Examining solvent effects on the ultrafast dynamics of catechol

We consider the effect of a polar, hydrogen bond accepting, solvent environment on the excited state decay of catechol following excitation to its first excited singlet state (S₁). A comparison of Fourier transform infrared spectroscopy and explicit-solvent ab initio frequency prediction suggests that 5 mM catechol in acetonitrile is both nonaggregated and in its "closed" conformation, contrary to what has been previously proposed. Using ultrafast transient absorption spectroscopy, we then demonstrate the effects of aggregation on the photoexcited S₁ lifetime: at 5 mM catechol (nonaggregated) in acetonitrile, the S₁ lifetime is 713 ps. In contrast at 75 mM catechol in acetonitrile, the S₁ lifetime increases to 1700 ps. We attribute this difference to aggregation effects on the excited-state landscape. This work has shown that explicit-solvent methodology is key when calculating the vibrational frequencies of molecules in a strongly interacting solvent. Combining this with highly complementary steady-state and transient absorption spectroscopy enables us to gain key dynamical insights into how a prominent eumelanin building block behaves when in polar, hydrogen bond accepting solvents both as a monomer and as an aggregated species.