J . Phys. Chem. 1990, 94, 2334-2344
2334
Electronic States of Heterospirenes. Linear Dichroism in Stretched Polyethylene Jens Spanget-Larsen, Roskilde University, Institute of Life Sciences and Chemistry, DK-4000 Roskilde. Denmark
Joachim Uschmann, and Rolf Gleiter* Organisch-Chemisches Institut der Uniuersitat Heidelberg, Im Neuenheimer Feld 270, 0 - 6 9 0 0 Heidelberg, FRG (Received: July 31, 1989)
The polarized absorption spectra for a series of heterocyclic spiro compounds with naphthylenic and phenylenic half-chromophores have been measured in stretched polyethylene at low temperature. Structural information is derived from the observed linear dichroism spectra, and the transitions are analyzed by comparison with the spectra of suitable reference compounds. The observed shifts are generally well predicted by calculations in the PPP a-electron model and can be rationalized in terms of inductive and field effects, spiroconjugation, and exciton coupling between the two half-chromophores.
1. Introduction The linkage of two cyclic *-electron systems via a spiro atom gives rise to what is known as spiropolyenes or spirenes.] The mutually perpendicular A systems exhibit a particular kind of homoconjugation which has been termed spiroc~njugation.~-~ Spiropolyenes are excellent model compounds for the study of intramolecular interactions between weakly conjugated chromophores. In the unique case of spiro[4.4]nonatetraene (i) it was
SCHEME I CH.
1
3
2
i
demonstrated by Batich et that spiroconjugation between the two cyclopentadiene moieties leads to a splitting of the first two bands in the electronic absorption spectrum by about loo00 cm-l. A practically identical splitting is observed for the first two peaks in the photoelectron s p e c t r ~ m This . ~ coincidence can be explained in part by the accidental near-degeneracy of locally excited 11-1 ) and charge-transfer ll+l’) configurations (which in turn depends on the relatively large exchange integrals for the former and the relatively small Coulomb integrals for the latter).5.6 In general, the impact of spiroconjugation on the electronic spectra of spirenes is less striking and does not lead to observable splitting of absorption bands. The absorption spectrum of 9,9’spirobifluorene (ii), where the influence of spiroconjugation is not
ii
immediately recognizable, is a matter of some controversy. The interpretation by Sagiv et al.’ was based on a pure exciton model, ( I ) Review: Diirr, H.: Gleiter, R. Angew. Chem. 1978, 90, 591; Angew. Chem., In?. Ed. Engl. 1978, 17, 559. (2) Simmons, H . E.; Fukunaga, T. J . Am. Chem. Soc. 1967, 89, 5208. (3) Hoffmann, R.; Imamura, A.; Zeiss, G.D. J . Am. Chem. SOC.1967, 89..~5215. (4) Hohlneicher, G. Habilitation, Techn. Hochschule Miinchen, 1967;Ber. Bunsen-Ges. Phys. Chem. 1967, 71, 917. (5) Batich, C . ; Heilbronner, E.; Rommel, E.; Semmelhack, M. F.; Foos, J. S. J . Am. Chem. SOC.1974, 96,1662. (6) Uschmann, J. Diss., Univ. Heidelberg, 1985. (7) Sagiv. J ; Yogev, A.; Mazur, Y J . Am. Chem. SOC.1977, 99, 6861.
CH,
6
7
8 cn.
9
10
11
involving only locally excited states. But in a more recent investigation by Spanget-Larsen et a1.,8 it was argued that the third strong absorption band around 41000 cm-I which has no counterpart in the spectrum of fluorene should be assigned to a charge resonance state involving the interchromophore transitions 114-1’) and I l ’ d l ) ; this state gains optical intensity by interaction with locally excited states and probably by vibronic interaction. In the present investigation, which is an extension of previously published photoelectron spectroscopic studies?JO the electronic absorption spectra for a series of 11 heterospirenes are discussed. The compounds are 0, N, and S functional orthocarbonic acid derivatives, involving phenylenic and naphthylenic ring systems. (8) Spanget-Larsen, J.: Gleiter, R.: Haider, R. Helv. Chim. Acto 1983, 66, 1441. (9) Gleiter, R.; Haider, R.; Quast, H. J . Chem. Res., Synop. 1978, 138. (10) Gleiter, R.; Uschmann, J. J . Org. Chem. 1986, 51, 370.
0022-3654/90/2094-2334$02.50/00 1990 American Chemical Society
Electronic States of Heterospirenes
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2335 E .lo",
SCHEME I1
I
FHJ
12
17
14
13
18
19
Y
Seven members of the series contain two similar polycyclic components and belong to type a, b, or c:
I
~ . . , . , . . . . , . . . . , . , 30
35
LO
LS
103~
Figure 1. (top) Absorption spectrum of spiro[ 1,3-benzodioxoIe-2,2'naphtho[2,3-d]-1,3-dioxole](9) in cyclohexane at room temperature. (bottom) Reduced linear dichroic (LD) absorption curves for 9 in stretched polyethylene at 77 K, indicating long-axis polarized absorption A,(;) (full line) and short-axis polarized absorption A,,(;) (dashed line).
The remaining four members consist of two different components. The molecular formulas for the spiro compounds 1-11 are given in Scheme I. In order to support the analysis of the intramolecular interactions in these compounds, the absorption spectra of a number of half-chromophore reference compounds 12-19 are also considered; their formulas are given in Scheme 11. Whenever possible, this investigation is based on the results of linear dichroic (LD) absorption spectroscopy in stretched polyethylene at 77 K.I1-l4 In contrast to ordinary liquid solution spectroscopy, LD spectroscopy has the important advantage of providing information on the transition moment directions of the absorption bands. Figure 1 is an adequate demonstration of the additional information which may be obtained by low-temperature stretched polymer LD spectroscopy as compared with routine solution spectroscopy (using the orthocarbonate 9 as an example). First should be noted the significantly improved resolution at low temperature. Second and more importantly, because of the resolution of differently polarized, overlapping transitions (e.g., B and C in Figure l), the LD spectrum uncovers a number of spectral features not revealed by the liquid solution spectrum. In addition, the observed orientation factors in stretched polyethylene contain information on the molecular structure, as discussed in section 4. ~~
~~~~~
(11) Thulstrup, E. W.; Michl, J.; Eggers, J . H. J . Phys. Chem. 1970, 74, 3868. Michl, J.; Thulstrup, E. W.; Eggers, J. H. [bid. 1970, 74, 3878. (12) Thulstrup, E. W. Aspects of the Linear and Magnetic Circular Di-
chroism of Planar Organic Molecules; Springer: Berlin, 1980. (13) Michl, J.; Thulstrup, E. W. Specrroscopy with Polarized Light. Solute Alignment by Photoselection, in Liquid Crystals, Polymers, and Membranes; VCH Publishers: Deerfield Beach, FL, 1986. (14) Michl, J.; Thulstrup, E. W . Acc. Chem. Res. 1987, 20, 192.
In the following, the observed spectra for the spiro compounds 1-11 are discussed and compared with those of the pertinent
reference compounds. The observed band shifts are rationalized in terms of intramolecular interactions between the two halfchromophores, using the results of model calculations as a guide. 2. Experimental Section Compounds 1-19 were synthesized and purified as described
in the literature.'O The electronic absorption spectra were recorded on a Cary 17D or a Shimadzu MPS-2000 spectrophotometer with cyclohexane and polyethylene as solvents. The polyethylene samples were prepared from pure sheet material, introducing the solutes by immersing the sheets in chloroform solutions of the compounds as previously described."-I3 For some compounds the solubility in polyethylene was so low that satisfactory samples could not be prepared. The LD spectra were measured at approximately 77 K by using uniaxially stretched (400%) polyethylene samples placed over liquid nitrogen in a quartz Dewar mounted on the spectrophotometer. Rotatable Glan prism polarizers were placed both in the sample and in the reference beam. Two different directions of the electric vector of the linearly polarized light relative to the stretching direction of the polyethylene sheet were used. In one experiment, producing the absorbance curve E=(;), the electric vector of the beam and the stretching direction were parallel; in the other, producing EA?) they were perpendicular. In all cases the propagation direction of the beam was perpendicular to the surface of the polyethylene sheet. The spectrum of an empty stretched polyethylene sheet was recorded independently for use as a reference, following the same procedure. The spectral curves
2336
The Journal of Physical Chemistry, Vol. 94, No. 6,1990
were recorded in digital form which was used for base-line subtraction and for construction of linear combinations of E,(;) and E d ; ) (see below), applying computer programs written by R. Bartetzko and J. Spanget-Larsen.
3. Calculations Quantum chemical calculations were performed using the a-electron model by Pariser, Parr, and Pople (PPP).15 The atomic parameters were taken as those suggested by Fabian et a1.,16except as noted below. The core energy for the methylamino centers, IN,was set equal to -26.3 eV.I0 In the spiro compounds 2,4, and 9, the core energy Io for the oxygen atoms was taken to be 1.2 eV lower than the value used for the reference compounds 13 and 15, in order to account for the inductive effect discussed in section 5. Spiroconjugation was considered by using the previously estimated &plro resonance integral^:^^^^^^ 0.38 (l), 0.62 (2,4,9), 0.32 (3. 5 ) , 0.56 ( 6 ) ,0.18 (7), and 0.35 eV (8). In the CI expansion, 64 singly excited configurations were included for the spiro compounds and 16 for the half-chromophore reference compounds. Oscillator strengthsfwere computed by using the dipole length formulation, and log values were estimated by the empirical relation log t = 4.74 + logf: The calculations on the spiro compounds were carried out first with and then without inclusion of spiro resonance integrals. The former calculations yield orbitals that can be considered as localized on each half-chromophore, and the computed transitions are influenced only by inductive and field effects and by exciton interactions between the half-chromophores. In the latter calculations, the effect of spiroconjugation is added, allowing the influence of the interaction between exciton and charge resonance (or charge transfer) configurations to be evaluated. 4. Orientation Factors and Transition Moment Directions The molecular alignment of solutes in uniaxially stretched polymers is only partial, and the interpretation of the LD spectra cannot be made directly. In an electric dipole experiment the orientational properties that can be determined are the orientation factors K j for the transition moments of the observed transitions fl2-14
K j = (cos2 ( M i , Z ) ) The pointed brackets indicate averaging over all solute molecules i, and ( M ] , Z ) denotes the angle between the moment of transition f i n molecule i and the polymer stretching direction Z . In symmetrical molecules, only some transition moment directions are possible. The spiro compounds of type a, b, and c have DZdmolecular point group symmetry (with some notable exceptions, see later), and symmetry-allowed transitions are polarized either along the molecular z axis (the S, axis) or in a plane perpendicular to it, i.e., the x,y plane. Hence, only two distinct Kf values are possible, corresponding to the orientation factor K , for the molecular z axis and the orientation factor K,,,, = K, = K,, for the two equivalent molecular axes x and y . The result we shall be looking for will thus be two absorption curves, A,(?), corresponding to transitions with Mfalong z , and the other, AX,(!), corresponding to transitions with Mf in the x,y plane. A z ( > )and A,,(;) are given by the expres~ionsl~-‘~ A,(;) = [(I - K x , ~ ) E z (-~ 2) K x , , , E d ~ ) 1 / ( K-2 K,,,,)
where the F?s are the molecular orientation factors discussed above and E,(>) and Ed?) are the experimental base-line-corrected absorption curves obtained with the electric vector of the beam parallel and perpendicular, respectively, to the stretching direction Z of the polymer. ( 1 5 ) Parker. R.; Parr, R. G . J . Chem. Phys. 1953, 21, 466. Pople, J. A. Trans. Faraday SOC.1953, 49, 1375. (16) Fabian. J.: Mehlhorn, A,; Zahradnik, R. Theor. Chim.Acta 1968, 12. 200, 247.
Spanget-Larsen et al. TABLE I: Observed Orientation Factors in Stretched Polyethylene at 77 K assumed molecular compd symmetry K. K, K, XK 0.30 0.41 1.01 1 0.30 D2d 1 .oo 2 0.21 0.21 0.58 D2d ( I .OO) (0.21) 0.58 3 (0.2 1 ) C D2d 4 0.07 0.86 I .oo 0.07 D2d 6 0.26 0.26 0.53 1 ,050 D2d 1.01 0.12 0.77 96 0.12 c 2 c 0.29 0.48 ( I .OO) 13 (0.23) C2” 0.23 0.63 15 c ( I .OO) (0.14) CZL. 0.63 0.23 ( I .OO) 17d (0.14) c 2 u I? The significant deviation from unity indicates that the molecular symmetry is less than the assumed Du. See the discussion in section 4. bRcdlike alignment assumed ( K , = K J . C Kvalues determined at room temperature. dThe orientation axis corresponds to the molecular y axis (Scheme 11). e Values in parentheses were determined indirectly
through the relation Z K = K ,
+ Ky + K, = I .
To determine the orientation factors K , and Kx,y,we use the TEM method.”-’4 In this, linear combinations are formed: E,(?) - dEy(Z) If a spectral feature due to transition f (say, a peak or a shoulder) disappears from the linear combination for d = df, the value of K, for the transition can be determined from the relation KJ = d J / ( 2
+ dJ)
Determination of K j for two differently polarized transitions gives K, and K,,,,. To decide which of the two K j values corresponds to K , and which to K,,, thereby making absolute assignments of transition moment directions, we may use the relation K,. + Ky + K, = 1 which follows from the definition of orientation factors; in the present case of DZdsymmetry we have K,. = KJ = K,,,,, leading to the relationship K, = 1 - 2K,.,. Besides, numerous experiments have shown that the molecular alignment of solutes in stretched polyethylene generally reflects the molecular shape.I2-l4 The “long” molecular axis z of the spiro compounds should thus be better aligned than the “short” axes x and y , and we naturally expect that K , > K,.,,,, corresponding to a “rodlike” distribution function.”-I4 Failure of the observed K values to fulfill the relationship K, = 1 - 2Kxs is an indication that the molecular symmetry is less than DZd(e.g., in the case of 6 ) . The LD data thus contain information on the molecular structure; several examples of structural applications of LD spectroscopy can be found in the 1 i t e r a t ~ r e . l On ~ ~ ’the ~ other hand, if the molecular symmetry can be assumed to be effectively DU,the relation between K, and Kxy is useful in those cases where only one K value can be deduced from the LD spectra (e.g., in the case of 3). The “asymmetrical” spiro compounds 8-11 as well as the reference compounds 12-19 have C, molecular symmetry (or less). In a molecule of C2, symmetry allowed transitions must be polarized along one of the molecular axes x , y , and z , but without additional assumptions it is generally impossible to determine the three corresponding absorption curves A,.(!), A,,(!), and A,(;) from the two observed LD curves E,(>) and Ey(ij). In the case of the phenylene-naphthylene orthocarbonate 9 we assume that the orientation is “rodlike”, Le., that K, > K,, = K,. This assumption is consistent with the molecular dimensions and with the two K values which can be determined from the LD curves, corresponding to K, = 0.17 and K,, = K, = 0.12. (The sum of K values is thus equal to unity within experimental error.) For the compounds 12-19 we shall make the assumption that the absorption in the near-UV region is polarized exclusively in the molecular y,z plane, Le., that A,.(;) = 0. The orientation factors K , and Ky are then determined by the TEM method in the usual manner, and the expressions for A,(;) and A,,(;) are identical with those given above for A,(:) and A,,(;) when the subscript x,y is replaced by J’. The observed orientation factors are listed in Table I, and the positions of the experimental points (K,,K,) in the “orientation
Electronic States of Heterospirenes
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2337
0.81 I
t
Figure 3. Qualitative frontier orbital energy level diagrams for the half-chromophores in 1-11, with indication of r-orbital shapes and symmetry labels in the C, point group. TABLE 11: Irreducible Representations of Exciton (ex) and Charge Resonance (cr) Confieurations in DW Sphenes‘ i bi a2 a2 b, -j bi a2 b~ a2 ex+ = 2-I/*(li-+-j) Ii’ej’)) B2 B ,2 \ E E ex- = 2-1/2(li-j) - li’+-j’)) A, A cr+ = 2-1/2(li-+-j’) li’---j)) A, B2\ E E cr- = 2-1/2(li4j’) - li’-j)) B1 AI ~~~
0 0.2 0.4 KY Figure 2. Orientation triangle showing the theoretical limits for the orientation factors (K&) with indication of experimental points. The horizontal axis in the formulas is the effective orientation axis z.
triangle” are shown in Figure 2. The triangle indicates the theoretical limits for the K values under the condition that K, I K y I KZ,i2-i4The bottom side of the triangle corresponds to Ky = K, (disklike alignment), the left-hand side to K, = Ky (rodlike alignment), and the right-hand side to K, = 0 (perfect alignment of the y , z plane). Points inside the triangle correspond to more general orientation distributions characterized by 0 < K , < K y < K , < 1. Inspection of Figure 2 shows that the points for approximately rod-shaped molecules lie close to the left-hand side of the triangle indicating rodlike alignment, and the longer and thinner the molecules, the closer they approach the limit (Ky,K,) = (0,l) which corresponds to perfect alignment of all molecular z axes with the stretching direction Z . The molecule 4 with (Ky,Kz)= (0.07,0.86) is one of the most perfectly aligned molecules observed; the “champion“ among aromatic compounds seems presently to be a derivative of p-quaterphenyl with (K,,K,) = (0.05,0.90).14 The K values for compounds 6 and 17 deserve some comments. The six-membered heterocyclic rings in these compounds are expected to be nonplanar because of pyramidalization at the nitrogen positions.I0 (Similar effects most likely operate in 7 and 19.10)
In the case of 17 this means that the molecular symmetry may be less than C2,, but this is not expected to affect significantly the transition moment directions of the naphthalene chromophore. However, the analysis indicates that the preferred orientation axis of 17 is the y axis; the other compounds considered in this investigation all tend to align their z axes with the stretching direction. The point for 17 in Figure 2 corresponds to a redefinition of the molecular axes according to the convention K , > Ky, in order to make the point fall inside the triangle as it should. Now to the case of 6: Under the assumption of D,, symmetry (Le,, K, = Ky = KxJ the sum of the observed orientation factors, X K , is equal to 1.05, which exceeds unity by more than the experimental uncertainty. This indicates that the molecular symmetry is less than DU, consistent with the expected distortion; the effective symmetry of 6 is probably not higher than C, which allows infinite possible transition moment directions (parallel to the C2 axis or any direction perpendicular to it). Hence, the K values for 6 given in Table I and plotted in Figure 2 do not correspond exactly to the principal values of the orientation tensor K,I3 and the “reduced” absorption curves in Figure 11 represent in principle unknown linear combinations of differently polarized
~
+ +
a i and -j indicate occupied and virtual orbitals of the two equivalent half-chromophores (local C, symmetry, see Figure 3); primed and unprimed indexes refer to orbitals of different half-chromophores (“left” and “right”).
contributions. Nevertheless, the LD spectral evidence is sufficiently unambiguous to allow resolution and assignment of the observed absorption bands in terms of predominantly long- and short-axis polarized transitions.
5. Results and Discussion The observed LD electronic absorption spectra are shown in Figures 1 and 4-1 1, and observed and calculated transitions are compared in Tables 111-IX. The observed log t values given in the tables correspond to band maxima measured in cyclohexane solution, but in several cases log t values for overlapping transitions resolved in the LD spectra have been estimated by assuming similar band shapes and intensities in cyclohexane and polyethylene solution. The transitions in the spiro compounds are analyzed in terms of those of the half-chromophores. Qualitative orbital energy level diagrams for the three different kinds of half-chromophores are shown in Figure 3 with indication of irreducible representation in the C2, point group; the orbitals have been previously discussed in detail.9J0 In the DZdspirenes, locally excited electronic configurations of the half-chromophores, say, li-j) and li’--j’), combine to form symmetry-adapted exciton configurations ex+ and ex- (Table II).’7*i* Charge-transfer configurations, say, I+-j’) and D-+-i’), involving transfer of an electron from one half-chromophore to the other, combine to form charge resonance configurations cr+ and cr- (Table The symmetry properties of the resulting exciton and charge resonance configurations in the DZdpoint group are given in Table 11. Transitions from the ground state to excited states of B2 symmetry are z-polarized, and transitions to degenerate states of E symmetry are x,y-polarized; transitions to other states are optically forbidden by symmetry. The transitions to charge resonance configurations are intrinsically forbidden, but cr configurations II),i79i8
(17) Murrell, J . N. The Theory of the Electronic Spectra of Organic Molecules; Methuen: London, 1963. (le) Suzuki, H. Electronic Absorption Spectra and Geometry of Organic Molecules; Academic Press: London, 1967.
Spanget-Larsen et ai.
2338 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
TABLE 111: Observed and Calculated Transitions for 12-14" obs calc symmetry 5 compd t pb log e A 32.1 12 3.8 (z) 'A, 30.6 B 37.5 3.9 (Y) 'B, 35.5 ,A, 45.5 '}D 46.0 4.6 (') IB, 43.6
A
cv)
13
14
A B
35.3 43.3c
3.5 3.5
'}D
50.8
4.6
cv)
A B
33.4 36.6
3.3 3.5
(z)
01)
E
42.2 48.8
4.5 s
(z?)
i} Figure 4. Reduced LD absorption curves for I , 1',3,3'-tetramethyl-2,2'spirobi[2,3-dihydro- 1H-benzimidazole] (1) in stretched polyethylene at 77 K. nm
OPO
250
40000
IA, 'Bi
y
3.5 4.0 4.8 4.5
,Al 'B, 'A, 'B, 'A,
34.2 38.4 44.5 46.1 52.8
3.4 4.0 4.7 4.1 3.5
IA,
t!
'The table indicates wavenumbers,,ri in units of 1000 cm-I, (log e) values (cyclohexane), and polarization directions p . For calculational details, see section 3. *Polarization directions p (see Scheme 11) determined by LD spectroscopy; entries in parentheses indicate calculated directions. cO-O transition observed at 41 700 cm-I.
45000
A
36.8
3.9
Z
B
44.6b
3.7
x,y
cm- 1
Figure 5. Reduced LD absorption curves for 1,3-benzodioxole (13) in stretched polyethylene at 71 K. nm qnn
IB,
33.5 40.3 49.1 48.6
TABLE IV: Observed and Calculated Transitions for 1, 2, and 3 (See Footnote a of Table 111) obs calc compd ij log t pa symmetry ij log e A 32.1 4.2 z 'B,(ext) 30.8 4.0 'A;(ex-) 31.3 'E(ex) 36.2 4.0 B 38.8 4.2 x,y 'A2(crt) 40.6 IBI(cr-) 40.6 IE(cr) 41.0 2.7 'B2(ext) 42.2 5.0 Z C 43.9 4.8 IE(ex) 42.8 4.0 D 44.3 4.4 x,y 47.6 lA,(ex-)
A
0
z
log t 3.7 4.1 4.8 4.0
210
A
33.4
3.7
Z
B C
37 40.3
4.0 4.6
x,y
D
41.2
3.9
x,y
E? F?
