2328
J. Phys. Chem. 1980, 84, 2328-2335
reactions, it is not unreasonable to propose that Cantrell’s two exciplex geometries14bare derived from the two nearly isoenergetic singlet states of benzonitrile. If, in fact, the bent excited state is responsible for azetine formation (eq l),questions remain as to how the bent state is populated and why it reacts only with very electron-rich alkenes. It is possible that the two states exist in equilibrium and that they react in a highly selective fashion. Alternatively, perturbation of the benzonitrile T,T* state upon exciplex formation with an alkene may induce state mixing and thus populate the bent state. Huang and Lombardi2 observed that very weak electric fields induce mixing of the T,H* and perturbing states. Aryl nitrile/ alkene exciplex stability and lifetime is known to increase with alkene electron d o n i ~ i t y . l ~Thus ~ J ~only alkenes capable of forming long-lived exciplexes with benzonitrile yield nitrile adducts. While there is ample precedent for fluorescence from two nearly isoenergetic singlet states,17we are unaware of previous postulates of divergent chemical reactions occuring from two such states. In the following paper we present quantitative experimental evidence for the involvement of two singlet states in the photochemical cycloaddition reactions of 1-naphthonitrile.
Acknowledgment. Support of this work by the National Science Foundation (CHE-7801120) is gratefully ac-
knowledged. We thank M. A. Ratner for encouragement and assistance with the molecular orbital calculations.
References and Notes (1) C. J. Seliskar, 0. S. Khalil, and S. P. McGlynn, “Excited States”, Vol. I, E. C. Lim, Ed., Academic Press, New York, 1974, p 265. (2) K.-T. Huang and J. R. Lombardi, J . Chem. Phys., 55, 4072 (1971). (3) J. E. Bloor, B. R. Gilson, and D. D. Shillady, J. Phys. Chem., 71, 1238 (1967). (4) J. Del Bene and H. H. Jaffe, J. Chem. Phys., 49, 1221 (1968). (5) W. von Niessen, Theor. Chim. Acta, 33, 185 (1974). (6) K. KroghJespersenand M. Ratner, J. Chem. Phys., 65, 1305 (1976). (7) G. M. Schwenzer, S. V. O’Niel, H. F. Schaefer, C. P. Baskin, and C. F. Bender, J. Chem. Phys., 60, 2787 (1974). (8) 2 . R. Grabowski, K. Rotkiewicz, A. Siemiarceuk, D. J. Cowley, and W. Bauman, Nouv. J . Chim., 3, 443 (1979). (9) J. A. Pople, D. L. Beveridge, and P. A. Dobosh, J . Chem. Phys., 47, 2026 (1967). (10) G. W. King, and A. A. G. vanputten, J . Mol. Spectrosc., 42, 514 (1972). (11) K. Rotkiewicz, K. H. Grellmann, and Z. R. Grabowski, Chem. Phys. Lett., 19, 315 (1973). (12) 0. S. Khalll, J. L. Meeks, and S. P. McGlynn, Chem. Phys. Lett., 39, 457 (1976). (13) (a) D. J. Cowley and A. H. Peoples, J. Chem. SOC.,Chem. Commun,, 352 (1977); (b) W. Rettigand and V. Bonacic-Koutecky, Chem. Phys. Lett.. 62, 115 (1979). (14) (a) T. S. Cantreli, d . Am. Chem. Sac.,94, 5929 (1972); (b) J . Org. Chem., 42, 4238 (1977). (15) F. D. Lewis, Acc. Chem. Res., 12, 152 (1979). (16) W. R. Ware, D. Watt, and J. D. Holmes, J . Am. Chem. Soc., 96, 7853 11974). (17) N. J. T h o , V. Ramamurthy, W. Cherry, and W. Farneth, Chem. Rev., 78, 125 (1978).
