A Detailed Analysis of Multiple Photoreactions in a Light-Harvesting Molecular Triad with Overlapping Spectra by Utrafast Spectroscopy

Abstract

The photoreactions that occur in a molecular triad composed of derivatives of boron-dipyrromethene (BOD), diketopyrrolopyrrole (DPP), and of triphenylamine (TPA) specifically designed for the use as a possible donor material for organic solar cells are studied by ultrafast fluorescence and transient absorption spectroscopy with hyper spectral coverage (300–900 nm). While the latter is often sufficient to reveal the reaction scenario in mono- or bicomponent organic molecules, the overlap of ground state absorption and fluorescence spectra of the components present in the TPA-DPP-BOD triad requires a careful and accurate characterization of the excited state differential absorption spectra and kinetics of the individual isolated moieties. In addition, the absorption spectra of the TPA cation and the DPP and BOD anions are determined by in situ spectroelectrochemical experiments. Picosecond fluorescence demonstrates efficient excited state quenching of both DPP and BOD, with downhill energy transfer from TPA occurring in a subpicosecond time scale, leading to its complete excited state quenching. Additionally, an analysis of the multiexponential fluorescence decay indicates the existence of reactive (quenched) and nonreactive (radiative) subpopulations of DPP and BOD, most probably due to structural heterogeneities. In order to treat this complexity of multiple reaction pathways, the transient absorption data are analyzed by a combination of global fitting, providing decay-associated differential spectra (DADS), and a reconstitution of these DADS by linear combinations of the characteristic difference spectra of all molecular species and excited states. This allows us to disentangle the photoreactions that the TPA, DPP and BOD excited state subpopulations undergo on time scales ranging from 200 fs to a few ns, and to clarify the contribution of TPA as an electron donor in the formation of an intramolecular charge transfer (CT) state. DPP has a dual role as an ancillary light-harvesting unit capable of performing ultrafast energy transfer to BOD, and as an electron acceptor for the CT state. The latter has a remarkably long lifetime of 0.5 ns. We conclude that the triad could act as a very efficient electron-donor material in a blend with commonly used acceptors such as phenyl-C61-butyric acid methyl ester (PCBM).