Intramolecular hydrogen bonding. 7. Effect of hydrogen bonding on

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J . Phys. Chem. 1989, 93, 2334-2336

2334

Intramolecular Hydrogen Bonding. 7. Effect of Hydrogen Bonding on the Radiative and Nonradlative Properties of 6-Aminobenzanthrone Gregory D. Gillispie,* Nagalingam Balakrishnan, and Douglas E. Johnson Department of Chemistry, North Dakota State University, Fargo, North Dakota 581 05 (Received: August I , 1988)

J. Phys. Chem. 1989.93:2334-2336. Downloaded from pubs.acs.org by UNIV LAVAL on 09/14/15. For personal use only.

Site-selected fluorescence,phosphorescence, and luminescenceexcitation spectra have been measured for 6-aminobenzanthrone in an n-hexane Shpol'skii matrix at 10 K. The S1-So transitions resemble those in the related compound 1-aminoanthraquinone, which also has a N-H-O=C intramolecular hydrogen bond. However, unlike 1-aminoanthraquinone,there is also measurable phosphorescence in 6-aminobenzanthrone. The phosphorescence vibronic pattern has little resemblance to the fluorescence and is markedly affected by deuteration of the hydrogen-bonded proton. The deuteration shifts the fluorescence origin 16 cm-I and the phosphorescence origin 6 cm-I, both to higher energy. Comparison is made to the spectroscopic behavior of the similar molecule 6-hydroxybenzanthrone.

Introduction Only a few molecules with intramolecular hydrogen bonds exhibit phosphorescence. The usual explanation is that the presence of the hydrogen bond enhances internal conversion processes to such an extent that the triplet manifold is effectively bypassed. Two exceptions to the rule, 6-hydro~ybenzanthrone'*~ (6-HBA) and 9-hydr~xyphenalenone~ (9-HPO), have been recently studied by this group. We now present the results of a similar investigation of 6aminobenzanthrone (6-ABA). Our previous work on amino- and

0

NH2

6-ABA

hydroxy anthraquinone^^^ has shown that the -OH and -NHz substituents have significantly different effects on the singlet electronic manifold when they are intramolecularly hydrogen bonded to a quinone oxygen atom. Thus, it is of interest to compare the 6-ABA spectra with those for 6-HBA. Similarly, both 6-ABA and 1-aminoanthraquinone ( 1-NH2-AQ) contain N-H..O=C intramolecular hydrogen bonds but with different aromatic nuclear frameworks. Experimental Section

The experimental apparatus is unchanged from previous work.2 6-Aminobenzanthrone was synthesized from 6-hydroxybenzanthrone by the method of Bradley and Jadhav.' The crude 6-ABA was first purified by recrystallization from ethanol to give yellow crystals with a melting point in the range 180-182 OC (literature 186-1 87 "C). Thin-layer chromatography and preliminary spectroscopic measurements revealed traces of 6hydroxybenzanthrone which were removed by column chromatography (chloroform eluant on alumina) to give spectroscopically pure 6-ABA. Deuteration was accomplished by shaking a hexane solution of 6-ABA with a few drops of D 2 0 prior to freezing of the solution. Control experiments with H 2 0 revealed no features ascribable to intermolecularly hydrogen bonded complexes.

Results A . Site Pattern. The site distribution of 6-ABA in n-hexane was surveyed with H g arc lamp excitation at 366 and 436 nm. Excitation at 436 nm gives the best signal-to-noise ratio (366 nm is in a valley of the room-temperature absorbance spectrum) but Author to whom correspondence should be addressed.

