Photophysics and photochemistry of syn- and anti-dipyrido-substituted

---

J . Phys. Chem. 1987, 91, 1366-1370

1366

Photophysfcs and Photochemistry of Syn- and Anti-Dipyrido-Substituted Tetraazapentalenes: Selective Photooxidation Involving Singlet Oxygen Arthur M. Halpern,* Christopher J. Ruggles, Xing-kang Zhang, Department of Chemistry, Northeastern University, Boston, Massachusetts 021 15

Michael P. Groziak, and Nelson J. Leonard Roger Adams Laboratory, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61 801 -3731 (Received: September 16, 1986)

The spectroscopic and photophysical properties of dipyrido[ 1,2-a:2’,1’-~-1,3,4,6-tetraazapentalene (3), with syn substitution on the central ring system, and the anti isomer, dipyrido[1,2-a:1’,2’-e]-1,3,4,6-tetraazapentalene (4). are described. Fluorescence lifetimes and quantum efficiencies are measured in acetonitrile, benzene, water (pH 7 phosphate buffer), and cyclohexane. Phosphorescence is observed only for 3 in EPA at 77 K. The assignment of the absorption spectra of 3 and 4 is performed with HAM/3 calculations. In cyclohexane solution, the lowest-lying electronic singlet states in 3 and 4 are assigned as ‘B2 and 3BU,respectively. For 3, the radiative rate constant (k,) increases with increasing solvent polarity while the nonradiative rate constant (k”,)decreases. For 4, k, is solvent independent (1.2 X 10’ s-I), and k , decreases with increasing solvent polarity. For both isomers and in all solvents studied, experimental k, values are in excellent agreement with those obtained from the Strickler-Berg relation. Compound 3 is photochemically labile in aerated solution; irradiation depletes the ground state and produces a fluorescent product (or products) whose absorption and fluorescence are red shifted. Isomer 4 does not show this behavior. This process is assigned as photooxidation involving the addition of singlet oxygen (‘Ag) to the ground state of 3.

Introduction There exists considerable interest, both synthetic and theoretical, in the class of compounds called aromatic azapentalenes.’ These are, generally, heteroaromatic analogues of the antiaromatic pentalene (1). Because they contain a 10 T-electron system, these

2

1

1

3

2

4

compounds are considered to be aromatic and are isoelectronic with the pentalene dianion (2). W e describe in this paper the spectroscopic, photophysical, and photochemical properties of an unusual and interesting pair of syn and anti isomers of the dipyrido-substituted, symmetrical 1,3,4,6-tetraazapentalene ring system. These compounds are dipyrido[ 1,2-~:2’,1’4-1,3,4,6(1) (a) Elguero, J.; Claramunt, R. M.; Summers, A. J. H. Adu. Heterocycl. Chem. 1978,22, 183-320. (b) Gimarc, B. M. J . A m . Chem. SOC.1983,105, 1979.

tetraazapentalene (3), the syn isomer, and dipyrido[ 1,2-a:1’,2’e]-l,3,4,6-tetraazapentalene, the anti isomer (4), the synthesis and properties of which have been reported recently.*s3 Previously, the synthesis of 1,3,4,6-tetraazapentaleneitself (5) was reported,4 and the only related dibenzo-substituted systems are zwitterionic compounds in which the two bridgehead positions (3a and 6a in 5) are heterosub~tituted.~ Particular interest in the photophysics and spectroscopy of 3 and 4 arises from the determination of the effects of the extrachromophoric (bis) pyrido substituents on these symmetrical molecules (they belong to point groups C,, and CZh,respectively, vide infra). For the anti isomer, which is centrosymmetric, the question arises as to the positions of the excited ‘A, states and hence the spectroscopic and photophysical implications vis-a-vis the syn isomer. Moreover, in view of the property of these fluorescent compounds to bind or intercalate with DNA,3 it is desirable to characterize the effects of solvent polarity on their absorption and fluorescence spectra and, as well, on their emission lifetimes and efficiencies.

