J. Phys. Chem. 1995, 99, 14659-14666
14659
Vibrational and Electronic Properties of Antiaromatic Systems: A Spectroscopic Study of 1,3,5-Tri-tert-butylpentalenet Anna Falchi, Cristina Gellini, and Pier Remigio Salvi* Laboratorio di Spettroscopia Molecolare, Dipartimento di Chimica, Universita ' di Firenze, via Gin0 Capponi 9, 50121 Firenze, Italy
Klaus Hafner Institut fur Organische Chemie der Technische Hochschule, Petersenstrasse 22, 0-6100 Darmstadt, Germany Received: May 22, 1995@
The vibrational and electronic spectra of the stable pentalene derivative 1,3,5-tri-tert-butylpentalene (TTBP) are reported, and their properties are discussed on the basis of QCW-PI MO semiempirical calculations, which allow geometry optimization and normal mode analysis for the ground and lowest excited states. The infrared and Raman spectra have intensities mostly arising from modes of pentalene origin. The observed frequencies are compared with those obtained from vibrational calculations on the parent molecule and on several tert-butyl derivatives. The nn* electronic transitions of TTBP in the visible and UV regions are satisfactorily predicted by our calculations, including the interaction between singly excited configurations. Optimization procedures show that bond altemation, a distinctive feature of antiaromatics, is reduced in the excited states and that ground and excited state potential surfaces have minima displaced one with respect to the other. Low-temperature absorption spectra in the S2 and S3 regions are interpreted in terms of FranckCondon vibronic transitions whose strength depends directly on the structural change upon excitation. The theoretical results on SO-. S2 and SO S3 band profiles, with consideration of the normal mode rotation in the excited state, that is, the Duschinsky effect, are in fair agreement with experiment.
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1. Introduction
There has been wide interest for many years in antiaromatic systems, that is, in planar conjugated cyclic systems with 4n n electrons, and attempts have been made to classify their molecular proper tie^.'-^ Most antiaromatics are n~naltemant;~ Le., the n centers of these species cannot be divided into two subsets, usually defined as starred and unstarred, such that members of one subset are linked only to those of the other subset. Strong correlations between n electrons occur in nonaltemant system^,^ giving origin to bond alternation, a distinctive feature of anti aromatic^,^-^ in contrast with the more symmetrical aromatic structure. Although the synthesis and the study of their chemical reactivity are a remarkable example of chemical expertise,6s8 unsubstituted antiaromatics are seldom stable. This may explain the scarcity of spectroscopic data, particularly when compared with the wealth of information on aromatic molecule^.^-^^ In the past, one exception has been biphenylene, for which a large amount of v i b r a t i ~ n a l ' ~and . ~ ~e l e c t r ~ n i c ' ~results - ~ ~ has been reported. More recently, our group has undertaken extensive studies on a stable indacene derivative, 1,3,5,7-tetra-tertbutylindacene, and novel spectral features have been observed.19-21 First, the molecule fluoresces from the second excited state,20s22 at variance with the emission behavior of most aromatics" (except a z ~ l e n e ~ ~ Second, - ~ ~ ) . the analysis of the absorption and resonance Raman spectra for the two lowest allowed states of s-indacene has shown that bond altemation strongly decreases in the excited states.21 Similar studies on other antiaromatic systems are necessary in order to draw + This work was supported by the Italian Minister0 Universita' e Ricerca Scientifica e Tecnologica (MURST) and Consiglio Nazionale delle Ricerche (CNR). @Abstractpublished in Advance ACS Abstracts, September 15, 1995.
0022-365419512099-14659$09.0010
conclusions of general applicability on the ground and excited state structures and on energy levels. In this paper we wish to report on theoretical considerations on a second nonaltemant antiaromatic system, pentalene (see Figure l), and to present experimental results of vibrational and electronic spectroscopy on the stable 1,3,5-tri-tert-butyl derivative. Although pentalene cannot be isolated, several derivatives have been s y n t h e ~ i z e d , * some ~ - ~ ~of which, however, are highly thermolabile and dimerizing even at low t e m p e r a t ~ r e . ~The ~*~~,~' first stable pentalene derivative, 1,3,5-tri-tert-butylpentalene (TTBP), was obtained by hindering the dimerization by steric effects.29 After a first proposal of aromaticity for ~ e n t a l e n e , ~ ~ it was soon realized by means of semiempirical calculation^^^^^^ that the polyenic structure of Figure la, corresponding to C2h symmetry, should have been more stable than the symmetrical D2h structure of Figure lb. Accordingly, the C-C bonds were calculated34as altemating in length between 1.358 and 1.461 8, for pentalene in the ground state. In this paper the theoretical study is addressed by using the approximate QCFF-PI Hamilt ~ n i a n . ~The ~ . ~corresponding ~ calculation method allows geometry optimization and normal mode calculations for the ground and the excited states and, due to its intemal consistency, is well-suited for discussion of properties depending on both Experimentally, electronic and vibrational properties have not been investigated in great detail in the past. The knowledge of spectroscopic data is a prerequisite for a useful discussion of the theoretical results. The paper is organized as follows. After the Experimental Section, MO calculations on pentalene are described in section 111. The optimized geometries and energies of the ground ( S O ) and of the lowest excited states, from SI to S5, have been obtained within the QCFF-PI approximation. In section IV normal mode calculations for the optimized ground state of pentalene and several other tert-butyl derivatives are illustrated.
0 1995 American Chemical Society
14660 J. Phys. Chem., Vol. 99, No. 40, 1995
1
Falchi et al.
6
(a) (b) Figure 1. Molecular structure of pentalene, according to C2h (left)and D2h (right) symmetry. Infrared and Raman spectra are reported and assigned with the help of the calculated frequencies. The electronic absorption spectra at room and low temperature are discussed in section V. Franck-Condon contributions to the second and third transition are discussed considering normal mode rotation in the excited states, Le., the Duschinsky effect, and the alkyl substitution. It is confmed, along with previous results2’ that bond altemancy is strongly reduced in the excited states, S, (n 1 l), of antiaromatic systems.
