Remarkable Solvatofluorochromism of a [2.2]Paracyclophane‐Containing Organoboron Complex: A Large Stokes Shift Promoted by Excited State Intramolecular Charge Transfer

Abstract

AbstractAn organoboron complex containing a [2.2]paracyclophanyl group (2 BF2) exhibits remarkable solvatofluorochromism associated with a large Stokes shift. With increased solvent polarity, the fluorescence (FL) color of 2 BF2 changes from sky blue (λFL=485 nm in cyclohexane (c‐C6H12)) to orange (λFL=576 nm in CH3CN). This behavior derives from the intramolecular charge transfer (ICT) character of the excited state 2 BF2* in which the [2.2]paracyclophanyl group serves as the electron donor while the remaining 2 BF2 structure (apart from the sub‐benzene ring) functions as the electron acceptor. Wave deconvolution of FL spectra in a solvent mixture of CH2Cl2/c‐C6H12, and a plot of the FL emission energy versus the CH2Cl2/c‐C6H12 ratio, suggest that FL of 2 BF2 occurs from two types of 2 BF2* excited states. The results of DFT calculations support this explanation and further emphasize that the ICT character of 2 BF2* is derived from a large conformational change associated with the electron‐donating character of the [2.2]paracyclophanyl group. Moreover, the fact that 2 BF2 is more sensitive than Reichardt's dye to polarity changes in a low polarity CH2Cl2/c‐C6H12 solvent mixture is also of importance.