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

Abstract

An 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.

“T‐Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane‐Containing Organoboron Complexes

“…As we previously described [ 32 ], τ FL of ( rac )-1BF 2 -H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing τ FL to that of the monomer FL in solution.…”

“T‐Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane‐Containing Organoboron Complexes

“…As we previously described [ 32 ], τ FL of ( rac )-1BF 2 -H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing τ FL to that of the monomer FL in solution.…”

“T-Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane-Containing Organoboron Complexes

“…As we previously described, [32] tFL of (rac)-1BF2-H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing tFL to that of the monomer FL in solution.…”

Remarkable Piezofluorochromism of an Organoboron Complex Containing [2.2]Paracyclophane

“…3a). Such an intermolecular p-stacking dimer was not observed in the pCP-H crystal, 25 being indicative of steric effects of the tert-butyl group. Inspection of Fig.…”

“T‐Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane‐Containing Organoboron Complexes

“…As we previously described [ 32 ], τ FL of ( rac )-1BF 2 -H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing τ FL to that of the monomer FL in solution.…”

“T-Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane-Containing Organoboron Complexes

“…As we previously described, [32] tFL of (rac)-1BF2-H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing tFL to that of the monomer FL in solution.…”

Remarkable Piezofluorochromism of an Organoboron Complex Containing [2.2]Paracyclophane

“…3a). Such an intermolecular p-stacking dimer was not observed in the pCP-H crystal, 25 being indicative of steric effects of the tert-butyl group. Inspection of Fig.…”

“T‐Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane‐Containing Organoboron Complexes

“…As we previously described [ 32 ], τ FL of ( rac )-1BF 2 -H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing τ FL to that of the monomer FL in solution.…”

“T-Shaped” Excimer Fluorescence of Chiral Crystals of [2.2]Paracyclophane-Containing Organoboron Complexes

“…As we previously described, [32] tFL of (rac)-1BF2-H in dilute solution depends significantly (3.9-16.8 ns) on the polarity of the solvent. Because of differences in excited-state dynamics, it is difficult to assign the FL species present in crystalline state by simply comparing tFL to that of the monomer FL in solution.…”

Remarkable Piezofluorochromism of an Organoboron Complex Containing [2.2]Paracyclophane

“…3a). Such an intermolecular p-stacking dimer was not observed in the pCP-H crystal, 25 being indicative of steric effects of the tert-butyl group. Inspection of Fig.…”