Singlet fission in thin films of metallo-supramolecular polymers with ditopic thiophene-bridged terpyridine ligands

Abstract

The singlet fission (SF) phenomenon is currently investigated for its potential to overcome the Shockley–Queisser energy conversion efficiency limit of single-junction photovoltaic (PV) cells. One of the hurdles of using SF for PV cells is the limited choice among the presently available SF materials. We compare the photophysical behavior of thin solid films of two novel compounds: a ditopic ligand bis(terpyridine-4′-yl)terthiophene (T) and a metallo-supramolecular polymer (MSP), prepared by the coordination of T to Zn²⁺ ions (PT). The transient absorption kinetics in PT after photoexcitation into its second electronic excited state was consistent with an ultrafast (SF) process with a time constant of 160 fs. The lifetime of such a formed triplet state reached only 1 ns, due to mutual bimolecular annihilation, which also hindered the determination of the yield of the triplet state formation. In contrast, the metal-free thin films of T showed no signs of SF, but rather profound exciton relaxation through an excimer formation. Powder WAXS diffractograms pointed out a lower degree of structural order in the PT than that in the T – which may be the probable reason for differences in energy relaxation pathways of these two materials. To the best of our knowledge, singlet fission has not yet been reported in the MSP class of materials.