Theoretical study of the relations between structure and photophysical properties of model oligofluorenes with central keto defect.

A systematic study of fluorenone and model oligofluorenes (trimer, pentamer, and heptamer) with a central keto defect was performed at ab initio Hartree-Fock (HF), density functional theory (DFT), configuration interaction singles (CIS), and time-dependent density functional theory (TD-DFT) levels. The main aim of this work was the investigation of the direct influence of the central keto defect on the optimal geometry, torsional potentials, and photophysical properties. From the structural point of view, the optimal all-trans electronic ground state geometries of studied oligomers exhibit a uniform torsion of ca. 44-45 degrees (HF) or 37-38 degrees (DFT). The optical excitation leads to the planarization of the fluorenone and fluorene fragments in the central part of the molecule (approximately 34 degrees for CIS and approximately 29 degrees for TD-DFT). The computed excitation and fluorescence energies show a good agreement with the experiment. These presented theoretical results can be useful in designing novel fluorene-fluorenone optical materials as well as understanding of excitation-relaxation phenomena which may occur in various time-dependent optical experiments.

Torsional potentials and full-dimensional simulation of electronic absorption and fluorescence spectra of para-phenylene oligomers using the semiempirical self-consistent charge density-functional tight binding approach.

A systematic study on the structural properties of para-phenylene oligomers based on the self-consistent charge density-functional tight binding approach (SCC-DFTB) and its time-dependent (TD) version is presented. Our goal is to investigate the applicability of DFTB for the present class of compounds and to use its computational efficiency for on-the-fly dynamics calculations and to perform in this way simulations of absorption and fluorescence spectra. For this purpose geometry optimizations have been performed for the ground state and for the electronically lowest excited state of oligomers containing two to seven aromatic rings. The torsional potential curves have been computed for para-biphenyl and para-terphenyl in the ground and lowest excited state. Agreement with previously computed DFT results is quite encouraging and DFTB seems to be well suited for the treatment of the class of conjugated pi systems investigated in this work. The intrachain vibrational broadening of absorption and emission spectra computed from dynamics simulations are presented and compared with experimental spectra.

Dependence of optical properties of oligo-para-phenylenes on torsional modes and chain length.

A systematic characterization of excited-state properties of para-phenylene oligomers constructed from two to eight aromatic rings is presented using density functional theory (DFT) and the coupled-cluster singles and doubles (CC2) method. Geometry optimizations have been performed for the ground state and for the electronically excited state. Vertical excitations and the fluorescence transitions have been calculated. Time-dependent DFT (TDDFT) method underestimates excitation and fluorescence energies systematically in comparison with experimental results. The computed TDDFT lifetime for the polymer limit (0.43 ns) is in agreement with the experimental value of 0.55 ns. The TDDFT torsional potential curves were investigated for biphenyl, terphenyl, and quarterphenyl oligomers in their electronic ground and excited states. Our calculations show an increase in the separation of the lowest excited state (S1) to the next higher one with increasing molecular size. No indication is found for state crossings of the S1 state with higher ones from planar structures up to torsional angles of 60 degrees to 70 degrees. Thus, an adiabatic description of the dynamics of the S1 state might significantly simplify any dynamics simulations of torsional broadenings.

Density matrix analysis, simulation, and measurements of electronic absorption and fluorescence spectra of spirobifluorenes.

The linear absorption and fluorescence spectra as well as the oscillator strengths of 2,2',7,7'-tetraphenyl-9,9'-spirobifluorene (A), 2,2',7,7'-tetrakis(biphenyl-4-yl)-9,9'-spirobifluorene (B), and 2,2',7,7'-tetrakis(9,9'-spirobifluoren-2-yl)-9,9'-spirobifluorene (C) are calculated on the basis of the collective electronic oscillator (CEO) approach of Mukamel et al. (see, e.g., Chem. Rev. 2002, 102, 3171). The graphical visualization and quantitative characterization of CEO modes allows one to extract the real-space distribution of electronic excitations of the molecules under study. Effects of the lengthening and branching of the oligophenylene segments have been analyzed. The influence of the lowest excited (S1) vs ground-state (S0) geometry changes on the CEO modes is investigated and related to the geometry changes of the molecular parts. The obtained theoretical results are in good agreement with experimental trends observed in absorption and fluorescence data.