2. Experimental Section TTBP has been synthesized according to the reaction scheme already reported.29 The sample (-1 g) was kept under vacuum and in the absence of light at -20 OC, and its purity was checked periodically by mass and gas chromatographic analysis. Infrared spectra were carried out at room temperature on a Fourier-transform infrared (FTIR) spectrometer (Bruker Model IFS 120 HR), operating at ~ 0 . 5cm-I resolution. For midinfrared measurements a thin polycrystalline film was prepared by heating a small quantity of the powder sample between two KBr windows to the melting point ( ~ 5 9 - 6 0 “C) and then slowly cooling at room temperature. Pellets of the pure powder or in a polyethylene matrix were used for far-infrared experiments. The Raman spectrum of the crystal powder was measured at 1.06 p m with the FT-Raman option of the Bruker instrument at el cm-’ resolution. Exciting in the visible range with the emission lines of the Ar and Kr lasers, no Raman spectrum was observed. As discussed in the next section, TTBP absorbs in this region, and as a consequence, the sample may undergo local buming even with few milliwatts excitation intensity. Absorption spectra in the visible and UV regions were taken at room temperature with freshly prepared solutions in cyclohexane or in a isopentane/diethyl ether (7:3) mixture on a Cary 05 spectrophotometer. Low-temperature absorption spectra were measured with the isopentane/ether solution in the concentration range of only 10-3-10-4 M. Applying thawand-freeze cycles for oxygen elimination and cooling the solution rapidly at 77 K, perfectly clear glassy samples were easily obtained. Lowering the temperature further to 15 K and slowly enough in a closed circuit He cryostat, the matrix remains transparent without any apparent interior cracking. On the contrary, TTBP solutions in cyclohexane become polycrystalline at low temperature, and the incident light is strongly scattered, reducing the instrumental detection capabilities. 3. Electronic States of Pentalene
(a) Preliminary Considerations. Pentalene may be formally derived from cyclooctatetraene (COT) by bond-linking opposite atoms of the cycle. The application of the Huckel theory to planar COT gives an open-shell configuration for the ground state of this molecule with two nonbonding MO’s being occupied by two electrons (see Figure 2).’ As a result, COT distorts from planarity to the “tub” conformation of DU symmetry. The planar geometry may be conserved in perturbed COT systems such as pentalene and benzocyclobutadiene (BCB),where all eight JC electrons occupy bonding orbitals, as can be seen from Figure 2. In the Huckel approximation, the
Figure 2. n energy levels of pentalene (a), planar cyclooctatetraene (b), and benzocyclobutadiene (c) in the Huckel approximation. The dotted horizontal line corresponds to the energy of the Coulomb integral.
two planar systems are not, however, more stable than the corresponding acyclic compound, Le., octatetraene. This may be easily understood with the help of the perturbational molecular orbital (PMO) approach42 and consideration of the HOMO orbital. Pentalene results are less stable by 2up (where j3 is the Huckel resonance integral and Q the HOMO electron density on the appropriate C atom) while BCB is stable as octatetraene. Accordingly, pentalene is classified as antiaromatic and BCB more properly as a nonaromatic system.’ The pentalene energy is lowered through bond altemancy. In an early two different Huckel integrals, Bs and p d , were defined for each carbon atom, depending on bond length, and the molecular energy was calculated as a function of their ratio, k = ps/Bd. As the energy minimum is found to be at k values different from unity (the “aromatic” case), the corresponding structure is bond alternating. We have found that for pentalene the energy minimum is at k = 0.6. This value is in good correspondence with those found in other calculations on similar molecules.44 (b) Ground and Excited State Equilibrium Structures. Antiaromatic nonaltemant hydrocarbons have been studied theoretically with s e m i e m p i r i ~ a l ~and , ~ ab ~ -i~n~i t i ~ ’methods ~*~~ in recent years, and in particular, the QCFF-PI calculation has been applied with success also to fit vibronic profiles in s-indacene.21 The ground state geometry of pentalene has been optimized by means of this latter p r o c e d ~ r e ? and ~ . ~the ~ results are reported in Table 1. The calculations show that bond alternation is pronounced in pentalene, “double” bonds being on the average m1.365 8, and “single” ranging from 1.47 to 1.50 A, in substantial agreement with previous MO estimates.34 In addition, the molecular symmetry converges to C2h freely, i.e., in the absence of constraints imposed during the optimization process. It should be noted that two C2h valence isomers are equally possible, the D2h structure of Figure l b corresponding to the transition state of the isomeric interconversion. The optimization procedure was also applied to several tert-butyl derivatives of pentalene, namely, 1-tert-butylpentalene(1-TBP), 5-tert-butylpentalene (5-TBP), 1,5-di-tert-butylpentalene( 1 3 DTBP), and TTBP (see Table 1). The calculated ground state structures of the derivatives have only minor changes with respect to that of pentalene around the substitution sites. The two C-C bonds of the pentalene unit closer to butyl increase their length by ~ 0 . 0 1A, while the Cpent-Cbutdistance is calculated around 1.48 8,. The effect of the substituent on the ring structure is independent of the substitution site, and each butyl behaves independently from the others. A careful analysis of experimental structural parameters is necessary before their comparison with calculation data. Both “ B P and 1,3-di-tert-butyl4,5dimethylc~xypentalene (DBMP,
Spectroscopic Study of 1,3,5-Tri-tert-butylpentalene
J. Phys. Chem., Vol. 99,No. 40, 1995 14661
TABLE 1: Optimized Bond Lengths (A) of Pentalene and Its tert-Butyl Derivatives According to QCFF-PI Calculations and Experimental Values from X-ray Diffraction Data
TABLE 2: Optimized Bond Lengths (A)of Pentalene, According to QCF'F-PI Calculations, in SOand in the Lowest Excited States, from SI to Ss
2&-55 I
a
1
calc" P rl.2 1-2.3 r 3 3 r3'3a,4 i-4,~ 1-5.6 r6,6a
r1.b r3a,6a 'I.Cbu;
1.496 1.377 1.471 1.360 1.496 1.377 1.471 1.360 1.450
2@55
6
so
expb
1-TBP 5-TBP 1,5-DTBP TTBP DBMP TTBP
r1,2
1.508 1.374 1.468 1.360 1.493 1.376 1.474 1.367 1.453 1.476
r2.3
1.495 1.377 1.471 1.358 1.510 1.384 1.469 1.360 1.448
1.508 1.374 1.469 1.357 1.506 1.383 1.473 1.368 1.450 1.477
'3,Cbur
1.480
r5.cbut
1.480
1.506 1.380 1.479 1.358 1.505 1.382 1.473 1.367 1.450 1.477 1.477 1.480
1.478 1.351 1.496 1.357 1.500 1.358 1.465 1.360 1.460 1.522 1.510
1.54 1.32 1.46 1.41 1.50 1.28 1.52 1.34 1.43 1.49 1.55 1.54
P, 1-TBP, 5-TBP, 1J-DTBP, and 'ITBP stand for pentalene, l-ferrbutylpentalene, 5-tert-butylpentalene, 1,5-di-fert-butylpentalene,and 1,3,5-tri-fert-butylpentalene,respectively. From ref 7. Cbut is the carbon atom of the butyl group directly bound to the pentalene ring at the substitution site. a
TTBP
DBMP
Figure 3. Molecular structure of 1,3,5-tri-terr-butylpentalene(TTBP, left) and 1,3-di-tert-buty1-4,5-dimethylcarboxypentene (DBMP, right).
