Theoretical and Experimental Investigations on the Aggregation‐Enhanced Emission from Dark State: Vibronic Coupling Effect

Abstract

Aggregation‐induced/‐enhanced emission (AIE/AEE) has aroused broad interest. The mechanism behind is understood as the aggregation restriction of the nonradiative decay from electronically excited state to the ground state, either through interconversion or through conical intersection, leaving the dipole‐allowed radiative decay channel relatively intact. Here, a report on an AEE phenomenon for 5,10‐diphenylphenazine (DPhPZ) compound is presented, for which the experiment shows to be AEE active but the lowest excited state has been long known to be “dark state,” namely with zero transition dipole with the ground state according to Kasha’s rule. The computational studies demonstrate that the optical emission stems from the “dark state” with emission intensity borrowed from higher‐lying “bright state” through Herzberg–Teller vibronic coupling, and the resultant spectra are in good agreement with the experiment in terms of both line shape and peak position. The vibronic‐coupling‐induced radiative decay and the restricted nonradiative decay synergistically bring about highly efficient luminescence of DPhPZ in the solid phase. The findings open a new avenue for the development of solid‐emissive luminophores.

A Multiple Excited-State Engineering of Boron-Functionalized Diazapentacene Via a Tuning of the Molecular Orbital Coupling

“…Furthermore, the weak MO coupling between S 3 and S 1 is consistent with the slow IC rates of the S 3 –S 1 transition observed in the time-resolved fluorescence spectroscopy experiments. Since the S 1 –S 0 transition is forbidden in N–B′ , the observed weak low-energy emission of N–B in solutions might be due to the vibronic coupling-induced radiative decay . According to the classical ICT models, the previous N–Ar–B systems have generally exhibited more twisted structures in more polar solvents. ,,− Therefore, with a switch from toluene to THF, it is reasonable to believe that the enhanced anti-Kasha emission (high-energy emission) of N–B stems from a weakened S 1 –S 0 coupling of its more stabilized and twisted structure in polar solvents.…”

Effects of Secondary Acceptors on Excited-State Properties of Sky-Blue Thermally Activated Delayed Fluorescence Molecules: Luminescence Mechanism and Molecular Design

“…When we calculate K ISC and K RISC between S 1 and T 1 , Δ G ji = E S1 – E T1 and Δ G ji = E T1 – E S1 respectively. All the abovementioned methods can be found in Peng’s, Shuai’s, and our previous works. − …”

Molecular mechanism of aggregation‐induced emission

“…[21,22,31] Moreover, a "dark" state with symmetry-forbidden transition is also found to emit bright light owing to the Herzberg-Teller vibronic coupling in 5, 10-diphenylphenazine (DPhPZ) aggregate. [23] As a result, the novel cores/backbones of AIEgens are exploited which provides a new stage to tremendously broaden the scope of excellent AIEgens. Several examples are presented in the following subsections.…”

“…The plausible intermolecular quantum phase led by aggregation will be hindered owing to the flexible intramolecular motions in these AIEgens. [9] As a result, a variety of explanations have been proposed for AIE phenomena for different organic systems, such as restriction of the intramolecular rotation (RIR), [8,10] restriction of intermolecular motion (RIM) [15] and blocking of nonradiative decay channels, [16] the restriction of E/Z isomerization process, [17] the excited-state intramolecular proton transfer, [18] the blockage of access to dark state via isomerization, [19] restricted access to conical intersection [20] crystalline-induced reversal from dark to bright state, [21,22] Herzberg-Tell vibronic coupling induced emission, [23] and so on. [9,10] Nevertheless, a clear and comprehensive picture…”