Phase transitions and structures of novel pyrenes potentially useful in photovoltaic applications.

A series of conjugated compounds, 6,7,15,16-tetrakis(alkylthio)quinoxalino[2',3':9,10]phenanthro[4,5-abc]phenazine (TQPP-SC(n)) (n = 6, 8, 10, and 12), which display p-channel characteristics was synthesized. These materials show promise for use in liquid crystalline photovoltaic applications. To determine their applicability, the different phase structures and transitions of these compounds were studied with differential scanning calorimetry (DSC), polarized light microscopy (PLM), wide-angle X-ray diffraction (WAXD), selected area electron diffraction (SAED), and Fourier transform infrared spectroscopy (FT-IR). Using TQPP-SC(12) as a model compound, DSC and 1D WAXD results showed that this compound possesses four crystalline, but no liquid crystalline, phases. Based on structural results obtained from 2D WAXD experiments on oriented samples and SAED patterns from single crystals, the unit cell of the lowest temperature TQPP-SC(12) crystalline phase (K(1)) was determined to be monoclinic with dimensions of a = 1.87 nm, b = 0.53 nm, c = 3.51 nm, and beta = 96.2 degrees . With increasing temperature, the K(1) phase transformed to other crystalline phases which all were monoclinic with different crystallographic parameters. The arrangement of the TQPP-SC(12) rigid fused rings changed only slightly in these crystalline phases, yet the conformation of the alkyl chains attached to the rigid cores changed significantly at the phase transitions. For the other TQPP-SC(n) compounds, only two phase transitions could be identified. It was determined that the transition temperature can be tuned by modifying the attached alkyl chains at the four corners of the rigid fused rings. One-dimensional WAXD studies indicated that the condensed state phase transitions of these compounds were all crystal-crystal transitions. Although single crystals will provide the highest charge carrier mobility, they are very difficult to grow and incur a high cost in production. On the other hand, liquid crystalline phases are preferred for the ease of processing and reasonable performance in change carrier mobility. Therefore, in order to achieve liquid crystalline phases in these compounds, as desired for their application as organic photovoltaic materials, additional modifications to the alkyl chains and their locations are necessary.