Theoretical Analysis of Properties of Ground and Excited States for Photodissociation of the C-O Bond in Polycarbonates

Quantum chemical calculations were carried out to investigate the properties of the ground state (GS) and the excited state (ES) of bisphenol-A polycarbonate (PC) with bisphenol-A hydrogen carbonate (BPAHC) as a model compound. Time-dependent density functional theory (TDDFT) was used to obtain the absorption spectrum and the corresponding transition natures of BPAHC. Furthermore, the ESs related to the transitions of the carbonate group and neighboring phenyl ring were optimized employing the TDDFT method for photodegradation. Our results showed that the carbonate group is broken at an ES with relatively high energy, which has a significant C-O bond cleavage within the carbonate group compared to that of GS geometry. The carbonate group C-O bond cleavage is caused by two reasons. One is the transition from the O lone pair to the carbonate π anti-bonding which is commonly known, and the other one is the transition from the O lone pair to the phenyl group (adjacent to the carbonate group) π anti-bonding that is newly proposed.

Dispersion-induced structural preference in the ultrafast dynamics of diphenyl ether

Dispersion interactions are omnipresent in large aromatic systems and influence the dynamics as intermolecular forces. The structural preference induced by dispersion interactions is demonstrated to influence the excited state dynamics of diphenyl ether (DPE) using femtosecond time-resolved transient absorption (TA) associated with quantum chemical calculations. The experimental results in aprotic solvents show that the S₁ state is populated upon irradiation at 267 nm with excess vibrational energy dissipating to solvent molecules in several picoseconds, and then decays via internal conversion (IC) within 50 ps as well as intersystem crossing (ISC) and fluorescence with a lifetime of nanoseconds. The polarity of the solvent disturbs the excited state energies and enhances the energy barriers of the ISC channel. Furthermore, the intermolecular dispersion interactions with protic solvents result in the OH–π isomer dominating in methanol and the OH–O isomer is slightly preferred in t-butanol in the ground state. The hydrogen bonded isomer measurements show an additional change from OH–O to OH–π geometry in the first 1 ps besides the relaxation processes in aprotic solvents. The time constants measured in the TA spectra suggest that the OH–O isomer facilitates IC. The results show that the OH–π isomer has a more rigid structure and a higher barrier for IC, making it harder to reach the geometric conical intersection through conformer rearrangement. This work enables us to have a good knowledge of how the structural preference induced by dispersion interactions affects excited state dynamics of the heteroaromatic compounds.

Dispersion interactions are omnipresent in large aromatic systems and influence the dynamics as intermolecular forces.

A three-state model for the photo-Fries rearrangement

Photo-Fries rearrangement plays a central role in synthesis, but it is still unclear how it works. A three-state model can explain it.

Solvent induced conformer specific photochemistry of guaiacol

Using a combination of ultrafast solution- and gas-phase spectroscopies, together with high-level theory calculations, we demonstrate that we are able to track conformer-specific photodissociation dynamics in solution through solvent choice. We reveal this phenomenon in guaiacol (2-methoxyphenol), a key subunit of the natural biopolymer lignin. In cyclohexane, the first electronically excited (1)ππ* (S1) state in guaiacol relaxes with a time-constant of τ = 4.5 ± 0.2 ns, mediated through intersystem crossing to lower lying triplet (Tn) states and internal conversion and fluorescence back to the ground state (S0). In contrast, in methanol, a further relaxation channel is also present; the S1 state relaxes with a time-constant of τ = 2.9 ± 0.1 ns, which is now additionally mediated through coupling onto a dissociative (1)πσ* (S2) state and subsequent O-H bond fission, evidenced through the appearance of a spectral signature for the guaiacoxyl radical after ∼250 ps. With the aid of complementary calculations, we attribute this to the now absent intramolecular H-bond between OH and OMe moieties, which now favours intermolecular H-bonding to methanol, lowering the barrier to O-H dissociation and facilitating H-atom loss via tunnelling.

Transient UV pump-IR probe investigation of heterocyclic ring-opening dynamics in the solution phase: the role played by nσ* states in the photoinduced reactions of thiophenone and furanone

The heterocyclic ring-opening dynamics of thiophenone and furanone dissolved in CH3CN have been probed by ultrafast transient infrared spectroscopy. Following irradiation at 267 nm (thiophenone) or 225 nm (furanone), prompt (τ < 1 ps) ring-opening is confirmed by the appearance of a characteristic antisymmetric ketene stretching feature around 2150 cm(-1). The ring-opened product molecules are formed highly vibrationally excited, and cool subsequently on a ∼6.7 ps timescale. By monitoring the recovery of the parent (S0) bleach, it is found that ∼60% of the initially photoexcited thiophenone molecules reform the parent molecule, in stark contrast with the case in furanone where there is less than 10% parent bleach recovery. Complementary ab initio calculations of potential energy cuts along the S-C([double bond, length as m-dash]O) and O-C([double bond, length as m-dash]O) ring-opening coordinate reveals insights into the reaction mechanism, and the important role played by dissociative (n/π)σ* states in the UV-induced photochemistry of such heterocyclic systems.