Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics

The electronic structure, spectra and linear and second-order polarizabilities of two symmetric ketocyanines, which are prototypic examples of D-A-D chromophores, have been investigated with two different toolsets: (i) the so-called 'essential-state model', here comprising three states, the ground and two lowest excited (1)ππ* states, has been adapted for these non-centrosymmetric, yet symmetric compounds to determine their permanent electric dipole moments, polarizabilities and first hyperpolarizabilities making use of experimental transition energies and moments; (ii) extensive TDDFT calculations have provided ground-state conformational results consistent with NMR-derived structural information, energies and dipole moments of up to 20 lowest-lying electronic states as well as, within the sum-over-states (SOS) scheme, the most relevant components of the polarizabilities and first hyperpolarizabilities. The two levels of description form consistent pictures of the ketocyanine excited states that provide the most relevant contributions to hyperpolarizabilities: extension of the SOS set beyond the three states of the basic model left unchanged (within ∼10%) the calculated vector component of the second-order polarizability tensor along the direction of the ground-state dipole moment (β(y)). Both approaches indicate that these D-A-D compounds, in spite of their quasi-linear structure, reminiscent of that of centrosymmetric quadrupolar chromophores, feature significant second-order molecular polarizabilities. These rapidly increase with the length of the polyenic bridges in the chromophores. About half of the total value of β(y) is predicted to come from the three-level-term part, β(y,3), most of which derives from the contribution involving the three electronic states of the essential-state model.

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The charge-transfer and spectral properties of two octithiophenes, namely 4',4''',3'(v),3(v)'-tetra(octylsulfanyl)-2,2':5',2'':5'',2''':5''',2'(v):5'(v),2(v):5(v),2(v)':5(v)',2(v)''-octithiophene and 4,3'',4(v),4(v)''-tetrabromo-4',4''',3'(v),3(v)'-tetra[(R)-2-methylbutylsulfanyl]-2,2':5',2'':5'',2''':5''',2'(v):5'(v),2(v):5(v),2(v)':5(v)',2(v)''-octithiophene, OT1 and OT2, respectively, are characterized by cyclic voltammetry and spectroelectrochemistry under ultradry conditions. The analysis of the voltammetric results shows the formation of up to the dication for both OT1 and OT2 and up to the tetraanion (OT1) and trianion (OT2) anions. The optical properties of the OT1 (2+, 1+, neutral, 1-, 2-) species were probed by in situ UV-vis-NIR spectroelectrochemistry. The calculated standard potentials at the B3LYP/cc-pVTZ level of the theory allowed the rationalization of the experimental electrochemical results. The UV-vis-NIR spectra were successfully compared with the theoretical electronic transitions and oscillator strength data obtained by time-dependent B3LYP/6-31G* calculations. Theoretical redox potentials and optical transitions properties are calculated including "the solvent effect" within the CPCM model. The consistency obtained between experimental and theoretical results indicates the existence of the hypothesized high-spin/high-charge p- and n-doped electronic states for the OT1 and OT2 octithiophenes here studied.