see molecular formulas in Figure 3) have been studied by X-ray diffraction at room tem~erature.~ Their ring structure is also reported in Table 1. It may be seen that, although DBMP has a geometry with nearly CZh symmetry and C-C distances in satisfactory agreement with calculated values, the l T B P structure is more asymmetric with the ring plane as the only symmetry element. In particular, the (3-3a) and (3a-4) bonds match bond alternation poorly, and all other ring bonds are unusually short and long (on the average, 1.31 and 1.52 A, respectively). The DBMP structure is more regular also in the region of the substituent^.^ For instance, the C4-Cbut and c6Cbut bonds are quite similar, around 1.515 A. On the contrary, the TTBP values are respectively 1.49 and 1.55 A. The CbutC,,, bond lengths are spread over the 1.49-1.56 8, range in TTBP but more constant in DBMP. The steric hindrance between the two ester groups of DBMP may be considered low, since the ester group in position 5 is nearly coplanar to the pentalene ring and the other is ~rthogonal.~ Keeping in mind these considerations, it is reasonable to assume that DBMP has a ring structure more representative of that of the parent molecule than lTBP. The view is qualitatively supported by crystal packing considerations. The crystal structure of TTBP (a = 9.77 A, b = 9.40 A, c = 19.98 A, space group P212121 (DZ4), Z = 4) is distorted along the c axis while that of DBMP has more balanced unit cell parameters (a = 11.13 8,b = 12.56 A, c = 13.81 A, space group P2dc (GhS), Z = 4).' The distortion could give rise to larger anisotropic interactions in the fist case and, plausibly, also to modifications of the molecular structure. Therefore, we will discuss spectroscopic properties concerning 'ITBP in highly diluted solutions using the calculation data of Table 1, relative to the isolated molecule cases. Excited state geometries of the pentalene molecule, from SI to S5, are collected in Table 2. Large structural changes with
r3,3a
r1.6~
na,b
SI 1.436 1.436 1.406 1.406 1.460
1.496 1.377 1.471 1.360 1.450
6
s2
s3
s 4
ss
1.413 1.450 1.427 1.461 1.384
1.432 1.478 1.395 1.441 1.438
1.482 1.424 1.457 1.404 1.449
1.437 1.502 1.376 1.431 1.467
TABLE 3: Vertical Transition Energies (cm-') of Pentalene at the Optimized SOGeometry, AE(So-S,), and Oscillator Strengthsf from QCFF-PI Calculations and Experimental Values from TTBP Absorption Data f 0.
AEs,-s.(So)
SI(A,) S2(Bu)
11 950 28 380 36 070 48 830 49 ooo
S3(Bu) S4('4g)
SdBu) (I
0.27 0.18
A E"
f
16 670 29 600 35 700
0.002 0.11 0.06
46 500
0.65
0. 1.26
Band maxima of the solution spectrum at room temperature.
respect to SOare found for all S, states. In more detail, it may be observed that (1) bond altemancy is conserved only in SZ and S4, although to a much lesser extent than in SO;(2) S 3 and S5 have similar structures, with nearly equal (1-6a), (1,2), (3a,4), and (45) bond distances; and (3) S I has a delocalized DU geometry, at variance with all other states, which are of CZh symmetry. We have calculated, in a singly excited configuration interaction (SECI) scheme involving all eight ~d MO's, vertical So-S, transition energies at the SO equilibrium configuration, Le., hE(So-S,;So), roughly corresponding to the absorption maximum of the electronic spectrum (see Table 3). Five electronic states occur below 50 000 cm-I, among which only SZ, S 3 , and Ss are one-photon-active, belonging to the B, symmetry species. The f i s t excited state, SI,is closer in energy to SOthan to SZ. In addition, the SO SZ transition is allowed with a relatively high calculated oscillator strength, but the SO S I transition is forbidden. These conditions are similar to those favoring the S 2 SOfluorescence in indacene.20*22Above Sz, the S 4 and S5 states have almost coincident energies. On the contrary, large energy gaps are predicted between S:! and S3 (e8000 cm-') and between S 3 and the SdSs pair ( e 1 3 000 cm-'1.
