Excited-state energy levels and photophysics of a short polyene 2-(4-phenyl-1,3-butadien-1-yl)thiophene

Article abstract of DOI:10.1016/j.jphotochem.2016.02.021

Emission, excitation and absorption spectra of a new and short polyene, 2-(4-phenyl-1,3-butadien-1-yl)thiophene (PBT), have been measured under different conditions by varying temperature and solvent and in the vapor phase, along with those of 2-(2-phenyl

Full text of DOI:10.1016/j.jphotochem.2016.02.021

Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Excited-state energy levels and photophysics of a short polyene
2
-(4-phenyl-1,3-butadien-1-yl)thiophene
Takao Itoh
Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 6 January 2016
Received in revised form 17 February 2016
Accepted 23 February 2016
Available online 26 February 2016
Emission, excitation and absorption spectra of a new and short polyene, 2-(4-phenyl-1,3-butadien-1-yl)
thiophene (PBT), have been measured under different conditions by varying temperature and solvent and
in the vapor phase, along with those of 2-(2-phenylethenyl)-thiophene (PET). The presence of a forbidden
excited singlet state, located at energies slightly below the strongly absorbing state, is indicated for PBT,
although the forbidden state was not identified for PET. The emission of PBTconsists of fluorescence from
the allowed S
from forbidden S
Quantitative analysis of the temperature dependence of the S
and S state energies and physical parameters that characterize the excited states of PBT.
2
and forbidden S
in the vapor phase and that from allowed S
and S
1
states in low polarizable solvents, while it consists of florescence mainly
state in high polarizable solvents.
fluorescence spectra provides the S
Keywords:
Short polyene
1
1
1
2
1
2
2
-(4-Phenyl-1,3-butadien-1-yl)thiophene
-(2-Phenylethenyl)-thiophene
2
S
2
fluorescence
ã 2016 Elsevier B.V. All rights reserved.
Forbidden S state
1
1
1. Introduction
out in 1972, followed by theoretical interpretation of the 2 A
g
state
[
2–4]. Since these findings, a number of studies have been carried
out concerning the excited states of polyenes [5,6], although the
presence of the forbidden S state of polyenes does not seem to be
well known for most of chemists even at present. Thus, it is of
necessity to accumulate the data showing the presence of the
forbidden lowest singlet excited state of short polyenes other than
the ones investigated heretofore. It is normally difficult to observe
Polyene structure plays important roles in photosynthesis,
visual pigments and other biological systems including vitamin A
as well as in conducting polymers [1]. In particular, clarification of
the excited-state electronic structures of short polyenes attracts
much attention in conjunction with the electronic structure of
1
diphenylbutadiene or unsubstituted butadiene. The S
1 2
and S
states are considered to be closely located to each other for shorter
polyenes. Most of symmetrically substituted all-s-trans polyenes
directly the transition to the forbidden S
temperature solution, because it is masked by strongly allowed
absorption to the S state, which is located at energies slightly
higher than the S state, and because it is dipole-forbidden. The
forbidden S state of polyenes is considered to obtain its one-
photon transition intensity mainly through the vibronic coupling
with the allowed S state.
Molecular structures of PBT (left) and PET (right)
1
state in room
such as
b
-carotene belong to the C2h point group. These C2h
2
polyenes exhibit an intense absorption band based on an allowed
1
1
1
electronic transition, 1 A
g
(S
0
) ! 1 B
u
(p
,
p
*), in the UV–vis region.
state for
1
1
The 1 B
u
(p
,
p
*) state had been considered to be the S
1
long time, but the existence of a forbidden excited singlet state,
2
1
1
2
g u
A , located at energies below an allowed 1 B state, was pointed
E-mail address: titoh@hiroshima-u.ac.jp (T. Itoh).
http://dx.doi.org/10.1016/j.jphotochem.2016.02.021
010-6030/ã 2016 Elsevier B.V. All rights reserved.
