Oligothiophene/graphene supramolecular ensembles managing light induced processes: preparation, characterization, and femtosecond transient absorption studies leading to charge-separation

Abstract

Advances in organic synthetic chemistry combined with the exceptional electronic properties of carbon allotropes, particularly graphene, is the basis used to design and fabricate novel electron donor-acceptor ensembles with desired properties for technological applications. Thiophene-based materials, which are mainly thiophene-containing polymers, are known for their notable electronic properties. In this frame moving from polymer to oligomer forms, new fundamental information would help for a better understanding of their electrochemical and photophysical properties. Furthermore, a successful combination of their electronic properties with those of graphene is a challenging goal. In this study, two oligothiophene compounds, which consist of three and nine thiophene-rings and are abbreviated 3T and 9T, respectively, were synthesized and noncovalently associated with liquid phase exfoliated few-layered graphene sheets (abbreviated eG), thus forming donor-acceptor 3T/eG and 9T/eG nanoensembes. Markedly, intra-ensemble electronic interactions between the two components in the ground and excited states were evaluated with the aid of UV-Vis and photoluminescence spectroscopy. Furthermore, redox assays revealed the one-electron oxidation of 3T accompanied by one-electron reduction due to eG in 3T/eG, whereas there were two reversible one-electron oxidations of 9T accompanied by one-electron reduction of eG9T/eG. The electrochemical band gap for the 3T/eG and 9T/eG ensembles were calculated and verified, in which the negative free-energy change for the charge-separated state of 3T/eG and 9T/eGvia the singlet excited state of 3T and 9T, respectively, were thermodynamically favorable. Finally, the results of transient pump-probe spectroscopy studies at the femtosecond time scale were supportive of charge transfer type interactions in the 3T/eG and 9T/eG ensembles. The estimated rates for intra-ensemble charge separation were found to be 9.52 × 10(9) s(-1) and 2.2 × 10(11) s(-1), respectively, for 3T/eG and 9T/eG in THF, which reveal moderate to ultrafast photoinduced events in the oligothiophene/graphene supramolecular ensembles.

Preparation, Photophysical and Electrochemical Evaluation of an Azaborondipyrromethene/Zinc Porphyrin/Graphene Supramolecular Nanoensemble

“…These spectra are very similart og raphene dispersions in organic solvents. [17,26] Upon excitation, we see instantaneous formationo fp hononp eaks of graphene. The near-IR signal shows at ime constant of 1.4ps.…”

“…Next, the ABDP/ZnTPP/GR nanoensemble was prepared (Scheme 2) as follows:asuspension of exfoliated graphene [17] in dichloromethane was added to an equimolar solutiono f9 and ZnTPP and the mixture was briefly sonicated and stirred overnight.A fter filtration over PTFE membrane filter (0.2 mm pore size) and washing, ABDP/ZnTPP/GR was isolated as a black powder.T he nanoensemble formss table suspensions in ortho-dichlorobenzene( oDCB), DMF and dichloromethane (Supporting Information, Figure S10), facilitating the spectroscopic characterization.…”

“…Once the binding properties of the ABDP/ZnTPP conjugate had been accessed, we investigated the photophysicalp roperties of the ABDP/ZnTPP/GR ensemble. The UV/Vis spectrum of ABDP/ZnTPP/GR shows ac ontinuous absorbance throughout the range of the electromagnetic spectrum, indicative of the presence of graphene, [17] as well as additional bands attributed to the ABDP and ZnTPP moieties ( Figure 4a). Importantly, while the absorption attributed to ABDP in ABDP/ZnTPP/GR is found practically at the same wavelengtha sc ompared to that of ABDP derivative 9,t hat is, 694 vs. 695 nm, the absorption attributed to ZnTPP appearsr edshifted by 30 nm as compared emission spectra of ZnTPP (red, l exc at 425 nm), ABDP derivative 9 (black), and ABDP/ ZnTPP/GR (blue, l exc at 450 nm).…”

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

“…A depleted band at 500 nm corresponding to stimulated emission of 1 TT * was observed. 61 The decay and recovery of the positive and depleted peaks was accompanied by a new peak in the 520-590 nm range that was assigned to the 3 TT * transitions from nanosecond transient absorption studies. 61 The decay rate constant of the 608 nm peak was found to be 1.37 × 10 9 s −1 .…”

“…61 The decay and recovery of the positive and depleted peaks was accompanied by a new peak in the 520-590 nm range that was assigned to the 3 TT * transitions from nanosecond transient absorption studies. 61 The decay rate constant of the 608 nm peak was found to be 1.37 × 10 9 s −1 . Figure 7a shows the femtosecond transient absorption spectra of the TT-C 60 dyad in benzonitrile.…”