Evolution of the electronic structure in open-shell donor-acceptor organic semiconductors

Abstract

Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship.

Fig. 1. Molecules and polymers with open-shell ground states.

a Electronic structure of open-shell Kekulé-type Tschitschibabin’s hydrocarbon with the resonance structure between the closed- and open-shell structures as well as the thermally excited triplet state. Examples of typical compounds with a similar mechanism drawn in closed-shell resonance form^,–. b The widespread cross-over from closed- to open-shell character through narrowing of the bandgap of the donor-acceptor molecules initially reported in our previous work. c Examples of donor-acceptor small molecules and polymers containing benzo[1,2-c;4,5-c]bis[1,2,5]thiadiazole and other similar acceptor units with open-shell character^–.

Fig. 2. Molecular structures and thin-film absorption profiles of donor-acceptor-donor small molecules containing TDPP, NTT, and NTC cores.

a, b Molecular structures of the DPP and NT-based small molecules with variations of the donor moieties. The optical bandgap (E_g^opt) as determined from the intersection of absorbance and emission curves of the thin-films and calculated y₀ at the PUHF/6-31 G(d,p) level of theory, are listed beneath the donor units. c, d Absorption spectra of thin-films cast from chloroform onto quartz substrates.

Fig. 3. ESR spectra of TDPP, NTC, and NTT derivatives and diradical index as a function of bandgap.

a, b ESR spectra of powder samples comparing signal intensity using the same molar quantity of each material and under the same test conditions. c Calculated y₀ is plotted against experimental E_g^opt obtained from the thin-film absorption profiles.

Fig. 4. Temperature dependent properties of TPAOMe-TTDPP and 2N-NTC.

a, d Stacked VT ¹H NMR spectra in CD₂Cl₂. b VT ESR spectra measured from 152–284 K with g = 2.0031 and (e) from 105–285 K with g = 2.0029. c, f SQUID magnetometry of the solid sample showing magnetic susceptibility times temperature χ_MT vs. T from 200–400 K, fit to the Bleaney−Bowers equation with g = 2.003, giving ΔE_ST (2 J/k_B) (red line).

Fig. 5. Solid-state structures by XRD.

ORTEP drawings with side-chains and hydrogen atoms omitted for clarity. Thermal ellipsoids are drawn at 50% probability. Selected bond lengths (Å) and dihedral angles (ϕ) for An-TDPP, Flu-TDPP, Py-TDPP, and TPAOMe-TDPP, respectively: ϕ_α (86.6°, 2.78°, 40.7°, 14.4°), ϕ_β (13.9°, 11.4°, 5.71°, 5.17°), a (1.475, 1.464, 1.469, 1.461), b (1.356, 1.369, 1.380, 1.367), c (1.403, 1.400, 1.396, 1.397), d (1.371, 1.372, 1.385, 1.376), e (1.442, 1.443, 1.432, 1.437), f (1.388, 1.384, 1.387, 1.404), g (1.408, 1.408, 1.412, 1.406), h (1.388, 1.386, 1.403, 1.404).

Fig. 6. Calculated NICS_iso(1) values, ACID, and FOD plots for An-TDPP, Flu-TDPP, Py-TDPP, and TPAOMe-TDPP.

NICS_iso(1) and ACID are calculated at RB3LYP/6-31 G(d,p) level of theory and basis set. The red and blue arrows indicate clockwise (diatropic: aromatic) and counterclockwise (paratropic: quinoidal) ring current, respectively. The applied magnetic field is perpendicular to the molecular backbone and points out through the plane of the paper. ACID plots were generated with an isovalue = 0.025 au. FOD plots (σ = 0.002 e/Bohr) are obtained from the FT-DFT at B3LYP/6-31 G(d,p) level of theory and basis set.