J. Phys. Chem. 1996, 100, 5217-5223
5217
Excited Electronic States of Arylbutatrienes Veeredej Chynwat, Tracy L. Coffin, Huifang Wang, and Robert E. Connors* Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 ReceiVed: August 10, 1995; In Final Form: January 11, 1996X
Electronic absorption spectra for 1,1,4,4-tetraphenylbutatriene (TPBT), 1,4-bis(9-xanthylidene)ethene (BXBT), and 1,1,4,4-dibiphenylenebutatriene (DBBT) have been measured and assigned with the aid of semiempirical INDO/S-SCI calculations. Geometries used for the INDO/S calculations were obtained by computing fully optimized structures for the arylbutatrienes with the AM1 Hamiltonian. The theoretical structure for TPBT is in good agreement with published X-ray data. Theory indicates that the nature of S1 and S2 depends upon the aryl substituent. For TPBT, S1 and S2 are both B1 and show considerable CI mixing of πfπ* (HOMOfLUMO) and π′fπ* (HOMO-1fLUMO). For BXBT, S1 (B1u) is πfπ* and S2 (B1g) is π′fπ*, whereas for DBBT the description of S1 and S2 is inverted from that of BXBT. These predictions are supported by several pieces of absorption and luminescence data. The effects of rotating the TPBT phenyl groups have been studied theoretically. Inclusion of doubly excited configurations in the INDO/S calculations inverts the order of S1 and S2 for TPBT, but does not alter the description of these states for BXBT and DBBT. INDO/ S-SCI and ab initio calculations have been carried out for unsubstituted butatriene and are in reasonable agreement. Both methods agree that S1 is B1g and that the first allowed electric dipole transition is 1B1urAg.
Introduction The concept of extended π bonding in organic molecules is of considerable practical and theoretical interest to chemists. Included in this important group of compounds are polyenes, cumulenes, acetylenes, and aromatics. Unlike the other members of the group, cumulenes beyond allene have had relatively little reported about their electronic structure and spectroscopy.1,2 Cumulenes are organic molecules with two or more double bonds in sequence. For cases where there are an even number of double bonds, such as allene, substituents lie in perpendicular planes. Whereas, for cumulenes with an odd number of double bonds, such as butatriene, substituents lie in the same plane. In this paper electronic spectral data are presented for three arylsubstituted butatrienes, 1,1,4,4-tetraphenylbutatriene (TPBT), 1,4-bis(9-xanthylidine)ethene (BXBT), and 1,1,4,4-dibiphenylenebutatriene (DBBT) (see Figure 1). An understanding of similarities and differences observed in the absorption and emission properties of these molecules is sought with the aid of semiemperical molecular orbital calculations. Butatriene molecular orbitals (MO’s) that are constructed from the overlap of p orbitals pointing perpendicular to the plane of the molecule are designated π-type orbitals and those that are constructed from the edgewise overlap of p orbitals pointing in the molecular plane are π′-type orbitals. The nodal planes for the two sets of orbitals are perpendicular and intersect along the cumulene chain. Our theoretical and experimental results for the molecules under study indicate that the orbital description of S1 depends upon the nature of the aryl substituent. Experimental Methods The butatrienes were synthesized using published procedures.3,4 Purification of the compounds was achieved by multiple recrystallization. Absorption spectra were measured with a Shimadzu UV2100U spectrometer. Low resolution fluorescence and fluorescence excitation spectra, as well as fluorescence quantum yields were obtained with a Perkin Elmer LS 50 luminescence spectrometer. Fluorescence quantum yields X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5217$12.00/0
Figure 1. Axis system and molecular structures of TPBT, BXBT, and DBBT.
