We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule all with distinctly different backbone conformations. The organic materials investigated here are planar methylated ladder-type poly para-phenylene, semi-planar polyfluorene, and nonplanar para-hexaphenyl. In the long-chain-polymers the photoluminescence transition energies blueshift with increasing temperatures. In contrast, in the conjugated molecules transition energies redshift with increasing temperatures. Empirical models that explain the temperature dependence of the band gap energies in inorganic semiconductors are extended to explain the temperature dependence of the transition energies in conjugated molecules.

The static polarizability properties and the Raman-scattering intensities in molecular ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ are found to be well reproduced by a bond polarizability model with parameters similar to those obtained from studies of hydrocarbons. For the Raman spectrum of ${\mathrm{C}}_{60}$ with off-resonance infrared laser excitation, a fit using first-principles vibrational eigenvectors yields ${\mathrm{\alpha }}_{\mathrm{\parallel }}^{\prime }$-${\mathrm{\alpha }}_{\mathrm{\perp }}^{\prime }$=2.30 A${\mathrm{̊}}^2$, 2${\mathrm{\alpha }}_{\mathrm{\perp }}^{\prime }$+${\mathrm{\alpha }}_{\mathrm{\parallel }}^{\prime }$=2.30 A${\mathrm{̊}}^2$, ${\mathrm{\alpha }}_{\mathrm{\parallel }}$-${\mathrm{\alpha }}_{\mathrm{\perp }}$=1.28 A${\mathrm{̊}}^3$ for single bonds and ${\mathrm{\alpha }}_{\mathrm{\parallel }}^{\prime }$-${\mathrm{\alpha }}_{\mathrm{\perp }}^{\prime }$=2.60 A${\mathrm{̊}}^2$, 2${\mathrm{\alpha }}_{\mathrm{\perp }}^{\prime }$+${\mathrm{\alpha }}_{\mathrm{\parallel }}^{\prime }$=7.55 A${\mathrm{̊}}^2$, ${\mathrm{\alpha }}_{\mathrm{\parallel }}$-${\mathrm{\alpha }}_{\mathrm{\perp }}$=0.32 A${\mathrm{̊}}^3$ for double bonds, with (${\mathrm{\alpha }}_{\mathrm{\parallel }}$-${\mathrm{\alpha }}_{\mathrm{\perp }}$) for the single bond arbitrarily set equal to its value in ethane, namely, 1.28 A${\mathrm{̊}}^3$. The transferability of these parameters to ${\mathrm{C}}_{70}$ is discussed in detail. © 1996 The American Physical Society.

We present experimental and theoretical findings on the geometry of polycrystalline para hexaphenyl via Raman scattering. The planarity of the molecule is affected by hydrostatic pressure and temperature. Our studies indicate that the potential energy curve which governs the torsional motion between neighboring phenyl rings is “W” shaped. We determine the activation energy to promote the molecule from a nonplanar to a planar state to be 0.04 eV, in good agreement with our quantum chemical calculations. From the relative intensities of the $1280{\mathrm{cm}}^{-1\mathrm{}}$ to the $1220{\mathrm{cm}}^{-1\mathrm{}}$ Raman modes we show that high pressure planarizes the molecules, modifying the “W”-shaped potential energy curve to a “U”-shaped one.

The $\beta$ phase in di-octyl substituted polyfluorene (PF8) exhibits a surprising level of conformational order at nanometer length scales compared to other phases. We present Raman scattering studies of PF8 as a function of thermal cycling, which establishes a connection between the conformational isomers and the side and main chain morphology. Density-functional calculations of the vibrational spectra of single chain oligomers in conjunction with the experimental results demonstrate the incompatibility of the $\beta$ phase with the overall $\alpha$ crystalline phase in PF8.

Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular disorder can be described very well by one-carrier space-charge-limited conduction for a discrete set of trap levels with energy $\sim 0.5\mathrm{eV}$ above the valence-band edge. Sweeping the bias above the trap-filling limit in the as-is polymer generates a set of exponential traps, which is clearly seen in the density of states calculations. The trapped charges in this set of traps have very long lifetimes and can be detrapped by photoexcitation. Thermal cycling the PF film to a crystalline phase prevents the creation of additional traps at higher voltages.