Photophysical modulation of D205 dye in acetonitrile-butanol mixtures: Interplay of solvent polarity, hydrogen bonding, and conformational dynamics

Understanding how local solvent environments influence dye photophysics is essential for improving the performance of dye-sensitized systems. In complex ionic liquid (IL)/molecular solvent mixtures, non-linear trends have been observed, raising questions about the underlying mechanisms. In this work, we investigate whether similar microenvironment-dependent behavior can be observed in conventional molecular solvent mixtures, using the indoline dye D205 in a binary 1-butanol (BuOH)/acetonitrile (ACN) system. This mixture allows systematic tuning of key solvent properties such as polarity, hydrogen-bonding, and viscosity, while preserving simpler and more interpretable solvent–solvent interactions. Combining Raman spectroscopy, steady-state UV–Vis and fluorescence spectroscopy, time-correlated single photon counting (TCSPC), femtosecond transient absorption spectroscopy (TAS), and TD-DFT calculations, we show that D205's excited-state dynamics are highly sensitive to changes in solvent composition. Increasing the BuOH fraction leads to blue shifts in emission, decreased Stokes shift, reduced quantum yield, and shorter fluorescence lifetimes at intermediate compositions. TD-DFT reveals solvent-dependent conformational changes and subtle (∼1 %) variations in donor–acceptor charge transfer character. However, these changes do not dominate excited-state deactivation pathways, suggesting that hydrogen bonding and local polarity fluctuations play a more decisive role. This study demonstrates that even in conventional binary solvents, local microheterogeneity can strongly modulate dye photophysics. Finally, our findings suggest that the effect of the mixture on the D205 photophysics observed trends is not exclusive to the IL/molecular solvent microenvironments but can also manifest in conventional binary solvent systems such as BuOH/ACN mixtures. • The distribution of acetonitrile and butanol in their mixture is inhomogeneous. • The photophysical properties of D205 behave in a no linear way relative to the mixture composition. • Combined effect of polarity and hydrogen bonding stabilize the excited state of D205 in butanol. • The effect of the BuOH/ACN mixture on the D205 photophysics has the same trend as in IL-solvent mixture.