Determination of Both Tilting and In-Plane Molecular Rotational Angles for Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene Using Near-Edge X-ray Absorption Fine Structure

We demonstrate that both in-plane molecular rotational and tilting angles of the molecular orientation can be determined using the sigma* resonance of sulfur (S) K-edge near-edge X-ray absorption fine structure. We examined dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) thin films on silicon dioxide (SiO2) and copper oxide (CuOx) substrates which are relevant to the cost-effective organic field-effect transistors. In-plane directed transition moments were attributed to the electron excitation from S is to the largely overlapped sigma* orbital related to the C-S bonding at the thiophene site. Under coexistence with a minor component of the amorphous region, it turned out that the DNTT domain with the single-crystal structure has the c-axis normal to the substrates; the DNTT molecules on SiO2 orient at a tilting angle (beta) of 85 degrees and an in-plane rotational angle (Phi(M)) of 77 and 103 degrees. In the case of the film on CuOx, values of 84 degrees for beta and 73 and 107 degrees for Phi(M) are acquired. The amount of the amorphous region in the film on CuOx, was larger than that on SiO2. Our approach is applicable to other sulfur-containing molecules with a small population of the lowest unoccupied molecular orbital on the S atoms.