1

T. Itoh / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
41
Since most of the all trans-polyenes with the polyene double
bond number over 3 are considered to posses a characteristic
thiophene (PET), also has been synthesized and the spectroscopic
properties were investigated under different conditions. Although
the symmetries of PBT does not belong strictly to the C2h point
forbidden S
1
state, it must be of interest to investigate the
electronic spectra of polyenes of various types. However, the
number of commercially available polyene molecules is extremely
limited. In the present study, a new short polyene, 2-(4-phenyl-1,3-
butadien-1-yl) thiophene (hereafter abbreviated by PBT) has been
synthesized and the spectroscopic properties were investigated
under different conditions. A close relative, 2-(2-phenylethenyl)-
group, the electronic structures of the
p
!
p* excited states are
expected to be similar to those of the C2h polyenes. The existence of
a forbidden lowest singlet excited state located at energies slightly
below a strongly absorbing state is indicated for PBT. It is shown
also that the emission of PBTconsists of dual fluorescence from the
2 1
S and S states in low polarizable solvents, while it consists mostly
Fig.1. (a) Absorption and corrected emission spectra of PBT in different solvents at room temperature and in the vapor phase. Vapor-phase emission spectrum was measured
ꢀ
at 95 C in the presence of 760 Torr N
2
. The emission spectra were obtained by excitation into the S
2
state (around 300–330 nm). All the spectra are normalized to a common
magnitude. Broken line indicates the location of the S
phase. Vapor-phase emission spectrum was measured at 80 C in the presence of 760 Torr N
1
state. (b) Absorption and corrected emission spectra of PET in different solvents at room temperature and in the vapor
ꢀ
2
. The emission spectra were obtained by excitation into the S
2
state (around
310 nm). All the spectra are normalized to a common magnitude.

4
2
T. Itoh / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
+
+
6 5
CH ), 77 (C H
+)
of florescence from the forbidden S
Quantitative analysis of the temperature dependence of the band
locations and the relative intensity of the fluorescence provides
1
state in the vapor phase.
including 212 (PBT ), 135 (C
4
H S ꢁꢁ CH
3
¼
CH ꢁꢁ CH
¼
+
and 45 (SCH ), which agreed with the chemical structure of PBT.
2-(2-Phenylethenyl)-thiophene (PET) was synthesized also by
means of Wittig reaction between benzaldehyde and triphenyl(2-
estimates of the S
1 2
and S state excitation energies and the physical
parameters that characterize the excited state of PBT.
thienylmethyl)-phosphonium bromide in
a similar way as
mentioned above, and was purified in the same way as that of
PBT. Purity of PET was checked in the same ways as those used for
PBT. Only a single peak was detected for the gas chromatography
measurement. The fragments in MS were seen at the positions
2
. Experimental
2.1. Materials
+
+
+
including 186 (PET ), 109 (C
4
H
3
S ꢁꢁ CH
¼
6 5
CH ), 77 (C H ) and 45
+
(
SCH ), which also agreed with the chemical structure of PET.
2-(4-Phenyl-1,3-butadien-1-yl) thiophene (PBT) was synthe-
Further, it was confirmed that the corrected fluorescence excita-
tion spectrum of purified PBT and PET agreed well with the
absorption spectrum in all the solvents used.
sized by means of Wittig reaction between trans-cinnamaldehyde
and triphenyl(2-thienylmethyl)-phosphonium bromide The syn-
thetic method of this molecule is described below in detail: Two
grams of a triphenyl(2-thienylmethyl)-phosphonium bromide and
sodium amide (1:1) molar mixture was dissolved in 30 ml of dry
2
.2. Measurements and computational
THF and the solution was stirred for 20 min under dry N
2
Emission and excitation spectra were measured with a Jobin
atmosphere. 0.6 ml of trans-cinnamaldehyde was then dropped
slowly into the reaction flask that was maintained in an ice bath.
After 20 min the reaction was stopped by adding aqueous 15%
NaOH solution to the flask and the aqueous layer was discarded
using a separatory funnel. The resultant reaction mixture was
extracted in hexane which was then chromatographed twice on a
silica gel (Merck silica gel 100) column using hexane as an eluant.
By irradiating UV light to the column, only the part showing blue
emission was collected during chromatography procedures. The
chromatographed hexane solution was evaporated under reduced
pressure to concentrate the solution. The resultant solution was
Yvon—Spex Fluorolog-3 (Model 21-SS) spectrophotometer
equipped with a double-grating excitation monochromator, a
high-pressure 450-W Xenon lamp as an excitation-light source,
and a photomultiplier tube (Hamamatsu R928-P) in an electric-
cooled housing operated in photon-counting mode. For most of the
emission measurement square 10-mm path-length quartz cells
were used. Two reflecting mirrors were placed beside the sample
cell so as to intensify the emission signals [7]. Temperature of the
sample cells was maintained by a thermostated cell holder during
the measurements. Absorption spectra were measured with a
Shimadzu UV-2550 spectrophotometer. A cylindrical quartz cell
with a 100-mm path length was also used for absorption
measurements of vapor samples at elevated temperatures.
