Charge Transfer Doping Induced Conformational Ordering of a Non-Crystalline Conjugated Polymer

3 years ago

Charge Transfer Doping Induced Conformational Ordering of a Non-Crystalline Conjugated Polymer

John K. Grey, Ryan Johnson, Hua Guo, Alan K. Thomas, Benjamin W. Stein, Martin L. Kirk

Charge transfer doping of a nominally disordered conjugated polymer induces long-range conformational ordering (stiffening) of backbone segments. Addition of [2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) to dilute solutions of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) results in quantitative charge transfer in the ground electronic state of the polymer. Following charge (hole) injection, greater MDMO-PPV monomer coplanarity is evident in electronic, Raman, and electron paramagnetic resonance (EPR) spectra over a broad range of dopant loadings. New transitions emerge at lower energies with spectral patterns distinct from pristine materials but closely resemble minority low energy conformers selectively that can be prepared by careful control of processing conditions. We further demonstrate that characteristic Raman patterns of PPV systems actually contain signatures of a minority ordered form that interacts preferentially with the dopant. Subsequent additions of dopant also show that most chains convert into the low energy form. This notion is consistent with greater backbone planarity and, hence, lower torsional reorganization energies required to access the cation form. Preresonant excitation of the emergent red-shifted optical transition reveals long overtone-combination progressions due to enhanced electronic delocalization along planarized backbone segments and diminished coupling the surroundings. We propose that planarity enhancements from doping also lead to the eventual formation of spinless bipolarons, evident from EPR spectra. Facile charge transfer doping induced conversion into the ordered MDMO-PPV conformer suggests that better control of polymer conformations and carrier levels could be harnessed to improve charge and energy transport efficiency in optoelectronic devices.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b08773

DOI: 10.1021/acs.jpcc.7b08773