Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Abstract

Photosensitizers that display “unusual” emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha’s rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe2 group around the core of benzo­[g]­coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states. For solvents with intermediate polarity, three of the four regioisomers exhibit fluorescence from two different excited states with bands that are well separated in the visible and the near-infrared spectral regions. Computational analysis, employing ab initio methods, reveals that the orientation of an ester on the pyrone ring produces two conformers responsible for the observed dual fluorescence. Studies with solid solvating media, which restricts the conformational degrees of freedom, concur with the computational findings. These results demonstrate how “seemingly inconsequential” auxiliary substituents, such as the esters on the pyrone coumarin rings, can have profound effects leading to “anti-Kasha” photophysical behavior important for molecular photonics, materials engineering, and solar-energy science.

Strongly emitting, centrosymmetric, ladder-type bis-coumarins with crankshaft architecture

“…The above mentioned photophysical characteristics corresponds well with the one observed for many aminocoumarins 10,19 and aminobenzo[ g ]coumarins 20 and the low-energy band that is typically assigned to the intramolecular charge transfer (ICT). The emission maxima of bis-coumarins 7 (Et 2 N) and 9 (Oct 2 N) in toluene displayed identical λ max em at 580 nm.…”

Poly(dimethylsiloxane) as a room-temperature solid solvent for photophysics and photochemistry

“…In addition to medium viscosity, solvent polarity can also strongly affect such conformationally driven anti-Kasha behaviour. 40 Moderate donor–acceptor electronic coupling in multicomponent dyes offers an alternative means for attaining broad fluorescence spectra comprising emissions from CT and LE states. 121…”

“…Thus, dissolving molecular (or other) species in the molten SOA and letting it cool down results in room-temperature solid-state solutions. 33–40 Due to the formation of the SOA crystalline phase, however, such glass samples lose their transparency when kept for extended periods. Also, the elevated temperature of the molten SOA may compromise the stability of the examined species.…”

The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor–donor–acceptor fluorophores

“…Slow relaxation to the lowest excited state and/or fast competing processes from the upper states lead to “anti-Kasha” behaviour. 82,83 Kasha–Vavilov rule, however, does not preclude the occurrence of the relatively slow processes from the upper states. Because of their relatively low probability, such slow processes usually have negligibly small and undetectable contributions to the observed photophysics.…”