Controlling Crystal Structures and Multiple Thermo‐ and Vapochromic Behaviors of Benzimidazole‐Based Squaraine Dyes by Molecular Design and Solvent Adjustment

Abstract

Organic micro- and nanostructures are expected to be promising candidates for micro- and nanophotonic materials with desirable properties owing to their low cost, flexible molecular design, and tunable self-assembly. Among these candidates, well-known squaraine dyes (SQs) have rarely been investigated because of their nonfluorescent properties in the solid state and because their optical behavior varies with changes in morphology. In this contribution, two novel 1,2-SQs, SQM and SQB, with strong bright-yellow to red fluorescence emission in the crystalline state, were designed and structured at the molecular level and by solvent adjustment. Their self-assembly behavior was studied, and it was revealed that the SQM assembly provided 1D microrods, whereas 1D microrods (Z-SQB⋅CH Cl ) and 2D microplates (E-SQB⋅2 CH OH) could be obtained from SQB assemblies through a solution-based self-assembly method. The varied assembly behaviors of these SQs were attributed to different π-π stacking interactions that resulted in different molecular conformations and packing modes. These assemblies exhibited distinct optical properties, and in particular, SQB⋅solvent assemblies showed multiple thermo- and vapochromic effects. Thus, the SQB assemblies are potential fluorescent sensors for organic solvent vapors. More importantly, favorable optical-waveguide properties were observed in these SQ-based microstructures.

First-principles studies of substituent effects on squaraine dyes

“…52,60,61 In the context of promoting stronger exitonic interaction, customizing squaraine dyes enables the introduction of substituents that may alter their electronic structures to make them more favorable for dipole interactions without detrimentally affecting photophysical properties. [62][63][64] There has also been a research interest in manipulating two key excitonic coupling factors, including exciton hopping (or exchange) energy, J m,n , and exciton-exciton interaction energy, K m,n , by controlling the transition dipole m and static dipole difference Dd of a dye monomer. 56,[65][66][67] The maximization of m within a single absorption band is also benetted by the minimization of vibronic coupling of dyes.…”

Emission-Tunable Soft Porous Organic Crystal Based on Squaraine for Single-Crystal Analysis of Guest-Induced Gate-Opening Transformation

“…The intrinsic oxygen atoms in the skeleton of squaric acid can act as natural functional sites for framework construction and selective molecular recognition through hydrogen bonds (H bonds) or Lewis acid/base interactions with special guest species, without the need for postsynthetic modification . In addition, a variety of squaraines (SQs) dyes derived from squaric acid exhibit favorable photophysical properties with intense absorption and emission in the wide range from visible to near infrared, which are sensitive to the surrounding environment. , Recently, a new type of SQs have been reported to exhibit unique solid-state tunable luminescence properties, which can be modulated through control of molecular excited-state vibration and electron behavior . Some of these SQs could achieve temperature-/solvent vapor-triggered SCSC reversible structure transformation associating with tunable luminescence behavior, , implying their structural flexibility and structure-related luminescence.…”

Multistimuli-Responsive Squaraine Dyad Exhibiting Concentration-Controlled Vapochromic Luminescence

“…Recently, our group reported a simple NCSQ molecule (CIEE-SQ, Scheme ) exhibiting tunable solid-state emissions and unique vaporchromic behavior . Several other squaraine derivatives exhibiting intriguing stimuli-responsive luminescence in the solid state have since been reported. − However, although most of these reported squaraines exhibit vapochromic behavior, none of the reports have investigated the effect of vapor concentration on vapor-induced luminescence switching.…”