hotochemical Cycloaddition of Two Nearly Isoenergetic Singlet States of 1-Naphthonitrile’ Frederick D. Lewis*2 and Bruce Holman, 111 Department of Chemistry, Northwestern University, Evanston, Illinois 6020 1 (Received: September 12, 1979; In Final Form: January 3 1, 1980)
Irradiation of 1-naphthonitrile in the presence of 1,2-dimethylcyclopenteneresults in the formation of 2 + 2 cycloadducts with both the nitrile group and aromatic ring. Both types of addition occur via singlet 1naphthonitrile. The ratio of nitrile/ring addition in ethyl ether solution is dependent upon dimethylcyclopentene concentration, oxygen concentration,and excitation wavelength. These results require kinetically nonequivdent singlet-state precursors for nitrile vs. ring addition. With the aid of a recent fluorescence polarization study by Suzuki, we assign ring adduct formation to the lL, state and nitrile adduct formation to either the l L b state or a hidden state with bent nitrile geometry. This assignment provides the first example of divergent reaction pathways for two nearly isoenergeticsinglet states. The chemical behavior of the I L b vs. ‘La states is discussed in terms of the electronic description of these states provided by a molecular orbital (INDO/§) calculation. The results and conclusions of the present investigation are shown to be compatible with the extensive photophysical investigations of the 1-naphthonitrile/1,2-dimethylcyclopentenesystem by Ware and co-workers. Introduction
Examples of fluorescence and chemical reactions from upper excited states of organic molecules in solution are becoming increasingly ~ o m r n o n .Fluorescence ~ from second singlet states has been reported both for molecules with large Sz-S1energy gaps (e.g., azulene, thiones) and small S2-Sl energy gaps (e.g., 3,4-benzopyrene, 1,2ben~operylene).~ In the former case slow Sz-S1 internal conversion is necessary for the observation of Sz fluorescence, whereas in the latter case thermal population (or repopulation) of S2 and S1is necessary for the observation of S2 fluorescence. The wavelength dependence of photochemical reactions can be used to provide evidence for
S2reactions when S2-S1 internal conversion is slow, but not when it is rapid. Wagner4 and Steel6 have presented convincing evidence for upper triplet reactions of aryl ketones in which a reactive n,T* T, state is populated thermally from an unreactive H,R* T1 state. However, there is to date no documented example of a photochemical reaction occurring from a thermally populated singlet state in solution. Several years ago Huang and LombardP reported the observation of electric-field-induced perturbation of the first singlet of benzonitrile due to a nearly isoenergetic singlet state, which is not observed spectroscopically. In the preceding paper we presented evidence that this per-
0022-3654/80/2084-2328$0 1.00/0 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 18, 7980 2329
Singlet States of l-Naphthonitrile
TABLE I: Solvent Dependence of 1-NN Fluorescence Yield and Lifetime 107x
solvent none (vapor) hexane
1.0 1.9
toluene diethylether
2.4 4.3
tetrahydrofuran methanol
7.6
acetonitrile l-methylnaphth ylene anthracene
Figure 1. Correctled fluorescence and fluorescence excitation spectra of M l-naphthonitrile in hexane (---), fer!-butyl alcohol (-), acetonitrile (---).
and
turbing state is a n,,,n * state with bent nitrile geometry. Recently, Suzuki ancf co-workers' have reported that several naphthalene derivatives including l-naphthonitrile (1-NN) fluoresce from both 'Lb and 'La states (Platt's notation8), the dominant emission being dependent upon solvent and temperature. We were struck by the possibility that the presence of nearly isoenergetic singlet states in benzonitrile and 1-NN might offer a clue to the unusual photochemical behavior of these r n ~ l e c u l e s .Cantrellg ~ ~ ~ ~ and Yang et al.1° have 2 cycloaddition of benzo- and nareported that 2 phthonitriles with certain electron-rich alkenes yields novel l-azetine products as well as the more common ring adducts (eq 1, 2).L'r12 We report here our investigations of
+
CN
the chemical behavior of singlet 1-NN with 1,2-dimethylcyclopentene (CP). The photophysics of the lNN/CP exciplex has been studied in great detail by Ware and co-worker~;'~ however, the chemistry of this system has not previously been reported. The results of our investigation are indicative of ring addition occurring from the 1-NN 'La state and nitrile addition occurring from either the 'Lb state or a hidden state with bent nitrile geometry. Spectroscopic and computational characterization of the nearly isoenergetic 'Lb and 'La states of 1-NN is presented prior to the photochemistry of the l-NN/CP system.
