Dynamic Excitons in Nitrile-Functionalized Ladder-Type Oligo(p-Phenylene)s By Pulse Radiolysis Coupled with Time-Resolved Infrared Spectroscopy

Abstract

Research and development efforts toward organic solar cells (OSCs) for large-scale power generation have continued unabated throughout the world. In recent years, OSCs based on π-conjugated polymers and small molecules have received increasing interest due to favorable electronic properties, component versatility, and low production and installation costs. Despite the progress, many improvements are still needed prior to widespread commercialization, involving device efficiency, lifetime, and cost. Of these, increasing device efficiency, particularly power conversion efficiency (PCE), is the most challenging task. The currently achievable PCE, for example, is only 17.3%.1 The key to achieving high PCE is the search for new organic conjugated molecules that are capable of harvesting light, and transferring and transporting charges promptly and efficiently. This requires a full understanding of how electrons and holes are localized and delocalized in organic conjugated molecules.Using time-resolved infrared (TRIR) detection combined with pulse radiolysis, Mani and co-workers2 have measured experimentally the degree of localization of an excess electron in a series of nitrile-functionalized oligofluorenes by the spectral shifts of nitrile vibrations. The nitrile vibrational bands in anions respond sensitively to the degree of electron delocalization (IR shifts) and the structural changes (IR linewidth). The electron is found to move back and forth within the oligomers, likely controlled by the movement of dihedral angles between monomer units.Unlike the non-coplanarity of the fluorenyl-fluorenyl backbone in oligofluorenes, the rigid and planar structure of ladder-type, oligo(p-phenylene)s (Scheme 1)3 would facilitate π-electron delocalization and thus improve the multiphoton absorption response. Such a structure would also increase photoluminescence efficiency with enhanced thermal and photochemical stability. To investigate the coupling between charge distribution and vibrational motion and vibronic coupling, we incorporate nitrile group in each compound as the infrared reporter group.In this talk, we report our molecular syntheses and characterizations of two of the title compounds (L3PCN and L4PCN, both neutral compounds and their radical anions) by TRIR followed by pulse radiolysis. Having no flexible dihedral angles, L3PCN and L4PCN exhibit sharper IR bands of the nitrile vibration than the oligofluorenes with the same number of benzene rings and some dihedral angles. Our results further demonstrate that the linewidth of the nitrile vibration, together with the IR shifts and intensities, reports on structural and accompanying electronic fluctuations,2 that is, on the dynamic excitons. References (1) Meng, L. X.; et al. Science 2018, 361, 1094.(2) Mani, T.; et al. Journal of the American Chemical Society 2015, 137, 10979.(3) Scherf, U.; et al. Makromolekulare Chemie-Rapid Communications 1991, 12, 489. Figure 1