Photoinduced electron transfer (PET) versus excimer formation in supramolecular p/n-heterojunctions of perylene bisimide dyes and implications for organic photovoltaics

Abstract

Foldamer systems comprised of two perylene bisimide (PBI) dyes attached to the conjugated backbones of 1,2-bis(phenylethynyl)benzene and phenylethynyl-bis(phenylene)indane, respectively, were synthesized and investigated with regard to their solvent-dependent properties. UV/Vis absorption and steady-state fluorescence spectra show that both foldamers exist predominantly in a folded H-aggregated state consisting of π-π-stacked PBIs in THF and in more random conformations with weaker excitonic coupling between the PBIs in chloroform. Time-resolved fluorescence spectroscopy and transient absorption spectroscopy reveal entirely different relaxation pathways for the photoexcited molecules in the given solvents, i.e. photoinduced electron transfer leading to charge separated states for the open conformations (in chloroform) and relaxation into excimer states with red-shifted emission for the stacked conformations (in THF). Supported by redox data from cyclic voltammetry and Rehm-Weller analysis we could relate the processes occurring in these solution-phase model systems to the elementary processes in organic solar cells. Accordingly, only if relaxation pathways such as excimer formation are strictly avoided in molecular semiconductor materials, excitons may diffuse over larger distances to the heterojunction interface and produce photocurrent via the formation of electron/hole pairs by photoinduced electron transfer.

Fluorinating π‐Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells

“…Excimer states have been observed in other NFAs [ 37,64,66 ] and act as low‐energy trap states that inhibit charge separation of bound electron–hole pairs at the donor–acceptor interface, reducing mobilities and PCEs. [ 36,67–70 ]…”

Controllable Electronic Structures and Photoinduced Processes of Bay‐Linked Perylenediimide Dimers and a Ferrocene‐Linked Triad

“…For example, on photoexcitation, heterogeneous excimer species are initially formed via the singlet excited states, and this is followed by structural reorganization associated with heterogeneous excimers to yield the lowest‐energy excimer state . Such an excimer is actually considered to behave as exciton trap, which prevents other processes such as fluorescence and electron transfer . Therefore, examination of the detailed photophysical process is essential for improving electronic‐device performance.…”

“…[14a] Such an excimer is actually considered to behave as exciton trap, which prevents other processes such as fluorescence [14b] and electron transfer. [16] Therefore, examination of the detailed photophysical process is essential for improving electronic-device performance.…”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…Further, the energetic relaxation may be even too high to enable charge carrier separation at the p-n-heterojunction interface. [267] More rigid columnar PBI aggregates were envisioned by enforcing a particular arrangement by additional hydrogenbonding connectivities (Figure 19). [268] Whilst a multitude of such PBI dyes has been synthesized and investigated particularly with regard to their gelator properties, [269,270] PBIs 3-6 are sufficient to illustrate the concept.…”