Results and Discussion Absorption and fluorescence spectra of compounds 3 and 4 in acetonitrile and cyclohexane solution are shown in Figures 1 and 2. It is evident that the effect of solvent polarity on the position and shape of these spectra is very small. Moreover, absorption and emission spectra similar to those observed in acetonitrile are seen in protic media, ethanol, and water (pH 7 phosphate buffer). Aside from a small red shift in the spectra of 3 and 4 in cyclohexane relative to acetonitrile (ca. 870 and 530 cm-’, respectively), there is a significant increase in the sharpness of the vibronic (2) Cruickshank, K. A,; Sumoto, K.; Leonard, N . J. Tetrahedron Left. 1985, 26, 2123.

(3) (a) Groziak, M. P.; Leonard, N . J. Book of Abstracfs, 191st National Meeting of the America1 Chemical Society, New York, NY; American Chemical Society: Washington, DC, 1986; Abstract No. 269. (b) Leonard, N . J. Abstracts, 10th International Congress of Heterocyclic Chemistry, Waterloo, Ontario, Canada, Aug 11-16, 1985; pp S6-33. (c) Groziak, M . P.; Wilson, S. R.; Clauson, G. L.; Leonard, N . J . J . Am. Chem. SOC.1986, 108. 8002. (4) Ferris, J. P.; Antonucci, F. R. J. Am. Chem. SOC.1974, 96, 2014, and references cited therein. (5) Carboni, R. A,; Kauer, J. C.; Castle, J. E.; Simmons, H. E. J . Am. Chem. SOC.1967,89, 2618.

0022-3654/87/2091-1366$01.50/00 1987 American Chemical Society

The Journal of Physical Chemistry, Vol. 91, No. 6, 1987

Substituted Tetraazapentalenes

,

,

:4

WAVELENGTH,

fi:,

NM

2 p

3j)O

1367

TABLE I: Spectroscopic and Photophysical Data for 3 and 4 Tfr

isomer solvent 3 watere acetonitrile benzene cyclohexane watere acetonitrile benzene cyclohexane

4

4

I

4 I

M

4

W

bf ns 0.97 4.94 0.81 6.1, 0.67 0.24

k,,“ k,b k,’ 0.06 1.96 0.31 1.31 1.6

5.38 0.61 5.23 1.45

0.54

4.40

0.48

4.10 1.2,

1.2,

0.47 3.48 0.21

1.1,

1.52 1.31 4.1

0.39 0.26 0.25

0.46 0.43 0.061 1.23 1.1, 1.37

1.00

1.7

f

1.1 1.2 1.0

0.27 0.22 0.21 0.16

Ok,, a (1 - @f)/Tf; units in IO8 s-I, *k, = @ f / ~ f ; units in s-I. Calculated from the “Strickler-Berg” relation; see ref 6. dDetermined from f = 4.32 X 10-9s~d’v for the S , So transition. cpH 7 phosphate buffer.