Fluorescence spectra were corrected for the spectral sensitivity
ꢀ
left at ꢁ5 C to generate white powder, which was then recystal-
1
lized twice in hexane to obtain purified PBT. H NMR data of the
purified PBT in CDCl
J = 12.0 Hz, 1H) 6.79 (s, 1H), 6.89 (ddd, J = 15.6, 12.0, 1.2 Hz, 1H),
.97–7.01 (m, 2H), 7.18 (dd, J = 6.0, 0.8 Hz, 1H), 7.23–7.36 (m, 3H),
.43 (d, J = 8.8 Hz, 1H), 7.44 ppm (d, J = 8.8 Hz, 1H). NMR peaks at
3
are d= 6.04 (d, J = 15.6 Hz, 1H), 6.78 (d,
of the detection system using
acetate buffer solution as the standard [8]. Excitation spectra were
corrected for the spectral intensity distribution of the exciting light
b
-naphthol in acetic acid ꢁ sodium
6
7
about 7.0–7.5 ppm correspond to the H atoms of the phenyl and
thienyl groups, while those at about 6.6–6.9 ppm correspond to the
H atoms on the butadiene frame. Purity of PBT was checked with a
Shimadzu GC-17A gas chromatograph using a column Agilent J&W
HP-5MS combined with a Shimadzu GP5000 mass spectrometer.
Only a single peak was detected for the gas chromatography
measurement. The fragments in MS were seen at the positions
1
with an aqueous solution of rhodamine B as a quantum counter. H
NMR data were obtained with a JEOL Alfa 400 with external field of
4
00 MHz.
Total energies and vibrational wavenumbers of the ground-
state molecules at optimized geometry were obtained by
DFT/B3LYP/6–311 + G(p,d) level calculations using Gaussian 03
program [9].
Fig. 2. Absorption and corrected fluorescence spectra of PBT in perfluorohexane at different temperatures. The emission spectra were obtained by excitation into the S
around 300–330 nm). All the spectra are normalized to a common magnitude.
2
state
(

T. Itoh / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
43
ꢀ
Fig. 3. Absorption (right) and corrected fluorescence (left) spectra of PBT in perfluorohexane–hexane mixtures with different ratios at 24 C: Red-colored spectrum, in
perfluorohexane; blue, in a perfluorohexane-hexane (9:1) mixture; green, in a perfluorohexane-hexane (7:3) mixture; and black, in hexane. The emission spectra were
2
obtained by excitation into the S state (around 300–330 nm). All the spectra are normalized to a common magnitude.
2
Fig. 4. Schematic energy-state diagrams including the relaxation processes of PBT in perfluorohexane (left) and in benzene or in CS (right). Broken lines indicate the
forbidden excited-state levels.
Fig. 5. An example of a pair of the observed and fitted fluorescence spectra of PBT in perfluorohexane.
3
. Results and discussion
in benzene, there is a sufficient overlapping between the
absorption and emission spectra, forming a sort of mirror image
relationship between the two spectra. Thus, the emission is likely
to originate from the absorbing state of PBT in benzene. With
decreasing solvent polarizability, the Stokes shift defined by the
difference between positions of the first absorption and emission
band increases. Further, there is only a little overlapping between
the absorption and emission spectra of PBT vapor. The emission
3.1. Emission and absorption spectra
Absorption and corrected emission spectra of PBT in different
solvents are shown in Fig. 1a along with those in the vapor phase.
The absorption and emission spectra of PBT show the features
characteristic to those of other typical polyenes. In the case of PBT

4
4
T. Itoh / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
Fig. 6. Fitted absorption and S
1
fluorescence origins of PBT (open and closed circles) and PET (open and closed triangles) plotted as a function of solvent polarizability a.
Solvents used for obtaining the plots of absorption origins are, from the left to the right, in the vapor phase, perfluorohexane, methanol, pentane, hexane, a hexane-benzene
1:1) mixture, benzene, a benzene-CS (1:1) mixture, and CS for PBT, and in the vapor phase, perfluorohexane, pentane, hexane, a hexane-benzene (1:1) mixture, benzene for
PET.
