Highly Polarized Coumarin Derivatives Revisited: Solvent‐Controlled Competition Between Proton‐Coupled Electron Transfer and Twisted Intramolecular Charge Transfer

Abstract

Linking a polarized coumarin unit with an aromatic substituent via an amide bridge results in weak electronic coupling that affects the intramolecular electron‐transfer (ET) process. As a result of this, interesting solvent‐dependent photophysical properties can be observed. In polar solvents, electron transfer in coumarin derivatives of this type induces a mutual twist of the electron‐donating and ‐accepting molecular units (TICT process) that facilitates radiationless decay processes (internal conversion). In the dyad with the strongest intramolecular hydrogen bond, the planar form is stabilized, such that twisting can only occur in highly polar solvents, whereas a fast proton‐coupled electron‐transfer (PCET process) occurs in nonpolar n‐alkanes. The kPCET rate constant decreases linearly with the energy of the fluorescence maximum in different solvents. This observation can be explained in terms of competition between electron‐ and proton‐transfer from a highly polarized (ca. 15 D) and fluorescent locally excited (1LE) state to a much less polarized (ca. 4 D) charge‐transfer (1CT) state, a unique occurrence. Photophysical measurements performed for a family of related coumarin dyads, together with results of quantum‐chemical computations, give insight into the mechanism of the ET process, which is followed by either a TICT or a PCET process. Our results reveal that dielectric solvation of the excited state slows down the PCET process, even in nonpolar solvents.

Quadrupolar Dyes Based on Highly Polarized Coumarins

“…This suggests an opening of the nonradiative decay pathway and requires the use of increasingly sophisticated models of solvent effects for CT systems. 26 − 28 …”

“…There is however no clear explanation for the observed decrease of fluorescence yield of Coum7 in polar DMSO. This suggests an opening of the nonradiative decay pathway and requires the use of increasingly sophisticated models of solvent effects for CT systems. − …”

Magnifying the ESIPT process in tris(salicylideneanilines) via the steric effect – a pathway to the molecules with panchromatic fluorescence

“…This differentiates weakly polarized TSANs from highly polar molecules for which changes of excited states energy and related to that substantial solvatochromism were well correlated with proton transfer rates. 64 Low temperature study At a temperature of 5 K the emission spectra of TSANs in n-hexane comprise of structured LE fluorescence of monomers and diffuse contribution of aggregates (Fig. S7, ESI †).…”

“…This differentiates weakly polarized TSANs from highly polar molecules for which changes of excited states energy and related to that substantial solvatochromism were well correlated with proton transfer rates. 64…”

Pyranone–Arylbenzene Molecules Controlled by the Competition of Local Excited State and Twisted Intramolecular Charge-Transfer State: Dual-State Emission, Polymorphism, and Mechanofluorochromism

“…For the LE state, the distribution positions of electrons and holes before and after electron transition are roughly the same, and this excited state often appears in conjugated rigid molecules, such as pyrenes and anthracenes. ,, In contrast, for the CT state, the electrons and holes are distributed on the different fragments of the molecule before and after electron transition, which often appears in the donor–acceptor (D–A) or D−π–A compounds. − The molecules dominated by the LE state generally have high radiation transition rates and photoluminescence efficiency, but the larger exciton binding energy affects the spin flip of electrons, resulting in a lower exciton utilization rate. Although the molecules dominated by the CT state can effectively improve exciton utilization, they are usually accompanied by weak luminous intensity. − Therefore, designing molecules with both LE and CT states can not only ensure the radiation efficiency of the energy but also increase the utilization rate of excitons. , Based on the above considerations, herein, four new D–A pyranone–arylbenzene molecules were designed and synthesized (Scheme ), in which pyranone ( Pr ) was selected as the acceptor to ensure that the molecules had the CT state, whereas benzene ( Ph ), naphthalene ( Np ), anthracene ( An ), and pyrene ( Py ) were used as donors to increase the LE state. Solvatochromic experiments, single-crystal structures, theoretical calculations, and low-temperature delayed emissions demonstrate that Pr – Ph and Pr – Np mainly have the twisted intramolecular charge-transfer (TICT) state in the excited state, and thus, these two compounds show very weak fluorescence emissions in solution.…”