-

is, of course, obscured by solvent absorption. For both 3 and 4, prominent and relatively narrow 0-0 bands (fhwm 300 cm-I) are observed in absorption and fluorescence in cyclohexane, and E,, values (the mean absorption and fluorescence 0-0 maxima) are at 26 500 and 23 950 cm-’, respectively. It is immediately apparent that the lowest-lying transition in 4 is one-photon allowed, and hence S1 is not ‘A,. The fluorescence spectra of 3 and 4 can each be interpreted in terms of progressions in two frequencies, 1580 and 940 cm-’ for 3,and 1470 and 560 cm-I for 4. A vibrational analysis of the SI So spectra of 3 and 4 also reveals progressions in two frequencies: 1500 and 1050 cm-I for 3,and 1430 and 590 cm-’ for 4. As would be expected for transitions between states of similar geometry, these excited-state frequencies (for both compounds) are close to the respective values observed in fluorescence (Le., the corresponding ground-state frequencies). In an EPA glass at 77 K, absorption and fluorescence spectra are considerably sharper relative to 295 K and reveal resolvable vibrational features. For 3, E , is at higher energy relative to cyclohexane solution at 295 K (27 760 cm-I), although the respective Franck-Condon patterns of the SI So transitions in the EPA glass are similar to those observed in cyclohexane at 295 K. Again, vibrational progressions in two frequencies can be discerned: 1530 and 850 cm-’ (absorption) and 1370 and 1000 cm-I (fluorescence). At longer wavelength, weak emission (0,015 relative to fluorescence) is also observed, the strongest and lowest energy component of which is at 20000 cm-I. Although the emission lifetime was not determined, this feature is assigned as the TI So origin. The remaining bands in the long wavelength spectrum can be analyzed in terms of two frequencies: 1530 and 820 cm-I. Under these low temperature conditions, the 0-0 band of 4 is noticably split, both in absorption and emission be ca. 380 and 430 cm-l, respectively, and might indicate the presence of a hydrogen-bonded species. Both spectra also exhibit vibrational progressions in similar frequencies, 1480 and 1000 cm-I in absorption and 1440 cm-’ in fluorescence. No long wavelength emission could be observed for 4. In order to determine the radiative and nonradiative rate constants, fluorescence lifetime and efficiencies of 3 and 4 were measured in the solvents mentioned above. The results are assembled in Table I. The fluorescence efficiency of 3, the polar isomer, shows a stronger solvent dependence than 4 which is nonpolar. The nonradiative rate constant of 3 is even more strongly solvent dependent than k, but in the opposite sense. For example, while k, decreases by about fourfold from water to cyclohexane solution, k,, increases by a factor of ca 140 (from 6 X lo6 to 1.4 X lo8 s-I) in these two solvents. The trend is k, is paralleled by changes in the SI So oscillator strength which decreases by ca. 6 in these two solvents. Apparently dipole coupling to the solvent in 3 (which presumably also involves hydrogen bonding in water) assists electric dipole-induced transitions (SI So), but retards nonradiative deactivation. The possibility of state (SI)inversion in polar/nonpolar solution is discussed below. That there is fluorescence enhancement in the polar solvents without an accompanying changes in the fluorescence spectrum is unusual, and

-

-

40 50 103 m-1 Figure 1. Absorption (-) and fluorescence spectra of 3 in degassed acetonitrile solution (upper) and cyclohexane (lower) at 295 K.

30

20

WAVENUMBERS,

(..e)

WAVELENGTH,

500

400

NM

200

250

300

-

>

-

t:

W z

r W

z w u m W

I .

0 K -1

WAVENUMBERS, 103 CM-1 Figure 2. Absorption (-) and fluorescence (. .) spectra of 4 in degassed acetonitrile solution (upper) and cyclohexane (lower) at 295 K.

.

features in absorption and fluorescence in the nonpolar solvent. Solvation by cyclohexane is very weak; e.g., the solubility of 3 is about 2 X 10” M at 295 K. The solubility of nonpolar 4 is slightly M. It thus appears that cyclohexane acts as larger, ca. 4 X a Spolskii medium and, aside from a presumably small polarizability effect, the spectra probably approach gas-phase characteristics. In benzene solution, spectra intermediate between those in Figure 1 are observed; however, information below ca. 280 nm

