The mechanism of high conductivity in organic semiconductors, tetrakis(alkylthio)tetrathiafulvalenes, TTCn-TTF, has been studied by the technique of solid-state high-resolution C-13 NMR. In the solid state of the long-chain compounds, significant downfield shifts, 3-4 ppm from those in the molten states, were observed for the alkyl carbon lines. It is shown that the energy difference between the trans/gauche rotational isomeric states are more than 10 times larger in the solid state than that in the molten state, and this provides evidence of the highly closely packed nature of the alkylthio chains in the solid state. It was also found that the solid-state chemical shieldings for both of the alkyl and the ethylene carbons gradually decreased as the chain length increased, suggesting that the electronic excitation energy gradually decreases due to the increasing intermolecular pi-pi overlap. Only the inner ethylene carbons exhibited upfield shift on crystallization, which has been interpreted as an enhancement of the pi-electron density in the central area of the TTF moiety. These findings provide evidence that the one-dimensional electrical conduction is enhanced by the intermolecular pi-pi overlap caused by the extremely closely packed nature of the long alkylthio chains, and therefore the electronic states of the organic semiconductors can be controlled by the length of the alkyl chains.