(
2
2
spectrum in the vapor phase is indicative of the presence of the
fluorescence. In view of these observations, it is not unreasonable
to assign the weak emission band seen at the onset of the emission
ꢁ
1
forbidden S
1
state located at about 26500 cm . Closer inspection
of the emission spectrum measured in perfluorohexane reveals the
spectrum in perfluorohexane as the fluorescence from S
2
, and the
band near 26500 cm (ꢂ376 nm) as the origin band of the S
fluorescence. The first strong absorption band can be assigned to
the origin of the S transition as in the case of diphenypo-
! S
lyenes. According to these assignments, the fluorescence bands
ꢁ
1
presence of a weak band at the blue side shoulder of the emission
1
ꢁ
1
spectrum (at about 27500 cm ). This weak band exhibits a sort of
mirror image relationship to the absorption spectrum, showing a
sufficient overlapping with the lowest strong band of the
absorption spectrum. The corrected excitation spectrum of the
weak emission agreed with the corresponding absorption spec-
trum. As will be mentioned later in detail, the weak emission band
shift in the same way as does the absorption spectrum when
solvent and/or temperature was changed. The spectral feature of
PBT in perfluorohexane resembles that observed for diphenylhex-
atriene in hexane, diphenyloctatetraene in benzene, dithienylbu-
tadiene in hexane, and methyl and pheny-substituted hexatriene
in hexane [10–13]. It is known for these polyenes that the observed
0
2
ꢁ1
located at near 26500 cm in perfluorohexane and in the vapor
phase can be regarded as the origin band of the S
well known that the location of the forbidden S
1
fluorescence. It is
state of polyenes
1
does not show a significant change by varying the solvent
polarizability and temperature, although that of the S state
2
changes significantly, as are seen in Fig. 1a. These assignments
make it possible to quantitatively fit the dependence of the
measured emission and absorption spectra on temperature and
solvent polarizability. This is discussed below in detail, along with
the data leading to the assignments.
1 2
emission consists of S fluorescence accompanied by weak S
Fig. 7. Values of ln (
F
2
/F
1
) ꢁ 2ln
DE plotted as a function of 1/T for PBT in perfluorohexane.

T. Itoh / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
45
In Fig. 1b, we show the absorption and corrected emission
spectra of PET in different solvents as well as those in the vapor
phase. The spectra of PET show a significant blue shift as compared
with that of PBT due to the shorter conjugation length. There is a
sufficient overlapping between the absorption and emission
spectra, forming a sort of mirror image relationship between
the two spectra. Further, the spectra clearly show that as the
solvent polarizability increases the emission bands shift to the red
in exactly the same way as does the absorption spectrum. These
observations indicate that the emission of PET is the fluorescence
originating from the absorbing state.
the 0–1 and 0–2 for each of the polyene C-C and C
¼
C stretching
modes and the combination bands of the two modes. In the case of
emission spectra, the S absorption spectrum reflected so that the
origin matched the origin of the S fluorescence and multiplied by
2
2
a fitted value was subtracted from the measured emission
spectrum [10]. The difference spectrum thus obtained was then
fit by six Gaussians to obtain the band position and intensity of the
Gaussians corresponding to the S
six Gaussians correspond to the S
0–2 for each of the C ꢁꢁ C and C C stretching modes in the ground
state, and the combination band of the two modes. The frequencies
1
fluorescence alone, where the
1
fluorescence origin, the 0–1 and
¼
of the vibrational modes appeared in the S
1
fluorescence were
ꢁ1
ꢁ1
3.2. Temperature and solvent polarizability dependence of the spectra
obtained from DFT outputs (1210 cm for the C ꢁꢁ C and 1610 cm
for C
fluorescence spectra of PBT are shown in Fig. 5. In order to confirm
the presence and shape of the S fluorescence, only the S
of PBT
¼C stretch modes). Examples of the measured and fitted
Fig. 2 shows absorption and corrected fluorescence spectra of
2
2
PBT in perfluorohexane at different temperatures. As the
temperature is raised, the relative intensity and energy of the
weak emission increases and the absorption spectra also show a
corresponding blue shift. Fig. 3 shows absorption and corrected
fluorescence spectra of PBT in perfluorohexane–hexane mixtures
with different ratios at fixed temperature. It is seen that the
absorption spectrum shifts to the red with increasing the solvent
polarizability, as was observed for a number of other polyenes. The
spectra show also that as the solvent polarizability increases the
fluorescence was extracted by subtracting the fluorescence
spectrum measured at lower temperature from that measured
at higher temperature. The obtained difference spectrum was
found to resemble the reflected S
Gaussian fitting procedures. In Fig. 6, fitted S
fluorescence origins are plotted as a function of solvent polariz-
2
absorption spectrum used in the
2
absorption and S
1
2
2
ability
refractive index of the solvent. It is seen that the energy of S
absorption origin decreases almost linearly with increasing
although that of S
changing for PBT. In the case of PET, however, the fluorescence
bands shift to the red in exactly the same way as does the
absorption spectrum with increasing for all the solvents used.
