Spectroscopic Studies of Charge-Transfer Character and Photoresponses of F₄TCNQ-Based Donor–Acceptor Complexes

Abstract

F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is used widely as a hole-doping agent in photoresponsive organic semiconducting materials, yet relatively little is known about the photoresponses of the F4TCNQ·– anion generated via doping. Furthermore, there is still relatively little systematic exploration of how the properties of the local material or chemical environment impacts the driving force for generating these charge-transfer complexes. Here we present spectroscopic and photophysical studies of F4TCNQ in charge-transfer complexes (CTCs) with the electron donor N,N′-diphenyl-N-N′-di-p-tolylbenzene-1,4-diamine (MPDA) both in dichloroethane solution and polystyrene matrices. Integer charge transfer (ICT) between donor and acceptor occurs readily in dichloroethane solvent to form F4TCNQ·–:MPDA+ CTCs, due to a ∼150 mV difference in MPDA+/MPDA and F4TCNQ/F4TCNQ·– reduction potentials. Ultrafast spectroscopic studies of the CTC as well as electrochemically generated F4TCNQ·– and MDPA+ reveal that the photoresponses of these CTCs are dominated by that of the dopant anion, including rapid deactivation (800 fs) after excitation to the anion D1 excited state, followed by slower (∼10 ps) vibrational cooling in the anion D0 state. Excitation to the higher-lying D2 state results in a rapid relaxation to the D1 state, in contrast to direct D2 → D0 relaxation previously observed for F4TCNQ·– in the gas phase. CTCs embedded in polystyrene (PS) matrices are observed to lose their integer charge-transfer character upon evaporation of solvent, as evidenced by changes to electronic and vibrational absorption features associated with F4TCNQ·–. This change is attributed to the loss of solvent stabilization of the ion pair formed through the charge-transfer reaction. Ultrafast spectral measurements reveal that the photoresponses of the partial charge-transfer (PCT) species embedded in PS are still highly similar to those of the ICT species and unlike that of neutral F4TCNQ, implying the electronic properties of the PCT state are likewise dominated by properties of the reduced acceptor molecule. We conclude that excitation of ICT or PCT states introduces optical losses for photoresponses of doped organic semiconductor materials due to the large anion absorption cross section and its rapid, dissipative deactivation dynamics.

Maximized Hole Trapping in a Polystyrene Transistor Dielectric from a Highly Branched Iminobis(aminoarene) Side Chain

“…The broad absorption from 650 to 950 nm is characteristic of the F 4 TCNQ anion; we also observe an additional broad feature above 950 nm, which we assign to the cation of the dendritic TPA. This is supported by previous electrochemical analysis of MPDA, which showed that the cation of the branched phenylenediamine has a broad absorption centered at 900 nm . In contrast, steady-state characterization of a mixture of MPDA and TCNE in dichloroethane (DCE), as shown in Figure S9, exhibited relatively little formation of CT complexes.…”

“…This is supported by previous electrochemical analysis of MPDA, which showed that the cation of the branched phenylenediamine has a broad absorption centered at 900 nm. 48 In contrast, steady-state characterization of a mixture of MPDA and TCNE in dichloroethane (DCE), as shown in Figure S9, exhibited relatively little formation of CT complexes. At the same concentrations used for the tricyanovinylation in DMF, we observe two different absorption features in DCE upon initial mixing of the reactants, one at 425 nm that corresponds to the TCNE anion 49 and another above 800 nm that corresponds to the MPDA cation.…”

“…At the same concentrations used for the tricyanovinylation in DMF, we observe two different absorption features in DCE upon initial mixing of the reactants, one at 425 nm that corresponds to the TCNE anion 49 and another above 800 nm that corresponds to the MPDA cation. Using the extinction coefficient previously determined for the MPDA cation, 48 we determine that less than 2% of the MPDA in solution forms CT complexes with excess TCNE in DCE. This is expected because TCNE has a much lower electron affinity than F 4 TCNQ, which can form CT complexes with MPDA at nearly 100% yield in DCE solution.…”

Isostructural Charge-Transfer Cocrystals Based on Triptycene End-Capped Quinoxalinophenanthrophenazine

“…A weak shoulder at ∼480 nm was observed in the spectra of QPP-OMe/TCNQ and QPP-OMe/F 4 TCNQ, which may belong to a photoreaction product of the TCNQ or F 4 TCNQ with solvent molecules. 114,115 All TCNQ and QPP-OMe/F x TCNQ mixtures showed similar, strongly red-shifted charge-transfer bands, with a main peak close to the near-infrared region (λ max = 837−854 nm) and two other peaks of decreasing intensity at λ abs = 741− 753 nm and λ abs = 677−787 nm. These bands could not be seen, when measured in CH 2 Cl 2 or CHCl 3 , suggesting that the polarized charge transfer state is stabilized in the polar solvent MeOH.…”

Facile Charge Transfer between Barbituric Acid and Chloranilic Acid over g-C3N4: Synthesis, Characterization and DFT Study

“…The charge transfer process (CT) has been extensively investigated in material science [1][2][3][4], biological systems [5,6], and crystal engineering [7,8]. The CT process is initiated between high electron donor molecules (Donors) and electron-deficient molecules (Acceptors) to form stable CT adducts [9,10].…”