Structure and Dynamics of the ¹(TT) State in a Quinoidal Bithiophene: Characterizing a Promising Intramolecular Singlet Fission Candidate

Abstract

Tetracyanoquinodimethane bithiophene (QOT2) has a long-lived (57 μs) photoinduced excited state that may correspond to triplets resulting from intramolecular singlet fission (SF). Since SF usually occurs through intermolecular processes, a detailed description of the excited states involved and their evolution is needed to verify this hypothesis. The photoresponse of QOT2 is investigated using high-level electronic structure methods and quantum dynamics simulations, which show ultrafast passage through a conical intersection from the bright 1 1 B u state to the dark 2 1 A g surface. Characterization of QOT2's 2 1 A g wave function found it to be composed of two strongly coupled triplets, leading to the first detailed electronic structure description of an intramolecular 1 (TT) state. The population of such a state upon excitation of QOT2 raises the possibility of SF through conformational changes that decouple the triplets. However, reaching an appropriate geometry for decoupled triplets appears unlikely given the energy cost of 1.76 eV. Consequently, the hypothesis that the long-lived excited state corresponds to 2 1 A g , a spin singlet, strongly interacting double triplet, was explored. Transition moment calculations to assign excited-state absorption signals and investigations into internal conversion and intersystem crossing decay pathways indicate that a long-lived 2 1 A g state with 1 (TT) character is consistent with the available experimental data.

Singlet Fission in Quinoidal Oligothiophenes

“…CASPT2 (10,8) calculations of the same authors yield excitation energies for this state (2.06 eV in absorption, 1.99 eV in emission) similar to our results, whereas the XMS-CASPT2 (10,8) calculations of Chien et al 19 place the 1 1 B u state at higher energies (2.69 eV in absorption, 2.44 eV in emission). All three theoretical studies agree in that the 1 1 B u state is nearly degenerate with the optically dark 2 1 A g state.…”

“…While Varnavski et al 18 as well as Chien et al 19 base their discussion of the vertical excitation energies of the electronic states on high-level CASPT2 (10,8) or XMS-CASPT2 (10,8) results, they employ very approximate RAS-SF quantum chemical methods, lacking essentially all dynamic correlation effects, for the potential energy scans. In an attempt to get a consistent picture, we performed potential energy scans and computed ESA spectra at the same level of theory; that is, we used DFT/MRCI-R throughout.…”

Design of Small Intramolecular Singlet Fission Chromophores: An Azaborine Candidate and General Small Size Effects

“…In this sense, 4 is very similar to tetracyanoquinodimethane bithiophene, which features a fast nonradiative decay from the lowest bright state to a "dark state" that contains a large portion of multi-exciton character 56 but also a substantial portion of other configurations. 57 Now let us review the problems of 4. The 35°dihedral angle around the interconnecting bond leaves large overlap between the frontier orbitals of the two 3 units.…”

Dithienoindophenines: p‐Type Semiconductors Designed by Quinoid Stabilization for Solar‐Cell Applications

“…Surprisingly, donorm aterials with purely quinoidal frameworks have rarely been reported to date, with low PCEs of 1.1 %. [3] Generally,q uinoidal oligothiophenes( QOT) [4] with rigida nd planarK ekulØ structuress how unique optoelectronic properties such as narrow optical bandgaps with large absorption coefficients, [5] singlet-fission property, [3,6] near-infrared fluorescence, [7] and excellent chargetransport properties, [8] which are highly desirable for OPV appli-cations.H owever,t he most well-known dicyanomethylene-ter-minatedQ OTss uch as quinoidal bithiophene( QBT) [9] are typically n-type semiconductors because of their high electron affinities. Indophenined erivatives (QBT with 1-alkylindolin-2-one terminals) known for over one century [10] have recently been found to show promising ambipolar properties (Figure 1).…”

“…Indophenined erivatives (QBT with 1-alkylindolin-2-one terminals) known for over one century [10] have recently been found to show promising ambipolar properties (Figure 1). [11] To demonstrate the potentialo fQ OTsfor OPV applications, p-type QOTsw ith suitable HOMO and LUMOe nergy levels should be developed preferentially to match [ 6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) acceptors.…”