TY - JOUR
T1 - Photochromic dibenzobarrlenes
T2 - Long-lived triplet biradical intermediates
AU - Sajimon, Meledathu C.
AU - Ramaiah, Danaboyina
AU - Suresh, Cherumuttathu H.
AU - Adam, Waldemar
AU - Lewis, Frederick D.
AU - George, Manapurathu V.
PY - 2007/8/1
Y1 - 2007/8/1
N2 - Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 ± 2 s (Ea for decoloration is 14.5 ± 0.7 kcal mol-1 and log A is 8.92 ± 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 ± 0.2 × 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular δ-H abstraction, to yield the triplet biradical intermediate 32. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The δ-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical 32, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical 32 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical 32 in the photochromism of appropriately substituted dibenzobarrelenes.
AB - Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 ± 2 s (Ea for decoloration is 14.5 ± 0.7 kcal mol-1 and log A is 8.92 ± 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 ± 0.2 × 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular δ-H abstraction, to yield the triplet biradical intermediate 32. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The δ-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical 32, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical 32 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical 32 in the photochromism of appropriately substituted dibenzobarrelenes.
UR - http://www.scopus.com/inward/record.url?scp=34547650509&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=34547650509&partnerID=8YFLogxK
U2 - 10.1021/ja0719125
DO - 10.1021/ja0719125
M3 - Article
C2 - 17625852
AN - SCOPUS:34547650509
SN - 0002-7863
VL - 129
SP - 9439
EP - 9445
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 30
ER -