Investigation of Aggregation-Induced Emission Mechanism of a Styrene Derivative: A Theoretical Unraveling of S ₁ /S ₀ -Conical Intersection

The mechanisms behind the fluorescence quenching of the styrene derivative, 4-dimethylamino-2-benzylidene malonic acid dimethyl ester (BIM) in solution are investigated computationally in the present study. Utilizing a penalty-constrained search algorithm of geometry optimization along with spin-flip long-range corrected time-dependent density functional theory (SF-LC-TDDFT), we examined the energetic and structural changes of the BIM molecule at the minimum energy conical intersection (MECI) between S₁ and S₀ states. Our results reveal that the S₁ excited state approaches the MECI through simultaneous rotation of the α-bond and the pyramidalization of the ethylenic CC bond. Notably, the S₁-S₀ energy gap at MECIs are 1.83 and 9.26 kJ/mol (nearly degenerate), and the oscillator strength (<i>f</i>) values are 0.000008 au, and 0.00003 au, indicating a nonradiative decay in solution state. Quantitatively, the radiative decay rate (<i>k</i> _<i>r</i> ) is found to be 2.44 × 10⁶ s^-1, whereas the nonradiative rates via MECIs reach 10¹¹-10¹² s^-1, underscoring the efficiency of internal conversion. The geometry observed at the MECI, which corresponds to a charge-transfer state, strongly indicates that the BIM molecule operates as a twisted intramolecular charge transfer (TICT) system (as evidenced from the response coefficients of the corresponding frontier orbitals), with quenching facilitated by α-torsional rotation. Our findings delineate a powerful approach to molecular design that effectively manipulates the aggregation effects for enhanced optical emission.