Highly fluorescent single crystals of a 4-ethoxy-1,3-thiazole

Abstract 4-Ethoxy-2-(pyridin-2-yl)-5-(3,4,5-trimethoxyphenyl)thiazole ( EPTT ) was prepared in a four-step synthetic protocol. Column shaped single crystals of EPTT were obtained from a saturated EtOH solution. Both single crystals and a THF solution of EPTT were investigated in terms of their photophysical properties and electronic structure. The crystals showed very intense fluorescence upon photoexcitation as well as a red shifted emission maximum compared to a THF solution of EPTT (single crystals: λ em  = 523 nm, Φ FL  = 0.82, THF solution λ em  = 477 nm, Φ FL  = 0.67). Quantum chemical methods were then employed to unravel the underlying fundamental transitions and explain the experimental results. Calculations for a small dye cluster modeled after the x-ray crystal structure predicted three dark intermolecular CT states, S 1 -S 3 , below the intramolecular CT state (S 4 ) for EPTT in the crystalline state. Photoexcitation and subsequent relaxation along the excited states relaxation pathways may populate the low-lying dark intermolecular CT states, which in consequence leads to an exciton moving along the stacked dyes in the crystal. This finding explains the significantly enhanced red-shift of the solid-state fluorescence of EPTT compared to the dissolved dye.