A Quaternary β -Cyclodextrin–Viologen Inclusion Complex with Fast Photochromism and Strong Fluorescence for UV Dosimetry and Information Encryption

A quaternary inclusion complex (TPV3+@β-CD3) was successfully assembled by encapsulating a viologen analogue bearing three pyridinium groups (TPV·Cl3) into three β-cyclodextrin (β-CD) hosts, as confirmed by NMR spectroscopy, fluorescence spectroscopic titration, and ESI-MS. For comparison, a ternary inclusion complex (DPV2+@β-CD2) was also prepared using a conjugation-extended viologen (DPV·Cl2). In the solid state, TPV3+@β-CD3 exhibited fast-response, high-contrast, and reversible photochromic behavior, with a coloration saturation time of only 90 s. Moreover, owing to the macrocyclic confinement effect that effectively suppresses aggregation-caused quenching (ACQ), TPV3+@β-CD3 displayed strong yellow fluorescence under UV light (λex = 365 nm) with a quantum yield of 15.71%, whereas the free guest TPV·Cl3 showed no detectable emission. Electron spin resonance (ESR) spectra and density functional theory (DFT) calculations revealed that the photochromism originates from photoinduced electron transfer from β-CD to the pyridinium units, generating free radicals. Notably, the fluorescence intensity of the inclusion complex can be dynamically modulated upon light irradiation, with up to 59.8% quenching upon reaching the colored state. Benefiting from these dual photoresponsive properties, TPV3+@β-CD3 was successfully applied as a colorimetric sensor for UV radiation dosage, as well as for multiple anti-counterfeiting and multilevel information encryption.