Intramolecular Long-Range Charge-Transfer Emission in Donor–Bridge–Acceptor Systems

Abstract

Charge recombination to the electronic ground state typically occurs nonradiatively. We report a rational design of donor−bridge−acceptor molecules that exhibit charge-transfer (CT) emission through conjugated bridges over distances of up to 24 Å. The emission is enhanced by intensity borrowing and extends into the near-IR region. Efficient charge recombination to the initial excited state results in recombination fluorescence. We have established the identity of CT emission by solvent dependence, sensitivity to temperature, femtosecond transient absorption spectroscopy, and unique emission polarization patterns. Large excited-state electronic couplings and small energy gaps enable the observation of intramolecular long-range CT emission over the unprecedented long distance. These results open new possibilities of using intramolecular long-range CT emission in molecular electronic and biomedical imaging probe applications.

An Electron‐Accepting aza‐BODIPY‐Based Donor–Acceptor–Donor Architecture for Bright NIR Emission

“…Because the excited‐state dynamics depend on the molecular structures and D–A interactions, the solvatochromic behaviors in the fluorescence spectra are slightly different between DAD‐2 and DAD‐3 . The broad fluorescence spectra of DAD‐2 in chloroform and benzonitrile can be ascribed to the emission from the CS state, which borrows intensity from the LE state [36] . On the other hand, the sharp fluorescence spectrum of DAD‐3 in benzonitrile, which is similar to those in toluene and chloroform, implies major contribution of the LE state for the emission, whereas the fluorescence quenching in benzonitrile is mainly governed by the dark CS state.…”

Excited state dynamics of BODIPY-based acceptor–donor–acceptor systems: a combined experimental and computational study

“…17,18 On the other hand, boron dipyrromethene (BODIPY) derivatives have been widely used as fluorophores as well as electron acceptors in fluorescent molecular probes 19,20 due to their large extinction coefficients, 21,22 long excited state lifetimes, 23 high photostability, 24 various modification possibilities 25 and their intrinsic electron-accepting ability. However, only a few red emissive BODIPY based D-A rotor molecules with large Stokes shifts have been reported, [26][27][28] and their quantitative photophysical characterizations regarding solvent dependence and steric restrictions on the ultrafast time scale, to our knowledge, have not been studied.…”

Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor–Bridge–Acceptor Molecules

“…However, the induced absorption at 530 nm (Pyr − ) is hidden due to the overlap with the strong SE band of B 1 , so that the positive signal at 510−560 nm is shifted relative to the ion signal in B 0 , suggesting that the excited-state evolution of the B 1 in THF is different from those in ACN and TOL, which can be explained by the presence of an equilibrium between the CT state and the CS state, as evidenced by high fluorescence quantum yields. 77 Considering that the emission process competes with the formation of the CS state, a similar equilibrium between the CT and CS states of B 0 in TOL could also exist. The fs-TA spectra of B 2 (Figure 3h) in THF, whose characteristic absorption peaks of CS state are also not observed, are similar to those of B 2 in TOL.…”