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.

Highlights from Faraday Discussion on Ultrafast Photoinduced Energy and Charge Transfer, Ventura, CA, USA, April 2019.

Following a tradition more than 100 years old, a Faraday Discussion meeting was held on the topic of 'Ultrafast Energy and Charge Transfer' in April 2019. While these meetings are historically held in the United Kingdom, the Royal Society of Chemistry (who organises the conference and publishes its proceedings) recognises the importance of finding the space and opportunity for the chemical sciences international communities to connect and exchange knowledge. As such, the meeting hereby referred to took place in Ventura, California, USA. This conference report, produced by early career researchers, covers the highlights of this meeting, focusing on brief summaries of the papers discussed as well as particularly interesting or recurring topics of the ensuing discussion.

Determination of Secondary Species in Solution through Pump-Selective Transient Absorption Spectroscopy and Explicit-Solvent TDDFT.

The measured electronic excitations of a given species in solution are often a composite of the electronic excitations of various equilibrium species of that molecule. It is common for a proportion of a species to deprotonate in solution, or form a tautomeric equilibrium, producing new peaks corresponding to the electronic excitations of the new species. One prominent example is alizarin in methanol, which at different temperatures, and in solutions with differing pH, has an isosbestic point between the two dominant excitations at 435 and 540 nm. The peak at 435 nm has been attributed to alizarin; the peak at 540 nm, however, more likely results from a species in equilibrium with alizarin. In this work, we were able to use both experimental and computational techniques to selectively examine electronic properties of both alizarin and its secondary species in equilibrium. This was achieved through use of transient electronic absorption spectroscopy, following selective photoexcitation of a specific species in equilibrium. The resulting transient electronic absorption spectra were compared to the known transient absorption spectra of potential secondary equilibrium species. The ground state absorption spectra associated with each species in equilibrium were predicted using linear-scaling time-dependent density functional theory with an explicitly modeled solvent and compared to the experimental result. This evidence from both techniques combines to suggest that the excitation at 540 nm arises from a specific monoanionic form of alizarin.