title: Ultrafast intersystem crossing and spin dynamics of photoexcited perylene-3,4:9,10-bis(dicarboximide) covalently linked to a nitroxide radical at fixed distances.
abstract: Time-resolved transient optical absorption and EPR (TREPR) spectroscopies are used to probe the interaction of the lowest excited singlet state of perylene-3,4:9,10-bis(dicarboximide) ((1*)PDI) with a stable tert-butylphenylnitroxide radical ((2)BPNO(*)) at specific distances and orientations. The (2)BPNO(*) radical is connected to the PDI with the nitroxide and imide nitrogen atoms either para (1) or meta (3) to one another, as well as through a second intervening p-phenylene spacer (2). Transient absorption experiments on 1-3 reveal that (1*)PDI undergoes ultrafast enhanced intersystem crossing and internal conversion with tau approximately = 2 ps to give structurally dependent 8-31% yields of (3*)PDI. Energy- and electron-transfer quenching of (1*)PDI by (2)BPNO(*) are excluded on energetic and spectroscopic grounds. TREPR experiments at high magnetic fields (3.4 T, 94 GHz) show that the photogenerated three-spin system consists of the strongly coupled unpaired electrons confined to (3*)PDI, which are each weakly coupled to the unpaired electron on (2)BPNO(*) to form excited doublet (D(1)) and quartet (Q) states, which are both spectrally resolved from the (2)BPNO(*) (D(0)) ground state. The initial spin polarizations of D(1) and Q are emissive for 1 and 2 and absorptive for 3, which evolve over time to the opposite spin polarization. The subsequent decays of D(1) and Q to ground-state spin polarize D(0). The rates of polarization transfer depend on the molecular connectivity between PDI and (2)BPNO(*) and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.
authors.affiliation: Department of Chemistry, Argonne-Northwestern Solar Energy Research Center, and International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Emilie M
authors.initials: EM
authors.lastname: Giacobbe
authors.forename: Qixi
authors.initials: Q
authors.lastname: Mi
authors.forename: Michael T
authors.initials: MT
authors.lastname: Colvin
authors.forename: Boiko
authors.initials: B
authors.lastname: Cohen
authors.forename: Charusheela
authors.initials: C
authors.lastname: Ramanan
authors.forename: Amy M
authors.initials: AM
authors.lastname: Scott
authors.forename: Sina
authors.initials: S
authors.lastname: Yeganeh
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski