Fluorescence, fluorescence excitation, and absorption spectra have been measured for diphenylpolyenes with the polyene double bond number (n) from three to seven in solvents with different polarizabilities at room temperature. These polyenes exhibit anti-Kasha S-2 (1 B-1(u)) fluorescence beside conventional S-1 (2 (1)A(g)) fluorescence in room temperature solution. It is shown that there are two different mechanisms for occurrence of the S-2 (1 B-1(u)) fluorescence of diphenylpolyenes. S-2 fluorescence occurs through the thermal population of the S-1 state for the shorter diphenylpolyenes (n=3 and 4), while it occurs as prompt S-2 fluorescence due to inefficient S-2-->S-1 internal conversion for the longer diphenylpolyenes (n=6 and 7). With diphenyldecapentaene (n=5), S-2 fluorescence based on both mechanisms is observed depending on the polarizability of the solvent used. The present results provide deeper insight into the mechanism for the occurrence of anti-Kasha S-2 fluorescence of polyenes. (C) 2004 American Institute of Physics.