12893
J. Phys. Chem. 1994, 98, 12893-12898
Geometry and Torsional Potential of 2,2'-Bithiophene in a Supersonic Jet Masao Takayanagi,tfsTatsuo GejoP,* and Ichiro Hanazaki'*+9$ Institute for Molecular Science and The Graduate University for Advanced Studies, Myodaiji, Okazaki 444, Japan Received: July 21, 1994@
Fluorescence-excitation, hole-burning, and dispersed-fluorescence spectra of 2,2'-bithiophene in a supersonic jet were measured. Two species were identified in the fluorescence-excitation spectra and were distinguished clearly in the hole-buring spectra. These species have the origin bands with the 91.4 cm-' shift to each other and show almost the same vibronic structures. They are assigned to the vibronic bands due to 2,2'-bithiophenes in the ground and low-lying excited vibrational states. Long and harmonic progressions due to the torsional vibration in the excited electronic state suggest a substantial change in the torsional angle upon the electronic excitation. The equilibrium structure of 2,2'-bithiophene in the excited electronic state is found to be trans planar with a deep, harmonic, and single-minimum torsional potential around the equilibrium structure. In the dispersed-fluorescence spectra, short and anharmonic progressions due to the torsional vibration in the ground electronic state were observed. By simulating the progressions, 2,2'-bithiophene in the ground electronic state is found to have a double-minimum torsional potential, whose equilibrium structures are twisted by about 21" from the trans-planar structure. The height of the barrier between the minima is estimated to be about 25 cm-'. Franck-Condon calculations were also performed to examine the observed intensity distributions in the fluorescence-excitation, hole-burning, and dispersed-fluorescence spectra.
Introduction Torsional vibrations and conformations of the molecules in which two aromatic rings are linked by a single bond have been the subject of numerous investigations. This kind of molecule is conformationally flexible along the torsional coordinate, so that its conformation depends strongly on the environment and on the electronic states in which the molecule stays. Biphenyl is a typical molecule belonging to this category.l s 2 The two phenyl rings in biphenyl are coplanar in the solid state. In solutions and vapor, it is twisted in the ground electronic (SO)state, while it becomes planar upon excitation to the lowest excited singlet electronic (SI) state. Similar conformational flexibility has been found for l,l'-bina~hthyl,~*~ 9-phenylant h r a ~ e n e ,9,9-bianthryl,' ~.~ and substituted anthracenes.8 Among them, 2,2'-bithiophene (BTP) has been studied extensively not only for structural interest but also for the fact that BTP constitutes the unit of polythiophenes, which attract profound interest as conductive polymers. Two conformers, planar trans and cis shown in Figure 1, can be considered as the limiting structures for BTP. It has been reported that the equilibrium angle between the two aromatic planes depends on the environmental conditions. BTP takes the trans-planar structure in the ~ r y s t a l while , ~ the angle between the two rings is twisted by 34" from the trans-planar structure in the gas phase at room temperature.l0 Two conformers, trans and cis, have been found in the liquid Very recently the cis conformer has been found also in the gas phase at around 100 "C.14 Although the stable conformations of BTP have been discussed t h e o r e t i ~ a l l y , l ~the - ~ ~ potential curve along the torsional coordinate has not been established; e.g., some calculations predict the existence of both trans and cis conformers, while others predict only a slightly twisted trans conformer. Institute for Molecular Science. The Graduate University for Advanced Studies. 8 Present address: Physikalisch-Chemishes Institut der Universit8.t Ziirich, Winterhurerstrasse 190, CH-8057Zurich, Switzerland. @Abstractpublished in Advance ACS Absrracts, November 1, 1994.
*
0022-3654/94/2098-12893$04.50/0
O i l
trona
Figure 1. Planar trans and cis conformers of BTF'.
