using methanol-glycerol mixtures, revealed that increasing viscosity weakens the visible emission intensity while causing a bathochromic shift. Concurrently, the ultraviolet emission band intensifies, with these spectral changes exhibiting an isosbestic point at 401 nm. This observation indicates the coexistence of two distinct emissive states in the excited state. The visible emission is assigned to a planar intramolecular charge transfer (PICT) state, while the ultraviolet emission arises from a locally excited (LE) state. The proposed PICT state decays more slowly in more polar solvents but faster in less polar solvents. This behaviour is consistent with stabilization of a polar charge-transfer excited state in polar media.