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.