Under these circumstances, it seems to be of great importance to investigate the structure of BTP under the isolated condition without any external intermolecular interaction. As far as we know, the only spectroscopic study of BTP in a supersonic jet reported so far is that by Chadwick and K ~ h l e r , *who ~ claim that the trans and cis conformers are found in the laser-induced fluorescence (LIF) excitation spectrum of BTP in a supersonic jet. However, the assignment of the observed species to trans and cis conformers does not seem to be the unique solution. The signal-to-noise ratios of Chadwick and Kohler's dispersedfluorescence spectra are too poor for any detailed analysis. They did not try to fit their observed torsional frequencies to model potentials. We believe that there is another, more reasonable, solution to this problem, namely, to assign two observed species to a torsionally twisted pair around the trans conformer. In this paper, we show that this conformational assignment is more plausible by examining newly measured fluorescence-excitation and dispersed-fluorescence spectra of BTP in a supersonic jet with a much higher signal-to-noise ratio. We have also measured the hole-burning spectra by probing the origin bands of both species to confirm the assignment. The torsional progressions in these three types of spectra have been fit to model potentials for the SO and S1 states, and the relative intensity distributions have been examined on the basis of the Franck-Condon analysis.
Experimental Section The LIF-excitation spectra were measured using the frequencydoubled output of an excimer-laser-pumpeddye laser (Lumonics HE-420-SM-B and Lambda Physik FL3002, -0.1 mllpulse, line 0 1994 American Chemical Society
12894 J. Phys. Chem., Vol. 98, No. 49, 1994 282.1
*
-z
151.5
111.9
i8K.7
185.2
,
181.1
185.9
18K.L
I-
0
181.1
I8K.I
155.2 185.1
155.5
181.9
l5K.O
1115.9
185.2
v)
L
L I F EXCITATION FREQUENCY /
Figure 2. LIF-excitation spectrum of BTP diluted by helium in a supersonic jet. Stagnation pressure is 2.5 atm.
width 0.2 cm-'). The frequency of the laser was calibrated with a "see-through" hollow cathode lamp (Hamamatsu L278313-NE-AL). The optical pump-probe absorption depletion technique (hole-burning spectroscopy) was also used to measure excitation spectra. This technique has often been applied to measure excitation spectra of a specific species among several produced in a molecular beam.26327The frequency of the probe laser is fixed in resonance with one of the transitions of the species of interest, and the fluorescence caused by the probe laser is monitored. The pump (saturation) laser is applied before the probe pulse, and its frequency is scanned. The fluorescence depletion is observed when the pump frequency is in resonance with the transition which has a common initial state with the transition being probed. The hole-buming measurements were performed with the above excimer-pumped dye laser system as the probe source and the frequency-doubled output of a dye laser (Quantel TDL50, -1 d l p u l s e , line width 0.2-0.5 cm-') pumped by the second harmonics of a Nd:YAG laser (Quantel YG571C) as the pump source. The delay between the pump and probe pulses was 100 ns. A monochromator (Nikon P-250, resolution 10-20 cm-l) was used for the dispersed-fluorescence measurements. The output of the Nd:YAG-laser-pumped dye laser system mentioned above was used for the excitation. BTP (Aldrich, reagent grade) was used as received. It was vaporized at 22 "C, diluted by helium, and expanded into a vacuum chamber with a pulsed nozzle (orifice diameter 0.8 mm, pulse duration 1 ms, stagnation pressure 2-2.5 atm). In the dispersed-fluorescence measurements, the sample reservoir and nozzle were heated up to 50 "C in order to get sufficient concentration of BTP in the expanded gas mixture. All measurements were performed at 20 mm downstream from the nozzle. The background pressure in the chamber was held below Torr throughout the measurements.
Results and Discussion 1. LIF-Excitation and Hole-Burning Spectra. Figure 2 shows the LIF-excitation spectrum of BTP in a supersonic jet. The abscissa shows the excitation frequency. The origin band is observed at 31 109 cm-' ( E . : ) with a number of vibronic bands on the higher energy side. No band is observed on the lower energy side of v:. The absorption maximum for the S1 SOtransition of BTP in CC4 solution was reported to be observed at a frequency about 5000 cm-l higher than that in the solid state.28 The shift has been argued in relation to the torsional angle and the degree of conjugation between two rings: a highly-conjugated planar structure in the solid state and a less-conjugated twisted structure in the solution. The origin band observed in the LIF-excitation
-
18K.K
I
186.2
t
117.7
cm-1
31000
31200
L
181.7
1
181.2
1
663.6
31600
31400
PUMP FREQUENCY
31800
32000
32200
/ cm-I
Figure 3. Hole-burning spectrum of BTP measured with the probe laser tuned at v: (the origin band of the main component).
