Excited State Charge Separation in Solution and in Electropolymerized Films of Terthiophene-Fullerene Dyad and Phenothiazine-Terthiophene-Fullerene Triad

Kurt Bodenstedt,Vladimir N Nesterov,Francis D'Souza,Tania Estrada,Navaneetha K Subbaiyan,Gary N Lim,Mark Vdouychenko

doi:10.1149/2.0021706jss

Abstract

A covalently linked molecular dyad comprised of terthiophene and C60, and a molecular triad comprised of phenothiazine, terthiophene and C60 have been newly synthesized to probe photoexcited events in solution and at the electropolymerized film. The X-ray structure of the dyad revealed spatial disposition of the donor-acceptor entities without significant intermolecular type interactions. Optical absorbance, fluorescence, and the electrochemical studies were consistent with the structural integrity of the dyad and triad. Further, the TT-C60 dyad (TT = terthiophene) was found to electropolymerize during multi-cycling of voltammograms. The frontier HOMO and LUMO orbitals of the phenothiazine-terthiophene-C60 triad (PTZ-TT-C60, PTZ = phenothiazine) were found to be located respectively on the phenothiazine-terthiophene and C60 entities. Free-energy calculations revealed that the excited state charge transfer resulting into the formation of TT•+-C60•− in the case of the dyad, and (PTZ-TT)•+-C60•− charge separated state in the case of triad to be thermodynamically feasible. Evidence for the occurrence of excited state singlet-singlet energy transfer from 1TT* to form 1C60* followed by charge transfer was secured from studies involving femtosecond transient absorption studies in polar benzonitrile. Charge stabilization to some extent in the triad compared to that in the dyad was obtained. Attempts were also made to secure evidence of charge separation in the terthiophene-fullerene drop-casted and electropolymerized films on FTO surface.

Highlights

As predicted by voltammetry data, the frontier LUMO was found to be exclusively on the C60 entity (Figure 6b). These results reveal the PTZ-TT entity to be the electron donor segment and C60 entity to be electron acceptor entity during photoinduced electron transfer events
Successful singlet state population of TT in the PTZ-terthiophene-C60 dyad (TT-C60) triad was clear with characteristic peaks of 1TT∗ at 600 nm
Photoinduced singlet-singlet excitation transfer followed by charge separation in the investigated molecular dyad and triad comprised of PTZ, TT and C60 entities has been demonstrated

Summary

Introduction

Excited State Charge Separation in Solution and in Electropolymerized Films of Terthiophene-Fullerene Dyad and Phenothiazine-Terthiophene-Fullerene Triad We have undertaken this task and report synthesis, X-ray structure and excited state events occurring in a terthiophene-C60 dyad (TT-C60), and a phenothiazine-terthiophene-C60 triad (PTZ-TT-C60) (see Figure 1 for structures). The dyad was electropolymerized to form thin films, and charge separation in these films and dropcasted film electrodes has been studied using femtosecond transient absorption spectroscopic technique.

Results

Conclusion