φf, corrected for differences in refractive indices, were determined in 3-methylpentane (3MP), relative to quinine sulfate in 0.1 N sulfuric acid. Sharp-lined Shpolskii spectra and polarization data were measured with an apparatus described previously.5 Computational Methods Owing to the limited amount of X-ray data that is available for aryl substituted cumulenes, we have used computed geometries as input for our spectral calculations. Fully optimized structures have been obtained with the AM1 Hamiltonian6 using the standard minimization techniques of the MOPAC program.7 Singlet excitation energies, oscillator strengths (f), polarization of transitions, and MO’s have been calculated with the ARGUS program,8 using the INDO/S Hamiltonian. Excitation energies for the substituted butatrienes were computed with configuration interaction (CI) using 1024 singly excited configurations (SCI). Theoretical spectra are presented in bar charts showing transitions predicted to have f values greater than 0.01. INDO/S © 1996 American Chemical Society
5218 J. Phys. Chem., Vol. 100, No. 13, 1996
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TABLE 1: Calculated and Experimental Geometry of TPBT bond lengths (Å) AM1 expta C1-C2 C2-C3 C4-C5 C5-C6 C5-C10 C6-C7 C7-C8 C8-C9 C9-C10
1.325 1.255 1.464 1.403 1.403 1.393 1.395 1.395 1.393
bond order: AM1
1.346 1.260 1.473 1.413 1.411 1.398 1.405 1.403 1.395
1.70 2.07 1.00 1.38 1.37 1.42 1.41 1.41 1.43
bond angles (deg) C1-C2-C3 C2-C3-C4 C3-C4-C5 C4-C5-C6 C4-C5-C10 C5-C6-C7 C6-C7-C8 C7-C8-C9 C8-C9-C10 C9-C10-C5
AM1
expta
180.0 180.0 120.4 120.1 120.7 120.3 120.2 119.8 120.2 120.3
176.1 176.0 118.3 121.4 120.2 120.7 120.4 119.2 120.5 120.9 torsion angles (deg)
R, β, γ, δ C5-C11-C17-C23 a
AM1
expta
33.0 1.4
33.9 (av) 8.8
From ref 11.
calculations with both SCI and doubly excited configurations (DCI) were performed with the ZINDO program using 25 SCI and 172 DCI.9 Ab initio calculations were performed using the Gaussian 92 program.10 DEC workstations were used for all of the computations. Geometries To obtain some idea as to the quality of the calculated geometries, a comparison of computed and X-ray structural parameters for TPBT11 is presented in Table 1. It is seen that the agreement between calculated and experimental results is reasonably good. For example, the X-ray data show that the bond lengths in the cumulene chain are 1.346 Å for the terminal (C1-C2) and 1.260 Å for the middle double bond (C2-C3). The shortening of the central double bond has been interpreted by Berkovitch-Yellin and Leiserowitz to imply some triple bond character.11 AM1 results yield values of 1.325 and 1.255 Å with bond orders of 1.70 and 2.07 for (C1-C2) and (C2-C3), respectively. It is interesting that AM1 has correctly predicted the relative order of the three chemically distinct aromatic C-C bond lengths in the phenyl ring with C5-C6 the longest, C6C7 the shortest, and C7-C8 intermediate. Steric interaction between the phenyl rings results in a nonplanar structure for the molecule. The experimental angles of rotation (R, β, γ, and δ) of the phenyl rings out of the plane containing the butatriene chain are 27.9, 27.4, 41.8, and 38.5° with an average angle of 33.9°. Crystal packing forces are thought to be responsible for the difference in torsion angles for the four phenyl rings.11 The AM1 calculated value of 33.0° (all torsion angles constrained to be equal) is in excellent agreement with the average experimental value. AM1 optimization predicts a planar structure for DBBT, whereas for BXBT the xanthylidene ring is predicted to have a small “butterfly” folding of approximately 5° along the C(chain)-O axis with the “wings” folded up for the xanth-
Figure 2. Room temperature electronic absorption spectrum of TPBT in n-hexane and INDO/S-SCI results. Left axis shows experimental extinction coefficients. Right axis shows oscillator strengths for INDO/ S-SCI calculations. Calculated polarizations are shown over the bars. Transitions predicted to have oscillator strengths less than 0.01 have been omitted from the plots.