spectrum of BTP in a supersonic jet is observed with a blue shift of 1506 cm-' from that in the n-hexane glass solution at 4.2 suggesting that the structure of BTP under isolated conditions is more twisted than that in the glass solution. The LIF-excitation spectrum of BTP in a supersonic jet is characterized by a long progression with an interval of about 93 cm-', as marked in Figure 2 with circles. It can be assigned to a series of transitions to the torsional vibrational levels in the S1 state. This progression is absent in the spectrum observed in the low-temperature glass solution.28 It extends over 2500 cm-' from the origin band, although it tends to be broad in the higher frequency region because of band congestion. In addition, intense bands are observed at v: 282.1, v: 372.0, and v: 650.0 cm-', though they happen to overlap with the 93 cm-' progression bands. They can be assigned to the ring deformation modes which have been observed also in the lowtemperature glass The relative intensities of the first two bands at vt and v: 91.4 cm-' are found to vary by changing the nozzle condition. By decreasing the stagnation pressure or by decreasing the distance between the nozzle and the point of the measurements, the relative intensity of the v: 91.4 cm-' band with respect to the v: band increases. The relative intensity decreases by adding 3% Ar to the diluent gas, while it increases slightly by increasing the nozzle temperature up to 110 "C. These results suggest that the two bands correspond to different origins. The band at v: 91.4 cm-' seems to be the origin band of the second component, which is a vibrationally hot BTP or the isomer that is less stable than the main component. Figure 3 shows the hole-buming spectrum measured with the probe frequency set at the v: band. The vibronic transitions of the main component are observed in this spectrum, while the transitions due to the second component starting at v: 91.4 cm-' band are absent. On the other hand, they appear in the hole-burning spectrum obtained by probing the vi 91.4 cm-' band (Figure 4). The transitions observed for the first and second species are summarized in Table 1 with tentative assignments. The bands corresponding to the second component are indicated by asterisks in the LIF-excitation spectrum in Figure 2. It has been confirmed that they change intensity along with the vi 91.4 cm-' band on changing the expansion conditions. The spacing of the progressions due to the main component should, therefore, be considered to be -186 cm-' instead of 93 cm-'. The band at v: 282.1 cm-' should then be assigned to the ring deformation mode from which another 186 cm-' progression starts. Additional 186 cm-' progressions also 117.7, vi 151.5, and v: 663.6 cm-'. The start at v: results in Figure 3 show that the torsional mode gives rise to
+
+
+
+
+
+
+ +
+
+
+
+
+
2,2’-Bithiophene in a Supersonic Jet 1282) IS2.I I
0
186.1
________
*!7.1
TABLE 1: Frequencies and Assignments of the Transitions Observed in the Hole-Burning Spectra of the First (Figure 3) and Second (Figure 4) Species and Frequencies of the Unassigned Transitions of the First Species below 31 600 cm-l
107.0
186.9
185.8
111.1
J. Phys. Chem., Vol. 98, No. 49, 1994 12895
106.7
186.6 117.2
first species frequencykm-l
r
116.9
0:
31 226.7 31 260.5 31 294.6
4
665.0
LL
31200
31 109.0 101.1
118.2
31000
31100
assignmenp
31600
PUMP FREQUENCY /
31800
32000
32200
cm-1
Figure 4. Hole-buming spectrum of BTP measured with the probe laser tuned at v: + 91.4 cm-’ (the origin band of the second
unassigned
long and harmonic progressions, which suggest that the corresponding potential in the S1 state has a deep, harmonic single minimum. Relatively deep dips are observed in Figure 3, presumably because the lifetime of the SIstate of BTP is shorter than the pulse width of the pump laser (8 ns). This is in accordance with the observation that the apparent fluorescence lifetime is almost the same as the laser pulse width, indicating that the lifetime of the SI state is much shorter than 8 ns. Comparison of Figures 2 and 3 reveals that the intensity distribution of the torsional progression is different for the LIFexcitation and hole-burning spectra. This is presumably caused by the competition with state-dependentradiationless transitions in the S1 vibronic states, which would decrease