Colorimetric and Fluorescent “Light‐Up” Probe to Investigate Intramolecular Charge Transfer and Photoinduced Electron Transfer Mechanisms for the Detection of Cu ²⁺ and a Solid‐State Emitter for the Visualization of Latent Fingerprints

We have reported perylene tetra-(alkoxycarbonyl) (PTAC) derivative PTE-EA functionalized with tetraoctylester chains at the peri-positions and an electron-donating group 2-aminoethanol (2-AE) at the mono bay-position. PTE-EA showed colorimetric and fluorescence turn-on response for detection of cupric ions (Cu²⁺) in binary aqueous-organic mixtures. Upon Cu²⁺ addition, the solution PTE-EA in 30% HEPES buffer (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)-CH₃CN (pH 7.3) showed a decrease in intramolecular charge transfer (ICT) absorption band at 536 nm with an increase in perylene's absorption at 460 nm. Significant fluorescence enhancement at 520 nm via inhibition of the acceptor-excited photoinduced electron transfer (PET) process from 2-AE to the electron-deficient perylene core was observed. Upon Cu²⁺ complexation, the increases in HOMO-LUMO energy gap from 2.689 to 2.824 eV and lowering of the HOMO energy from -5.350 to -10.766 eV supported the experimental bathochromic shift in the absorption maxima and thermodynamically unfavorable PET enabled fluorescence turn-on response, respectively. The formation of the PTE-EA-Cu²⁺ complex was further corroborated by dynamic light scattering (DLS) studies, and cyclic voltammetric (CV) profile exhibited marked alterations. Also, PTAC as mono-brominated PTE-II with inherent solid emission was used to visualize latent fingerprints (LFPs) both under 365 nm and daylight to achieve second level of information.