Heteroatom and side chain effects on the optical and photophysical properties: ultrafast and nonlinear spectroscopy of new Naphtho[1,2-b:5,6-b ']difuran donor polymers

Abstract

The photophysical and electronic properties of four novel conjugated donor polymers were investigated to understand the influence of heteroatoms (based on the first two member chalcogens) in the polymer backbone. The side chains were varied as well to evaluate the effect of polymer solubility on the photophysical properties. The donor–acceptor polymer structure is based on naptho[1,2-b:5,6-b′]difuran as the donor moiety, and either 3,6-di(furan-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole or 3,6-di(thiophen-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole as the acceptor moiety. Steady-state absorption studies showed that the polymers with the furan moiety in the backbone displayed a favorable tendency of capturing more solar photons when used in a photovoltaic device. This is observed experimentally by the higher extinction coefficient in the visible and near-infrared regions of these polymers relative to that of their thiophene counterparts. The excitonic lifetimes were monitored using ultrafast dynamics, and the results obtained show that the type of heteroatom π-linker used in the backbone affects the decay dynamics. Furthermore, the side chain also plays a role in determining the fluorescence decay time. Quantum chemical simulations were performed to describe the absorption energies and transition characters. Two-photon absorption cross sections (TPA-δ) were analyzed with the simulations, illustrating the planarity of the backbone in relation to its torsional angles. Because of the planarity in the molecular backbone, the polymer with the furan π-linker showed a higher TPA-δ relative to that of its thiophene counterpart. This suggests that the furan compound will display higher charge transfer (CT) tendencies in comparison to those of their thiophene analogues. The pump–probe transient absorption technique was employed to probe the nonemissive states (including the CT state) of the polymers, and unique activities were captured at 500 and 750 nm for all of the studied compounds. Target and global analyses were performed to understand the dynamics of each peak and deduce the number of components responsible for the transient behavior observed respectively. The results obtained suggest that the furan π-linker component of a donor and acceptor moiety in a conjugated polymer might be a more suitable candidate compared with its more popular chalcogenic counterpart, thiophene, for use as donor materials in bulk heterojunction photovoltaic devices.