Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor鈥揵ridge鈥揂cceptor Molecules

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• 作者：Michael T. Colvin ; Annie Butler Ricks ; Amy M. Scott ; Dick T. Co ; Michael R. Wasielewski
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：March 1, 2012
• 年：2012
• 卷：116
• 期：8
• 页码：1923-1930
• 全文大小：449K
• 年卷期：v.116,no.8(March 1, 2012)
• ISSN：1520-5215

文摘

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor鈥揵ridge鈥揳cceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ鈥揂n) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ^{+鈥?/sup>鈥揚I鈥撯€?/sup> or DMJ+鈥?/sup>鈥揘I鈥撯€?/sup> quantitatively in 1鈥?b>4 (蟿_CS 鈮?10 ps), and that charge recombination occurs with 蟿_CR = 268 and 158 ps for 1 and 3, respectively, and with 蟿_CR = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ+鈥?/sup> and PI鈥撯€?/sup> or NI鈥撯€?/sup>, giving 2J > 600 mT for 1鈥?b>3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of 3*An upon charge recombination occurs by spin鈥搊rbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1鈥?b>3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1鈥?b>4 depend on the donor鈥揳cceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.}