Deeper insight into the multifaceted photodynamics of a potential organic functional material emphasizing aggregation induced emission enhancement (AIEE) properties

Abstract

The multifaceted photodynamics of a simple azine based organic functional molecule namely 2-((Z)-((E)-(pyren-1-ylmethylene)hydrazono)methyl)quinolin-8-ol (PHQ) emphasizing its brilliant fluorescence emission redemption properties in aggregation state (AIEE) have been primarily explored through absorption and steady state emission techniques. The governing role of photo-induced electron transfer (PET) rates and active intramolecular motions of non-interacting PHQ monomers at lower water fraction comprising mixed solvent systems have been identified to be the prime reasons for non-radiative annihilation of photoexcited states. The transition from weakly emissive to highly emissive state has been substantiated through elaborate study using time-resolved photoluminescence (TRPL), fluorescence quantum yield and variation of external control experiments. In the current study, the increased rotational relaxation time of aggregated hydrosol, responsible for AIEE, is investigated using time-resolved anisotropy measurement (TRAM) of different PHQ microenvironments, which is unprecedented to the best of our knowledge in AIEE research. Interestingly, the high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image shows one dimensionally grown molecular entity of a single nano-sheet like structure of PHQ, which is a unique observation. The average particle size of PHQ aggregates is also increased from 84.5 nm to 814 nm corresponding to 10% and 90% fw (water volume %) system respectively. The energy-dispersive X-ray spectroscopy (EDX) has also been employed, for the first time in AIEE research, which reveals a gradual increase in the amount of carbon within the aggregated microstructure with addition of water. The present molecular system PHQ, being a molecular rotor system, provides future prospect for probing local microenvironmental viscosities within biological systems.