The lowest excited triplet (T-1) states of diphenylacetylene and several alpha,omega -diphenylpolyynes (DPY) having two, three, four and six triple bonds were studied using continuous wave time-resolved electron paramagnetic resonance (CW-TREPR), pulsed EPR, and phosphorescence spectroscopy. Linear and planar molecular structures in the T-1 states of DPY were suggested from the magnetophotoselection experiments and observation of a strong 0-0 band in the well-resolved phosphorescence spectra. The spin density distributions, which were obtained by electron spin echo envelope modulation measurements and semi-empirical molecular orbital calculations, and the phosphorescence polarization normal to the long axis of molecule for the 0-0 bands suggested that the T-1 states of DPY have a B-3(1u) (pi (x)pi (x)*) electronic configuration. DPY showed an abnormally large \D\ value of the zero-field splitting parameters that increased with increasing molecular size. Such an unusual behavior of the D values is interpreted in terms of the spin-orbit interaction between the T-1 and (3)A(u)(pi (x)pi (y)*) states mainly due to their energy proximity which is probably characteristic of the excited states for the polyynes. The estimated energy gap between the (3)pi (x)pi (y)* and (3)pi (x)pi (x)* states for DPY ranges from 2900 cm(-1) to 1400 cm(-1). There existed a good correlation between the acceleration of the nonradiative decay rate constant from the T1 state and appearance of a ring twisting vibronic band with b(1g) symmetry in the phosphorescence spectra for DPY. Therefore, we concluded that the vibronic interaction between the (3)pi (x)pi (y)* and (3)pi (x)pi (x)* states promoted by the b(1g) vibration leads to a broadening of the potential surface of the T-1 state which results in the fast radiationless decay Co the ground state, (C) 2001 American Institute of Physics.