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-
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4. Vibrational Spectroscopy of TTBP
(a) Normal Mode Calculations. Vibrational calculations on pentalene and its derivatives (1-TBP, 5-TBP, 1,5-DTBP, and TTBP) have been made starting from their optimized ground state structures. In the C2h symmetry, pentalene (CsH6) has 13 a, 5 b, 6 a, 12 b, modes, among which those of a, and b, symmetry are infrared-active and the remaining, of a, and b, symmetry, Raman-active. We further observe that the 13 a, and 12 b, modes correspond to in-plane and the 6 a, and 5 b, modes to out-of-plane vibrations. In the derivatives the molecular symmetry is lower than CZh, and assuming that the butyl groups are freely rotating around the connection bond to the pentalene moiety at room temperature, the only surviving symmetry element is the ring plane. Normal modes must therefore be classified only as in-plane or out-of-plane and are all infrared- and Raman-active. If the interaction energy between the pentalene and the butyl substituents is small, most vibrational modes of the whole molecule will be associated with
+
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Falchi et al.
14662 J. Phys. Chem., Vol. 99, No. 40,1995 atomic displacements on one or on the other fragment (P and B modes, respectively). Mixed (BP) modes originate from hindered translational or rotational motions of one unit with respect to the other. In the simplest case, i.e., the monoderivatives, it is relatively easy to follow the frequency changes of the P modes with the monosubstitution. The pentalene modes have almost unaltered atomic displacements in the derivatives, and their frequencies are only slightly affected by the substitution. In few cases of out-of plane frequencies large shifts occur, resulting from the substitution of the vibrating light hydrogen atom with a heavy butyl group. The results are summarized in Table 4. For 1-TBP it has also been possible to determine reasonably the B and BP modes. Stretching in-plane modes of the tert-butyl group with respect to the pentalene unit, of BP type, occur at 1189 and 1151 cm-I, and out-of-plane BP vibration occurs at 118 cm-I. Rotational motions transform into torsional oscillations calculated at 591, 219, and 44 cm-l. On the contrary, no clear distinction between B and BP assignment may be proposed for a large number of 5-TBP modes. Increasing the number of substituents up to TTBP, a correlation of P modes from the parent molecule to TTBP may be attempted as an aid in the assignment of infrared and Raman bands proper of the pentalene ring (see Table 4). Most in-plane frequencies increase with the triple substitution while out-ofplane frequencies decrease, as expected considering the inductive and mass effect of the butyl group. In-plane modes above loo0 cm-I, of C-C stretching and CCH bending nature, are quite satisfactorily correlated from pentalene to TTBP. On the contrary, due to the large number of B and BP modes in TTBP, it becomes difficult to make a similar correlation for these modes from the monoderivatives. For the purpose of the present paper, some of them are discussed in the next subsection in comparison with the experimental data. (b) Vibrational Spectra. Infrared and Raman spectra of solid TTBP samples have been measured at room temperature in medium-resolution conditions. Being relative to the ground state, they are important pieces of information to test the vibrational calculations on the isolated molecule. The spectra are shown in Figures 4-6, and the experimental results are summarized in Table 5. Solid state effects, such as the occurrence of multiplet bands, are observed in both spectra, which should therefore be discussed on the basis of crystal considerations. We recall that the TTBP crystal structure is orthorhombic, space group P212121(DZ4),with four molecules per unit cell in general p ~ s i t i o n .Each ~ molecular mode, a' (inplane) or a" (out-of-plane), is split into four crystal components (a bl bz b3), according to the correlation diagram C,Cl -02, all of which are Raman-active. On the other hand, only the bl, bz, and b3 components are infrared-active. In most cases, however, the number of observed components is less than predicted and with small energy splitting. In addition, no intermode mixing occurs between different molecular modes due to intermolecular interactions. It is reasonable to neglect crystal effects for assignment purposes and to follow the molecular approach already summarized in Table 4. In the range 1700-1500 cm-', five P modes are expected, four of which are approximately described as in- and out-of-phase combinations of C-C stretchings and the fifth as the interannular symmetric stretching. Three of them are observed in both spectra at 1612, 1553, and 1533 cm-I. The remaining two are weak, around 1693 cm-I in the infrared and 1637 cm-I in the Raman spectrum. The additional bands or shoulders associated with the main peaks are assigned to less intense crystal components. Other P modes above loo0 cm-' correspond to CCH bendings (1399, 1386, 1326 cm-'; calculated values) and to mixed bending and C-C stretching modes
+ + +
TABLE 4: Normal Mode Frequencies (cm-9, Except Those Corresponding to C-H Stretchings, of Pentalene, 1-TBP, 5-TBP, zpd TTBP (see Table 1 for Symbols), from QCFF-PI Calculabons, in the Optimized Ground State Structure 1-TBP
Dentalene
634 67 1 905 1021 1106 1223 1378 1504 1547 1679 511 846 1018 1082 1149 1308 1381 1501 1622 235 307 499 784 1010 1118 498 633 75 1 1012 1110
a'
a"
a'
a"
a'
668 733 881 1054 1122 1244 1384 1527 1547 1692 579 869 1014 1083 1161 1328 1399 1509 1649 260 288 501 793 697 1111 437 600 782 1007 1107 1501 1499 1492 1483 1471 1464 1453 1449 1447 1031 920 444 359 294 244 1154 1037 1028 910 376 312 236 1189 1151 59 1 219
a" 118 44
5-TBP P modes 614 684 915 1022 1120 1225 1383 1532 1573 1683 777 922.1 1093 1154 1321 1380 1505 1637 247 308 500 76 1 1009 960 427 63 1 620 1021 1110
TTBP 576 749 95 1 1129 1172 1227 1386 1581 1612 1694 630 822 1093 1147 1195 1326 1399 1533 1663 137 29 1 434 606 730 968 436 542 629 1011 998
B modes 1500 1499 1492 1480 1468 1464 1453 1449 1448 1034 922.4 443 243 1153 1038 1028 905 283 BP modes 1246 1167 870 559 335 31 1 209 363 232 94 28b
P, B, and BP modes refer to normal modes of tert-butyl derivatives having atomic displacements mostly on the pentalene ring (P),on the butyl groups (B), or comparable on both molecular fragments (BP). Negative frequency.
