Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules

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• 作者：Michael T. Colvin ; Annie Butler Ricks ; Michael R. Wasielewski
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：March 8, 2012
• 年：2012
• 卷：116
• 期：9
• 页码：2184-2191
• 全文大小：345K
• 年卷期：v.116,no.9(March 8, 2012)
• ISSN：1520-5215

文摘

The impact of donor鈥揳cceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PE_nP, where n = 1鈥?) 1鈥?b>3 or oligo(2,7-fluorenone) (FN_n, where n = 1鈥?) 4鈥?b>6. Photoexcitation of 1鈥?b>3 and 4鈥?b>6 produces DMJ^{+鈥?/sup>-An-PE_nP-NI鈥撯€?/sup> and DMJ+鈥?/sup>-An-FN_n-NI鈥撯€?/sup>, respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both 3*An and 3*NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. 3*NI is produced by hole transfer from DMJ+鈥?/sup> to NI鈥撯€?/sup>, while 3*An is produced by electron transfer from NI鈥撯€?/sup> to DMJ+鈥?/sup>, using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of 3*NI and 3*An in 1鈥?b>6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PE_nP, while it occurs by preferential electron transfer when the bridge is FN_n. Over time, the initial population of 3*NI decreases, while that of 3*An increases, indicating that triplet鈥搕riplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from 3*NI to An is weakly exponential with a decay parameter 尾 = 0.08 脜鈥? for the PE_nP series and 尾 = 0.03 脜鈥? for the FN_n series. In the PE_nP series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FN_n series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.}