Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules

Michael T. Colvin, Annie Butler Ricks, Amy M. Scott, Dick T Co, Michael R Wasielewski

Research output: Contribution to journalArticle

25 Citations (Scopus)

Abstract

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)- julolidine (DMJ-An) 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 +•-PI -• or DMJ +•-NI -• quantitatively in 1-4 (τ CS ≥ 10 ps), and that charge recombination occurs with τ CR = 268 and 158 ps for 1 and 3, respectively, and with τ 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 +• and PI -• or NI -•, giving 2J > 600 mT for 1-3 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-orbit 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 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor 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.

Original languageEnglish
Pages (from-to)1923-1930
Number of pages8
JournalJournal of Physical Chemistry A
Volume116
Issue number8
DOIs
Publication statusPublished - Mar 1 2012

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spin exchange
Charge transfer
Orbits
Ions
Molecules
charge transfer
Angular momentum
molecules
orbits
ions
Magnetic field effects
Exchange coupling
Absorption spectroscopy
angular momentum
Light absorption
Paramagnetic resonance
orbitals
Spectroscopy
polarization (charge separation)
naphthalene

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules. / Colvin, Michael T.; Ricks, Annie Butler; Scott, Amy M.; Co, Dick T; Wasielewski, Michael R.

In: Journal of Physical Chemistry A, Vol. 116, No. 8, 01.03.2012, p. 1923-1930.

Research output: Contribution to journalArticle

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