Substituent‐controlled aggregate luminescence: Computational unraveling of S₁/S₀ surface crossing

Abstract

We computationally investigated the molecular aggregation effects on the excited state deactivation processes by considering both the direct vibrational relaxation and the S0/S1 surface crossing, that is, the minimum energy conical intersection (MECI). Taking classical AIEgens bis(piperidyl)anthracenes (BPAs) isomers and the substituted silole derivatives as examples, we show that the deformation of MECI always occurs at the atom with greater hole/electron overlap. Besides, the energetic and structural changes of MECI caused by substituent has been investigated. We find that effective substituent such as the addition of the electron‐donating groups, which can polarize the distribution of hole/electron density of molecules, will lead to the pyramidalization deformation of MECI occurring at the substituent position and simultaneously reduce the required energy to reach MECI. And MECI is sterically restricted by the surrounding molecules in solid phase, which remarkably hinders the non‐radiative decay through surface crossing. Through quantitative computational assessments of the fluorescence quantum efficiency for both solution and solid phases, we elucidate the role of MECI and its dependence on the substitutions through pyramidalization deformation, which give rise to the aggregation‐induced emission (AIE) phenomenon for 9,10‐BPA, to aggregation‐enhance emission (AEE) behavior for 1,4‐BPA, and to conventional aggregation‐caused quenching (ACQ) for 1,5‐BPA. We further verify such mechanism for siloles, for which we found that the substitutions do not change the AIE behavior. Our findings render a general molecular design approach to manipulating the aggregation effect for optical emission.

Mechanistic insights into diversified photoluminescence behaviours of BF2 complexes of N-benzoyl 2-aminobenzothiazoles

“…The optimized MECI geometry of BFBB-1 was obtained by using a spin-flip time-dependent density functional theory (SF-TDDFT) method (Table S5, ESI †). 27,29 The energies of the key points involved in both vibrational relaxation and S 0 /S 1 surface crossing channels were calculated using the Gaussian 16 package. As shown in Fig.…”

“…In the past few years, theoretical investigations based on a twochannel model, 24,25 which takes account of both the vibronic relaxation and S 0 /S 1 surface crossing pathways, have revealed that minimum energy conical intersection (MECI) induced by the structural deformation in different types of polyheteroaromatic dyes, 26,27 including BF 2 -containing polyheterocycles, 17,28,29 is actually the underlying channel for their nonradiative decay. Inspired by these reports, we herein specifically synthesized four BF 2 complexes of N-benzoyl 2-aminobenzothiazoles (BFBB-1-4) with or without EDGs as model compounds to address the issues mentioned above.…”

Aggregation-Induced Emission (AIE), Life and Health

“…This photophysical behavior has been termed aggregation-induced emission (AIE). After more than 20 years of development in the field of AIE, the working mechanism of restriction of intramolecular motion (RIM) has been established and well-recognized. , RIM can lead to conformational rigidification, thereby suppressing nonradiative decay pathways, e.g., vibronic coupling, conical intersection, and photochemical reactions, thus contributing to the enhanced luminescence process. − The discovery and exploration of AIE has also greatly advanced the field of precision medicine and light-based healthcare. , For fluorescence imaging, AIE luminogens (AIEgens) and their formed NPs are endowed with the inherent advantages of low background noise, fair photostability, − large Stokes shift, − biocompatibility, − high brightness, ,− and deeper tissue penetration and higher spatial resolution. , In particular, the tetraphenylethylene (TPE) skeleton is the most versatile AIEgene because it can be designed to have various desired functions by introducing functional groups or π-extensions …”

Dense and Acidic Organelle-Targeted Visualization in Living Cells: Application of Viscosity-Responsive Fluorescence Utilizing Restricted Access to Minimum Energy Conical Intersection

“… 41 , 43 − 46 They showed by a theoretical study that this molecule adopts a Dewar-benzene-like nonflat structure at the MECI due to a large structural change from the planar geometry at the Frank–Condon state, together with experimentally confirmed viscosity responsiveness of this molecule. 41 , 47 We expected that its smaller molecular size makes it possible to design highly biocompatible molecules.…”

“…In this study, we focused on viscosity-responsive fluorescence caused by restricted access to minimum energy conical intersection (MECI). − If a large conformational change is required to access the MECI in the excited state, fluorescence of the molecules should be responsive to viscosity since high viscosity of the surrounding environment restricts the transition to the MECI. To demonstrate the application of this strategy for developing viscosity-responsive fluorescent probes that can work in cellular systems, we chose 9,10-bis­( N , N -dialkylamino)­anthracene, reported by Konishi and co-workers. ,− They showed by a theoretical study that this molecule adopts a Dewar-benzene-like nonflat structure at the MECI due to a large structural change from the planar geometry at the Frank–Condon state, together with experimentally confirmed viscosity responsiveness of this molecule. , We expected that its smaller molecular size makes it possible to design highly biocompatible molecules.…”