Spectroscopic Study of 1,3,5-Tri-tert-butylpentalene
J. Phys. Chem., Vol. 99, No. 40, 1995 14663
'I
TABLE 5: Observed Infrared and Raman Frequencies (cm-') and Intensities (I) of TTBP and Proposed Assignment obs
A
IR P 1693 m 1639 w
.4
1612 1593 1553 1546 1534 0 '
I
600
800
1000
1200
14OC
wavenumbers ( c m
1600
1800
-')
Figure 4. Infrared spectrum of TTBP as a polycrystallinefilm between KBr windows at room temperature in the frequency range 1800-500 cm-I.
A
.to0
200
150
250
wovenumbers
300
350
(cm -')
Figure 5. Infrared spectrum of a TTBP pellet at room temperature in the frequency range 400-100 cm-I.
1478 1457 1446 1393 1364 1359 1343 1305 1297 1250 1218 1200 1106 1081 1074 1032 1021 1011 1000 934 922 901 883 852 850 825 815 804 797 739 729 673 637 578
s
sh m mw w s s
sh m s s w
w w s
m s m m m
Raman
1637 1625 1612 1594 1553 1544 1533 1521 1477 1459 1446 1386 1370 1340 1307 1298 1242 1198 1104 1082
Ia
sh m s
sh
1581 P
sh s
1533 P
sh w
B
mw m w w
200
A00
600
800
10'00 12'00 1400 1 6 0 0
wovenumbers
(cm
-'I
Figure 6. FT-Raman spectrum of TTBP crystal powder at room temperature with 1.06 pm excitation wavelength. (1195, 1129,1093 cm-'; calculated values). They are in fairly good agreement with observed bands in the corresponding energy regions (see Table 5). Butyl (B) modes are calculated and observed around 1500-1450 and 1030-1020 cm-I. More interesting, the highest BP modes, in the range 1250- 1150 cm-I in the monoderivatives, shift to 1310 and 1275 cm-' in TTBP. A doublet, 1307/1298cm-', and a single band at 1250 cm-' are found experimentally. Below 1000 cm-' B modes are expected only in the narrow range 920-900 cm-' and below 500 cm-I. Lowering the energy of the vibrational excitation, the displacements on the pentalene unit interact more effectively with those on the butyl groups, and the vibrational modes are
B B
1399 P 1386 P
vw
1326 P 1310 BP
vw m
1275 BP
w
m
w w
w mw m
I
CCH bending
1227
1195 P 1129 P 1093 P
} CCH bending } C-c stretching
B B
1010
br
lo''
998 P BP(B)
932
w
900 881
w w
905 BP 879 P(BP) 857 BP
824 810
sh
822 P(BP)
778 734
w
640 575 537 472
w
397
vw
280
vw
243
vw
m sh w s
C-C stretching
B-P stretching
w
sh mw w w
assigntb
1
1612 P
s
} out-of-planebending C-C stretching
s
m mw vw vw m s vw
w w
w
439 w 357 318 284 267 245 200
calc 1694 P 1663 P
e w w w w w
749 P 730 P 629 606 576 522 463 434 396
P P P BP BP BP BP BP BP
out-of-planebending C-C stretching out-of-plane torsion
B B B
BP a s = strong, m = medium, w = weak, sh = shoulder, and br = broad. Calculated QCFF-PIfrequencies of TTBP; see Table 4 for P, B, and BP assignment. more frequently of BP nature. For instance, the infrared bands at 883, 850, 815, and 804/798 cm-' and the intense Raman band at 815 cm-' are assigned to C-C stretching modes strongly coupled to butyl motions. Out-of-plane P modes occur at 1011 and lo00 cm-' (1010,998 cm-I; calculated) and at 673 and 636 cm-' (629,606 cm-l; calculated). Below 500 cm-' the infrared and Raman spectra weaken considerably. It was necessary to use pellets of pure crystal powder to determine the spectrum in the far-infrared region (400-100 cm-I; see Figure 5). The observed bands are related, according to our calculations, to modes with large atomic displacements on the
Falchi et al.
14664 J. Phys. Chem., Vol. 99, No. 40, 1995
A
36870 I
0
10000
20000
30000
wavenumbers
40000
SO000
0 ' I 26000
( c m -')
30000
34000
wovenumbers
Figure 7. Absorption spectrum of TTBP in cyclohexane solution: c = M, 25 000-7000 cm-I. M, 50 000-25 000 cm-'; c =
38000 (cm
42000
-')
Figure 8. Absorption spectrum of 'ITBP 4 x M in isopentane/ ether solution at 15 K in the energy range 42 000-25 000 cm-l. The (0-0)band of the SO SZ transition is indicated.
-
butyl groups. It may be concluded, considering also the low intensity of others B modes, that the pentalene unit is responsible for most infrared and Raman activity.
5. Electronic Absorption Absorption TTBP spectra have been measured in the visible and UV regions at room temperature in cyclohexane solution and at 15 K in a glassy isopentane/ether matrix. (a) Room Temperature Spectrum. Four broad absorption bands of varying intensity are observed in the electronic spectrum of TTBP. The spectrum shown in Figure 7 is essentially similar to that already reported.& A weak absorption region is centered around 16 670 cm-I (600 nm) with a very low oscillator strengthJ- 0.002, indicative of a weakly allowed or a vibronically induced transition. Below 400 nm, three absorption regions are found with maxima at ~ 2 600 9 cm-' (338 nm), -35 700 cm-' (280 nm), and -46 500 cm-I (215 nm). The last has the largestfvalue, 0.65. It is more difficult to estimate the oscillator strength of the other two bands, due to their extensive overlap. The fit with two Gaussian bands gives for the fiist, around 29 600 cm-I, f 0.11 and for the second, around 35 700 cm-l, f 0.06. We may satisfactorily account for the experimental data on the basis of our QCFF-PI calculations. There is a fair agreement between calculated vertical transition energies at the SOequilibrium configuration, i.e., AE(Sn-So;So), and absorption maxima. Also, the oscillator strengths are in the correct order of magnitude. The three absorption regions above 25 OOO cm-I may therefore be assigned to allowed transitions, SO S2(Bu), SO S3(Bu),and SO S5(B,). The SO SI(A,) transition on the other hand, is forbidden and may acquire strength only through vibronic coupling by means of b, modes. This is again in agreement with the low experimentalflso-SI) value. The energy gap between the fiist and the second absorption maxima, -12 930 cm-', is comparable, although smaller, to the calculated value, -16 430 cm-I, of AE(SZ-SI;SO). Since experimentally no other absorption bands are seen between those at 16 670 and 29 600 cm-I, it must be concluded that in T l B P the first excited state, SI, is roughly equidistant from SO and
-
sz.
