Temperature-Dependent Photophysical Analysis of Coumarins 6, 7 and 30 in Alkane Solvents: Role of Solvent Viscosity...

The temperature-dependent photophysical behavior of Coumarins 6 (C6), 7 (C7), and 30 (C30) was systematically investigated in non-polar alkane solvents (n-heptane, cyclohexane and n-hexadecane) across 303-343 K. Using steady-state absorption and photoluminescence together with time-resolved fluorescence (TCSPC), we quantified emission maxima shifts, quantum yields (Φ), fluorescence lifetimes (τ) and derived first-order rate constants (k_f, k_nr) with propagated uncertainties. Small blue shifts in absorption and emission with heating indicate reduced solute-solvent stabilization. C6 in n-hexadecane showed increased radiative contribution at higher temperature, C7 exhibited high thermal robustness with minimal k_nr activation, and C30 was most sensitive to thermal activation in cyclohexane. Arrhenius analysis of lnk_nr versus 1/T yields activation energies in the range 0.3-5.7 kJ·mol⁻¹ (reported with standard errors). All Φ values were corrected for temperature-dependent refractive index (n(T)) and Förster-Hoffmann analysis lnk_nr vs. ln(η/T) confirms viscosity as a primary control parameter. These quantitative results clarify how solvent viscosity and molecular structure modulate LE-ICT balance and inform the design of thermally stable coumarin fluorophores for sensing and optoelectronic applications.