ylidene ring on one end of the chain and down for the xanthylidene ring on the other end of the chain. Spectra and Calculations TPBT. Figure 2 presents the room temperature electronic absorption spectrum for TPBT in n-hexane and the Argus INDO/ S-SCI calculated results. The first region of absorption is found at 340-470 nm and is characterized by a strong, broad, structureless band with an experimental f ) 0.65. Weaker absorption is found between 300-330 nm followed by a strong absorption band in the 210-290 nm region. INDO/S-SCI results suggest that there are two separate electronic transitions lying within the first absorption band. S1 is calculated to appear at 450 nm with f ) 0.22 and S2 at 396 nm with f ) 1.14. Experimental support for this finding is provided by the electrooptical absorption measurements for TPBT made by Liptay et al., who interpret their data in terms of two different electronic transitions in the interval of measurement along the long wavelength side of the first absorption band.12 Since INDO/S-SCI theory predicts only one electric dipole-allowed absorption band in the long wavelength region for DBBT and BXBT (see below), it was decided to study more closely the nature of the two allowed bands predicted for TPBT. The important TPBT MO’s for this discussion are shown in Figure 3. These are the highest filled π orbital (HOMO), the highest filled π′ orbital (HOMO-1) and the lowest unfilled π* orbital (LUMO). The major structural difference between TPBT and the other two butatrienes (DBBT, BXBT) is deviation from planarity due to rotation of the phenyl groups. Thus TPBT is classified under the D2 point group while DBBT and BXBT are classified under D2h. A series of INDO/S-SCI calculations were performed for TPBT where the phenyl torsion angles (R, β, γ, δ) were varied incrementally while the remaining geometric parameters were kept at their AM1 optimized values. Results showing calculated values of f, wavelengths, and CI coefficients for the first and second transitions predicted for TPBT with varied phenyl torsion angles are presented in Tables 2 and 3. For the special case where all phenyl torsion angles are equal to 0° (planar molecule), TPBT belongs to the D2h point group. Direct products are shown in Table 4 for the πfπ* (HOMOfLUMO) and π′fπ* (HOMO-1fLUMO) excitations under the D2 and D2h point groups. For D2 both of these excited state configurations give rise to electric dipole-allowed B1 (z)
Excited Electronic States of Arylbutatrienes
J. Phys. Chem., Vol. 100, No. 13, 1996 5219
Figure 4. Fluorescence and fluorescence excitation spectra of TPBT in methylcyclohexane at 77 K.
TABLE 4: States Arising From HOMOfLUMO and HOMO-1fLUMO Excitations For TPBT under D2h and D2 Point Groups HOMOfLUMO (πfπ*) HOMO-1f LUMO (π′fπ*)
Figure 3. Molecular orbital plots derived from INDO/S theory for planar TPBT. Dark lobes indicate positive sign, and light lobes indicate negative sign. HOMO and LUMO shown with 0.5 Å plane offset.
TABLE 2: INDO/S-SCI Excitation Wavelengths and Oscillator Strenghts as a Function of Phenyl Torsion Angle for TPBT first transition
second transition
torsion angle (deg)
wavelength (nm)
oscillator strength
wavelength (nm)
oscillator strength
0 5 10 20 25 33 45
548 507 505 494 486 450 445
0.00 0.33 0.38 0.39 0.39 0.22 0.29
463 430 424 404 393 396 355
1.12 0.95 0.91 0.92 0.94 1.14 1.06
TABLE 3: Absolute Values of Configuration Interaction Coefficients as a Function of Phenyl Torsion Angle for TPBT torsion angle (deg) 0 5 10 20 25 33 45
first transition πfπ* π′fπ* 0.00 0.62 0.67 0.69 0.69 0.48 0.64
0.97 0.62 0.70 0.65 0.64 0.80 0.67
second transition πfπ* π′fπ* 0.98 0.75 0.72 0.69 0.69 0.85 0.73
0.00 0.51 0.64 0.64 0.63 0.45 0.57
states that are capable of mixing with each other through CI. For planar TPBT, πfπ* gives rise to an electric dipole-allowed B1u (z) state and π′fπ* gives rise to an electric dipole-forbidden
D2h
D2
b2g × b3u ) B1u (z)
b2 × b3 ) B1 (z)