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-
-
-
-
(b) Low-Temperature Spectrum. At 15 K the SO SZand S3 absorption regions of 'ITBP in a glassy isopentane/ ether matrix present vibronic structures, as seen from Figure 8. The origin of the SO S 2 band system is observed at 27 440 cm-' (-364.4 nm). Two other bands occur at 28850 and 30020 cm-I, -1410 and 2580 cm-' at higher energy. The upper transition has four vibronic bands at 34 620, 35 750, 36 870, and 38 560 cm-I. The SO S,(A,) spectrum (not SO
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24000
42600 ' wavenumbers (cm-I) Figure 9. Observed (dotted line) and calculated (full lines) spectra of TTBP in the region of the second and third electronic transitions. The calculations include Franck-Condon contributions from a, fundamentals and combinations up to 10000 cm-' above (0-0), considering also the Duschinsky rotation of normal modes in the excited states. '
shown), on the contrary, is quite unstructured even at 15 K, the only change with respect to the room temperature spectrum being a weak shoulder at 11 720 cm-I on the broad band, whose maximum shifts from 16 670 to 17 800 cm-I. Due to the allowed nature of the SO SZ and SO S3 transitions, the vibronic profiles of Figure 8 should be interpreted as mostly due to the Franck-Condon absorption mechanism. Following this approach, the absorption intensity of the 1O;O) (n;vi)vibronic transition, with 'u quanta of the ith mode excited in addition to the purely electronic SO Sn excitation, is proportional to
-
-
-
-
where PO,, is the electronic transition moment and IOo) and Ivy) are vibrational wave functions of the ith mode in the ground and in the nth excited state. Since the Franck-Condon integral, (@lvr), vanishes for all final wave functions except those of a, symmetry, vibronic transitions may be induced (i) through a, vibrations (or their combinations) for any 'u value, (ii) even overtones of non-totally symmetric modes (or their combinations), and (iii) mixed combinations between these two sets. In the following, for the sake of simplicity, we limit our considerations only to transitions arising from the fust possibility. As the molecular equilibrium configuration shifts upon electronic excitation, the equilibrium position QOof all a, modes is displaced as well. Assuming that the vibrational frequencies
J. Phys. Chem., Vol. 99, No. 40, 1995 14665
Spectroscopic Study of 1,3,5-Tri-tert-butylpentalene
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TABLE 6: Calculated SO SZ Adimensional Displacement Factors B and Vibronic Intensities (I)Relative to the (0-0) Intensity for the Totally Symmetric Modes (Y, cm-') of Pentalene in the Optimized SOand SZ Geometrief
so
s2
2,
B
I
634 67 1 905 1021 1106 1223 1378 1504 1547 1679
-0.006 1.994 0.285 0.502 0.446 0.563 -0.971 0.516 -1.344 1.984
1.8E-5' 1.988 0.040 0.126 0.099 0.158 0.471 0.133 0.903 1.968
TABLE 8: Calculated Frequencies (cm-') of Pentalene (Except C-H Stretchings) in the Ground (So) and Excited States, from SI to SS, According to QCFF-PI Calculations a,
V
B
I
630 658 932 1060 1095 1224 1386 1454 1553 1610
-1.071 1.710 -0.184 0.415 -0.549 0.497 1.010 1.682 0.956 1.175
0.573 1.462 0.017 0.086 0.150 0.123 0.510 1.414 0.456 0.690
b,
A, modes of the ground (left) and of the excited state (right) have been used in the calculations. Read as 1.8 x
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TABLE 7: Calculated So S3 Adimensional Displacement Factors B and Vibronic Intensitied (I)Relative to the (0-0) Intensity for the Totally Symmetric Modes (Y, cm-') of Pentalene in the Optimized So and S3 Geometrief
so
a,
s3
V
B
I
V
B
I
634 67 1 905 1021 1106 1223 1378 1504 1547 1679
-0.700 1.757 0.056 -0.048 0.622 0.458 -0.560 1.442 -1.455 1.791
0.245 1.543 0.001 0.001 0.193 0.104 0.157 1.039 1.058 1.604
637 660 911 1036 1088 1211 1367 1427 1546 1623
-0.624 -1.860 -0.022. 0.872 0.552 0.616 1.321 0.763 0.005 1.728
0.194 1.730 2.0E-4' 0.380 0.152 0.189 0.872 0.291 %l.OE-5 1.493
a A, modes of the ground (left) and of the excited state (right) have been used in the calculations. Read as 2.0 x
do not appreciably change from SO to S,, as is true for most of the a, modes in S2 and S3 (see Table 8), the Franck-Condon integral (Oolvin)depends only on the dimensionless displacement parameter37
Bj = 0.172~~"~(X,,,, - Xo,eq)mlRLi where (Xn,eq - Xo,,,) is the 3r-dimensional displacement vector (with r being the number of atoms) of the equilibrium structure upon excitation, mil2the 3r x 3r diagonal matrix of the square root of the atomic masses, and Li the 3r-dimensional vector of the ith mode with frequency oi (cm-I) in terms of massweighted Cartesian coordinates. The relative Franck-Condon intensity of the 1O;O) 1n;v;)transition with respect to (0-0) is given by
-
(3)
-
while that of the combination transition )O;O) In;vivj) depends on the product of those of component modes. The Bi parameters of all pentalene a, fundamentals, except C-H stretchings, have been calculated, according to eq 2,for the SO Sz and the SO S3 transitions with displacement vectors Li relative to the ground state ag modes. The B and intensity values are reported in Tables 6 and 7. Taking into consideration the SO S2 transition, it can be noted that three a,&) modes, 671,1547,and 1679 cm-I, have vibronic activity 2 1. The calculated Franck-Condon profile, including a, overtones and combination modes due to a, vibrations up to x 10 000 cm-I, grossly overestimates the activity of transitions