47 48
3.1 4.4
x,y
Z
Z
IB,(ext) IA, (ex-) IE(ex) 'Az(cr') BI (cr-) 'B,(ext) 'E(ex) IE(cr) 'A, (ex-)
34.9 35.1 42.5 49.3 49.3 48.4 51.0 49.5 52.1
'B2(ext) 'A, (ex-) IE(ex) 'BZ(ext) 'Al (ex-) 'E(ex) 'A,(crt) 'B, (cr-) IE(cr) 'Bz(ext) 'A I (ex-)
34.3 34.4 38.6 43.1 45.1 45.1 46.9 46.9 47.1 50.9 51.0
3.7 3.8 5.1 4.5 4.0 3.7 4.0 5.0 4.0 2.2 3.0
'Polarization directions p (see Scheme I) determined by LD spectroscopy; entries in parentheses indicate calculated directions. 0-0 transition. Figure 6. Reduced LD absorption curves for 2,2'-spirobi[ 1,3-benzodioxole] (2) in stretched polyethylene at 77 K.
of B2 and E symmetry may gain optical intensity by spiroconjugation, which enables them to interact with (and thereby borrow intensity from) optically allowed ex configurations. Spirenes of Type a. The LD absorption spectra of the spiro compounds 1-3 and the reference compound 13 are shown in Figures 4-7. Observed and calculated data for 12-14 and 1-3 are listed in Tables 111 and IV. According to the calculated results, the near-UV absorption spectra of the reference compounds 12-14 are characterized by four T-T* transitions, and the observed bands A, B, C, and D
are easily assigned to them (Table 111). The two medium-intense bands A and B correspond to benzene ' L b and 'Latransitions, respectively, and are fairly well described by 3bl 4bl and 3bl 3a2 promotions in the theoretical model (see Figure 3). The strong band toward higher energy is assigned to two overlapping transitions C and D, corresponding to the doubly degenerate lB,,b state of benzene; these transitions involve primarily the promotions 2a2 4bl and 2a2 3a2. The shifts of the bands relative to those of benzene are well-understood in terms of conjugative interactions between the benzene chromophore and the auxochromic groups
-
-
-
-
in 12-14.'7-20 (19) Gleiter, R.;Gubernator, K.; Eckert-MaksiE, M.; Spanget-Larsen, J.; Bianco, B.; Gandillon, G.; Burger, U. Hela. Chim. Acra 1981, 64, 1312.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2339
Electronic States of Heterospirenes 50
nm
250
300
A
-/ ________------BOOM)
-
35000
400w
45000
cm-1
Figure 7. Reduced LD absorption curves for 2,2'-spirobi[ 1,3-benzodithiole] (3) in stretched polyethylene at 77 K.
The assignment of the transitions is supported by the LD spectrum of 13 (Figure 5) which clearly indicates that band A is purely long axis polarized and band B predominantly short axis polarized. The long-axis polarized component of the B band is most likely of vibronic origin, involving nontotally symmetric vibrations.20 The spectra of the spiro compounds 1-3 are dominated by four low-energy bands A-D, closely corresponding to the spectra of the half-chromophore reference compounds. As indicated in Table IV, the four main transitions are easily assigned to calculated IB2(ex+) and IE(ex) states. This assignment is supported by the observed polarization data; in particular, the LD spectra of 1 and 3 (Figures 4 and 7) confirm the assignment of the strong absorption in the 40000-45 000-cm-' region to two mutually perpendicularly polarized transitions C and D. The bands of the spiro compounds are shifted relative to those of the reference compounds (cf. Tables I11 and IV). Bands A and B are thus blue-shifted in the case of 1 and 2; in the case of 3 the shifts of these bands are insignificantly small. The strong band which is primarily due to the C transition is red-shifted in the case of 1 and 3 but blue-shifted in the case of 2. In a qualitative sense, these shifts are well-described by the theoretical results. Within the framework of the PPP model, the calculated shifts can be discussed in terms of (1) exciton coupling, (2) interaction with charge resonance (cr) configurations, and (3) inductive and field effects. (1) Electrostatic first-order exciton ~ o u p l i n g ' ~ shifts ~ ' * z-polarized transitions to 'B2(ex+) states, Le., transitions A and C, toward lower energies (red shift); xppolarized transitions are unaffected by electrostatic dipoledipole interaction. The energy shift is approximately proportional to the square of the transition moment and to the cube of the inverse distance between the half-chromophores. The red shift of band A should thus be much smaller than that of band C. The predicted red shifts of band A due to exciton coupling are 300, 100, and 50 cm-l for 1, 2, and 3, respectively; the corresponding shifts of band C are 2100, 1750, and 800 cm-', respectively. But the observed A bands are blue-shifted rather than red-shifted, and even the C band of 2 is blue-shifted. Obviously, exciton coupling alone does not qualitatively explain the observed band shifts. (2) As indicated above, spiroconjugation allows ex and cr configurations to interact, provided they involve orbitals that are capable of spiro interaction (Le., fragment orbitals of local a2 symmetry). In the PPP model this interaction is related to Pspiro, the "resonance integral" between the spiro centers. As mentioned in the Introduction, spiro[4.4]nonatetraene (i) presents a special case of this interaction, since low-energy ex and cr configurations are accidentally near-degenerate, leading to first-order interaction. (20) Spanget-Larsen, J.; Gubernator, K.; Gleiter, R.; Thulstrup, E. W.; Bianco, B.; Gandillon, G.; Burger, U. Helu. Chim.Acra 1983, 66, 676.
In this case the distinction between ex and cr states breaks down; for a detailed discussion, see, e.g., ref 6. In case of the spiro compounds discussed in this investigation, the calculated cr configurations are generally about 1 eV (-8000 cm-I) higher in energy than the corresponding ex configurations, and the interaction between them tends to be quite small. The calculated transitions A and B for 1,2, and 3 are thus essentially unaffected by interaction with cr configurations. The C and D transitions are predicted to be red-shifted by about 1000 cm-l as a result of interaction with cr configurations at higher energies. (An exception is the D transition of 2; interaction with a cr configuration at lower energy shifts the calculated transition 350 cm-' toward higher energies.) In the case of 3, the shifts predicted on the basis of exciton coupling and interaction between ex and cr configurations account quite well for the observed shifts, but not so in the case of 1 and 2. (3) When passing from 13 to 2 the effective electronegativity of the heteroatoms is expected to increase, because of the replacement of the C-H bonds of the methylene group by C - 0 bonds involving more electronegative atoms. Similar, though smaller, inductive effects2' are expected in the case of 1 and 3. The inductive effect stabilizes the occupied orbitals more strongly than the unoccupied because of the larger amplitude of the former on the heteroatoms, resulting in a general blue shift of the electronic transitions. Field effects1' may contribute to this trend. (Within the PPP a-electron model, the heteroatoms carry a net positive charge as a result of donation of electrons to the aromatic a systems.) As the inductive effect operates largely through the u bonds, a a-electron model like PPP does not adequately account for it. But the effect can be simulated by lowering the core energy parameter, I,. In particular, lowering of the Io value by about 1 eV (see section 3) leads to predicted overall blue shifts of bands A and B for 2 which are consistent with the observed shifts. The calculated blue shifts are then 1400 (A) and 2200 cm-' (B), to be compared with the observed ones, 1500 (A) and 2900 cm-I (B, ~ shift 0-0 transition). On the other hand, the 5 0 0 - ~ m -blue observed for band C is still underestimated; a red shift of 700 cm-l is predicted. This could easily be explained by an overestimate of the red shift due to exciton coupling; the standard PPP model usually predicts too large transition moments and thereby too strong exciton couplings. All things considered, the observed blue shifts for all bands A, B, and C in the case of 2 can be explained by the assumption that the red shifts caused by exciton coupling (A and C) and by spiroconjugation ( C ) are overcompensated by a strongly blueshifting inductive effect, thereby accounting also for the relative magnitude of the observed overall blue shifts. In the case of 1 only bands A and B are (slightly) blue-shifted, while band C is red-shifted, consistent with the assumption of a smaller inductive effect. Finally, in the case of 3, the inductive effect seems to be negligible. The blue shifts of the bands A and B of 2 have been previously discussed by Smolinski et aLz2 They assume that hyperconjugative interactions involving the u* orbitals of the C-0 bonds stabilize the highest occupied a orbital, leading to the observed blue shift of bands A and B. However, for reasons of symmetry,6 this assumption does not explain the observed blue shift of transition C, which is essentially 2a2 3a2 (Figure 3). Hence, the explanation of the blue shifts in terms of inductive effects as outlined above seems more plausible to us. Relative to the spectra of 12-14, no additional or ynew" bands are clearly observed in the spectra of the spiro compounds 1-3. This is consistent with the calculated results, which indicate that allowed transitions to cr states occur at high energies, where they are likely to be masked by much stronger ex bands. The lowest optically allowed cr state is predicted to be of IE symmetry, +
(21) Gleiter, R.; Kobayashi, M.; Spanget-Larsen, J.; Gronowitz, S.; Konar, A.; Farnier, M. J . Org. Chem. 1977, 42, 2230. (22) Smolinski, S.; Balazy, M.; Iwamura, H.; Sugawara, T.; Kawada, Y. Bull. Chem. SOC.Jpn. 1982, 55, 1106.
