TIME-RESOLVED AND PULSED EPR STUDIES ON THE LOWEST EXCITED TRIPLET-STATE OF 1,4-DIPHENYLBUTADIYNE

The short-lived T-1 state of 1,4-diphenylbutadiyne (DPB) has been inve stigated in various glassy matrices at low temperatures using time-res olved EPR and pulsed EPR methods. Zero-field splitting parameters of D = -0.250 cm(-1) and E = +0.018 cm(-1) were determined. The negative D value was verified from the M-S dependence of the ENDOR frequencies. The hyperfine splitting constants of ring protons were obtained from t he electron spin-echo envelope modulation with three-pulse stimulated echo. The spin density distribution on the phenyl groups was estimated from the analysis of the hyperfine splittings at each of the canonica l orientations. The moderate spin densities on the phenyl rings indica ted that the T-1 state was (3)pi(x)pi(x) in electronic character. The origin of the large negative D value of DPB has been discussed in ter ms of the spin-orbit interaction between the close lying (3)pi(x)pi(x) and (3)pi(x)pi(y)* states.