the intensity of a vibronic band in the LIF-excitation spectrum. On the other hand, the intensity of vibronic bands in the hole-burning spectrum is not affected by the relaxation as long as the spectrum is measured far from the saturation regime. The fast relaxation in the S1 vibronic levels has been suggested recently by Buma et al.29 They have found that the delayed REMPI (resonance-enhanced multiphoton ionization) measurements of BTP indicate much longer lifetimes for the intermediate states than the fluorescence lifetime, which they have attributed to the triplet state. The relative intensities of the bands in the torsional progression observed in their REMPI spectra differ appreciably from those in our LIF-excitation spectrum, suggesting the effect of state-dependent radiationless transitions in the S1 vibronic states. In Figure 4, which corresponds to the excitation spectrum 91.4 cm-l, positive of the second component starting at v: bands are caused by the fluorescence of the main component due to the pump laser. It is too intense to fade out completely within the delay of 100 ns between the pump and probe pulses. This may also be one of the reasons for the shallower dips in Figure 4 than those in Figure 3. The hole-burning spectrum of the second component is found to be very similar to that of the main component except for the shift of the origin band by 92.4 cm-’. The 186 cm-l progressions are observed also for the second component starting at vi 118.4, vi 152.3, vi 467.6, and v: 665.2 cm-’ (v: = vt 91.4 cm-I; as shown below, the origin band of the second component is assigned to the v’ = 1 v” = 1 transition of the torsional mode). An origin band expected to appear at -v: 282 cm-l is unobservable for the second component, though the bands corresponding to v’ = 1 and 2 of this progression are observed at v: 558.8 and v: 745.8 cm-l. If the main and second components correspond to different conformers, e.g. trans and cis conformers, such a similarity in the progressions can not be expected, for the excited-state torsional force constant should be different for two conformers.
31 391.1 31 413.3 31 446.9 31 449.5 31 481.0 31 496.9
c:,
31 531.0
unassigned
31 545.2 31 566.7
unassigned unassigned
31 576.8 31 599.5 31 632.8 31 667.3
CAT;
31 762.1 31 772.6 31 818.9 31 852.8
CAG
+
+
+
-
+
+
+
+
31 200.4
Tt
31 318.6 31 352.5
4Tt
31 386.7
T:
Ti
31 380.6
+
assignment“
BA
component).
+
second species frequencylcm-I
BAT;
4.G
BAT:
unassigned
G unassigned 31 505.5 31 538.6 31 572.5
31 947.3 31 959.3 32 004.7 32 038.7 32 132.5 32 145.5 32 190.7 32 223.9 “A, mode.
4G BAG
el
D:, B:,% T:
31 667.8 31 691.2 31 725.5 31 759.2
31 854.8 31 865.4 31 912.1 31 946.4
C Z
DAT: BAT:
TF
CAT: DAG
B~TF
Tf
B, C, and D dnote the ring modes. T denotes the torsional
The second component is, therefore, most reasonably assigned to the vibrationally hot BTP in the same conformation as the main component. 2. Dispersed Fluorescence Spectra. In order to obtain information on the torsional potential in the SOstate, dispersedfluorescence spectra were measured with the excitation of the origin bands of the main and second components, respectively, at v: and v:. As shown in Figure 5a and b, short and anharmonic progressions due to the torsional vibration in the SO state are observed. Their frequencies are summarized in
#:
12896 J. Phys. Chem., Vol. 98, No. 49, 1994
Takayanagi et al.
.......
s1
-
.... .. .. .. ... ... ,.. ... ... ... .. .. .. so ....................... .. .. .. ...................... ... * ...--ST.: -.. ...... I . .
I
.
.
I . . .
.
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0
500
1000
FREQUENCY
1500
2000
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2500
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/ cm-I
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1,-
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-
9-
.
.
,
-
-21
0 21 t r r r l o n r l ingle / d a g .
v)
z w
I-
z
Figure 6. Schematic potentials along the torsional coordinate of BTP in the SO and SI states. Symmetric (+) and antisymmetric (-) vibrational levels are indicated by solid and broken lines, respectively. Transitions observed in electronic and dispersed-fluorescence spectra are shown by solid and broken arrows corresponding to the symmetric symmetric and antisymmetric antisymmetric transitions, respectively.