+-

-

1368 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987

the mechanism responsible for this effect is unclear. Because 3 is photostable (in rigorously degassed solution), it appears that the dominant nonradiative process in SI is the return to the ground state directly via internal conversion, or through an intermediate triplet state (or states). As mentioned above, the radiative rate constant for 4, the anti isomer, is rather solvent insensitive ( k , = 1.2 X 10' s-I); accordingly, the oscillator strength of the SI So transition is nearly constant in the solvents studied, Le., 0.2. For this isomer, there is only a slight solvent dependence, however, of knr;it increases by about fourfold from water to cyclohexane. Table I also contains values of the radiative rate calculated from the relation developed by Strickler and Berg6 for electronically allowed transitions between similar potential energy surfaces which are not too widely displaced from each other. The SI So transitions shown in Figure 1 for 3 and 4 clearly satisfy these criteria, and it is gratifying, but not too surprising, that agreement between the measured and calculated k, values is so close. It is worth commenting that the good agreement also gives credence to the absolute fluorescence quantum efficiencies (and lifetimes) reported in Table I. Thus, the value of & = 0.55 for quinine sulfate in aerated 1.O N H2S04,the standard used in these measurements,' is nicely, although indirectly, confirmed. A more detailed assignment of the electronic transitions in 3 and 4 was attempted with the use of HAM/3, a semi-empirical MO-SCF method developed by Lindholm and co-workers.' Configuration interaction based on 20 microstates was performed. The fully optimized geometries used for these calculations were obtained from M N D O cal~ulations;~ all bond lengths and bond angles were varied in the search for the energy minimum. The structures thus obtained are in good agreement with those obtained from the X-ray data reported by Groziak et aL3 The results indicate, for example, that both compounds are planar and that the peripheral C-C bond lengths in the six-member rings alternate (1.37 and 1.44 A). The latter characteristic may be relevant in connection with the photochemical properties of 3 and 4 (vide infra). These isomers belong to point groups Cz0and CZh,respectively. Although the H A M / 3 results indicate electronic transitions which are too low (by ca. 0.3 eV) compared with the observed spectra of 3 and 4, the general patterns revealed by these calculations are nevertheless useful in suggesting assignments. The calculations predict, for example, that the lowest transition in 3 is IB2 'A, and is 1500 cm-I higher in energy than the lowest 'Al, The lowest transition in 4 which is dipole allowed 'B, transitions in 3 and 4 are both observed to be one-photon allowed (see Figures 1 and 2), the former being 2550 cm-' higher in energy than the latter. The H A M / 3 results further indicate that, in 4, the first three allowed transitions (r,r*), each to 'B, states, have oscillator strengths of 0.35,0.11, and 2.0. The observed transitions, having origins a t 24 000, 3 1 700, and 36 500 cm-I, correspond reasonably well to this prediction (see Figure 2). Moreover, within this energy range, the calculation shows two one-photon-forbidden 'A,-IA, transitions and one very weakly allowed cf = 0.008) ~A,-'A, (n,r*) transition. Because of its lower symmetry in 3, the absorption spectrum is more complex than that of 4. The HAM/3 results indicate two transitions, the lowest to a IB2 state and nearby, low-lying I(r,r*) the next, separated by only 440 cm-I, to an 'Al state. The calculated oscillator strengths of these transitions are 0.05 and 0.25, respectively. In addition, the calculations, when performed on slightly different geometries for 3, show a considerable variation in the relative positions of these transitions, IAl 'Al, however,

Halpern et al. WAVELENGTH, NM

60C

!OC

403

500

I

I

.

+ -

-

-

-

+ -

(6) (a) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814. (b) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; pp 87-88. (7) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229. (8) (a) Asbrink, L.; Fridh, C.; Lindholm, E. Chem. Phys. Lett. 1977, 52, 72. (b) Chong, D. P. J. Mol. Sci. 1982, 2, 5 5 . (c) Asbrink, L.; Fridh, C.; Lindholm, E.; Chong, D. P. QCPE No. QCMP 5 . (9) (a) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. (b) Stewart, J. J. P. QCPE No. 455.

I

25

??

WL\.ElrUMEEPS,

J

I

30

7

,:

UC

1iI3

Figure 3. (-)

Absorption spectra of 3 (3.0 X M) in aerated acetonitrile solution a t 295 K irradiated a t 312 nm. Irradiation times: (A) 0 min, (B) 5 min, (C) 10 min, (D) IS min. Fluorescence spectrum of photoproduct (after 1 5 min) excited a t 436 nm. (.-e)