Results and Discussion Spectroscopy lof 1-Naphthonitrile. The solution phase fluorescence and fluorescence excitation spectra of low5M 1-NN in hexane., tert-butyl alcohol, and acetonitrile are shown in Figure 1. The two lowest energy absorption maxima (322 and 317 nm in hexane) are assigned to a 'Lb lA transition on the basis of the observed frequency ~ the recently reported separation (ca. 500 ~ r n - l )and magnetic circular dichroism spectrum.lq The maxima at 308, 295, 283, aijtd 273 (sh) nm constitute a vibrational progression with the same frequency separation (ca. 1400
-
E
32.7 37.5
r , ns
26.5 19.1. 18:3a 17.5b 15.1, 15.0a 9.6, 9.g. 8.2, 9.Ba 9.0a 67' 4.9'
@f
iovx
k f , s-' k d , S'
0.30
1.6
3.7
0.2gb 0.32
1.6 2.1
4.1 4.5
0.37
3.9
6.6
0.54
6.6
5.2
0.47 0.25'
5.2 0.37
5.9 1.1
0.36'
7.3
Data from ref 17a. a Data from ref 13c. ref 1 6 for cyclohexane solutions.
13.1
' Data from
cm-') as the second singlet (lLJ of n a ~ h t h a l e n e . ~While J~ the prominent absorption maxima are insensitive to solvent polarity, the onset of the ILa band shifts to longer wavelength with increasing solvent polarity from E = 1.9 (hexane) to 12.5 (tert-butyl alcohol), partially obscuring the weaker 'Lb band. Further increases in solvent polarity have little effect on the appearance of the absorption spectrum. Scrutiny of Figure 1 reveals that the fluorescence spectrum of 1-NN is virtually the same in polar and nonpolar solvents except for the presence of the relatively sharp 322- and 328-nm maxima in nonpolar solvent. The intensity of these maxima vs. the broad maxima at 331 and 338 nm decreases continuously with increasing solvent polarity and increases with decreasing temperature. The 322-nm maximum cannot be detected in l-butanol solution at room temperature, but is well resolved at -84 "C. The 322- and 328-nm maxima bear a mirror image relationship to the two lowest energy absorption maxima and display the same frequency separation as the highly structured fluorescence of the lowest singlet state ('I+,) of naphthalene (ca. 500 cm-').16 Suzuki et al.7have assigned the structured 322-nm fluorescence to the lLb state and the broad, longer wavelength fluorescence to the lLa state on the basis of differences in the fluorescence excitation polarization spectra. Adopting this assignment, the temperature dependence of the fluorescence spectrum provides an energy separation between the 'Lb (lower energy) and 'La (higher energy) states of 0.8 f 0.2 kcal/mol in both hexane and diethyl ether solution. Singlet lifetimes (7)and fluorescence quantum yields (af)for 1-NN in several solvents are summarized in Table I. Ware et aI.l3 report single exponential decay in all solvents and temperature-independent lifetimes in hexane solution over the range -40 to 45 "C. The concomitant decrease in T and increase in af with increasing solvent polarity results in a fourfold increase in the observed fluorescence rate constant (h,) and a smaller increase in the nonradiative decay rate constant (hd).13cJ7a Also included in Table I are parameters for the lowest singlet state of l-methylnaphthalene (%b) and anthracene ('La). The 1-NN parameters for polar solvents are similar to those of anthracene, supporting the assignment of ILa 1-NN fluorescence in polar solvents. The 1-NN parameters in nonpolar solvents are intermediate between those of anthracene and l-methylnaphthalene and are consistent with dual lLb, lLa fluorescence in nonpolar solvents. The fluorescence of 1-NN has previously been assigned to the 'Lb state with solvent effects attributed to strong vibronic
2330
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
Lewis and Holman
TABLE 11: Calculated and Experimental Properties of l-NNa
energy, eV INDO/S LCAO-SCF~ exptl oscillator strength INDO/S ( r ) INDO/S ( v ) LCAO-SCF exptld dipole moment, D INDO/S exptle a
0
3.94 3.95 3.84
4.30 4.06 4.22
5.32 5.66 5.49
0.006
0.30
0.001
0.05
0.030
0.42
1.66' 0.59' 1.4' 0.9'
3.85
3.05
0.15 4.07 4.16
4.28
This work except as noted.