b2u × b3u ) B1g (f)
b2 × b3 ) B1 (z)
B1g state. Because of the difference in parity, CI mixing of B1u and B1g does not occur. For planar TPBT, S1 is B1g, corresponding to an essentially pure HOMO-1fLUMO (π′fπ*) excitation and S2 is B1u, corresponding to a pure HOMOfLUMO (πfπ*) excitation. Twisting of the phenyl groups lowers the symmetry and results in a mixing of S1 and S2. As the phenyl torsion angle increases from 0° to the optimized value of 33°, the contribution of HOMO-1fLUMO to S1 decreases and the contribution of HOMOfLUMO increases, with the opposite trend occurring for S2. At the optimized geometry S1 and S2 are mixed considerably with S1 being primarily HOMO1fLUMO and S2 primarily HOMOfLUMO. Thus the two allowed transitions calculated by INDO/S-SCI and presumably observed by Liptay’s electrooptical experiments in the region of the first absorption band can be attributed to the nonplanarity of TPBT which lowers the symmetry from D2h to D2 and permits transitions to both S1 and S2 to be electric dipole-allowed. It is also seen from Table 2 that the wavelengths predicted for the transitions to S1 and S2 vary with phenyl torsion angle. At room temperature, TPBT in solution is expected to have a broad distribution of phenyl torsion angles, given the relatively low frequency and high amplitude for vibrational motion along this coordinate. This lack of geometric homogeneity is likely to be responsible for the broad and structureless nature of TPBT’s room temperature electronic absorption spectrum. In a frozen methylcyclohexane (MCH) glass at 77 K (Figure 4), where the distribution of torsion angles is expected to be narrower, the fluorescence and fluorescence excitation spectra are red shifted and exhibit resolved vibronic structure. El-Bayoumi has interpreted a similar red shift of λmax upon cooling for tetraphenylmethylbutadiene in terms of deexcitation of torsional modes, arguing that thermal population of these modes at higher temperature results in increased nonplanarity and poorer conjugation for a large fraction of molecules in the sample.13 TPBT, along with other arylbutatrienes, exhibits a dramatic temperature dependence in fluorescence intensity. In 3MP, φf for TPBT increases from 2 × 10-4 at room temperature to 0.90 (10% error) at 77 K. This temperature effect correlates with solvent viscosity and is related to twisting about the central
5220 J. Phys. Chem., Vol. 100, No. 13, 1996 cumulene chain in the excited state.14 The fluorescence lifetime of TPBT measured in MCH at 77 K is 1.81 ns. The intrinsic fluorescence lifetime τ0(f) ) 2.0 ns calculated from the 77 K fluorescence properties via τ0(f) ) τ/φf is in good agreement with τ0(a) ) 2.2 ns calculated from the low-temperature absorption and emission spectra following the method of Strickler and Berg.15 This agreement indicates that the emitting state (S1) contributes the major portion of the measured oscillator strength between 390 and 450 nm, suggesting that the intense (primarily) πfπ* transition is S1 and lies lower in energy than the (primarily) π′fπ* transition. Theoretical support for this alternative assignment of the relative order of πfπ* and π′fπ* is provided below in the discussion of INDO/S calculations that include doubly excited configurations. Although it is tempting to assign the two bands that are observed in the fluorescence excitation spectrum of TPBT between 390 and 450 nm at 77 K as separate transitions to S1 and S2, the mirror image relation with the fluorescence spectrum of TPBT and the similarity in appearance to the absorption spectra of planar aromatic butatrienes (see below) suggests that the second band is likely to be a vibronic component of the intense πfπ* transition. The vibronic band is separated from the origin band by 1305 cm-1 in the fluorescence excitation spectrum and by 1170 cm-1 in the fluorescence spectrum. This mode is believed to correspond to the C-phenyl stretching vibration