Intramolecular ring current in (4n + 2)π electronic states in the neutral acenes

The mechanism of the intramolecular diamagnetic ring currents in the (4n + 2)π electronic states of neutral acenes with microscopic sizes such as naphthalene (10ac), anthracene (14ac), and tetracene (18ac) is discussed, and compared with that of electrical conductivity in normal metals and conventional superconductors with macroscopic sizes. Diamagnetic intramolecular ring current in the (4n + 2)π electronic states of neutral acenes with microscopic sizes can be explained in terms of the electron pairing caused by attractive Coulomb interactions between two electrons with opposite spins occupying the same nondegenerate π molecular orbitals. The critical temperature Tcneutralacenes at which diamagnetic intramolecular ring current states are destroyed for the (4n + 2)π electronic states of each neutral acene with microscopic sizes such as 10ac, 14ac, and 18ac are estimated to be much larger than the critical temperature (Tc,BCS) for the conventional superconductors with macroscopic sizes. Energy lowering in the exponential factor in the equation for Tc,BCS, originating from continuous energy levels of electronic states in the conventional superconductors with macroscopic sizes, and the energy lowering factor which is proportional to the Tcneutralacenes values, originating from discrete energy levels of electronic states in acene molecules with small sizes such as 10ac, 14ac, and 18ac, are the main reason that the Tcneutralacenes values for the (4n + 2)π electronic states of neutral acenes with microscopic sizes are estimated to be much larger than the Tc,BCS values for the conventional superconductors with macroscopic sizes. In bulk system, electrical conductivity in the (4n + 1)π electronic states of the monocation crystal is compared with that in the (4n + 2)π electronic states of neutral acene crystals formed by small acene molecules.