By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we havecomprehensively studied the low-lying excited singlet states of
,
-diphenylpolyynes (DPY) having 1-6triple bonds. The a
g vibrational modes of the C
C stretching and of the phenyl ring motion were observedin the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in thefluorescence spectra of the long DPY with the triple-bond number (
N) more than two, the phenyl ring motionwith a
g symmetry disappeared and the b
1g modes of the phenyl ring twisting (~400 cm
-1) and of the C-Hbending (~900 cm
-1) were detected. The observed fluorescent states of DPY with
N 2 and
N 3 areassigned to the 1
1B
1u (
xx*) and 1
1A
u (
xy* and/or
yx*) states, respectively, based on the vibronicstructures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not onlythe allowed transition of 1
1B
1u S
0 but also the forbidden transition of 1
1A
u S
0 was detected in thefluorescence excitation spectra of the long DPY with
N 3. The low-lying excited state with A
u symmetryis characteristic in polyyne, which does not exist in polyene. The oscillator strength (
f) of the first absorptionband in DPY decreases with an increase in
N, which is the opposite behavior of the
all-
trans-,
-diphenylpolyenes. The
N-dependence of the
f value is understood by the configuration interaction betweenthe 1
1B
1u and 2
1B
1u (
yy*) states, which is consistent with the reduction of the nonlinear optical responseof polyyne.