which has been found to appear with similar frequencies in the fluorescence and absorption spectra of transstilbene.16 The higher frequency in the excited state is consistent with the MO’s shown in Figure 3 where the C-phenyl bond is antibonding in the HOMO and bonding in the LUMO. The weaker π′fπ* transition that is also predicted to appear in this region appears to be obscured by overlap with the πfπ* transition. Attempts to obtain a sharp-lined spectrum for TPBT using a number of Shpolskii matrices at 15 K were unsuccessful. Broadness in spectral features of bulky nonplanar aromatic guests such as TPBT in Shpolskii hosts is usually attributed to a failure of the molecule to incorporate substitutionally into the hydrocarbon crystalline matrix. BXBT. As shown in Figure 5, the room temperature absorption spectrum of BXBT in chloroform is comprised of three absorption regions. The first region, 17500-25600 cm-1 (570-390 nm), is strong and has a prominent vibronic band. The similarity in appearance to the first absorption region of TPBT at 77 K is consistent with the view that phenyl torsion influences the appearance of the TPBT spectrum at room temperature. Bonding of the phenyl groups to oxygen restricts BXBT to a nearly planar structure and prevents any significant torsional distortion at room temperature. The second region, 29000-39200 cm-1 (344-255 nm), is weaker and is followed by strong absorption in the third region, 39200-45500 cm-1 (255-220 nm). Fluorescence excitation and polarized excitation spectra for BXBT in 3MP at 77 K and INDO/S-SCI results are also shown in Figure 5. The first transition has positive polarization and is calculated to be a strong, z-polarized 1B r1A excitation, arising primarily from HOMOfLUMO 1u g which is b2g(π)fb3u(π*). S2 is predicted to be a symmetry forbidden 1B1grAg transition, arising primarily from HOMO3fLUMO which is b2u(π′)fb3u(π*). The molecular orbital description of S1 and S2 calculated by INDO/S-SCI for BXBT correlate with S2 and S1, respectively, for planar TPBT with the exception that π′ is HOMO-3 for BXBT. In the second region of absorption for BXBT, the degree of polarization is seen to be negative between 29000 and 36000 cm-1 and positive between 36000 and 40000 cm-1, indicating overlapping transi-
Chynwat et al.
Figure 5. Top: Room temperature electronic absorption spectrum of BXBT in chloroform and INDO/S-SCI calculated results. The diamond on the x axis indicates the location predicted for the forbidden 1B1gr1A1g transition. Bottom: Fluorescence excitation and polarized fluorescence excitation spectra of BXBT in 3MP at 77 K.
Figure 6. Fluorescence spectrum of BXBT in n-octane at 15 K.
tions. This agrees well with the calculation which places moderately intense (f ) 0.2) B2u (y) and B1u (z) states in this region. The third band system is seen to be negatively polarized and is assigned as an excitation to a strongly absorbing B2u state calculated to occur at 40240 cm-1 (248 nm) with f ) 1.01. Sharp-lined fluorescence and fluorescence excitation spectra have been obtained for BXBT in n-octane at 15 K and are presented in Figures 6 and 7. The spectra are relatively simple in appearance with a minor site appearing 43 cm-1 below the major site. A small Stokes shift of 20 cm-1 is observed for the 0-0 bands and the major vibronic features have comparable frequencies and relative intensities in the fluorescence and fluorescence excitation spectra, giving rise to a mirror image relationship. The prominent band that is separated from the origin band by 1250 cm-1 in the fluorescence spectrum and by 1305 cm-1 in the fluorescence excitation spectrum, is similar to the 1170/1305 cm-1 vibrational mode observed in the 77 K spectra of TPBT. Other strong bands are observed at 635 and 2026 cm-1 in the fluorescence spectrum and at 628 and 2124
Excited Electronic States of Arylbutatrienes
Figure 7. Fluorescence excitation spectrum of BXBT in n-octane at 15 K.