W
0
z W
0
-
+.
v)
W
a
0 3 -1
LL
0
I000
1500
FREQUENCY
500
SHIFT
2000
2500
/ cm-'
Figure 5. Dispersed-fluorescence spectra of BTP measured (a) with the v: (main component; resolution -10 cm-') excitation and (b) with the v: 91.4 cm-' (second component; resolution -20 cm-l) excitation. The abscissa shows the shift from the excitation frequencies. The bands at 0 cm-I were recorded with lowered sensitivity to prevent saturation of the detector. In the insets are shown the spectra around 0 cm-I with an expanded scale.
+
TABLE 2: Observed and Calculated Torsional Frequencies (cm-') of BTP in the Ground Electronic State observed calculated symmetric antisymmetric symmetric antisymmetric 0
0 1.6
20
1.6
19 33
49
31 48
67
66
86
86 108
129
107
130 154
153
Table 2. The spacing increases with the quantum number, suggesting that BTP in the SO state has a double-minimum torsional potential with a low barrier between two potential minima and with high barriers at both sides. The progressions observed in the LIF-excitation,hole-burning, and dispersed-fluorescence spectra are consistent with the torsional potentials shown in Figure 6 for the SO and S1 states. The SI potential is deep and harmonic with a minimum at a planar structure. The potential then results in the vibrational states with regular intervals, whose symmetries are altemately symmetric (+) and antisymmetric (-) with respect to the torsional motions. On the other hand, the potential in the SO state is of double-minimum type with two equivalently twisted equilibrium structures. When the barrier between the two equilibrium structures is reasonably high, the ground and firstexcited vibrational states of torsion are nearly degenerate. Their character is, respectively, symmetric and antisymmetric. Above them, there exist symmetric and antisymmetric torsional vibrational states altemately, with increasing spacing with increasing quantum number. The allowed vibronic transitions are symmetric symmetric and antisymmetric antisymmetric. If BTP molecules in a supersonic jet are populated in the ground and fist-excited vibrational states, the LIF-excitation spectrum consists of the
-
-
100
200
FREQUENCY SHIFT / o m - l
Figure 7. Dispersed-fluorescence spectra of BTP measured with the vi excitation (resolution -10 cm-1). The intensity of the fluorescence is so weak that the first member of the progression with the 20 cm-I shift from the excitation frequency is observed on the tail of the intense band due to the scattered light. progressions due to BTP in the ground vibrational state (solid upward arrows in Figure 6) and those due to hot BTP (broken upward arrows in Figure 6). The latter should appear blueshifted compared to the former because of the selection rule. This situation is consistent with the experimental results. Therefore, the main and second components observed in the LIF-excitation spectrum can be attributed to BTP in v f f = 0 and 1 of the torsional mode, respectively. The v' = 1 vtf = 1 transition appears as the band at vi (=v: 91.4 cm-l). On the other hand, in the dispersed-fluorescence spectra measured with the excitation to v' = 0 and 1, the transitions to the symmetric and antisymmetric vibrational states in the SO state are observed, as indicated by the solid and broken downward arrows, respectively, in Figure 6. In Figure 7 is shown the dispersed-fluorescence spectrum of 185.6 cm-' BTP measured with the excitation of v: (=vi), which is the second member of the symmetric torsional progression in the LIF-excitation spectrum. The same progression due to the torsional motion in the SO state is observed as that for the v: excitation (inset of Figure 5a), although their relative intensity distributions are appreciably different (see discussion below). This result supports the assignment of the progressions observed in the LIF-excitation and dispersedfluorescence spectra to the torsional motion. The splitting between the lowest symmetric and antisymmetric vibrational states in SOmay be calculated as follows. It is seen from Figure 6 that v: = v: v' - v", where v' and v" are the vibrational frequencies of the v = 1 torsional states in S1 and SO,respectively. Since v: - v: = 91.4 cm-l and v f = 186/2 = 93 cm-l, v" is calculated to be 1.6 f 0.2 cm-'.