2340
The Journal of Physical Chemistry, Vol, 94, No. 6, 1990 350
400
nm
250
300
Spanget-Larsen et al. TABLE V: Observed and Calculated Transitions for 15 and 16 (See Footnote a of Table 111)
A
~
obsd
15
) :=I
compd 15
pa
S
log c
A B C
308 350' 434
39 37 42
z
'A,
y y
'B2 'B, 'A,
D,
444
47
z
'A,
'B2
E
16
A B
E} Figure 8. Reduced LD absorption curves for naphtho[2,3-d]-1,3-dioxole (15) in stretched polyethylene at room temperature. ,o
nm
250
300
calcd symmetry S
28.3 32.3' 37.5
3.7 3.7 4.9
44.6
4.2
(z)
'A,
(y)
'BZ
(z)
'AI
(y) (y) (z) (y)
'A,
'B2 'B, 'B2
297 338 422 41 4 433 460 30.1 33.4 38.7 40.6 43.9 44.1 46.6 48.7
log t 36 38
45 48 47 34 3.7 3.7 4.8 4.4 3.4 4.3 3.0 4.4
H? 48.2 4.5 ( z ) 'A, Polarization directions p (see Scheme 11) determined by LD spectroscopy; entries in parentheses indicate calculated directions. bO-O transition.
TABLE VI: Observed and Calculated Transitions for 4 and 5 (See Footnote a of Table 111)
A
obsd A,:
-
A, y ;
_---
compd 4
s
log c
A
31.5
4.1
B
35.2c 4.0
'}D
42'4
ub z
calc symmetry s log t 'B,(ext) 30.6 3.7 IA,(ex-)
x,y
"I
lB2(ext)
'A (ex-)
A
30000
350W
40000
45000
5' cm-1
Figure 9. Reduced LD absorption curves for 2,2'-spirobi[naphtho[2,3d]-1,3-dioxole] (4) in stretched polyethylene at 77 K.
46.0
4.5
x,y
A
28.2 (33)
3.9
(z) (x,~)
IB,(ex+)
(z)
36.9
5.0
'Bdex+) 'Efex) IE(cr) 'B2(ext) 'Efex) 'E(ex)
c'r
(X#)
(x#)
involving the configurations 13bl+3a{) and 13bI'-3a2) (see Table 11). This state gains x,y-polarized intensity through interaction with locally excited configurations, but the transition tends to overlap with the much stronger bands B and D. In the case of 3, the transition is predicted close to 47 000 cm-' and it is tempting to assign band E to it (Figure 7 ) . However, the assignment of the bands in this region is not straightforward; even saturated spirocyclic orthothiocarbonates absorb with medium intensity between 40 000 and 50000 cm-1,23indicating that transitions in this region are not necessarily of T--a* character. Spirenes of Type b. The LD spectra of 15 and 4 are shown in Figures 8 and 9. Calculated and observed transitions for 15, 16, 4, and 5 are compared in Tables V and VI. The spectra of the reference compounds are characterized by at least four main absorption bands below 50000 cm-I. The medium-intense bands A and B can be assigned to naphthalene-type 'Lb and 'La transitions, respectively, and involve primarily 4bl 5b, (A) and 3a2 5b, (B) promotions in the PPP model (see Figure 3). This assignment is supported by the LD data for 15 (Figure 8, Table V). The stronger bands toward higher energies must be assigned to several overlapping transitions. In the case of 15, the LD spectrum (Figure 8) reveals a "hidden" short-axis polarized band C below the strong long-axis polarized absorption between 40000 and 47 000 cm-I. This band can be assigned to a transition predicted at 42 200 cm-l involving essentially the 4bl 4a2 promotion. The long-axis polarized absorption in this region must be assigned to at least two electronic transitions D and E,
-
-
-
(23)Williams, D.R.;Coffen, D. L.; Garrett, P. E.; Schwartz. R. N . J . Chem. SOC.B 1968. 1 ! 32.
'E(cr) 'A,(cr+) 'A2(crt) 'E(ex) ' A I (ex-)
E
€3
(z)
30.8
34.1 'E(ex) 'B2(ex+) 40.6 44.2 43.0 43.3 43.7 43.7 44.9 45.4 29.9 33.1 37.7 40.7 41.2 42.9 45.2 45.9 48.8
3.9 5.3 4.3 1.0
4.5 4.1 3.7 5.2 4.0 4.2 4.4 3.0 2.3 4.6
H? 46.7 4.7 ( z ) 'B, In the case of 5, symmetry-forbiddentransitions are omitted from the calculated results. 'Polarization directions p (see Scheme I) determined by LD spectroscopy; entries in parentheses indicate calculated directions. cO-O transition.
-
which involve 3a2 4a2 and several other promotions. In the case of 16, the degree of complexity is even higher; unfortunately, the analysis is complicated by the absence of LD data for this compound. The bands A and B exhibit considerable fine structure. The LD curves for 15 (Figure 8) show that band B consists of two mutually perpendicularly polarized vibronic progressions, strongly reminiscent of the B bands of 13 and 2 (Figures 5 and 6). The origin close to 35 000 cm-' is short axis (y) polarized, consistent with the electronic symmetry of the transition, while the long-axis ( z ) polarized progression can be assigned to vibronic components involving nontotally symmetric vibrations and probably gaining intensity by vibronic coupling with the intense transitions at higher energies. The situation is very similar in the case of 4 (Figure 9) and seems to be characteristic for the ]La type bands of many benzene and naphthalene chromophores.20 The spectrum of the spiro compound 4 is quite similar to that of 15 (Figures 8 and 9), and the blue shifts of the bands observed when passing from 15 to 4 are generally much smaller than those encountered when passing from 13 to 2. This is understandable in view of the larger delocalization of the fragment wave functions in the case of 4. The most significant shift seems to be that of
Electronic S t a t e s 400
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2341
of Heterospirenes nm
250
300
350
TABLE VII: Observed and Calculated Transitions for 17 and 19 (See Footnote a of Table 111) obs calc compd t log c pa symmetry t log c 17 A 29.0 3.9 y 'Bz 29.7 3.9 B 31.3 3.6 z 'Al 30.8 4.4 'B2 35.3 3.4 IA, 38.9 2.7 'Bz 40.8 2.3 C 42.4 4.8 y IB, 44.1 4.9 'Al 48.4 2.2 'Al 48.8 3.6 D 51.8 4.5 (2) 'Al 51.0 4.5 'A, 53.4 4.8
A
l9 00000
25000
35000
40000
cm-l
45000
350
250
300
4.1
(z)
IAl
C
3.5
(y) (y)
IB2 'Bz
36.1
'Al
Figure 10. Reduced LD absorption curves for 1,3-dimethyl-2,3-dihydro-IH-perimidine (17) in stretched polyethylene at 77 K. Note that in 17 the effective orientation axis is t h e y axis. 400
t } 28.5 D
40.0
4.4
(y)
'B2 'B2 IA, 'A,
nm
'A,
E F
A
47.9 51.0
4.5 4.6
(y)
IB2
(2)
IA,
31'4 31.6 35.7 42.3 42.9 44.2 46.4 50.1 51.2 53.4 53.6
4'4 3.5 3.3 2.4 3.6 4.9 4.0 3.3 3.6 4.4 4.8
Polarization directions p (see Scheme 11) determined by LD spectroscopy; entries in parentheses indicate calculated directions.
n Figure 11. Reduced LD absorption curves for 1,1',3,3'-tetramethyI2,2'-spirobi[2,3-dihydro-lH-perimidine] (6) in stretched polyethylene at 77 K. The molecular symmetry of 6 is probably less than D2d; see the discussion in the text.