-
-
-
b,
so
SI
sz
53
54
SS
634 67 1 905 1021 1106 1223 1378 1504 1547 1679 511 846 1018 1082 1149 1308 1381 1501 1622 235 307 499 784 1010 1118 498 633 75 1 1012 1110
646 652 904 1058 1088 1223 1367 1432 1519 1594 520 846 1046 1084 1188 1315 1360 1463 1563 256 305 503 842 934 1066 502 638 710 994 1043
630 658 932 1060 1095 1224 1386 1454 1553 1610 497 84 1 1051 1075 1152 1305 1359 1455 1513 146 299 48 1 72 1 85 1 1036 484 569 687 830 1034
637 660 911 1036 1088 1211 1367 1427 1546 1623 507 833 1029 1079 1161 1298 1357 1433 1559 220 297 454 722 847 927 457 606 682 879 913
630 655 898 1036 1093 1214 1368 1436 1514 1589 504 832 1029 1077 1149 1296 1360 1436 1531 160 28 1 438 652 816 919 45 1 534 685 827 910
638 659 887 1018 1092 1210 1359 1440 1503 1656 511 830 1012 1082 1164 1296 1366 1436 1585 232 293 442 446 687 923 610 662 845 856 903
TABLE 9: Duschinsky Matrix for the ag Modes in the SO and SZStates of Pentalene (Only Coefficients Larger Than 0.05 Are Indicated) so Sz
634
630 -0.84 658 -0.54 932 1060 1095 1224 1386 1454 1553 1610
671
905
1021 1106 1223 1378 1504 1547 1679
-0.53 -0.07 -0.08 0.84 -0.08 0.06 -0.07 -0.09 0.12 -0.1 1 -0.98 -0.08 -0.06 0.62 0.66 0.31 -0.13 0.19 -0.17 -0.05 0.66 -0.72 0.15 -0.14 -0.28 -0.17 0.84 0.32 0.22 -0.17 -0.08 0.05 -0.11 -0.15 -0.28 -0.23 0.70 -0.58 -0.06 0.11 -0.27 0.30 -0.81 -0.05 0.14 0.37 -0.06 0.06 -0.08 0.37 0.28 0.15 0.86 0.12 0.06 -0.59 -0.74 0.28
of high total v value, predicting the intensity maximum around x5500 cm-' above the (0-0) transition. However, normal coordinates mix after electronic excitation, and the correspondence between modes in different electronic states is expressed through the mode-mixing (or Duschinsky) matrix. The calculation may be easily carried out once the Cartesian displacement vectors of all the modes in the two states are known.37 Therefore, a normal mode analysis has been performed also for the lowest excited states of pentalene, from SI to S5, on the basis of the optimized geometries of Table 2. The excited state frequencies are compared with those of SOin Table 8. In-plane modes have approximately constant frequencies in the ground and the excited states, except the pairs 1504/ 1679 and 1501/1622cm-I, of ag and b, symmetry, respectively. These latter are in- and out-of-phase combinations of C=C stretchings in the two fused rings of pentalene and strongly favor the change from the bond alternating SOgeometry to the more delocalized S, structure. The Duschinsky matrix of ag modes for the SdS2 pair of states (see Table 9) shows that the C=C stretching modes, 1547 and 1679 cm-I, contribute to SZ vibrations 1224,1386, 1454, 1553, and 1610 cm-' and that a strong mixing occurs also between the two lowest modes,
Falchi et al.
14666 J. Phys. Chem., Vol. 99, No. 40,1995
calculated at 634 and 671 cm-' in SO. If excited state ag coordinates are used, the vibronic activity therefore redistributes over additional fundamentals, as shown in Table 6. The result is that the four a&) fundamentals above 1300 cm-' and the two lowest at 629 and 658 cm-' have more balanced displacement B factors, and the Franck-Condon maximum shifts to e3000 cm-I above (0-0), in better agreement with experiment. The same general conclusions apply also to the SO S3 transition (see Table 7). In addition to normal mode rotation in the excited state, the effect of alkyl substitution must be considered. In the case of s-indacene2' branched alkyl substituents such as terr-butyl may affect the SO-S, intensity profile of the parent molecule. In the derivative the P modes may have nonvanishing displacement components also on the substituent, thus providing a second mechanism for intensity redistribution. In addition, B modes, localized mostly on the terr-butyl group, or mixed (BP) modes may show appreciable intensity. The vibronic SO SZ and SO S3 spectrum of the actual molecule, TTBP, has been simulated using eqs 1-3 and then convoluting the stick spectrum with Gaussian band shapes. The following r(fwhm) values were used for best fit to experiment: 600 cm-' for all SO S2 vibronic transitions (except (0-0) S O - S ~for which r = 300 S3 cm-I) and 300 cm-' for all those belonging to the SO system. The overall agreement with experiment is fair, noting S5 transition contributing to the total that the strong SO absorption profile in the SO S3 region is not taken into account. It should be finally noted that, although the (0-0) So-Sz transition energy may be confidently assigned from the experimental spectrum, no clear evidence of (0-0) So-S3 is found in Figure 8, due to the extensive overlap with the absorption wing of SO S2. Our simulated spectrum indicates that (0-0) SOS3 should occur at 32 650 cm-'.
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6. Conclusions
Acknowledgment. The authors thank Prof. C. Taliani and Dr. G . Ruani from Laboratorio di Spettroscopia Molecolare (CNR, Bologna, Italy) for the use of the Raman facility and for valuable assistance during the measurements. References and Notes (1) Garrat, P. J. Aromaticity; McGraw-Hill: London, 1971. (2) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Ya. Aromaticity and Antiaromaticity; Wiley-Interscience: New York, 1994. (3) Bohm, M. C.; Schutt, J. J. Phys. Chem. 1992, 96, 3674. (4) Coulson, C. A.; Longuet-Higgins, H. C. Proc. R . SOC. London, A 1947, 192, 16.