-
+
+
+
2,2'-Bithiophene in a Supersonic Jet
J. Phys. Chem., Vol. 98, No. 49, 1994 12897
n obs. (LIF) a o b s . IHB) Ical.
0
0
0
0
A L i
b -
0
b O
0
1 N
t o r a l o n a l i n g l e / des.
Figure 8. Torsional potentials calculated with two different parameter sets. Solid line: VI = 1200 cm-I, VZ= 650 cm-I, and V4 = -333 cm-I. Broken line: V I = 1800 cm-I, V2 = 450 cm-I, and V4 = -320
cm-I. Two potential curves overlap completely with each other around q5 = 0".
c
1
c
It is clear from the above discussion that BTP observed in the excitation and dispersed-fluorescence spectra is in either the trans-planar or cis-planar conformation in S1 and is twisted around the trans or cis conformation in SO. However, the above spectroscopic analysis does not give any information on whether it is trans or cis. In the following discussion, we assume it to be trans, since all t h e ~ r e t i c a l ' ~and - ~ ~experimental1°-14 results reported so far suggest that trans-BTP is more stable than cisBTP. 3. Analysis of the Torsional Potential in the SOState. Frequencies of the large-amplitude torsional motion can be calculated with the method given by Lewis et al.,30where the following one-dimensional potential is assumed along the torsional coordinate, Cp.
c
n 1
'i
h
c
b
m
-b
3
Figure 9. Observed and calculated intensity distributions in the
torsional progressions in the LIF-excitation and hole-burning (HJ3) spectra. (a) Progression due to the transitions from v" = 0. (b) Progression due to the transitions from v" = 1. The intensity of the band denoted by N could not be determined because of bandoverlapping or low intensities.
at 4 = O", as shown in Figure 6. This potential, together with the single-minimum potential around Cp = 0" for the SI state, reproduces the observed spectroscopic data with sufficient accuracy. The equilibrium torsional angle in BTP is determined by the balance between the steric hindrance and/or electrostatic repulsion and the conjugation of n-electrons in the thiophene rings. The former operates between the sulfur atoms and hydrogen atoms to increase the dihedral angle. On the other hand, the planar structure is preferable for the conjugation. In the SOstate, the equilibrium structure is twisted because the steric hindrance andor electrostatic repulsion prevails. Upon electronic excitaThe torsional angles Cp = 0" and 180" correspond, respectively, tion, the equilibrium structure becomes planar, presumably to the trans and cis conformers. The frequencies of torsional because of increased n-electron conjugation. vibrational levels in the potential represented by eq 1 can be 4. Franck-Condon Analysis. Relative intensities of the calculated by diagonalizing the matrix given in the l i t e r a t ~ r e . ~ ~ bands in the torsional progressions observed in the LIFValues of V,, are determined by fitting the calculated frequencies excitation and dispersed-fluorescence spectra are calculated to to observed ones. Only VI, V2 and V4 are considered in this c o n f i i the assignment proposed here. The Franck-Condon analysis; a double-minimum potential around Cp = 0" is calculation has been performed following the method reported introduced by V2 and V4, while VI determines the energy p r e v i ~ u s l y .The ~ ~ harmonic torsional potential in SIaround difference between the cis and trans conformers. The rotational the trans conformer was expressed as constant for the intemal rotation, which is included in the matrix elements, was calculated to be 0.334 cm-' on the basis of the 1 geoemtry of BTP in the solid state reported by Nagashima et V(4)= ~(-1)"-l(v/2"-')[1 - cos(n4)] (2) al.31 This value was assumed to be independent of the torsional n=l angle. A basis set composed of 80 wave functions of free The torsional frequencies were calculated in a manner similar rotations was used. to that used for the SO state. By adjusting the spacing of each It is impossible to determine a unique set of parameters by of the symmetric and antisymmetric torsional progressions to fitting the calculated frequencies to the observed ones. Figure 8 illustrates the potential cuves calculated for two such parameter 186 cm-', V was determined to be 51 800 cm-l, although this sets VI = 1200 cm-', V2 = 650 cm-', V4 = -333 cm-l and VI value itself has no significance, since frequencies of only the limited number of torsional levels at the bottom of the potential = 1800 cm-', V2 = 450 cm-l, V4 = -320 cm-', which give (12000 cm-l) were fitted. With eigenvectors obtained in the the same potential around Cp = 0". The corresponding frequencalculations of the SOand S1 torsional frequencies, the relative cies are almost the same as those shown in Table 2. A unique values of the Franck-Condon factors have been c a l c ~ l a t e d . ~ ~ parameter set can not be determined unless the torsional Figure 9 shows the calculated intensity distributions of the frequencies for the cis conformer become available. Neverthetorsional progressions in the excitation spectra due to BTP in less, the SO potential around the trans conformer can be v" = 0 and 1. The calculated intensity distribution of the v" = determined uniquely to be double minimum with the potential 0 torsional progression (Figure 9a) reproduces reasonably the minima at I$ = & 21" separated by a barrier of about 25 cm-'
c(Cp)
2
12898 J. Phys. Chem., Vol. 98, No. 49, 1994
Takayanagi et al.
