Structural and electronic properties of the ground and lowest excited states of the electron-donor conjugated copolymer poly (thieno[3,4-b]thiophene benzodithiophene) (PTB1) are reported based on high-level theoretical investigations. PTB1 combined with phenyl-C 61butyric acid methyl ester (PCBM) results in a bulk heterojunction blend with promising properties for use in organic solar cells. The ab initio algebraic diagrammatic construction method to second order, ADC(2), was used to obtain benchmark data for excited state energies, oscillator strengths, and bond length alternation (BLA) analysis. Time dependent density functional theory (TDDFT) calculations using the exchange-correlation functionals PBE, B3LYP, BHandHLYP, CAM-B3LYP, and LC-wPBE were also performed and compared with ADC(2) calculations. It was shown that a minimum of 20% Hartree−Fock exchange in the functional is necessary to reproduce the major features of the ADC(2) results. Analysis of the BLA results indicates the possibility of exciton trapping by geometry relaxation occurring in the middle of the polymer chain. The corresponding exciton binding energy is about 0.4 eV. Charge distributions in the ground and lowest excited singlet state were analyzed as well. The natural population analysis (NPA) confirms the electronegative character of the benozodithiophene group and the corresponding positive one of the thienothiophene moiety leading to an alternant chain polarization of positive and negative charges. Electron density differences between the S 0 and S 1 states show a transfer of electron density from double bond regions to areas of single bonds, a feature which parallels nicely the lengthening of double bonds and shortening of single bonds in the S 1 state.

The first three valence transitions of the two nitromethane conformers (CH3NO2) are two dark n → π* transitions and a very intense π → π* transition. In this work, these transitions in gas-phase and solvated in water of both conformers were investigated theoretically. The polarizable continuum model (PCM), two conductor-like screening (COSMO) models, and the discrete sequential quantum mechanics/molecular mechanics (S-QM/MM) method were used to describe the solvation effect on the electronic spectra. Time dependent density functional theory (TDDFT), configuration interaction including all single substitutions and perturbed double excitations (CIS(D)), the symmetry-adapted-cluster CI (SAC-CI), the multistate complete active space second order perturbation theory (CASPT2), and the algebraic-diagrammatic construction (ADC(2)) electronic structure methods were used. Gas-phase CASPT2, SAC-CI, and ADC(2) results are in very good agreement with published experimental and theoretical spectra. Among the continuum models, PCM combined either with CASPT2, SAC-CI, or B3LYP provided good agreement with available experimental data. COSMO combined with ADC(2) described the overall trends of the transition energy shifts. The effect of increasing the number of explicit water molecules in the S-QM/MM approach was discussed and the formation of hydrogen bonds was clearly established. By including explicitly 24 water molecules corresponding to the complete first solvation shell in the S-QM/MM approach, the ADC(2) method gives more accurate results as compared to the TDDFT approach and with similar computational demands. The ADC(2) with S-QM/MM model is, therefore, the best compromise for accurate solvent calculations in a polar environment.

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