(5) Hafner, K.; Knaup, G. L.; Lindner, H. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 633. (6) Hafner, K. Pure Appl. Chem. 1982, 54, 939. (7) Kitschke, B.; Lindner, H. J. Tetrahedron Lett. 1977, 29, 2511. (8) Breslow, R. Acc. Chem. Res. 1973, 6, 393. (9) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; McGraw-Hill: London, 1971. (10) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1971. (11) Berlman, J. D. Handbook of Fluorescence Spectra of Aromatic Molecules: Academic Press: New York, 1971. (12) Pecile, C.; Lunelli, B. J. Chem. Phys. 1968, 48, 1336. (13) Girlando, A,; Pecile, C. J . Chem. Soc., Faraday Trans. 2 1973, 69, 818. (14) Hochsuasser, R. M. J . Chem. Phys. 1960, 33, 950. (15) Zanon, I. J. Chem. SOC., Faraday Trans. 2 1973, 69, 1164. (16) Elsaesser, T.; Laermer, F.; Kaiser, W.; Dick, B.; Niemeyer, M.; Luttke, W. Chem. Phys. 1988, 126, 405. (17) Nickel, B.; Hertzberg, J. Chem. Phys. 1989, 132, 219. (18) Bich, V. T.; Bini, R.; Salvi, P. R.; Marconi, G. Chem. Phys. Lett. 1990, 175, 413. (19) Gellini, C.; Cardini, G.; Salvi, P. R.; Marconi, G.; Hafner, K. J . Phys. Chem. 1993, 97, 1286. (20) Gellini, C.; Salvi, P. R.; Hafner, K. J. Phys. Chem. 1993,97,8152. (21) Gellini, C.; Angeloni, L.; Salvi, P. R.; Marconi, G. J. Phys. Chem. 1995, 99, 85. (22) Klann, R.; Bauerle, R. J.; Laermer, F.; Niemeyer, M.; Luttke, W. Chem. Phys. Lett. 1990, 169, 172. (23) Beer, M.; Longuet-Higgins, H. C. J. Chem. Phys. 1955,25, 1390. (24) Binsch, G.; Heilbronner, E.; Kankow, R. Chem. Phys. Lett. 1967, 1, 135. (25) Birks, J. B. Chem. Phys. Lett. 1972, 17, 370. (26) Murata. S.: Iwanaea. C.: Toda. T.: Kokubun. H. Chem. Phvs. Lett. 1972, 15, 152. (27) Le Goff. E. J. Am. Chem. SOC. 1962. 84. 3976. (28) Hafner, K.; Donges, R.; Goedecke, E.; Kaiser, R. Angew. Chem. 1973, 85, 362. (29) Hafner, K.; Suss, H. U. Angew. Chem., Int. Ed. Engl. 1973, 12, 575. (30) Bloch, R.; Marty, R. A.; de Mayo, P. J. Am. Chem. SOC.1971, 93, 3071. (31) Bloch, R.; Marty, R. A.; de Mayo, P. Bull. SOC. Chim. Fr. 1972, 2031. (32) Craig, D. P.; Maccol, A. J . Chem. SOC. 1949, 1 1 , 964. (33) Dewar, M. J. S.; de Llano, C. J. Am. Chem. SOC. 1969, 91, 789. (34) Baird, N. C.; West, R. M. J . Am. Chem. SOC. 1971, 93, 3072. (35) Warshel, A.; Kaplus, M. J . Am. Chem. SOC. 1972, 94, 5612. (36) Warshel, A,; Levitt, M. QCPE 1974, No. 247. (37) Zerbetto. F.: Zgierski, M. Z. Chem. Phys. 1986, 110, 421. (38) Zerbetto, F.; Ziierski, M. Z.; Orlandi, G.; Marconi, G. J. Chem. Phys. 1987, 87, 2505. (39) Orlandi, G.; Zerbetto, F. Chem. Phys. 1988, 123, 175. (40) Negri, F.; Zgierski, M. Z. J. Chem. Phys. 1992, 97, 7124. (41) Gustav, K.; Storch, M. Znt. J . Quantum Chem. 1990, 38, 1, 25. (42) Dewar, M. J. S.; Dougherty, R. C. PMO Theory of Organic Chemistry; Plenum Press: New York, 1975. (43) Nakajima, T.; Katagiri, S. Bull. Chem. SOC. Jpn. 1962, 35, 910. (44) Nakajima, T.; Saijo, T.; Yamaguchi, H. Tetrahedron 1%4,20,2119. (45) Hertwig, R. H.; Holthausen, M. C.; Koch, W.; Maksic, Z. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 1192. (46) Bischof, P.; Gleiter, R.; Hafner, K.; Knauer, K. H.; Spanget-Larsen, J.; Suss, H. U. Chem. Ber. 1978, 111, 932. I
The vibrational and electronic properties of pentalene, a model antiaromatic system, have been investigated from the experimental and theoretical point of view. Infrared and Raman spectra of the stable tri-tert-butyl derivative, TTBP,show that most of the vibrational activity is due to modes of pentalene origin, by comparison with normal mode calculations. This allows extraction of information on modes proper of the pentalene unit. The electronic absorption spectra of TTBP are in good agreement with QCF'F-PI results of calculation. The profiles of the SO SZand SO S3 absorption bands at low temperature are discussed in terms of Franck-Condon transitions between displaced ground and upper potential energy surfaces. It is confirmed, along with previous observations on other antiaromatic systemsl6Sz1that bond alteration decreases in the excited state structures. As a result, the electronic absorption bands are extended over a large energy range and their maxima are shifted largely with respect to the (0-0) band. As a final interesting point, we note that pentalene has a sparse distribution of excited states up to S5, and in particular, SI is roughly equidistant from S2 and SO. The last condition favors the emission from S2. The fluorescence properties of TTBP will be the object of a forthcoming paper.
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JP95 1406E
