Tunable Non-Kasha Behaviors and Excited-State Dynamics of Quadrupolar Squaraine Aggregates

Abstract

Versatile coupling theories have been developed for rationalizing unusual aggregation phenomena of multipolar chromophores. Here, diverse excitonic couplings of a quadrupolar squaraine dye protonated by trifluoroacetic acid could be achieved and tuned unprecedentedly in different solvation media. Subtle changes of the solvent and ion pair influenced the aggregation of the donor–acceptor–donor (D–A–D)-type SQC6 and led to significant variations in optical properties. In contrast to conventional H/J aggregates, strong spectroscopic evidence of nonfluorescent and red-shifted hJ aggregation was obtained. Assumptions of the excitonic interplay with variable strength stabilized by the synergic contributions of π–π stacking and electronic interaction were addressed. Comparative excited-state dynamics in the aggregates clarified the distinctive excitonic coupling of adjacent quadrupolar molecules and the nature of the excited state beyond the dimers. Meanwhile, dominant two-photon absorption transitions could be elucidated by a resonance-enhanced mechanism. The present unusual molecular interplay provides a strategy to fine tune the optical properties of multipolar aggregates.

Comparative Observation of Distinct Dynamic Stokes Shifts in Diaryl BODIPY Triads with Broadband Two-Photon Absorption

“…In detail, the δ 2PA maximum for AN-BDP was found to be 488 GM (GM  10 –50 cm 4 ·s·photon –1 ·molecule –1 ) at 670 nm (Figures c and S2). Based on the two-photon excited fluorescence method, the quadratic dependence of the fluorescence intensity on the incident laser power was verified to be around 1.91, indicating the intrinsic 2PA nature (Figure d). − Meanwhile, the difference between 1PA and degenerate 2PA maxima could be resonance enhancement dominated by the detuning energy and large μ ge values, in agreement with the previous findings. ,,, …”

“…Therefore, the red-shifted DSS with a lower energy transition was modeled (Figure b). We know that rapid conjugation perturbation through symmetry breaking and subsequent charge separation generally stabilized the resultant CT state, , while as it was evident from Figure b, the energy levels for CF 3 Ph-BDP and CH 3 OPh-BDP with a relatively weak CT character should be converted properly. The structural planarization might first dominate the relaxation momentarily and facilitate the ensuing CT process.…”

“…34−37 Meanwhile, the difference between 1PA and degenerate 2PA maxima could be resonance enhancement dominated by the detuning energy and large μ ge values, in agreement with the previous findings. 22,29,38,39 Structural Characterization and Ultrafast Transient Absorption (TA) Measurements of the Diaryl BODIPY Triads. As confirmed by the crystallographic data of AN-BDP, the dihedral angles between the end-capping anthracenyl units and the diaryl BODIPY core might be 59.8 and 65.4°due to the steric repulsion (Figure S3 and Table S2).…”

Structure–activity relationships of aniline-based squaraines for distinguishable staining and bright two-photon fluorescence bioimaging in plant cells

“…The degenerate δ 2PA values of SQs were investigated in THF and acquired by a standard open-aperture Z-scan method employing the excitation of 200 femtosecond pulses. 35–37 Z-scan profiles of the SQs showed intense transmission valleys indicative of typical reverse saturable absorption behaviors (Fig. 1d and Fig.…”

“…1c). 35 We speculated that despite the structural distortion because of the close-core substituents, the squaraine skeleton still retained a certain amount of overall planarity which would facilitate the intermolecular π–π stacking interaction for aggregation. 22,30 A low Φ F value of ∼0.03 and a short fluorescence lifetime of ∼2.18 ns for SQ3 in the aqueous medium could be attributed to its efficient interactions with surrounding solvent, resulting in the non-radiative decay processes (Fig.…”