Conclusion
O
ot
W
ot
+
ot
U
a
ot
(
ot
D
i
O
0
t
N 0
The LIF-excitation, hole-burning, and dispersed-fluorescence spectra have been measured for BTF’ under the isolated condition in a supersonic jet. The torsional potential in the SO state was found to be double minimum by simulating the progressions observed in the dispersed-fluorescencespectra. The equilibrium structure is twisted by about 21” from trans planar, with a barrier of 25 cm-’ between the two potential minima. On the other hand, BTP in the SI state is found to be trans planar, with a single-minimum, deep, and harmonic torsional potential around the equilibrium structure. The Franck-Condon analysis also supports the present assignments and torsional potentials. We have analyzed all the observed bands as belonging to the trans conformer. It is then required to search for the cis conformer. An experimental search is under way in our laboratory together with the determination of the whole torsional potential by ab initio calculations.
References and Notes Figure 10. Observed and calculated intensity distributions in the torsional progressions in the dispersed-fluorescence spectra. In each figure,the observed intensities are scaled so that the differences between the calculated and observed intensities are minimized. The intensity of the band denoted by N could not be determined experimentally: (a) excitation at v$ (b) excitation at vt; (c) excitation at vi.
one observed in the LIF-excitation spectrum (Figure 2) except for the 0-0 band. Figure 9a shows also the intensity distribution for the v“ = 0 progression observed in the hole-burning spectrum (Figure 3). The calculated distribution reproduces it reasonably well. Figure 9b shows a similar comparison for the torsional progressions due to BTP in v” = 1. Reasonable agreements between the observed and calculated relative intensities are obtained again, although the band intensity could be determined for a limited number of bands in the observed spectra (Figures 2 and 4) because of their low intensities. Some of the observed bands in Figures 2-4 are congested by accidental overlapping of progressions belonging to different origins. In estimating the intensities given in Figure 9, we tried to deconvolute them to determine the intensity of the proper component. However, in some cases, especially at higher v’, it was impossible to separate the congested components. They are denoted by N in Figure 9. The intensities of the origin bands for the v” = 0 and v” = 1 progressions in the excitation spectra can not be reproduced by the calculation. As mentioned above, a fast nonradiative transition to the triplet excited state is expected, which may be more or less vibronic-state dependent. It is probable that the origin bands are less affected by this relaxation process than those with v‘ 2 2. The origin band in the hole-burning spectra seems to be reproduced reasonably by the calculated Franck-Condon factors. The hole-burning intensity is not affected by the relaxation as long as it is measured far from the saturation regime. Figure 10 shows the observed and calculated intensity distributions of the progressions in the dispersed-fluorescence spectra measured with the v:, vi, and vt excitations. In these cases, the relaxation in the upper state does not affect the intensity distribution and the calculated intensity patterns reproduce the observed ones well. In spite of some difficulties in determining the vibronic band intensities, the results in Figures 9 and 10 seem t o support the validity of the torsional potential given above for SO and S1 of BTP and the assignment of the two components to the transitions from the v’ = 0 and v” = 1 states in the double-minimum potential around the trans conformer of BTP.
