Fluorescence lifetimes and radiative rate constants of alkanes at 25°C

Fluorescence lifetimes of 33 alkanes were measured at 25°C using a 0.7 ns pulse radiolysis system and the relationship between molecular structure, fluorescence lifetime (τ), quantum yield (Φ_f) and radiative rate constant (R = $\text{Φ}{}_{\mathrm{f}}\text{τ}$) is discussed. The τ's showed a large dependence on the molecular structure, however, the R-values were approximately constant for groups of alkanes: R = 1.3×10⁶s^?1 for n-alkanes, R ≈ 4×10⁶ s^?1 for aliphatic vicinally substituted dimethyl alkanes, R = 7×10⁶s^-1 for cyclohexane and alkylcyclohexanes, and R = 10×10⁶s^?1 for di- and trimethylcyclohexanes. The excited state lifetimes of some non-fluorescing alkanes were also determined from steady-state photolytic quenching data and found to be very low (? 0.3 ns).