always being more intense. The fourfold decrease in the oscillator strength of the SI So transition of 3 in cyclohexane relative to water may result from an inversion of the ]A, and IB2 states in these two solvents. It is possible that hydrogen bonding sufficiently distorts 3 to stabilize the 'A, state relative to IB,. With reference to the observed spectrum of 3, it is possible to identify the feature at 27 590 cm-l (in cyclohexane) either as the 'Al transition (about 1000 cm-' higher in origin of the ]Al energy than the lowest transition origin) or as the first member of a 1000-cm-' progression. On the basis of the Franck-Condon pattern observed in the fluorescence spectrum of 3, however, where the first intense feature after the origin belongs to a ca. 1580-cm-' progression, we suggest that the 27 590-cm-I band is the origin of a nearby electronic transition. Without knowing the polarizations of these transitions, a more definitive assignment cannot be sustained. The remaining members of the absorption bands in this region can be analyzed in terms of superimposed progressions in ca. 1500 and 1050 cm-'. The calculations reveal four additional, but weaker, transitions at higher energies; the stronger of these (f 0.01) might correspond to the feature at 3 1 800 cm-'. The very weak (f 0.005) 'A, transition is expected to lie in this vicinity. l(n,r*) 'B, Above this region, two strong (f=0.25) and one intense (f= 1.0) ,A,, are transitions are predicted. The former two, both 'B, 'Al. As can be separated by only 700 cm-'; the latter is 'Al seen in Figure 1, the absorption spectrum of 3 reveals a number of higher lying transitions, and several suggestions can be made regarding their tentative assignments. the features at 35 100, 36 520, and 41 940 cm-' might be the origins of the three strongly allowed transitions just discussed. The assignment of these features as electronic origins is clearly complicated by the involvement of a vibrational progression in ca. 1500 cm-I in the spectra of 3. Finally, we discuss the interesting photochemical properties of 3 and 4. A solution of 3 in aerated acetonitrile or benzene solution rapidly turns bright yellow; in addition, the fluorescence changes from blue to green. This effect is shown in Figure 3. The presence of the isosbestic point indicates that the photochemical transformation of 3 involves the depletion of the ground state and the concomitant and exclusive formation of a new species. This product which absorbs at longer wavelengths than 3 is also fluorescent;1° its spectrum is also portrayed in Figure 3. This photoreaction is not observed in rigorously degassed solution (Le., + -

+-

-

-

-

+ -

+ -

(10) The fluorescence decay of the photoproduct of 3, measured in aerated acetonitrile immediately after irradiation, was nonexponential. The decay curve obtained with A,, = 425 nm ( x 2 = 1.5) indicated a major component having a subnanosecond lifetime (0.5 ns). This is thought to be a consequence of light scattering in a slightly turbid solution. The long-lived component had a lifetime of 5.3 ns.

The Journal of Physical Chemistry, Vol. 91, No. 6, 1987

Substituted Tetraazapentalenes five freeze-pump-thaw cycles). Moreover, it is not observed in aerated ethanol. The anti isomer, 4, was found to be photochemically stable under all conditions to which 3 was exposed. The excited states of both 3 and 4 are quenched by oxygen at the diffusion-controlled rate in aerated benzene and acetonitrile and 1.9 X M, solution. For O2 solubilities of 3.0 X respectively, second-order quenching rate constants of 2.0 X 1Olo and 3.6 X 1OloM-I s-I are determined from fluorescence lifetime measurements. Fluorescence lifetime data indicate that in aerated solution the efficiencies of fluorescence quenching by oxygen are 0.15 and 0.26 for 3 in benzene and acetonitrile, and 0.29 for 4 in both solvents.]’ The absolute disappearance efficiency of 3 in aerated acetonitrile is 0.14 and is independent of excitation wavelength. Thus, while oxygen quenching of S1 is about twice as efficient in 4 relative to 3, only the latter undergoes the photochemical oxidation reported above. The mechanism of this photooxidation was further explored with the aim of establishing the role of singlet oxygen. It was observed that the photobleaching of the ground state of 3, and the concomitant appearance of an absorbing species at ca. 420 nm, was nearly completely quenched in the presence (0.01 M) of 1,4-diazabicyclo[2.2.2]octane(DABCO), a known singlet oxygen quencher. While DABCO was found to quench the fluorescence of 3 only slightly under these conditions ( N lo%), it is possible that the triplet state of 3 is intercepted by the quencher. More definitive evidence of singlet oxygen involvement was obtained from experiments utilizing the singlet oxygen sensitizers Rose Bengal (RB) and methylene blue (MB). Both dye sensitizers when irradiated at 547 nm (at which wavelength, the absorption of 3 is negliglble) were found to effect the same photochemical reaction observed as in the direct irradiation of 3. Also, RBsensitized photobleaching of 3 takes place in aerated ethanol solution. The rate of this reaction is slower than in acetonitrile, presumably because of the shorter lifetime of ‘0, in ethanol.l2 Significantly, the irradiation of RB or MB in aerated acetonitrile solution of 4 failed to produce photobleaching. Furthermore, the presence of singlet oxygen reaction inhibitors DABCO or 2,3dimethyl-2-butene (0.01 M) in solutions of 3 and RB or MB was found to quench the photobleaching. The following conclusions are drawn from the above observations: (1) singlet oxygen is formed from the oxygen quenching of the excited singlet state of 3, possibly through the intermediacy of a triplet state; (2) singlet oxygen attacks the ground state of 3 to form the photoproduct; (3) the ground state of 4 is unreactive toward singlet oxygen. A possible mechanism for the photooxidation reaction is as follows:

3

hv

13*

in which the formation of singlet oxygen might occur directly or, as suggested above, through the intermediacy of the triplet state of 3. The fact that 3 and not 4 c a n be photosensitized with RB or MB seems to rule out the failure of the triplet state of 4 to produce singlet oxygen as the cause of the reaction selectivity. Thus differences in intersystem crossing efficiencies of 3 and 4 and/or triplet-state lifetimes cannot be the sole explanation of the unreactivity of 4 vis-a-vis 3 toward photooxidation. The absence of photobleaching of 3 in aerated ethanol despite the fact that the singlet is quenched and the ability to photosensitize the reaction with RB imply that the triplet state of 3 is involved in the direct process and moreover that 33 is short lived in ethanol. On the basis of the measured oxygen-induced photochemical disappearance efficiency of 3 in acetonitrile, Le., 0.14, as well as the (1 1) The efficiency of oxygen quenching was determined as +bq = rd(l/ra - 1/rd),where rd and r, are the fluorescence lifetimes in degassed and aerated

solutions, respectively. (12) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978; pp 5 8 8 et seq.

1369

measured oxygen quenching efficiency of l3* by oxygen (0.26, vide infra), the reaction efficiency between ground-state 3 and singlet oxygen is deduced to be 10.54. Regarding the nature of the photooxidation of 3 and the photoproduct, [302],an interesting parallel can be drawn between the selective photoreactivity of 3 and the reaction between 3 and dimethyl acetylenedicarboxylate (DMAD). Pereira and Leonardi3 have found that 3, but not 4, undergoes an [r8+ r2]cycloaddition reaction with DMAD in benzene solution. The initial attack of acetylenic carbon atoms was postulated to occur at the C l ( l 0 ) and C l l a positions in 3. This type of reaction was taken as an indication of the conjugated double bond (hence enophilic) nature of the terminal rings of 3 and thereby accounts for the established structure of the product. It is possible that the photooxidation of 3 by IO2 involves the formation of a cyclic peroxide analogous to DMAD. It is unclear why this species would absorb at longer wavelengths than 3 unless it undergoes rapid rearrangement. Another possibility is that IO2 adds directly across the N 5 and N 6 positions in 3. This would rationalize the nonreactivity of 4. The initial adduct would be a diradical, presumably having unpaired electrons centered on the C4a and C6a positions, and this species could revert to the zwitterionic bis(N-oxide). Attempts to isolate the photooxidation product of 3 have not been successful. This species, while stable for several hours in dilute acetonitrile (or benzene) solution M) forms insoluble, apparently polymeric, material on workup. Irradiation of more concentrated solutions of 3 also forms the yellow photoproduct, but insoluble material is formed more rapidly. Spin resonance studies of the photoproduct or CIDNP experiments might help to elucidate the mechanism of the photoreaction and the structure of the photoproduct.