Since the S absorption energy decreases almost linearly with
a
, which is defined by (n ꢁ1)/(n + 2) with n denoting the
2
a
,
weak emission band (S
almost the same way as does the absorption spectrum. Further, the
relative intensity of the S fluorescence increases as the energy
difference between the highest energy band of the S fluorescence
and the lowest energy band of the S absorption decreases.
2
fluorescence) shifts to lower energy in
1
fluorescence origin is almost invariant with
a
2
1
a
2
2
a
Schematic energy-state diagrams showing the relaxation process-
es of PBT in different solvents are shown in Fig. 4.
which is approximately proportional to the reciprocal of the
density, and since the density decreases nearly linearly with
increasing temperature, we expect that the excitation energy of the
In order to quantitatively determine band positions and relative
intensities for the S
1
and S
2
fluorescence and S
2
absorption spectra
S
2
state will increase approximately linearly with increasing
of PBT, all the spectral data were fitted by sum of Gaussians using
temperature. Thus, we obtain,
2
2
least square fit, I(
n
,
) = exp[ꢁ(
n
ꢁ
0
n ) /s ], where I(n) is the intensity
D
E ffi + sT
D
E
0
(1)
at wavenumber
n
n
0
is the Gaussian center and
s
is the width.
ꢁ
When six Gaussians were fit to the first 4000 cm ' of the
absorption spectra, the measured and calculated spectra were
almost indistinguishable to each other. In the case of absorption
where E is the expected S –S energy difference at 0 K and s is a
D
0
1
2
ꢁ1
constant. We obtained the s value of 3.3 cm /K.
Temperature dependence of the fluorescence spectrum of PBT
in perfluorohexane indicates that the S1 and S2 states are in
2
spectra, the six Gaussians correspond to the S absorption origin,
thermodynamic equilibrium. Therefore, the S
2 1
/S fluorescence
quantum yield ratio ( ) is given approximately by
2 1
F /F
F
2
/
F
1
ffi k /k  exp(ꢁ T)
2
1
D
E/k
B
(2)
and S
2
Table 1
ꢁ
1 a
Locations (in cm
tadiene), DPB (diphenylbutadiene), MPB (methylphebylbutadiene) and MPH
methylphenylhexatriene) estimated from the fluorescence, absorption and/or
excitation spectra in different environments.
) of the low-lying excited states of PBT, PET, DTB (dithienylbu-
where k and k are the radiative rate constants for the S
1
2
1
states, E is the energy difference between these two excited
states, k is the Boltzmann constant and T is the absolute
temperature. Basically, Eq. (2) can explain all the spectral change
shown in Figs. 2 and 3. That is, the S fluorescence in this case
occurs through the Boltzmann distribution of the S state. The ratio
/k in Eq. (2) is related to the ratio of the oscillator strength for
the S transition, f , to that for the S ! S transitions, f by k
! S
/f [14]. If we assume that all of the radiative strength for the
fluorescence originates from coupling between the S
D
(
B
Molecules
PBT
Excited states
Vapor
In perfluorohexane
In hexane
2
S
S
2
(Allowed)
(Forbidden)
29000
27000
28500
27000
27500
–
1
1
k
2
1
2
/
PET
S
2
(Allowed)
30500
28500
29700
28800
2
0
2
1
0
1
k
1
ffi f
2 1
DTB
S
S
2
(Allowed)
(Forbidden)
27500
25300
26500
25200
S
1
1
and S
2
b
1
25471
states, the oscillator strengths should obey a formula (3) [3],
DPB
S
S
S
2
1
1
(Allowed)
(Forbidden)
(Allowed)
30700
29653
33200
29600
28500
–
 V122/
E2
f
1
= f
2
D
(3)
b
c
ꢂ29300
MPB
MPH
32400
32000
where V12 is the matrix element connecting the two excited states.
