Scheme 1. Design of model molecule HBT-DPI with multi-dimensional H-bonds.
Excited-state intramolecular proton transfer (ESIPT) is known as an H-bond dependent photophysical process that requires an intramolecular H-bond to trigger the enol to keto conversion upon photoexcitation, leading to a relatively large emission shift and high quantum yield.26-29 The strategy of shutting intramolecular H-bond via isomerization has been utilized to design smart ESIPT materials for anti-counterfeiting,30sensing,31,32 and force-response luminescent material.33 Notably, a recently reported (2-(((1H-benzo[d]imidazol-2-yl)imino)methyl)-4-methoxyphenol system, which contains multiple H-bond donor and acceptors, exhibited a remarkable sensitivity in distinguishing heavy water from the water via the regulation of intermolecular H-bonds.34 Despite the successes in their specific applications, these ESIPT chromophores possess limited emission alterations due to insufficient diversity in the regulation of intra- and intermolecular H-bonds. It is worth noting that intermolecular H-bond governed molecular packing reconstruction is a major strategy to accomplish emission alterations in many fluorescent materials35,36 other than ESIPT fluorophores, which suggests a potential but challenging route to extend the tunability in photochemical properties and application scenarios of ESIPT systems via regulating intra- and intermolecular H-bonds.
Surfaces are fundamental and prevalent in nature and industry, whose surface properties, especially hydrophobicity/hydrophilicity, are of great importance in many practical applications, such as biological adhesion,37,38 coating, 39,40anti-fogging, 41 oil-water separation,42,43, and catalysis. 44Thereby, characterizations of surface hydrophobicity/hydrophilicity are critical in understanding and controlling the surface behaviors. However, the conventional contact angle method falls short in offering a comprehensive measurement on such surfaces, while the heterogeneity might also lead to uncertainties in measurements. 45Considering that the abundance of interfacial H-bond acceptors is closely relevant to the surficial hydrophobicity/hydrophilicity,46-48 fluorescence-based method that is sensitive to H-bonding might offer a solution to complement the contact angle method in the scenario of large-scale heterogeneous surfaces.
In this study, we explored the potential of multi-dimensional regulation of intra- and intermolecular H-bonds in single molecule ESIPT systems to expand the variety of emissions. We designed and synthesized a model ESIPT chromophore HBT-DPI by incorporating a diphenylimidazole (DPI ) group into HBT (Scheme 1). Unlike conventional ESIPT chromophores, the presence of two H-bond accepting groups (HBAs), benzothiazole (BT ) (HBA-1) and DPI (HBA-2), provides an extra choice for the intramolecular H-bond with the hydroxyl group in the phenol core. Moreover, the N-H in imidazole, as an H-bond donor, along with the bulky diphenyl group could offer flexibility in subtle modulation of intermolecular H-bond formation. In response to different solvent environments, four types of HBT-DPI crystal/cocrystals showed diversity in the contents of structural isomers, molecular packing modes, and photophysical properties, resulting from the varied behaviors in intra- and intermolecular H-bonds formation. Notably, we successfully employed HBT-DPI to visualize the surficial hydrophobicity/hydrophilicity distribution along with their quantification on heterogeneously modified PVDF membranes, demonstrating a new approach for hydrophobicity/hydrophilicity monitoring and measurement on a large-scale surface with heterogeneous modification. Overall, this study provides a new strategy to construct ESIPT-inspired chromophores whose single molecular emissions are regulated by multiple-dimensional H-bonds, and demonstrated the unique application in hydrophobicity/hydrophilicity mapping on a large-scale heterogeneous surface.