Experimental Section All solvents were spectroscopic grade (Burdick and Jackson) and used without further purification. Fluorescence spectra were obtained with a dc fluorimeter equipped with a 200-W Hg(Xe) or, in certain cases, a 150-W Xe lamp. These lamps were used in the irradiation experiments; the fluorimeter excitation monochromator isolated the incident radiation. The fluorescence detector was a 300-nm-blazed grating coupled to an EM1 9558-QB photomultiplier tube. Fluorescence efficiencies were measured relative to a value of 0.55 for quinine sulfate in 1.0 N H2S04.’ Optical densities of the reference and the sample were kept below 0.2 and were matched. The actual absorbance values were measured in the fluorimeter cavity by using a transmitted light detector. Lifetimes were measured with a time-correlated photon-counting apparatus. A thyratron-triggered D,-filled flashlamp (0.5 atm) was run at 30 kHz. Radiation was isolated by using a 0.25-m Ebert monochromator (3.2-nm bandpass). The time base was calibrated with an Ortec Model 462 time calibrator. Absorption spectra were acquired on a Varian 2300 spectrophotometer. Absorption, fluorescence, and fluorescence decay spectrometers were coupled to LSI 11/03 and 11/23 microcomputers. Decay curves were analyzed by using a reconvolution procedure previously de~cribed.’~The absolute disappearance yield of 3 in acetonitrile was determined by using a calibrated thermophile (Eppley Laboratories). M N D O calculations were performed on a DEC VAX 11-780 computer; the HAM/3 calculations were carried out on a Zenith 148 with 640K and equipped with a math coprocessor. Running times were about 15 min. The compounds dipyrido[ 1,2-a:2’,1’-fl-1,3,4,6-tetraazapentalene (3) and dipyrido[ 1,2-~:1’,2’-e]-l,3,4,6-tetraazapentalene (4) were made as described p r e v i o ~ s l yand ~ ~ ~were purified by vacuum sublimation followed by recrystallization. Acknowledgment. A.M.H. acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work and Professors (13) Periera, D. E.; Leonard, N. J. Tetrahedron Lett. 1985, 27, 4129. (14) Halpern, A. M.; Fye, S. L.; KO,J.-J. Photochem. Photobiol. 1984, 40, 5 5 5 .

1370

J . Phys. Chem. 1987, 91, 1370-1374

D. A. Forsyth and L. D. Ziegler for several suggestions. Dr. T. Wilson is also acknowledged for helpful discussions. The research at the University of Illinois was supported by the National Science Foundation (NSF C H E 84-16336) and in part by an unrestricted

grant from Eli Lilly and Company. Registry No. 3, 100460-12-6;4, 104716-50-9;acetonitrile, 75-05-8; water, 7732-18-5; cyclohexane, 110-83-8; benzene, 71-43-2; oxygen, 7782-44-7.

Molecular Potentlals from CARS Photofragment Spectroscopy: Spectroscopic Constants and Potential Energy Curve for O,(a' A,) Jong-Chen Nieh and James J. Valentini* Department of Chemistry, University of California, Irvine, California 9271 7 (Received: October 2, 1986)

We describe the use of CARS spectroscopy of photofragments as a means of obtaining extensive spectroscopic constants and accurate potential curves. UV photodissociation of ozone yields 02(a'A,) in u = 0-6 and J = 0-50. CARS spectra of the alAgphotofragment under collision-freeconditions are analyzed to yield vibrational constants through ageand rotational constants through yo. The constants are used to compute classical turning points for the vibrational motion, via the RKR analysis. The RKR points are fit to a simple analytical potential function, the extended Rydberg function, that provides an excellent description of the 02(a'A,) potential energy curve.