band C. This band is observed a t a b o u t 4 3 000 cm-' in t h e LD spectrum of 15 b u t is blue-shifted by 2000-3000 cm-' in t h e corresponding spectrum of 4 (where it is labeled E). This blue shift is well-predicted by the calculation and can be explained in part by a relatively large impact of t h e inductive effect on this transition, which is essentially 4bl 4a2 (large amplitude on the oxygen atoms in t h e occupied orbital 4bl, small amplitude in the unoccupied orbital 4a2; see Figure 3). T h e shape of t h e intense composite z-polarized band in t h e 4 0 000-47 000-cm-' region changes dramatically when passing from 15 to 4 the long-wave part of the band gains intensity relative
-
t o t h e short-wave part. This is in good agreement with t h e intensities of the calculated transitions (D and E for 15, C a n d D for 4; see Tables V and VI). T h e theoretical results indicate that the half-chromophore states D and E are strongly mixed by firstand second-order exciton coupling in 4, resulting in 'B2(ex+) states at 40600 and 44 200 cm-' (C and D) and IA,(ex-) states at 43 000 and 45 400 cm-I; a detailed analysis is presented in the literature.6 T h e spectra of t h e orthothiocarbonate 5 and t h e reference compound 16 a r e almost identical in t h e region of band A, b u t in the region of band B the situation is quite different. While band B of 16 shows pronounced fine structure, similar to band B of 15, t h e corresponding band of 5 is markedly diffuse a n d merges continually with t h e intense third band. Also, t h e B band of orthothiocarbonate 3 is quite diffuse (Figure 7). I t cannot be excluded that transitions other than m*transitions are significant in this energy region. As mentioned above, saturated spirocyclic orthothiocarbonates absorb in the near-UV region; in fact, weak
TABLE VIII: Observed and Calculated Transitions for 6 and 7 (See Footnote a of Table 111) obs caka*b compd t log c pa,c symmetry t log t 27.4 4.7 6 A 27.0 4.7 z 'B,(ex+) 29.4 3.9 B 27.9 4.4 x,y IE(ex) 32.0 'A I (ex-) 'E(ex) 36.2 3.3 IB,(cr+) 37.1 3.0 39.8 2.7 'E(ex) 40.0 2.7 'B,(ex+) 'A, (ex-) 40.1 41.0 3.9 )E(cr) 43.1 4.9 C 42.8 4.9 x,y IE(ex) 2.2 'E(cr) 43.2 7b
A} B
27.5
4.4
40.9
4.7
(x,y)
C?
D
(x,y)
'Bz(ex+) 'E(ex) IE(ex) 'B2(cr+) 'B,(ex) 'E(ex) 'E(ex) 'E(cr) IE(cr) )B,(ex)
30.7 31.6 36.2 38.8 42.7 43.2 44.0 45.2 46.2 46.3
4.7 3.5 3.3 1.7 2.6 3.7 4.8 1.8 4.2 4.3
the case of symmetry assumed; see discussion in the text. 7, transitions that are symmetry forbidden in the D2d point group are omitted from the calculated results. cPolarization directions p (see Scheme 1) determined by LD spectroscopy; entries in parentheses indicate calculated directions. transitions may occur significantly below 40000 ~ m - ' .In~ t~h e spiro compounds 3 a n d 5 these transitions m a y mix with F A * transitions a n d thereby gain intensity. T h e strong band of 16 a t 37 500 cm-I can be assigned t o t h e calculated transitions C and D, which correspond to the transitions D a n d C of 15, respectively. This band is red-shifted in t h e spectrum of 5, in good agreement with the theoretical results. In addition to the intense z-polarized transition C, which is red-shifted a s a result of exciton coupling, t h e calculation of 5 predicts numerous excited states between 4 0 000 and 50 000 cm-' (some of which a r e not indicated in Table V I ) . T h e 'E(cr) state E is predicted to gain considerable intensity by mixing with t h e near-degenerate IE(ex) state D. But in view of t h e complexity of t h e observed a n d calculated spectra, t h e lack of LD data, and the probable presence of non-r--P* states in t h e case of 5, a
2342 The Journal of Physical Chemistry, Vol. 94, No. 6,1990
detailed assignment of individual electronic states in the highenergy region is impossible at this time. Spirenes of Type c. The observed LD spectra of 17 and 6 are shown in Figures 10 and 1 1. Observed and calculated transitions for 17, 19, 6, and 7 are compared in Tables VI1 and VIII. As discussed in section 4, the six-membered heterocyclic rings in these compounds are probably nonplanar. This means that the molecular symmetry may be less than Cb (17, 18, 19) or D u (6,7). In particular, the K values observed for 6 indicate that the symmetry of this species deviates somewhat from DZd (section 4). Nevertheless, the theoretical calculations were performed under the assumption of effective C2, or D2d symmetries, and we shall discuss the excited states of these compounds within this assumption. In contrast to the case of 15 and 16 where transitions A and B are well-separated in energy, the first two electronic transitions in 17, 18, and 19 are essentially near-degenerate. In fact, they are not resolved in the liquid solution spectra, but the presence of two mutually perpendicularly polarized transitions around 30000 cm-' is evident from the LD spectrum of 17 (Figure 10). The near-degeneracy of these transitions which correspond to 'Lb and ,Latransitions of the naphthalene chromophore is well-reproduced by the theoretical results (Table VII) and tends to be characteristic for naphthalene derivatives with electron-donating substituents in the a-positions. In the high-energy region the spectra of 17, 18, and 19 seem more complex than those of 15 and 16. Generally, 1 &disubstituted naphthalenes have a higher density of states in this region than the corresponding 2,3-disubstituted ones; this trend has been previously discussed in detail.20 Numerous transitions are thus predicted above 40000 cm-l (Table VII). For 17 and 18, the relative intensities of the composite structures observed in this region differ considerably, possibly as a result of a larger deviation from planarity in the case of 18.10,24 A s indicated in Figure 3, the H O M O as well as the LUMO of these half-chromophore reference compounds has a2 symmetry, which means that both frontier orbitals are capable of spiro interaction. The impact of spiroconjugation on the low-energy absorption bands of type c spirenes should thus be relatively large, especially in the case of 6 where this interaction is comparatively strong. (The first two bands in the photoelectron spectrum of 6 are split by 0.7 eV as a result of spiro interaction.10) The theoretical results predict that the two low-energy transitions of 17, 'B2 (A) and ' A , (B), are both red-shifted in 6, primarily because of spiroconjugation. However, the predicted shift of the 'A, transition which is primarily 3a2(HOMO) 4a2(LUMO) is very much larger than that of the 'B2 transition, with the consequence that the order of resulting states is reversed; in other words, band A of 17 should correspond to band B of 6, and vice versa. This prediction is confirmed by the experimental LD spectra (Figures 10 and l l ) , which show that the bands A and B are y - and z-polarized, respectively, in the case of 17, but z- and xppolarized, respectively, in the case of 6. (Note that the effective orientation axis of 17 is they axis; the broken-line absorption curve in Figure 10 corresponds to the full-line curve in Figure 11, and vice versa.) The observed red shift of the z-polarized band (B in 17, A in 6) is of the order of 3000 cm-I, which compares well with the predicted shift. 3400 cm-'. The shift can be described largely as the result of interaction between the 3a2 4a2 type ]B2(ex+) configuration and the corresponding 'B,(cr+) configuration, which is predicted at relatively low energy. This interaction which is a consequence of spiroconjugation accounts for about 75% of the predicted shift; the remaining 25% is due to exciton coupling. This picture is simplified and does not account well for the predicted intensities which are influenced by small admixtures of high-energy B2 configurations. These contributions increase the intensity of the B2(ex+)transition (but insufficiently to explain the hyperchromic shift observed when passing from 17 to 6). The resulting intensity of the 'Bz(cr+) transition predicted at 37 100 cm-' is weak, and the transition is not observed in the experimental spectrum.
