Ultrafast Intersystem Crossing and Spin Dynamics of Photoexcited Perylene-3,4:9,10-bis(dicarboximide) Covalently Linked to a Nitroxide Radical at Fixed Distances

• Published in: Journal of the American Chemical Society Vol. 131; no. 10; pp. 3700 - 3712
• Main Authors: Giacobbe, Emilie M, Mi, Qixi, Colvin, Michael T, Cohen, Boiko, Ramanan, Charusheela, Scott, Amy M, Yeganeh, Sina, Marks, Tobin J, Ratner, Mark A, Wasielewski, Michael R
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 18.03.2009; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; 3,4,9,10-PERYLENETETRACARBOXYLIC DIANHYDRIDE DYES; BRIDGE-ACCEPTOR MOLECULES; TRIPLET PAIR MECHANISM; STABLE RADICALS; ELECTRON-PARAMAGNETIC-RESONANCE; TIME-RESOLVED EPR; TRANSIENT ABSORPTION-SPECTRA; QUINTET S=2 STATES; EXCITED SINGLET-STATES; INTRAMOLECULAR CHARGE-TRANSFER
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja808924f

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 (2BPNO•) at specific distances and orientations. The 2BPNO• 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 τ ≅ 2 ps to give structurally dependent 8−31% yields of 3* PDI. Energy- and electron-transfer quenching of 1* PDI by 2BPNO• 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 2BPNO• to form excited doublet (D1) and quartet (Q) states, which are both spectrally resolved from the 2BPNO• (D0) ground state. The initial spin polarizations of D1 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 D1 and Q to ground-state spin polarize D0. The rates of polarization transfer depend on the molecular connectivity between PDI and 2BPNO• and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species.