I
V_aluesfrom-ref 19. states. d Values
I- 8.0911
' Sum of values for overlapping S,-S, from ref 14.
e
coupling with the IL, ~ t a t e , The ~ ~ operation ~ J ~ of a solvent effect on vibronic coupling only for solvents with dielectric constants < 10 D would be fortuitous indeed. We concur with the dual 'Lb, IL, assignment of Suzuki et al. and find it to be consistent with the photochemical kinetics of the l-NN/CP system (vide infra) as well as the fluorescence behavior of 1-NN. Molecular Orbital Calculations. The screened INDO (INDO/S) model developed by Krogh-Jespersen and R a t n e P for the analysis of optical spectra and excited state properties of organic molecules has been applied to 1-NN. The results given in Table I1 are for INDO/S calculations employing extensive configuration interaction (49 lowest singly excited configurations) parameterization identical with that used by Krogh-Jespersen and R a t n e P for INDO/S calculations on benzonitrile, and the atomic coordinates used by Suzuki et al.19 The results of a LCAO-SCF calculation by Suzuki et al.I9 employing more limited configuration interaction are included for purposes of comparison. Both calculations provide excitation energies for the first three singlet states of 1-NN which are in good agreement with experimentalvalues. Experimental values were assigned by Suzuki et al.19from band maxima and probably overestimate the 'La and Bb energies. Oscillator strengths were calculated by the use of dipole length ( r ) and dipole velocity (V) operators. The former method tends to overestimate and the latter method to underestimate experimental values.18120The overlap of 'Lb and IL, absorption bands prevents measurement of separate experimental values. Clearly, the oscillator strength of the lL, transition is much lower than that of the 'Lb transition, the value for which lies between the values calculated by INDO/S. The calculated and experimentalz1 values for the dipole moment of ground state 1-NN are in excellent agreement. The calculated dipole moments of the and lL, states are 5% larger and smaller than that of the ground state, thus accounting for the insensitivity of the energy of the absorption and fluorescence maxima to solvent polarity. The dipoles of both the ground and lLb states are aligned within a few degrees of the naphthalene short axis, whereas the dipole of the IL, state is aligned at an angle of 29' from the short axis and away from the nitrile. Atomic orbital coefficients and energies for the two highest occupied molecular orbitals (SHOMO and HOMO) and two lowest unoccupied molecular orbitals (ILJMU and SLUMO) of 1-NN are shown in Figure 2. The major configuration involved in the IL, lA transition is the LUMO HOMO (93%). The 1-cyano substituent extends conjugation in the direction of polarization causing
-
HOMO
Value from ref 21.
+-
Atomic orbital coefficients and energies (eV)for the frontier molecular orbitals of I-naphthonitrile. Figure 2.
a bathochromic shift; however, perturbation of the naphthalene orbital coefficients is minimal. Application of frontier molecular orbital (FMO) theory leads to the prediction of 2 + 2 cycloaddition to the aromatic ring (eq 1). The principal configurations contribution to the 'Lb 'A transition are HOMO SLUMO (56%) and SHOMO .+ LUMO (41%). Inspection of the orbitals involved does not allow unambiguous prediction of the locus of cycloaddition; however, the nitrile ring charge transfer character of this transition is reflected in its calculated dipole. In view of the results for benzonitrile described in the preceding paper, it is possible that bending the nitrile group might alter the frontier orbitals so as to favor nitrile addition. Photochemical Cycloaddition of 1-Naphthonitrile with 1,2-Dimethylcyclopentene.Irradiation of 1-NN with CP in nonpolar solvents results in the formation of three cycloadducts (eq 3, quantum yields are limiting values for
-
-
-
1-NN
0.017
M
Ia (0.088)
CP
h
Ib (0.032)
11(0.10)
degassed hexane solutions irradiated to
