Intramolecular Long-Distance Electron Transfer and Triplet Energy Transfer. Photophysical and Photochemical Studies on a Norbornadiene−Steroid−Benzidine System

Abstract

A bichromophoric compound 3β-((2-(methoxycarbonyl)bicyclo-[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17β-yl-2,2′,6,6′,N,N,N′,N′ heptamethylbenzidine (N-S-B) was synthesized, and its photochemistry was examined by using both steady-state and time-resolved techniques. Intramolecular electron transfer from the singlet excited state of benzidine to the norbornadiene chromophore in N-S-B occurs with efficiency (Φ ET ) of ca. 12% and rate constant (k SET ) of ca. 1.1 × 10 7 s -1 , resulting in the singlet radical ion pair 1 (N •--S-B •+ ) followed by intersystem crossing to the triplet state 3 (N •--S-B •+ ). Recombination of 3 (N •--S-B •+ ) yields triplet norbornadiene group. The efficiency (φ RIPISC ‚φ RIPRC ) of the formation of the triplet norbornadiene from the radical ion pair is ca. 17%. The singlet excited state of benzidine group in N-S-B undergoes intersystem crossing to its triplet state with efficiency (Φ ISC ) of ca. 56%. Intramolecular triplet energy transfer leading to the formation of the triplet norbornadiene chromophore is efficient (φ TT ) ca. 65%, k TT ) ca. 5.2 × 10 5 s -1 ). Selective excitation of the benzidine chromophore results in isomerization of the norbornadiene group to quadricyclane. This isomerization proceeds either via intramolecular triplet sensitization or radical-ion pair recombination mechanism. The long-distance intramolecular triplet energy transfer and singlet electron transfer are proposed to proceed via a through-bond mechanism.

Photoinduced Intramolecular Electron Transfer and Triplet Energy Transfer in a Steroid‐Linked Norbornadiene–Carbazole Dyad

“…This value is consisted with that reported in the literature. [35] The reduction potential (E red (A)) of MNBD has been reported in previous work [15,16] to be about À 1.67 V versus SCE. Calculation according to Equation (1) reveals that electron transfer from the singlet excited state of the carbazole portion of NBD-S-CZ to the norbornadiene group is exothermic by about 21 kcal mol…”

“…[3, 7, 9, 14±16] We have utilized the androstene skeleton as the bridge, mounted an antenna chromophore and a norbornadiene (or quadricyclane) group on the steroidal framework, and investigated intramolecular triplet energy transfer and electron transfer within these systems. [15,16] Although the donor and acceptor are separated by as much as about 17 ä, electron-transfer and triplet energy-transfer processes occur with reasonably large rate constants by a through-bond mechanism. Our study on a norbornadiene ± steroid ± benzophenone system [15] has shown that the benzophenone chromophore harvests photon energy and transfers its triplet energy to the norbornadiene group with a rate constant of about 1.5 Â 10 5 s À1 resulting in the isomerization of the later into quadricyclane group.…”

“…For norbornadiene ± steroid ± benzidine, [16] both intramolecular triplet energy transfer and singlet electron transfer occur, and the isomerization of the norbornadiene to quadricyclane group proceeds both by triplet sensitization and radical-ion pair recombination mechanisms. In the present work we have synthesized the bichromophoric compound norbornadiene ± steroid ± carbazole (NBD-S-CZ).…”

Intramolecular triplet energy transfer in benzophenone-{crown ether}-naphthalene system

“…[1][2][3][4][5][6][7][8][9][10] For rigid bridge-linked donor-acceptor molecules, the through-bond mechanism appears to be favored. [1][2][3][4][5][6][7][8][9] Steroid, [11][12][13][14][15][16] fused norbornenyl 6,17 and many other groups 18 have been used to constitute the bridge. In contrast, in flexible bridge-linked donor-acceptor molecules triplet energy transfer process has been regarded as a through-space interaction.…”

Triplet energy management between two signaling units through cooperative rigid scaffolds

“…[12][13][14][15][16][17][18][19][20][21] In these systems, the observed TTET happening TB is facilitated by the  and * orbitals of the bridge and, in general, is strongly dependent on the conformations. [22][23][24] Exciplex formation constitutes an alternative channel for excited states deactivation, 1 whose involvement has been reported in the intramolecular singlet quenching of flexible systems.…”