Flexible control of excited state transition under pressure/temperature: distinct stimuli-responsive behaviours of two ESIPT polymorphs

Abstract

Realizing fexible and tunable emission conversion in organic solid-state luminescent materials is always fascinating. Herein, we successfully achieved fexible photochemical conversion under pressure and temperature on two types of 4-methyl-2-(5-[4-dimethylaminophenyl]-1H-pyrazole-3-yl)phenol (4MPP) single crystals with distinct aggregation form and geometry showing as green (GC) and blue-violet (BVC). These conversions were studied in detail by fluoresence and Raman spectroscopies, followed with Hirshfeld surface analysis, and this study provides a significant material system for understanding the mechanisms of structural conversion between normal emission (E*) and excited state intramolecular proton transfer (ESIPT, K*) comprehensively under pressure and temperature. Owing to having a proton transferred tautomer, BVC exhibits remarkable changes in its emission feature with pressure increasing to 4.79 GPa, along with the blue-shift and fluorescence enhancement according to a conversion from E* to K*. This compression-induced conversion is proved to be caused by the contraction of the intermolecular distances, the increase of molecular planarization degree, and the enhanced O–H⋯N intramolecular interaction, along with the reduced distance of H⋯N and the elongation of the bond length of O–H. Differently, GC shows rapidly weakened fluorescence and slight blue-shift at a relatively lower pressure (1.01 GPa) based on its more restricted structure. Moreover, GC can realize an excited state transition from K* to E* when lowering its temperature. BVC experiences an abrupt phase transformation to GC at a high temperature of 190 °C. This work discloses the photo-physical properties and stimuli-responsive behaviours of two 4MPP polymorphs, which provides deep insight into the relationship between molecular structure and photoluminescent nature, especially in the crystals with the ESIPT property.