-
-
'
(24) Pozharskii. A . F.; Suslov, A. N.; Starshikov, N. M.; Popava, L. L.: Klyuev. U .4.; Adanin, V. A. Z h . Org. Khim. 1980. 16. 2216.
Spanget-Larsen et al.
30
calc.
40
1 0 3 ~ ~ 4
Figure 12. Correlation of observed and calculated wavenumbers for transitions A and B in 1, 2, 4, 9, 12, 13, and 15.
The intensity observed between 35 000 and 45 000 cm-' in the spectrum of 6 is predominantly short axis polarized and can easily be assigned to the several ' E states predicted in this region, including a fairly intense IE(cr) state at 41 000 cm-'. Apart from the red shifts discussed above, the shape and intensity of the A and B bands are significantly affected when going from 17 to 6 (Figures 10 and 11). The PPP calculations offer no simple explanation for these changes; but when two electronic states are near-degenerate like A and B, with completely overlapping absorption bands, effects due to vibronic coupling or even more severe breakdowns of the Born-oppenheimer approximation are likely to be important. Also, different deviations from the assumed idealized geometries may play a role. The spectra of the sulfur compounds 7 and 19 seem quite similar, at least below 45 000 cm-I. In contrast to the case of 6, spiroconjugation is rather insignificant in 7.1° This can probably be explained by the strong deviations from planarity of the heterocyclic rings which must be expected for these compound^.^^^^^ These deviations may also explain why the theoretical calculations based on idealized structures are only moderately successful in the case of 7 and 19 (Tables VI1 and VIII). Spirenes Containing Two Different Chromophores. The LD spectrum of 9 is shown in Figure 1, and observed and calculated transitions for 8,9, 10, and 11 are listed in Table IX. (For reasons indicated below, calculated data for 10 and 11 are omitted.) In these compounds, exciton interactions are second rather than first order and the calculated transitions tend to localize in one of the two chromophores as indicated in Table IX. Similarly, chargetransfer rather than charge resonance transitions are predicted, involving a net transfer of charge from one chromophore to the other. R-R* transitions to locally excited states are all symmetry allowed within the C2" point group and will be polarized along the x , y , and z axes, corresponding to trar.d:ions to lB1, lB2, and 'A, states, respectively. Charge-transfer (ct) states may belong to any irreducible representation of the C, point group; transitions to 'B', 'B2, and IA, ct states may gain intensity by spiroconjugation, but IA2 states are optically forbidden by symmetry. Because of the reduction in symmetry, the complexity of the computed spectra is considerably increased relative to the case of D2d spirenes. A particularly large number of transitions are predicted for 10 and 11, but the results are in poor agreement with the observed spectra, probably, at least in part, because of inadequacy of the assumed C2, symmetry. For this reason, we have chosen to exclude the numerous states computed for 10 and 11 from Table IX. The most precise experimental data are available for 9 for which a well-resolved LD spectrum could be measured (Figure 1). The ( 2 5 ) A d a m , W. J.; Bartell, L. S. J . Mol. Srrucf. 1977, 37, 261. (26) Kobayashi, M.; Gleiter, R.; Coffen, D. L.; Bock, H.; Schulz. W.; Stein. I-. Tetrahedron 1977, 33. 433.
Electronic States of Heterospirenes
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2343
TABLE IX: Observed and Calculated Transitions for Compounds 8-11 (SeeFootnote a of Table 111)
compd
i
8
E
} }
F" )
obs log e
32.4
4.0
38.9
4.1
42.8
4.7
31.5 35.OC 37.0e
3.6 3.7 3.7
G
9
A
B C
D
10
11
E
(43) 45.0
4.7
F
45.0
4.4
A
28.4
4.4
} C
30.0
4.3
D E F G
38.5 43.4 47.7 51.3
4.0 4.7 4.7 4.7
A
29.2
4.2
}
30.6
4.2
;}
36.4
4.0
43.5
4.9
G
48.3
4.6
C D
p4
cakb symmetry i
log z
30.9 34.7 35.6 39.1 39.3 39.5 42.9 43.1 45.1 46.4 49.3 49.1
3.1 3.3 4.0 3.9 2.7
30.5 33.9 35.0 42.1 42.4 42.1 43.9 44.5 45.3
3.4 4.0 3.5
4.0 5.0 3.9 3.7 3.6
3.8 5.0 3.9 4.5 3.3
'Polarization directions p (see Scheme I) determined by LD spectroscopy; entries in parentheses indicate calculated directions. bThe labels s and r indicate the localization of the transition in the "left" and "right" half-chromophore,respectively, referring to the molecular formula in Scheme I. cO-O transition. transitions A and B are easily assigned to transitions in the naphthodioxole moiety, polarized in the z and x direction, respectively, while transition C is assigned to a z-polarized transition in the benzodioxole moiety (Table IX). As expected, these transitions are more or less blue-shifted relative to the corresponding transitions in 13 and 15. The observed shifts of band A, B, and C are 700,0, and 1700 cm-l, respectively, in excellent agreement with the calculated shifts 800, 100, and 1500 cm-', respectively. According to the theoretical results, these shifts are due to inductive and field effects, as previously discussed for 2 and 4. The intense absorption of 9 in the 41 000-50000-~m-~region can be assigned to several transitions, polarized along all three axes and localized in both half-chromophores (Table IX). The lowest ct transition is predicted at 42 100 cm-' and corresponds to excitation of an electron from the benzodioxole HOMO to the naphthodioxole LUMO (Figure 3). Since the transition is forbidden by symmetry, it cannot easily be identified in the experimental spectrum; the short-axis polarized shoulder D around 43 000 cm-l can probably be assigned to the y-polarized transition in the benzodioxole fragment predicted at 42 400 cm-l (corresponding to band B of 13; see Figure 5). D is thus probably polarized perpendicular to the stronger short-axis polarized peak F which can be assigned to the x-polarized transition in the naphthodioxole fragment predicted at 44 500 cm-l (corresponding to band C of 15, see Figure 8). But since the orientation of 9 in stretched polyethylene is rodlike, it is not possible to distinguish between x- and y-polarized transitions in the LD spectrum.
In the case of 8, six states are predicted below 40 000 cm-l but the computed transitions are in poor agreement with the observed spectrum (Table IX). This can be explained in part by the prediction of too low transition energies for the phenylenediamine chromophore and too high energies for the benzodithiole chromophore. (This is evident by inspection of the results for 12 and 14 in Table 111.) The experimental spectrum of 8 consists of three broad bands with no fine structure; a possible assignment to the several calculated transitions is indicated in Table IX. The lowest ct states are predicted around 39 000 cm-I, but they are weak or symmetry forbidden and are thus likely to be hidden under neighboring much stronger transitions. The spectra of 10 and 11are of similar diffuse nature with broad overlapping band structures. Comparison with the spectra of the pertinent reference compounds indicates that the intensities of the low-energy bands of 10 and 11 are significantly increased. A similar increase is observed for 6 and 8. The results of the simple model calculation offer no explanation for these hyperchromic shifts; it cannot be excluded that geometrical factors are invol~ed.~,~~~~~ 6. Concluding Remarks The shifts of the observed electronic transitions of the heterospirenes relative to those of the pertinent half-chromophore reference compounds are in most cases well-reproduced by calculations in the PPP *-electron model and can be rationalized in terms of spiroconjugation, exciton coupling, and inductive and field effects. The transitions in the orthocarbonates 2, 4, and 9 are blueshifted as a result of the predominance of inductive and field effects, and a similar situation applies to the first two transitions of 1. Low-energy transitions in these compounds are described admiringly well in the PPP model (see Figure 12), provided that the Io parameter for the orthocarbonates is adjusted to account for the strong, inductive effect prevailing in these compounds. The shifts of the transitions in the orthothiocarbonates are generally small, but the intense long-axis polarized transitions in the high-energy region tend to be red-shifted, similar to the corresponding transition in 1, indicating the significance of exciton coupling. The observed bands are frequently broad and diffuse, possibly with contributions from transitions of other than PA* origin. As expected from the results of the previously published photoelectron spectroscopic investigation,I0 the most pronounced influence of spiroconjugation is found in the case of 6 where it is predicted to invert the order of the first two near-degenerate excited states. This result is consistent with the observed LD data, but the analysis is complicated by the presence of a symmetrylowering molecular distortion. Similar distortions probably occur in 7, 10, and 11. In no case was it possible to observe significant transitions to charge resonance or charge-transfer states. This is in keeping with the theoretical results which indicate that these transitions generally are weak and likely to be hidden under much stronger transitions. The hydrocarbons spiro[4.4]nonatetraene (i) and 9,9'-spirobifluorene (ii) so far remain the only spiro compounds where the influence of charge resonance transitions on the observed absorption spectrum is p r o f o ~ n d . ~ ~ ~ * * This investigation demonstrates the usefulness of LD spectroscopy in stretched polyethylene for complex organic chromophore~."-'~ The stretched sheet method is both simple and inexpensive, and the "reduced" spectral curves allow assignment of transition moment directions and reveal details that would have been otherwise unobserved. In addition, structural information can be extracted from the observed LD spectra. The orientation factors determined for the spiro compounds 1, 2,4, and 9 indicate a rodlike orientation distribution in stretched polyethylene and are consistent with the assumption of DZdmolecular symmetry (C, for 9). In contrast, the K values observed for 6 are compatible (27) Nelsen, S. F.; Grezzo, L. A,; Hollinsed, W. C. J . Org. Chem. 1981, 46, 283.