Combination of Eqs. (2) and (3) provides
S
S
2
(Allowed)
(Forbidden)
32500
28500
31000
28300
30000
28200
1
ln (
F
2
/
F
1
) ꢁ 2ln
It follows from Eqs. (1) and (4) that,
) ꢁ 2ln E = ꢁ /k T ꢁs/k ꢁ 2ln V12
D
E = ꢁ
D
E/k
T ꢁ 2ln V12
(4)
(5)
B
In perfluorohexane and hexane locations at room temperature are shown.
a
b
c
ꢁ1
Accuracy of about Æ200 cm except for the values obtained in a jet.
In a jet taken from Ref. [15–17].
ln (
F
2
/
F
1
D
D
E
0
B
B
1
Estimated from the extrapolation of the S
1
(2 Ag) state energies versus solvent
polarizability obtained from Ref. [18].

4
6
T. Itoh / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 40–46
Thus, the slope and intercept of the values of ln(
E plotted as a function of 1/T in a fixed solvent should provide the
values of ꢁ 2ln V12, respectively. The spectra such as
and ꢁs/k
the ones shown in Fig. 2 were fitted to obtain the and
values at each temperature, and these were analyzed on the basis
of Eq. (5). The S and S fluorescence origin band centers were
estimated by the Gaussian fitting procedure and the energy
differences between the fitted Gaussian band centers of the S and
fluorescence origins were taken as the E values. The relative
quantum yields of the S fluorescence were obtained by integrating
the S fluorescence spectrum obtained by subtracting the S
fluorescence spectrum from the measured emission spectrum and
those of the S fluorescence were obtained by integrating the
reflected S absorption spectrum. Fig. 7 shows the values of ln (
) ꢁ 2ln E plotted as a function of 1/T for PBT in perfluorohex-
ane. The plot provides a straight line as is expected from Eq. (5). We
F
2
/
F
1
) ꢁ 2ln
4. Conclusions
D
D
E
0
B
It is indicated for PBT that there is a forbidden excited state with
the oscillator strength of approximately 0.01, located at energies
slightly below the strongly absorbing state, although the forbidden
excited state was not identified for PET. Emission from PBT in low
polarizability solvents such as perfluorohexane is a mixture of the
2
F /
F
1
DE
1
2
1
1 2
S fluorescence and weak thermally-activated S fluorescence at
S
2
D
temperatures near room temperature. All the available informa-
tion on band positions and intensities evaluated as a function of
temperature and solvent properties can be well interpreted by
assuming that the S
equilibrium. Quantitative analysis of the temperature dependence
of the relative fluorescence intensity and band positions provides
1 2
estimates of the S and S state excitation energies and physical
parameters that characterize the excited state of PBT.
1
1
2
2 1
and S levels are in thermodynamic
2
2
2
F /
F
1
D
ꢁ
1
obtained the E
0
and s/k
B
+ 2lnV12 values of 380 cm
and 14.5,
respectively, which leads to the V12 value of 130 cmꢁ1. The V12 value
thus obtained is of a reasonable order of magnitude for vibronic
coupling energies of polyatomic molecules [15]. The molar
extinction coefficient of the absorption maximum near 342 nm
Acknowledgment
Help from Dr. Hisashi Omura of Hiroshima University in
collecting mass and NMR spectral data reported here is noted with
gratitude.
ꢁ1
ꢁ1
(
29240 cm ) of PBT in hexane is evaluated to be 63345 (mol
3
ꢁ1
dm cm ), which corresponds to the oscillator strength (f
about 0.6. Based on Eq. (3), the f
2
) of
1
value of PBT is evaluated to be
References
approximately 0.01 in perfluorohexane at room temperature.
[
1] V.R. Preedy, Vitamin A and Carotenoids: Chemistry, (2012) RSC Publishing; T.,
Polivka, H., Hashimoto, V., Sndstroem, (Ed.) Carotenoid photophysics, Chem.
Phys. 373 (2010); 1–152
It is known that the emission of diphenylbutadiene consists
1
mostly of the S
2
(1 B
u
) fluorescence in the static vapor phase in the
presence of a buffer gas. [11] In the case of PBT vapor, however, the
emission is considered to consist mostly of the S fluorescence.
