Relationship between radiative characteristics and molecular geometry: Photophysical analysis of 2-phenylnaphthalene-appended ortho-carboranyl luminophores

Abstract

This study investigates the photophysical features of aromatic conjugated o-carboranes and provides strong evidence for the relationship between their molecular geometry and photoluminescence efficiency. Four 2-phenylnaphthalene appended o-carboranyl luminophores, each with a different 2-phenyl ring distortion angle (1F, 2P, 3M, and 4I) were prepared and characterized. In particular, the geometric features around the 2-phenyl ring in the crystalline state were fully determined using single-crystal X-ray diffraction. All compounds in rigid states (solution at 77 K and solid states) demonstrated strong intramolecular charge-transfer (ICT)-based emissions in the yellowish-green region. Intriguingly, the quantum efficiency of the ICT-based emission gradually decreased as the distortion of the terminal 2-phenyl ring increased. Furthermore, the radiative decay constant of 1F was five times that of 4I, whereas the nonradiative decay of 1F was less than one-third that of 4I. In addition, theoretical modeling of the low-energy transition in the first excited state of 2P upon the rotation of the 2-phenyl ring revealed that the distorted geometry strongly affects the ICT-based radiative process. This model strongly suggests that the ICT-based radiative efficiency of π‒conjugated o-carborane is significantly affected by the geometry of the appended aromatic group.