(1) Takei, Y.; Yamaguchi, T.; Osamura, Y.; Fuke, K.; Kaya, K. J . Phys. Chem. 1988, 92, 577 and references therein. (2) Im, H.-S.; Bemstein, E. R. J . Chem. Phys. 1988, 88, 7337. (3) Jonkman, H. T.; Wiersma, D. A. Chem. Phys. Lett. 1983,97,261. (4) Jonkman, H. T.; Wiersma, D. A. J . Chem. Phys. 1984, 81, 1573. (5) Werst, D. W.; Londo, W. F.; Smith, J. L.; Barbara, P. F. Chem. Phys. Lett. 1985, 118, 367. (6) Werst, D. W.; Gentry, W. R.;Barbara, P. F. J . Phys. Chem. 1985, 89, 729. (7) Yamasaki, K.; Arita, K.; Kajimoto, 0.;Hara, K. Chem. Phys. k r r . 1986, 123, 277. (8) Werst, D. W.; Brearley, A. M.; Gentry, W. R.;Barbara, P. F. J . Am. Chem. SOC.1987, 109, 32. (9) Visser, G. J.; Heeres, G. J.; Wolters, J.; Vos, A. Acta Crystallogr. B 1968, 24, 467. (10) Almenningen, A,; Bastiansen, 0.;Svendsas, P. Acta Chem. Scand. 1958, 12, 1671. (11) Ter Beek, L. C.; Zimmerman, D. S.; Elliot Bumell, E. Mol. Phys. 1991, 74, 1027. (12) Bucci, P.; Longeri, M.; Veracini, C. A.; Lunazzi, L. J . Am. Chem. SOC. 1974, 96, 1305. (13) Khetrapal, C. L.; Kunwar, A. C. Mol. Phys. 1974, 28, 441. (14) Samdal, S.; Samuelsen, E. J.; Volden, H. V. Synth. Mer. 1993,59, 259. (15) Skancke, A. Acta Chem. Scand. 1970, 24, 1389. (16) Galasso, V.; Trinajstic, N. Tetrahedron 1972, 28, 4419. (17) Abu-Eittah, R.;Al-Sageir, F. Znt. J . Quantum Chem. 1978,13,565. (18) Abu-Eittah, R. H.; Al-Sageir, F. A. Bull. Chem. SOC. Jpn. 1985, 58, 2126. (19) Barone, V.; Lelj, F.; Russo, N.; Toscano, M. J . Chem. Soc., Perkin Trans. 2 1986, 907. (20) Jones, D.; Guerra, M.; Favaretto, L.; Modelli, A.; Fabrizio, M.; Distefano, G . J . Phys. Chem. 1990, 94, 5761. (21) Lopez Navmete, J. T.; Tian, B.; Zerbi, G. Synrh. Mer. 1990, 38, 299. (22) Bredas, J. L.; Heeger, A. J. Macromolecules 1990, 23, 1150. (23) Novak, I.; Ng, S. C.; Huang, H. H.; Mok, C. Y.; Ehor, E.; Kovac, B. J. Phys. Org. Chem. 1991, 4, 675. (24) Quattrocchi, C.; Lazzaroni, R.; Bredas, J. L. Chem. Phys. Len. 1993, 208, 120. (25) Chadwick, J. E.; Kohler, B. E. J . Phys. Chem. 1994, 98, 3631. (26) Lipert, R. J.; Colson, S . D. J . Phys. Chem. 1989, 93, 3894. (27) Takayanagi, M.; Hanazaki, I. Chem. Phys. Lert. 1993, 208, 5 and references therein. (28) Bimbaum, D.; Kohler, B. E. J. Chem. Phys. 1991, 95, 4783. (29) Buma, W. J.; Kohler, B. E.; Shaler, T. A. J . Phys. Chem. 1994, 98, 4990. (30) Lewis, J. D.; Malloy, T. B., Jr.; Chao, T. H.; Laane, J. J . Mol. Struct. 1972, 12, 427. (31) Nagashima, U.;Fujimoto, H.; Inokuchi, H.; Seki, K. J . Mol. Srmct. 1989, 197, 265. (32) Okuyama, K.; Mikami, N.; Ito, M. J . Phys. Chem. 1985.89, 5617.
