8-(1H-Benzo[d]imidazol-2-yl)quinolin-7-ol (HQB) has structural asymmetry and contains double hydrogen bonds (HBs), which undergo excited state intramolecular charge transfer and double proton transfer (ESICT/ESIDPT). HQB exhibits three absorption peaks at 360, 315, and 305 nm. On photoexcitation, HQB displays dual emission at 420 and 570 nm, with Stokes’ shifts of 60 and 210 nm, respectively. The dual emission with large Stokes’ shifts and growth of an absorption shoulder peak at ∼400 nm in high polarity environments depict excited state intramolecular charge transfer (ESICT) coupled with excited state intramolecular proton transfer (ESIPT). The calculated Franck–Condon (FC) vertical excitation peaks and shifts in electron density in the frontier molecular orbitals (FMOs) involved in the transition endorse the observed absorption peaks and ESICT phenomenon. Atoms in molecules (AIM) calculations establish dual HBs in the form of O₁₆–H₁₇⋯N₂₉ and N₃₀–H₂₈⋯N₁₅. The four tautomeric forms (HQB, SPT(OH), SPT(NH) and DPT) establish an energy relationship of SPT(OH) > DPT > SPT(NH) > HQB for the S₀ state and HQB > SPT(OH) > DPT > SPT(NH) for the S₁ state. On S₀ → S₁, HQB displays a red-shift of 212 cm⁻¹ for N₃₀–H₂₈ vibrational stretching, which favors the tautomeric conversion HQB* → SPT(NH)*. Once HQB* → SPT(NH)* conversion takes place, the O–H vibrational stretching displays a red-shift of 178 cm⁻¹, which strengthens the O₁₆–H₁₇⋯N₂₉ intramolecular HBs and thus promotes dual proton transfer in the S₁ state. A small energy barrier in the proton transfer (PT) pathway and resemblance of predicted emission peaks with experimental results support the ESIDPT phenomenon.

Chemosensors that demonstrate a ratiometric change in fluorescence are in demand, but are scarce for nitroaromatic compounds owing to fluorescence quenching by nitro-aromatic compounds (NACs). Here, we report that the aggregates of the tricationic fluorescent-red chemosensor DMAS-TP in buffer undergo simultaneous fluorescence quenching at 615 nm and an increase in fluorescence intensity between 510–540 nm, revealing an unprecedented ratiometric behaviour for 2,4,6-trinitrophenol (TNP). The amplified fluorescence change in the FI_615nm/FI_521nm ratio from 57.67 to 30.18 (∼50%) with only 0.2 equivalents (2 μM) of TNP allows determination of TNP at a concentration as low as 2 nM. Paper strips coated with DMAS-TP differentiate TNP from other NACs through a fluorescence colour change from pink-red to blue with TNP only and provide unprecedented recognition of TNP from other NACs. In comparison to the only reported ratiometric probe for TNP, DMAS-TP provides advantages in terms of its usability in water and lower limit of detection. Furthermore, DMAS-TP adsorbed on silica nano-particles (10–20 nm) reveals a high fluorescence quantum yield (ϕ = 10.72%) with an emission maxima at approximately 615 nm, in the far-red region. These fluorescent silica nanoparticles NPs can be used to visualize latent fingerprints on a variety of substrates such as aluminium foil, metal sheets, coins, ceramic tiles and so on, with a high resolution and could be used in modern forensic applications.

Latent fingerprints (LFPs) identification is of paramount importance for national security and criminal investigations. We designed and synthesized fluorescent organic molecule HPBI for the rapid fluorescence imaging of latent fingerprints on various surfaces such as aluminium foil, currency paper, brick, and glass, which did not require any post treatment. The HPBI exhibits a combination of ESIPT and AIE properties and forms green fluorescent nano-aggregates in CH₃CN–H₂O (1 : 1) mixture, as evaluated by DFT, SEM, TEM, and fluorescent microscopic studies. Spraying the solution of spherical HPBI nanoaggregates (50–100 nm size) on various surfaces resulted in fluorescence display of LFPs in <2 minutes. LFPs clearly revealed level-2 (ridge ending, islands, bifurcation) and level-3 (ridges path deviation, edge contours, sweat pores) details with high contrast and no background interference. The relocation of fluorescent fingerprints on cello tape provides an efficient and clean method for carrying fingerprints to the forensic laboratory.