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J . Phys. Chem. 1990, 94, 2344-2347
only with a molecular symmetry less than D2d These results confirm previous suggestions that the half-chromophore fragments are essentially planar in the case of 1 but nonplanar in the case of 6.1°
Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the BASF Aktiengesellschaft in Ludwigshafen, and the Danish Natural Science Research Council for financial support.
Structures, Relative Stabilities, Barriers to Internal Rotation, and Vibrational Frequencies of Isomeric HNOH and HPOH Suk Ping So Chemistry Department, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received: August 2, 1989; In Final Form: October 10, 1989)
The geometries of isomeric HNOH and HPOH have been obtained at the SCF level by using various basis sets and their state energies have been corrected for electron correlation computed by MP3/6-3 1G**//6-31G** method. Vibrational frequencies for various isotopic isomeric HNOH and HPOH have been calculated. The trans conformers are predicted to be more stable than the cis conformers by 27.9 kJ mol-' for HNOH and by 6.1 kJ mol-' for HPOH. In contrast, reported MIND0/3 results indicate that cis-HNOH lies 23.0 kJ mol-] below rrans-HNOH. The cis and the trans barriers to internal rotation have been calculated to be 29.7 and 57.6 kJ mol-l for HNOH and 12.2 and 18.3 kJ mol-' for HPOH, respectively, and are all attractive dominant. In agreement with general experimental observations, the NO bond but not the PO bond has been found to have some double bond character.
Introduction Experimental studies of simple oxides and oxyhydrides of phosphorus are few. The combustion reactions of phosphorus and phosphorus-containing compounds are often accompanied by the emission of visible light. Species identified or suggested as giving rise to these emissions include PO, HPO, POz, HOPO, and (PO)z.1-5 The novel H 3 P 0 and H2POH isomeric species as well as HPO and phosphoric acid (HO)zHPO have been shown by infrared spectroscopy6 to be produced in the phosphine-ozone complex photolysis. Besides, HOOPO, H P ( 0 2 ) 0 , and metaphosphoric acid, HOP02, were also formed as secondary photolysis products in the reaction.6 Recently, Withnall and Andrew$ conducted an infrared investigation of argon/phosphine samples codeposited at 12 K with argon/oxygen samples passed through a microwave discharge. Their work confirms the earlier observation of PO and HPO by Lazilliere and J a c ~ x .It~ also provides an infrared spectroscopic evidence for the formation of additional species such as PO2, PO3, HOPO, P2OS,H3P0, and HPOH. The last five species were in fact observed for the first time. Among the various above-mentioned species, the structures of only PO, POz, and HPO have been determined experimentally.'"-12 ( I ) Fraser, M . E.; Stedmen, D.H. J . Chem. Soc., Faraday Trans. I 1983,
79, 527.
(2) Fraser, M. E.; Stedman, D.H.; Dun,T. M. J . Chem. SOC.,Faraday Trans. I 1984, 80, 285.
(3) Henchman, M.; Vigianno. A. A.; Paulson, J. F.; Freedman, A.; Wormhoudt, J. J . A m . Chem. Sot. 1985, 107, 1453. (4) Harris, D. G.; Chou, M. S.;Cool, T. A. J . Chem. Phys. 198582, 3502. ( 5 ) Hamilton, P. A.; Murrells, T. P. J . Chem. SOC.,Faruday Trans. 2 1985, 81, 1531; J. Phys. Chem. 1986, 90, 182. (6) Withnall, R.; Hawkins, M.; Andrews, L. J . Phys. Chem. 1986, 90. 575. (7) Whitnall, R.; Andrews, L. J . Phys. Chem. 1987, 91, 784. (8) Withnall, R.; Andrews, L. J . Phys. Chem. 1988, 92, 4610. (9) Lazilliere, M.; Jacox, M. E. NBS Spec. Publ. 1978, No. 561; J . Mol. Spectrosc. 1980, 79, 132. (IO) Butler, J. E.; Kawaguchi, K.; Hirota, E. J . Mol. Spectrosc. 1983, 101, 161. ( I I) Kawaguchi, K.; Saito. S.; Hirota, E. J . Chem. Phys. 1985.82, 4893. (12) Hirota, E.. unpublished work cited in: Lohr, L. L. J . Phys. Chem. 1984. 88, 5569
0022-3654/90/2094-2344$02.50/0
On the contrary, the geometries, energies, and vibrational frequencies of PO, POz, HPO, HOP (triplet state), PO3, H 2 P 0 , HOPO, HPO,, and HOPOz have been computed theoretically by using 6-3 1G plus polarization basis sets.I3J4 About 10 years ago, Loew et aLis performed MO calculations using the M I N D 0 / 3 method on a series of small molecules (namely NO, NO+, H3, and isomeric cations and neutral molecules of [H, N, 01and [H, H, N, 01)to explore ion-molecule reactions involving NO and NO+ which could lead to the formation of interstellar molecules. They found that the geometry-optimized but yet-unknown planar cis-HNOH was more stable than the trans conformer by 23.0 kJ mol-', contradicting the general observation that cis conformers are higher in energy owing to the positive steric effects. There has been no experimental or theoretical study on the structures and energies of HPOH reported. Withnall and Andrews8 thus suggested that cis-HPOH was more likely stable than its trans conformer by analogy to HNOH. Experimentally, trans-HONO has been foundi6to be lower in energy than cis-HONO by 2.1 f 0.8 kJ mol-I. However, ab initio calculation^^^^^^ on this molecule have shown that the energy difference AE (=Ecis- E,,,,,) between the cis and the trans conformers depends strongly on the levels of sophistication of theory and the basis sets used, with values ranging from -18.6 to + ] O S kJ mol-]. It is interesting to note that only the medium-sized economical 4-31G basis set gives the best agreement with experiment ( A E = +2.69 kJ mol-I). Thus, a theoretical study on the geometries, relative stabilities, barriers to internal rotation, and infrared spectra of isomeric HNOH and HPOH was carried out in the hope of getting an insight to their electronic structures as well as supplying some (13) Yabushita, S.; Gordon, M. S . Chem. Phys. Lett. 1985, 117, 321. (14) Lohr, L. L.; Boehm, R. C. J . Phys. Chem. 1987, 91, 3202, and references cited therein. (15) Loew, G. H.; Berkowitz, D. S.; Chang, S.Astrophys. J . 1978, 219, 458. (16) Cox Peter, A.; Brittain, A. H.; Finnigan, D. J. J . Chem. SOC.,Faraday Trans. 1971, 67, 2179. (17) Turner, A. G. J . Phys. Chem. 1985, 89, 4480. (18) Darsey, J. A.; Thompson, D. L. J . Phys. Chem. 1987, 91, 3168.
0 1990 American Chemical Society