Figure 8. Room temperature electronic absorption spectrum of DBBT in n-hexane and INDO/S-SCI calculated results. The diamond on the x axis indicates the location predicted for the forbidden 1B1gr1A1g transition.
cm-1 in the excitation spectrum. The 2026/2124 cm-1 mode is assigned to the (ag) high-frequency butatriene chain stretching mode.17 Little change in geometry between S0 and S1 is indicated by the observation of strong origin bands combined with the lack of identifiable Frank-Condon progressions in the fluorescence and excitation spectra. DBBT. The room temperature absorption spectrum of DBBT in n-hexane is shown in Figure 8 along with the results of the INDO/S-SCI calculations. As seen with BXBT, the long wavelength region is similar in appearance to the low-temperature spectrum of TPBT. Unlike TPBT and BXBT, we have not been able to observe fluorescence from DBBT at room temperature or at low temperatures down to 15 K (detection limit: φf ∼ 10-5). This result is surprising, especially when comparing DBBT with TPBT, since it is usually observed that the quantum yield of fluorescence increases with rigidity for molecules of similar structure.18 Because a fluorescence excitation spectrum could not be measured, the low-temperature absorption characteristics of DBBT were obtained by passing light from a tungsten lamp through a sample of the compound embedded in a Shpolskii matrix. It is seen from Figure 9 that the absorption spectrum of DBBT in n-heptane at 15 K reveals features that are not observable in the room temperature absorption spectrum and that differ markedly from results obtained with TPBT and BXBT. An extended series of closely spaced, relatively sharp, weak lines superimposed upon a broad background is seen. The broad features of the background correspond to the first two strong bands appearing in Figure 8 which we have assigned as 1B1ur1Ag, πf π*. A tentative vibrational analysis for the weak band system, a portion of which
J. Phys. Chem., Vol. 100, No. 13, 1996 5221
Figure 9. Electronic absorption spectrum of DBBT in n-heptane at 15 K. Solid circles indicate reproducible sharp features in the spectrum.
Figure 10. Expanded portion of the absorption spectrum of DBBT in n-heptane at 15 K.
TABLE 5: Vibrational Analysis of the Electronic Absorption Spectrum of DBBT in n-Heptane Matrix at 15 K λ (nm)
ν (cm-1)
522.60 518.65 515.12 511.20 507.49 503.88 500.40 496.77 493.56 490.08 486.97 484.33 480.47 478.58 472.70 469.70 466.44 463.71 460.64 456.93
19135 19281 19413 19562 19705 19846 19984 20130 20261 20405 20535 20647 20813 20895 21155 21290 21439 21565 21709 21885
∆ν 146 278 427 570 711 849 995 1126 1270 1400 1512 1678 1760 2020 2155 2304 2430 2574 2750
analysis 0-0 fundamental fundamental fundamental 427 + 146 427 + 278 2 × 427 2 × 427 + 146 2 × 427 + 278 3 × 427 fundamental fundamental fundamental fundamental fundamental fundamental 2155 + 146 2155 + 278 2155 + 427
error
3 -6 5 5 6 11
-3 3 8
is shown in greater detail in Figure 10, is presented in Table 5. The majority of the bands can be assigned to a progression in a 427 cm-1 mode with 146 and 278 cm-1 modes built upon the origin band and the 427 cm-1 progression. Several bands are found 1400-2000 cm-1 from the origin and may be single quanta of fundamental vibrations; however, this region of the spectrum is difficult to analyze due to strong absorption by the broad background. When n-hexane or n-octane is used as the solvent at 15 K, a spectral pattern identical to that shown in Figures 9 and 10 is observed with the bands shifted uniformly by -480 and +20 cm-1, respectively. This observation argues
5222 J. Phys. Chem., Vol. 100, No. 13, 1996
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TABLE 6: Excited Electronic States of Butatriene Obtained by ab Initio and INDO/S Calculations state
wavelength (nm)
oscillator strength
B1g B1u B1g
Ab Inito 6-311g**//AM1 273 201 197
0.00 0.76 0.00
B1g B1g B1u
INDO/S-SCI//AM1 387 263 234
0.00 0.00 0.96
