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

Abstract

Abstract An organoboron complex containing a [2.2]paracyclophanyl group ( 2 BF 2 ) exhibits remarkable solvatofluorochromism associated with a large Stokes shift. With increased solvent polarity, the fluorescence (FL) color of 2 BF 2 changes from sky blue ( λ FL =485 nm in cyclohexane ( c ‐C 6 H 12 )) to orange ( λ FL =576 nm in CH 3 CN). This behavior derives from the intramolecular charge transfer (ICT) character of the excited state 2 BF 2 * in which the [2.2]paracyclophanyl group serves as the electron donor while the remaining 2 BF 2 structure (apart from the sub‐benzene ring) functions as the electron acceptor. Wave deconvolution of FL spectra in a solvent mixture of CH 2 Cl 2 / c ‐C 6 H 12 , and a plot of the FL emission energy versus the CH 2 Cl 2 / c ‐C 6 H 12 ratio, suggest that FL of 2 BF 2 occurs from two types of 2 BF 2 * excited states. The results of DFT calculations support this explanation and further emphasize that the ICT character of 2 BF 2 * is derived from a large conformational change associated with the electron‐donating character of the [2.2]paracyclophanyl group. Moreover, the fact that 2 BF 2 is more sensitive than Reichardt's dye to polarity changes in a low polarity CH 2 Cl 2 / c ‐C 6 H 12 solvent mixture is also of importance.