These seemingly contradicting observations can be explained by
[2] B.S. Hudson, B.E. Kohler, A low-lying weak transition in the polyene
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a,v
1
[
3] B.S. Hudson, B.E. Kohler, Polyene spectroscopy: lowest energy excited singlet
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(1973) 4984–5002.
1
1
using Eq. (2). The S
1
(2 A
g
) ꢁ S
2 u
(1 B ) energy difference is about
ꢁ
1
1000 cm for diphenylbutadiene vapor [11], while that of PBT
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ꢁ
1
polyenes and related molecules, Chem. Phys. Lett. 14 (1972) 305–309.
vapor is estimated to be about 2000 cm or perhaps more (see
[5] B.S. Hudson, B.E. Kohler, K. Schulten, Linear polyene electronic structure and
Figs. 1 and 5). Thus, the ( ) values are calculated to be 0.04
F
2
/F
1
potential surfaces, Excit. States 5 (1982) 1–95.
and 2.1 for PBT and diphenylbutadiene vapors, respectively, when
[6] T. Itoh, Fluorescence and phosphorescence from higher excited states of
ꢀ
organic molecules, Chem. Rev. 112 (2012) 4541–4568.
the f
2
/f
1
ratios are assumed to be 100 at 100 C, which agrees with
[
7] T. Itoh, Intensification of the emission signals using the reflection mirrors
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von fluorescenzspektren mit hilfe von spektralphotometern und
vergleichsstandards, Z. Anal. Chem. 170 (1959) 1–18.
the observations.
[
3.3. Energy levels of PBT, PET and other shorter polyenes
[9] M.J. Frisch, et al., Gaussian 03, Revision B.05, Gaussian, Inc., Pittsburgh, PA,
Considering the Stokes shift between absorption and fluores-
2003.
[
10] T. Itoh, B.E. Kohler, Dual fluorescence of diphenylpolyenes, J. Phys. Chem. 91
cence bands, extrapolation of the S
shown in Fig. 6 to higher solvent polarizability indicates that the S
and S states cross at
= ꢂ0.25. The data shown in Figs. 1 and 6
suggests that the forbidden S state of PBT is probably located at
energies slightly lower than the strongly absorbing S state also in
1
fluorescence origin of PBT
(
1987) 1760–1764.
1
[
11] T. Itoh, B.E. Kohler, Fluorescence from diphenylpolyene vapors, J. Phys. Chem.
92 (1988) 1807–1813.
12] T. Itoh, Excited electronic states and spectroscopy of unsymmetrically
2
a
[
1
substituted polyenes, J. Chem. Phys. 139 (2013) 094304-1––094304-7.
2
1
1
1
[
13] T. Itoh, M. Yamaji, 1 Bu (S
-dithienylbutadiene and
(2008) 13413–13418.
2
) and 2 Ag (S
a,v
1
) fluorescence and the 2 Ag-state of
methanol, hexane and pentane at temperatures in the neighbor-
a,v
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hood of room temperature. It is known that the fluorescence of
1
1
[14] S.J. Strickler, R.A. Berg, Relationship between absorption intensity and
diphenylhexatriene originates from the S
2 u 1 g
(1 B ) and S (1 A )
fluorescence lifetime of molecules, J. Chem. Phys. 37 (1962) 814–822.
states in room temperature solution, but it originates solely from
[
15] R.M. Hochstrasser, Analytical and structural aspects of vibronic interactions in
the ultraviolet spectra of organic molecules, Acc. Chem. Res.1 (1968) 266–274.
16] B.E. Kohler, T.A. Spiglamin, Sructure and dynamics of excited singlet states of
isolated diphenylhexatriene, J. Chem. Phys. 80 (1984) 5465–5471.
1
S
1
(1 A
similar to those of diphenylhexatriene were observed also for PBT
11]. The locations of low-lying excited states of PBT and PET
g
) in the vapor phase [10,11]. Thus, fluorescence properties
[
[
[
17] J.F. Shepanski, B.W. Keelan, A.H. Zewail, Diphenylbutadiene in supersonic jet:
estimated from the fluorescence and absorption spectra in
different environments are summarized in Table 1, along with
those of other short polyenes, dithienylbutadiene, diphenylbuta-
diene, methylphebylbutadiene and methylphenylhexatriene.
spectroscopy and picosecond dynamics, Chem. Phys. Lett. 103 (1983) 9–13.
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1
[
perfluorohexane and perfluoropentane, Chem. Phys. Lett. 342 (2001) 550–554.