against assigning the weak bands to secondary sites in the Shpolskii matrix. It is generally found that the structure of multiple guest/host sites varies from one alkane host to another for aromatic guest molecules. Guided by the INDO/S-SCI calculations, we believe that the strong 1B1ur1Ag transition observed in the room temperature spectrum under low resolution is to S2, not S1, and that the sharp features observed under conditions of high resolution starting on the low-energy side of the first strong band correspond to a separate weak transition to S1. The INDO/S-SCI calculations indicate that the transition to S1 is a symmetry forbidden 1B1gr1Ag, π′fπ* excitation, similar to the first transition predicted for planar TPBT. Lack of an origin band that dominates in intensity and the presence of extended vibrational features suggest that the molecule may undergo a change in geometry upon excitation to this weakly absorbing state. Such a geometry change coupled with the small fluorescence rate constant associated with a weak transition is likely to result in radiationless decay being the dominant path of deactivation, thus explaining the failure to observe fluorescence from DBBT. The vibrational assignment offered in Table 5 assumes that the transition to S1 which is parity forbidden in the isolated molecule has become weakly allowed in the frozen alkane matrix due to environmental influences such as small distortions of the guest molecule and crystal field effects. An alternative view is that the lowest energy feature in the spectrum is a false origin and that the transition is vibronically induced. Little more can be said about these two possibilities until a complete description of the normal modes of vibration is obtained. Inclusion of Doubly Excited Configurations. Contrary to the situation found with polyenes,19,20 inclusion of doubly excited configurations in INDO/S-CI calculations does not systematically alter the ordering of the low lying excited states for aryl butatrienes. For BXBT and DBBT the descriptions of S1 and S2 remain the same, whereas for TPBT there is an interchange between S1 and S2, where S1 becomes the stronger of the two allowed transitions and there is less difference in their oscillator strengths (f(S1) ) 0.72, f(S2) ) 0.45). The absolute values of the important CI coefficients are 0.83 (πfπ*) and 0.54 (π′fπ*) for S1; 0.54 (πfπ*) and 0.84 (π′fπ*) for S2. In general, the agreement between calculated and observed band positions for the strong transitions of the compounds studied in this work appears to be slightly better when DCI is omitted. Butatriene ab Initio and INDO/S Calculations. Unsubsituted butatriene is a small molecule suitable for study by ab initio techniques, thus allowing a comparison between semiemperical and ab initio theory for a molecule containing three cumulative bonds. The results of 6-311g** (CIS)//AM1 and INDO/S-SCI//AM1 calculations (100 configurations) are presented in Table 6.21 Reasonable qualitative agreement is found for the two theoretical methods. It is seen that both predict that S1 is B1g. This π′fπ* excitation correlates with S1 calculated by INDO/S-SCI for planar TPBT and DBBT. Both
theoretical methods predict that the first electric dipole-allowed transition is to a B1u singlet state (HOMOfLUMO) which is S2 (nearly degenerate with S3) for 6-311g** and S3 for INDO/ S-SCI. Although the experimental data is sparse, Schubert et al. report observing a weak transition at 310 nm (max ) 250 M-1 cm-1) and a strong transition at 241 nm (max ) 20300 M-1 cm-1) for butatriene in ethanol which may correspond to the calculated 1B1g and 1B1u states, respectively.22 However, in view of the extreme reactivity of butatriene, it is also possible that decomposition products contribute to the spectrum observed in ethanol. Conclusions The AM1-optimized geometry of TPBT was found to be in good agreement with the experimental structure determined by X-ray diffraction. INDO/S-SCI calculations of the electronic properties of TPBT, BXBT, and DBBT were performed employing AM1-optimized geometries. The agreement between calculated and observed electronic spectral properties is good, providing useful insight for understanding the electronic spectroscopy of these compounds. The calculations indicate that the nature of S1 and S2 depends upon the aryl substituent. For planar TPBT, S1(B1g) is described as an electric dipole-forbidden HOMO-1fLUMO (π′fπ*) excitation and S2 (B1u) as an allowed HOMOfLUMO (πfπ*) excitation. Rotation of the phenyl groups to their optimized value of 33° lowers the symmetry from D2h to D2, thus allowing CI mixing of HOMO1fLUMO (π′fπ*) and HOMOfLUMO (πfπ*). S1 (B1) and S2 (B1) are both optically allowed for the AM1-optimized structure of TPBT and exhibit considerable CI mixing. In the case of BXBT it is found that S1 (B1u) is πfπ* and S2 (B1g) is π′fπ*. However, for DBBT, as with planar TPBT, S1 (B1g) is π′fπ* and S2 (B1u) is πfπ*. Several pieces of experimental data support these predictions. Liptay’s electrooptical absorption studies indicate that there are two separate transitions in the region of the first broad room temperature absorption band for TPBT. For BXBT the positions, relative intensities, and polarization of absorption bands agree well with the theoretical results. The mirror image relationship and small Stokes shift between absorption and emission in a Shpolskii matrix at 15 K indicate that the strong transition observed at 570-390 nm is to S1 for BXBT which can be assigned with confidence as 1B1ur1Ag. In the case of DBBT, the observation of weak bands to the red of the strong 1B r1A transition and the lack of measurable fluorescence 1u g are consistent with the theoretical prediction that S1 is B1g. Inclusion of doubly excited configurations in the INDO/S calculations does not significantly alter the theoretical predictions. The relative order of S1 and S2 is unchanged for BXBT and DBBT; however, there is a state inversion for TPBT with the stronger of the two 1B1r1A transitions becoming S1 and the weaker S2, which appears to be consistent with the fluorescence lifetime data. In general, there appears to be a somewhat better fit with the overall features of the experimental spectra when doubly excited configurations are omitted. A comparison was made between the semiempirical INDO/ S-SCI method and ab initio theory for unsubstituted butatriene. Both methods agree that S1 (B1g) is π′fπ* and that the first allowed electric-dipole transition is πfπ* (B1u) which is S2 for ab initio and S3 for INDO/S-SCI. Acknowledgment. We thank Dr. Arthur M. Halpern for measuring the fluorescence lifetime of TPBT in his laboratory. References and Notes (1) Mehlhorn, A. J. Prakt. Chem. 1986, 328, 784.
Excited Electronic States of Arylbutatrienes (2) Connors, R. E.; Mochel, J.; Chynwat, V. J. Phys. Chem. 1988, 92, 1792. (3) Hopf, H. In The Chemistry of Ketenes, Allenes, and Related Compounds; Patai, S., Ed.; John Wiley & Sons: New York, 1980; Part 2, p 779. (4) Chynwat, V. Thesis, Worcester Polytechnic Institute, 1992. (5) Connors, R. E.; Sweeney, R. J.; Cerio, F. J. Phys Chem. 1987, 91, 819. (6) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (7) Stewart, J. J. P. QCPE Program 455, v.5. (8) Thompson, M. A. Molecular Science Research Center, Battelle Pacific Northwest Laboratories, Richmond, WA 99352. (9) Ridley, J.; Zerner, M. Theor. Chim. Acta 1973, 32, 111. (10) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J; Stewart, J. J. P.; Pople, J. A. Gaussian 92, ReVision G.4; Gaussian, Inc.: Pittsburgh, PA, 1992. (11) Berkovitch-Yellin, Z.; Leiserowitz, L. Acta Crystallogr. 1977, B33, 3657.
J. Phys. Chem., Vol. 100, No. 13, 1996 5223 (12) Liptay, W.; Wortman, R.; Bo¨hm, R.; Detzer, N. Chem. Phys. 1988, 120, 439. (13) Kordas, J.; Avouris, P.; El-Bayoumi, A. J. Phys. Chem. 1975, 79, 2420. (14) Unpublished results from our laboratory. (15) Strickler, S. J.; Berg, R. A. J. Chem Phys. 1962, 37, 814. (16) Dyck, R. H.; McClure, D. S. J. Chem. Phys. 1962, 36, 2326. (17) Miller, F. A.; Elbert, W. F.; Pingitore, W. J. Mol. Struct. 1977, 40, 25. (18) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, CA, 1978. (19) Hudson, B. S.; Kohler, B. E.; Shulten, K. In Excited States; Lim, E. C., Ed.; Academic Press: New York, 1982; Vol. 6, p 1. (20) Connors, R. E.; Burns, D. S.; Farhoosh, R.; Frank, H. A. J. Phys. Chem. 1993, 97, 9351. (21) Problems with convergence were encountered when diffuse functions were added to the ab initio basis set. (22) Schubert, W. M.; Liddicoet, T. H.; Lanka, W. A. J. Am. Chem. Soc. 1952, 74, 569.
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