Conformationally Dynamic π-Conjugation: Probing Structure–Property Relationships of Fluorescent Tris(N-salicylideneaniline)s

Abstract

We recently reported the design and synthesis of a series of conformationally dynamic chromophores that are built on the C3-symmetric tris(N-salicylideneaniline) platform. This system utilizes cooperative structural folding–unfolding motions for fluorescence switching, which is driven by the assembly and disassembly of hydrogen bonds between the rigid core and rotatable peripheral part of the molecule. Here, we report detailed time-resolved spectroscopic studies to investigate the structure–property relationships of a series of functionalized tris(N-salicylideneaniline)s. Time-resolved fluorescence decay spectroscopy was applied to determine the main relaxation mechanisms of these π-extended fluorophores, and to address the effects of hydrogen bonding, steric constraints, and extension of the π-conjugation on their relaxation dynamics. Our results agree well with the conformational switching model that was previously suggested from steady-state experiments. Notably, extension of the π-conjugation from peripheral aryl groups resulted in the stabilization of the excited states, as evidenced by longer lifetimes and lower non-radiative decay constants. As a consequence, an increase in the fluorescence quantum yields was observed, which could be explained by the suppression of the torsional motions about the C–N bonds from an overall increase in the quinoid character of the excited states. A combination of time-resolved and steady-state techniques also revealed intermolecular interactions through π–π stacking at higher concentrations, which provide additional de-excitation pathways that become more pronounced in solid samples.

Theoretical studies on charge transport and optical properties of tris(N-saclicylideneanilines)

“…TSANs exhibit multiple proton transfer process, and this property makes them a promising material for molecular electronics applications such as switches. 24,25 In general, most of the reported discotic liquid crystalline molecules, such as triphenylene and hexabenzocoronene, are p-type materials, and only a few studies on n-type materials are reported in the literature. 6 A few discotic molecules, such as hexaazatriphenylene and perylene diimide, 6,7,26 were reported to be n-type materials.…”

“…TSAN5 and TSAN6 exhibit the same tautomerism as TSAN2 and TSAN3 but with three OCH 3 groups at the end phenyl rings. Although many experimental studies 18,19,[21][22][23]25 have been performed on TSANs, only a few theoretical studies have been reported. 24,28 Previous experimental studies on TSANs have shown that these molecules have uorescent properties due to the extended p-conjugation.…”

The Rich Tautomeric Behavior of Campestarenes

“…[27][28][29] In the particular case of the enol-iminem otif shown in Figure 1, as trong intramolecular hydrogen bond is involved in keepingt he molecule nearly planar,a nd this conformation facilitates intramolecular protont ransfer. Only in af ew cases has the keto-enamine form been reported as the majori somer in salicylaldimines, [32][33][34][35][36] but it is known to be more stable in other polyaromatic o-hydroxyimine compounds. Only in af ew cases has the keto-enamine form been reported as the majori somer in salicylaldimines, [32][33][34][35][36] but it is known to be more stable in other polyaromatic o-hydroxyimine compounds.…”

“…Thee nol-iminef orm is generallyt he mosts table at room temperature, [30,31] even if it is in rapid equilibrium with the keto-enamine form. Only in af ew cases has the keto-enamine form been reported as the majori somer in salicylaldimines, [32][33][34][35][36] but it is known to be more stable in other polyaromatic o-hydroxyimine compounds. [37,38] The equilibrium between the enol-imine and the keto-enamine forms, togetherw ith the resonance contribution of az witterionic intermediate, explains the unusual stability of this imine, and the locked conformation (trans for the imine, cis for the enamine)o ft his motif.…”

Stabilization of a Strained Heteroradialene by Peripheral Electron Delocalization

“…Surprisingly, these new tris­(salicylaldimine) molecules 2 (TSANs) exist exclusively as the keto-enamine tautomer 2k rather than the expected enol-imine tautomer 2e . While the keto-enamine motif has been exploited in a variety of discrete and extended structures, − little work has been done to explain its dominance over the canonical enol-imine structure. These new molecules challenge our notions of aromaticity and beg the following questions: What stabilizes the TSANs to compensate for the loss of aromaticity in 2k vs 2e ?…”