Vibronic spectra of jet-cooled 1-methyl-2(1H)-quinolinone studied by Fluorescence spectroscopy and Quantum chemical calculation

Electronic Relaxation Dynamics in 2-Quinolinones with Extended Conjugation

The 2-quinolinone family of molecules, also known as carbostyrils, have been proposed as light absorbing donor molecules in energy transfer based sensing schemes and as possible photocatalysts. Both of these applications make use of electronic excited states, but the photophysics of 2-quinolinones have not yet been examined closely. This study applies static and dynamic spectroscopy, with supporting density functional theory calculations, to reveal the electronic relaxation dynamics of a family of five 2-quinolinones with extended conjugated rings. These modifications lead to red-shifted absorbance and emission maxima, relative to unmodified 2-quinolinone. Optical excitation of these molecules with near UV light resulted in transitions with strong π → π* and HOMO → LUMO character. Time-correlated single photon counting measurements yielded fluorescence lifetimes ranging from 849.3 (±0.6) ps to 4.586 (±0.002) ns. Transient absorption spectroscopy revealed relaxation dynamics of the S₁ excited state formed by photoexcitation at 350 nm, along with formation of a long-lived signal assigned as excited state absorption by a triplet excited state. Vibrational relaxation in the S₁ state was also characterized in some compounds. Overlapping signals of S₁ decay and triplet growth in the transient absorption data set could not be fully disentangled. These results demonstrate a highly competitive relaxation scheme following multiple simultaneous pathways, a promising situation for establishing chemical control of electronic relaxation in the 2-quinolinone family.

Ab initio and NBO studies of methyl internal rotation in 1-methyl-2(1H)-quinolinone: effect of aromatic substitution to 1-methyl-2(1H)-pyridone

1-methyl-2(1H)-quinolinone (MeQone) forms the framework of several hundred alkaloid molecules both natural and synthetic being used for various biological applications. From chemical structure point of view, the molecules can also be seen as an aromatic ring fused to 1-methyl-2(1H)-pyridone (1-MPY). In this work, we present theoretical investigations on internal rotation of methyl group in MeQone in light of 1-MPY. We looked into the change in the three-fold (V₃) methyl internal rotation barrier resulted from the aromatic ring substitution to 1-MPY. The V₃ term in two molecules were calculated using density functional theory and Hartree-Fock theory with different basis sets. MeQone has calculated V₃ term (in S₀ state) three times higher in magnitude compared with that of 1-MPY. The role of aromatic substitution in increase of V₃ term is investigated using natural bond orbital (NBO) analyses. In the NBO analysis, it is found that the aromatic ring as highly delocalized π-system lowers the magnitude of hyperconjugation energy in MeQone compared with 1-MPY. This is due to the extension of delocalization of π-electrons to pyridone ring which lowers the orbital overlap. However, the Lewis energy increases substantially and make the overall barrier energy higher in MeQone compared with 1-MPY. From our study, we conclude that in the molecules such as 1-MPY and MeQone where the methyl group has two single bonds vicinal to it, the overall hyperconjugation energy is always barrier forming with nonlocal interactions playing significant role. Also, the Lewis energy plays the decisive role in barrier formation, and its magnitude can be tuned by tuning the π-electron delocalization. We have also looked into the change in methyl group conformation upon electronic excitation to S₁ state. In 1-MPY, the methyl group rotated by 60° upon excitation whereas in MeQone, there was no conformational change. Strong π*-σ* interaction in LUMO in top-of-barrier conformation is responsible for the change in the methyl group conformation in 1-MPY, whereas same π*-σ* interaction in LUMO of minimum energy conformation results in unchanged excited state conformation in MeQone. Graphical abstract.