Magnetic Field-Induced Switching of the Radical-Pair Intersystem Crossing Mechanism in a Donor−Bridge−Acceptor Molecule for Artificial Photosynthesis

• Published in: Journal of the American Chemical Society Vol. 133; no. 5; pp. 1240 - 1243
• Main Authors: Colvin, Michael T, Ricks, Annie Butler, Scott, Amy M, Smeigh, Amanda L, Carmieli, Raanan, Miura, Tomoaki, Wasielewski, Michael R
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 09.02.2011; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; ENERGY; REACTION CENTERS; ELECTRON-PARAMAGNETIC-RESONANCE; TRIPLET-STATE; KINETICS; LIQUID-CRYSTALS; SPECTRA; POLARIZATION; CHARGE-RECOMBINATION; DEPENDENCE
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja1094815

A covalent, fixed-distance donor−bridge−acceptor (D−B−A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin−spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T+1) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D−B−A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of 3*D−B−A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of 3*D−B−A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T0 eigenstate.