Effect of Thermal Motions on the Structure and UV−Visible Electronic Spectra of Stilbene and Model Oligomers of Poly(p-Phenylene Vinylene)

Abstract

In the present study, we investigate the influence of temperature on the width and location of bands in the UV-visible absorption spectra of oligomer chains of poly(p-phenylene vinylene) (PPV). These spectra have been computed by means of molecular dynamics (MD) simulations along with the classical MM3 force field, in conjunction with (Z)INDO/S-CIS calculations of vertical excitation energies and transition moments. In addition, the MD(MM3) computations enable consistent insights into the average structures of cis-and transstilbene, at temperatures ranging from 0 to 500 K. For trans-stilbene and larger PV-n oligomer chains converging to PPV, thermal motions at room temperature in the vacuum result in a constant broadening of bands by 24 nm (0.20 eV) at half the maximum. Compared with the trans-isomer, thermal broadening intensifies to 33 nm (0.44 eV) for cis-stilbene, due to enhanced steric effects. When accounting in addition for the outcome of vibronic broadening, the width of the first absorption band increases at 300 K to 33 nm (0.48 eV) for trans-stilbene and ∼46 nm (0.40 eV) for the largest PV-n chains. At last, upon studying a large molecular cluster reproducing the crystalline structure of trans-stilbene, it has been found that, although they strongly impede the rotations of phenyl rings and out-of-plane distortions, packing effects and steric hindrances in the solid phase lead to a limitation of the width of the first absorption band by 6 nm (0.07 eV) only at 300 K.

Theoretical investigation of the structure and the electron-vibrational dynamics of 9,9′-spirobifluorene

“…Moreover, due to their great importance, ultrafast phenomena in photochemistry and photobiology have been investigated recently by molecular dynamics (MD) simulations [8]. Despite the impressive growth of computer performance and software development, these studies are based on the classical molecular mechanics force field [9], which are followed by ZINDO-CIS computations of vertical electron transitions on structures extracted from MD snapshots [10][11][12]. In addition, the application of MD based on semiempirical Hamiltonians or DFT approaches for large organic systems in the lowest excited state for picosecond time scales is still very restricted and not commonly used.…”

Second quantized reduced Bloch equations and the exact solutions for pairing Hamiltonian

“…We find that the usual Fermi-Dirac statistics for the free fermion gas holds also for the interacting fermions, and that the usual Hartree-Fock equation at zerotemperature can be extended to the case of any finite temperature. The mean force field that every orbital or particle is subject to will be from the contribution of both coherent and incoherent superpositions of the wavefunctions of other orbitals or particles, and is therefore temperature-dependent [5][6][7][8]. This opens a way for investigating the interplay between the microscopic structure and macroscopic thermodynamic quantities for molecules and solids [5][6][7][8][9].…”

“…The mean force field that every orbital or particle is subject to will be from the contribution of both coherent and incoherent superpositions of the wavefunctions of other orbitals or particles, and is therefore temperature-dependent [5][6][7][8]. This opens a way for investigating the interplay between the microscopic structure and macroscopic thermodynamic quantities for molecules and solids [5][6][7][8][9]. In addition, the Fermi-Dirac distribution for interacting particles demonstrates that there exists an effective one-particle description for the interacting many-body system [2,10].…”

TORSIONAL POTENTIALS AND FULL-DIMENSIONAL SIMULATION OF ELECTRONIC ABSORPTION SPECTRA OF para-PHENYLENEVINYLENE OLIGOMERS USING SEMIEMPIRICAL HAMILTONIANS

“…With respect to the computational capabilities, the recently published MD calculations 22 for PPV oligomers are based on the classical molecular mechanics (MM) force field, followed by ZINDO/S-CIS computations of vertical electronic transitions or molecular orbital energies on structures extracted from the MD structures. Although the MM force field enables the evaluation long timescale (ps and ns) dynamics, the steric interaction is here usually overemphasized, in particular in the case of the coplanar conformations.…”