Restriction of Conformation Transformation in Excited State: An Aggregation-Induced Emission Building Block Based on Stable Exocyclic C=N Group

Abstract

Summary The development of aggregation-induced emission (AIE) building block and deciphering its luminescence mechanism are of great significance. Here a feasible strategy for the construction of AIE unit based on E - Z isomerization (EZI) of exocyclic C=N double bond is proposed. Taking [1,2,4]thiadiazole[4,3- a ]pyridine (TZP) derivative as an example, its aryl-substituted derivative (TZPP) shows obvious AIE character. The analysis of spectral data and theoretical calculations indicates that fast structural relaxation of TZPP in the emissive state plays a key role in a low fluorescence quantum yield in dilute solution, which should be caused by the small energy gap between locally excited (LE) state and twisted intramolecular charge transfer state. When in solid state, the bright emission with LE state characteristic reappears due to the large shift barrier of geometry transformation. As a potential building block for AIEgens with special heterocyclic structure, these findings would open up opportunities for developing various functional materials.

Restriction of Intramolecular Motion(RIM): Investigating AIE Mechanism from Experimental and Theoretical Studies

“…For excited-state evolution of most double bond-based AIEgens, photoisomerization and photocyclization, which are based on low-frequency intramolecular motion, always contribute a lot to the non-radiative decay in quenching state [90,91] . Photoisomerization corresponds to excited-state double-bond torsion, double-bond elongation and phenyl-ring twisting, while photocyclization usually means photocyclized intermediate with newly formed chemical bond.…”

A peroxo‐Mo(VI)/Mo(VI)‐mediated redox synthesis of quinazolin‐4(3H)‐ones and their aggregation‐induced emission property and mechanism

“…To elucidate the AIE mechanism of non‐propeller‐shaped quinazolin‐4(3 H )‐ones, the potential energy curves along the dihedral angle of N1–C2–C1′–C2′ ( φ ) in both S 0 and S 1 states of 3a and 3c were obtained (Figure 5A,C). [ 40,41 ] The theoretical calculation results of 3a showed that φ = 25.9° in the optimized S 0 geometry and the phenyl ring is rotatable due to the low energy barrier ( E a = 3.8 kcal/mol). Once 3a is excited to the S 1 state, the phenyl ring can only twist within the ranges of 0–50° and 130–180°, which have relatively low energy barrier ( E a < 6.6 kcal/mol).…”

“…To elucidate the AIE mechanism of non-propeller-shaped quinazolin-4(3H)-ones, the potential energy curves along the dihedral angle of N1-C2-C1 0 -C2 0 (φ) in both S 0 and S 1 states of 3a and 3c were obtained (Figure 5A,C). [40,41] F I G U R E 3 Molecular orbital amplitude plots of highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels of 3a, 3f, 3h, and 3c calculated by using B3LYP/6-311G(d,p) basis set…”

Coordination Polymerization-Induced Emission Based on a Salicylaldehyde Hydrazone AIEgen toward Zn2+ Detection

“…Interestingly, the Schiff base as the AIE-active compound is due to excited-state intramolecular proton transfer (ESIPT) mechanism, which can be further functionalized with the free rotation motif to enhance the AIE responsiveness. − In contrast to TPE derivatives, the Schiff base AIEgen has become another candidate structure for probe design because of its rich source of synthetic raw materials, simple preparation, and strong coordination ability. , Especially for metal-ion sensing, the coordination interaction can restrict the intramolecular rotation and affect the ESIPT procedure, thus leading to an emission switching. , To promote the coordination polymerization effect, in this work, we design a Schiff base AIEgen (H 2 hbih ) with the strategy of multibinding site by condensation of the isonicotinic moiety and salicylaldehyde (Scheme b). H 2 hbih demonstrates high ability in polymerization with Zn 2+ , leading to aggregation and fluorescence enhancement by detecting Zn 2+ as low as 2.8 × 10 –7 M. The mechanism of Zn 2+ sensing was proved by the single-crystal structure of the corresponding 2D (two-dimensional) coordination polymer of the Zn 2+ complex ( 1 ).…”