Introduction Optical spectroscopy with spectral resolution sufficient to distinguish transitions involving different rotational and vibrational states is a powerful means for characterizing the structure of molecules and determining potential energy curves. Due to the anharmonic nature of molecular vibrations and vibration-rotation interactions, an accurate description of the molecular structure and the potential energy curve requires determination of a wide range of vibrational and rotational eigenenergies. In general, this necessitates spectroscopic observation of high u, high J energy levels, levels not appreciably populated under thermal equilibrium conditions at temperatures accessible in the laboratory. For excited electronic states, high-energy vibrational levels are accessible in absorption, provided Franck-Condon factors are favorable. For the ground electronic state, high u levels can be observed in emission, again given adequate Franck-Condon factors. However, for electronic states, ground or excited, for which the electronic transitions are strongly dipole forbidden or for which the transitions are allowed but involve continuum, rather than bound, states, conventional electronic spectroscopy is not useful for determination of rotational-vibrational energy levels or potential energy curves. Moreover, even when spectroscopically convenient electronic transitions are available, thermal rotational populations and dipole selection rules limit the range of rotational states that are accessible. When dipole-allowed bound-bound electronic transitions are not accessible, vibrational spectroscopy (IR or Raman) can be used to acquire the desired spectroscopic information, if a method for producing non-thermal-equilibrium rotational and vibrational state distributions can be implemented. One such way is to use chemical reactions or photochemistry. Chemical reactions, either exothermic or at high collision energies, and photodissociation generally yield products with nonthermal state distributions. Provided that these products can be observed spectroscopically on a time scale short compared to the collisional relaxation time, this provides an avenue to high v,J states. In this paper we describe the use of such an approach to obtain accurate and detailed spectroscopic and potential energy curve information about a very common, but poorly characterized, diatomic species, O2 in its lowest excited state, a'A,. Pulsed laser UV photodissociation of O3at wavelengths between 240 and 3 1 1 nm is used to produce 02(a1A8)in vibrational states u = 0-6 and rotational states J = 0-50. Coherent anti-Stokes Raman scattering (CARS) is used to detect the O,(a'A,) photofragments under 0022-3654/87/2091-1370$01.50/0

collision-free conditions. Vibrational constants through quartic in v and rotational constants through quadratic in both u and J are directly determined, while the centrifugal distortion coefficient is indirectly determined. The directly determined constants yield 14 vibrational turning points, through the RKR analysis, and a simple potential energy function, the extended Rydberg function, is fit to the RKR turning points to yield an analytic potential energy curve. The 'A state of molecular oxygen has been of interest to chemists since Mulliken] predicted the existence of low-lying excited states of oxygen. The a'A, state is important in the atmosphere of the earth and other planetsS2 It has been used extensively in organic chemistry as a reagent for oxidative transfor~nations~*~ and may be involved in biological processes such as photocarcinogenesis and enzymatic oxidation.2 Because it can be produced by purely chemical means, Oz(a'A,) has been used as an energy-transfer species in a chemical laser.2 The O2 electronic configuration K K ( ~ S C T , ) ~ ( ~ S U , , ) ~ ( ~ ~ C T ~ ) ~ (2pII,,)4(2pIIg)2gives rise to the alAgstate, as well as the ground X32; state and the second excited state, b'Z,+. The a' A, state lies only 0.977 eV above the ground state,4 yet the spectroscopy of the a'Ag X32; transition is not extensive, since the transition is strongly forbidden by dipole selection rules. The metastability of the ala, state is evidenced by its extremely long radiative lifetime, about 65 min, as well as by its resistance to collisional dea~tivation.~ Emission from the a ' h state can be observed in the night sky. Herzberg and Herzberg4 dispersed this emission, observing a rotationally resolved 0-0 band at 1.27 pm and also a barely detectable 1-0 band at 1.07 pm. From this they determined Be = 1.4264 cm-l and ae = 0.0171 cm-l and estimated we and w$c, as 1509 and 13 cm-I. Slanger6s7 has identified bands in the 300-500-nm region of the oxygen afterglow as belonging to C3A, alAgand has determined B, = 1.374 cm-', we = 1510.23 (34) cm-l, and w&, = 13.368 (1 2) cm-I. More recently, laser magnetic resonance* and microwave spectroscopyg have been used to de-

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(1) Mulliken, R. S . Phys. Reu. 1928, 32, 186. (2) Frimer, A. A., Ed. Singlet 02; CRC: Boca Raton, FL, 1985. (3) Ranby, B.; Rabek, J. F., Eds. Singlet Oxygen; Wiley: New York, 1978. (4) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978.

(S) Herzberg, L.; Herzberg, G. Asrrophys. J . 1947, 105, 353 (6) Slanger, T. G. J . Chem. Phys. 1978, 69, 4779. (7) Slanger, T. G. Chem. Phys. Lett. 1979, 66, 344.

0 1987 American Chemical Society