TY - JOUR
T1 - Effect of Conjugation Length on Photoinduced Charge-Transfer in π-Conjugated Oligomer-Acceptor Dyads
AU - Jiang, Junlin
AU - Alsam, Amani A.
AU - Wang, Shanshan
AU - Aly, Shawkat Mohammede
AU - Pan, Zhenxing
AU - Mohammed, Omar F.
AU - Schanze, Kirk S.
N1 - KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: This work was partially supported by the National Science Foundation (Grant No. CHE-1504727). KSS also acknowledges the Welch Foundation for support. We also acknowledge many insightful discussions with John R. Miller (Brookhaven National Laboratory) which in part stimulated this work.
PY - 2017/6/21
Y1 - 2017/6/21
N2 - A series of -conjugated oligomer-acceptor dyads were synthesized that feature oligo(phenylene ethynylene) (OPE) conjugated backbones end-capped with a naphthalene diimide (NDI) acceptor. The OPE segments vary in length from 4 to 8 phenylene ethynene units (PEn-NDI, where n = 4, 6 and 8). Fluorescence and transient absorption spectroscopy reveals that intramolecular OPE NDI charge transfer dominates the deactivation of excited states of the PEn-NDI oligomers. Both charge separation (CS) and charge recombination (CR) are strongly exothermic (G0CS ~ -1.1 and G0CR ~ -2.0 eV), and the driving forces do not vary much across the series because the oxidation and reduction potentials and singlet energies of the OPEs do not vary much with their length. Bimolecular photoinduced charge transfer between model OPEs that do not contain the NDI acceptors with methyl viologen was studied, and the results reveal that the absorption of the cation radical state (OPE+•) remains approximately constant ( ~ 575 nm) regardless of oligomer length. This finding suggests that the cation radical (polaron) of the OPE is relatively localized, effectively occupying a confined segment of n 4 repeat units in the longer oligomers. Photoinduced intramolecular electron transfer dynamics in the PEn-NDI series was investigated by UV-visible femtosecond transient absorption spectroscopy with visible and mid-infrared probes. Charge separation occurs on the 1 – 10 ps timescale, with the rates decreasing slightly with increased oligomer length (βCS ~ 0.15 Å-1). The rate for charge-recombination decreases in the sequence PE4-NDI > PE6-NDI ~ PE8-NDI. The discontinuous distance dependence in the rate for charge recombination may be related to the spatial localization of the positive polaron state in the longer oligomers.
AB - A series of -conjugated oligomer-acceptor dyads were synthesized that feature oligo(phenylene ethynylene) (OPE) conjugated backbones end-capped with a naphthalene diimide (NDI) acceptor. The OPE segments vary in length from 4 to 8 phenylene ethynene units (PEn-NDI, where n = 4, 6 and 8). Fluorescence and transient absorption spectroscopy reveals that intramolecular OPE NDI charge transfer dominates the deactivation of excited states of the PEn-NDI oligomers. Both charge separation (CS) and charge recombination (CR) are strongly exothermic (G0CS ~ -1.1 and G0CR ~ -2.0 eV), and the driving forces do not vary much across the series because the oxidation and reduction potentials and singlet energies of the OPEs do not vary much with their length. Bimolecular photoinduced charge transfer between model OPEs that do not contain the NDI acceptors with methyl viologen was studied, and the results reveal that the absorption of the cation radical state (OPE+•) remains approximately constant ( ~ 575 nm) regardless of oligomer length. This finding suggests that the cation radical (polaron) of the OPE is relatively localized, effectively occupying a confined segment of n 4 repeat units in the longer oligomers. Photoinduced intramolecular electron transfer dynamics in the PEn-NDI series was investigated by UV-visible femtosecond transient absorption spectroscopy with visible and mid-infrared probes. Charge separation occurs on the 1 – 10 ps timescale, with the rates decreasing slightly with increased oligomer length (βCS ~ 0.15 Å-1). The rate for charge-recombination decreases in the sequence PE4-NDI > PE6-NDI ~ PE8-NDI. The discontinuous distance dependence in the rate for charge recombination may be related to the spatial localization of the positive polaron state in the longer oligomers.
UR - http://hdl.handle.net/10754/623760
UR - http://pubs.acs.org/doi/abs/10.1021/acs.jpca.7b03547
UR - http://www.scopus.com/inward/record.url?scp=85024396638&partnerID=8YFLogxK
U2 - 10.1021/acs.jpca.7b03547
DO - 10.1021/acs.jpca.7b03547
M3 - Article
C2 - 28541674
SN - 1089-5639
VL - 121
SP - 4891
EP - 4901
JO - The Journal of Physical Chemistry A
JF - The Journal of Physical Chemistry A
IS - 26
ER -