TABLE I: Site Energies of 6-Aminobenzanthrone in n-Hexane" species state site 1 site 2 6-ABA Si 22480 (444.84) 22491 (444.63) 6-ABA TI 18 116 (551.99) 18056 (553.83) 6-ABA-db S, 22496 (444.51) 22507 (444.30) 6-ABA-d TI 18 122 (551.78) 18061 (553.67)

" In cm-' and uncorrected to vacuum wavenumbers; wavelengths (nm) in parentheses. bForm of 6-ABA in which intramolecularly hydrogen bonded proton has been replaced with deuteron; see text. is too close to the fluorescence origin to allow for observation of the 0-0 band. As is typical of the benzanthrones and phenalenones in n-hexane, two major sites exist; origin wavenumbers for these sites are given in Table I. Note that there is a site inversion between the SI and TI states; Le., the longer wavelength site in fluorescence is the shorter wavelength site in phosphorescence. All spectra and analysis in this paper are for site 1, the longer wavelength site in the singlet manifold. We verified that the site 2 spectra give satisfactory agreement with those for site 1. In n-heptane solvent there are three major sites of nearly equal population. The phosphorescence origin bands for these three sites are 552.4, 553.5, and 555.68 nm. B. Substituent Effects on the Electronic Origin Position. The 0-0band of the SI-Soelectronic transition is observed at 444.84 nm for 6-aminobenzanthrone in n-hexane solvent. The SI state energy of 22480 cm-I is approximately 500 cm-' less than in 6-hydroxybenzanthrone, consistent with the expectation that -NH2 is a better electron donor to the aromatic ring system than is an -OH group. A more extensive comparison is possible for the anthraquinone nuclear framework since many -NH2 and -OH substituted derivatives are available. We have presented a correlation which shows that an -NHz substituent at the a-position reduces the SI state energy in anthraquinone by about 3000 cm-l, while the effect of an -OH substituent is only about half as great.* The S1-T, electronic energy gap is 4400 cm-I, compared to 5100 cm-I in 6-HBA and 5400 cm-I in 9-HPO. Note that the TI state energy in 6-ABA is actually some 200 cm-' greater than in 6HBA. This is but one of many indications that the -OH and -NHz substituents influence the T I state much differently than they do the SI and So states. (1) Van Benthem, M. H.; Gillispie, G. D.; Haddon, R. C. J . Phys. Chem. 1982,86, 4281. (2) Gillispie, G. D.; Van Benthem, M. H.; Vangsness, M. J . Phys. Chem. 1986, 90, 2596. ( 3 ) Gillispie, G. D. J . Chem. Phys. 1986, 85, 4825. (4) Carter, T. P.; Gillispie, G. D.; Connolly, M. A. J . Phys. Chem. 1982, 86, 192. (5) Carter, T. P.; Van Benthem, M. H.; Gillispie, G. D. J . Phys. Chem. 1983, 87, 1891. (6) Van Benthem, M. H.; Gillispie, G. D. J . Phys. Chem. 1984,88,2954. (7) Bradley, W.; Jadhav, G. V. J . Chem. SOC.1948, 1622. (8) Gillispie, G. D.; Van Benthem, M. H.; Balakrishnan, N. Electronic Energy Shifts in cup'-Substituted Anthraquinones. J . Phys. Chem., in press.

0022-365418912093-2334$01.50/0 0 1989 American Chemical Society

Intramolecular Hydrogen Bonding

The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2335

TABLE 11: Deuterium Isotope Effects on Electronic Energy Gaps'

molecule

fluorescence shiftb

phosphorescence shift

6-HBA 9-HPO 6-ABA 1-NHZ-AQ

+I30 +128 +16 +49

-28 -60 +6 c

FLUORESCENCE

m P

I

J. Phys. Chem. 1989.93:2334-2336. Downloaded from pubs.acs.org by UNIV LAVAL on 09/14/15. For personal use only.

OAll shifts in cm-'. b A positive shift indicates origin band shifts to shorter wavelength when proton of hydrogen bond is isotopically replaced. CUnobserved.

C. Deuterium Isotope Effects. In principle, there should be four distinct species in the spectra of deuterated samples of 6-ABA: the -do (normal isotopic species), the -d2 (both amino protons exchanged for deuterons), and two -dl species, since the isotopically labeled atom could either be hydrogen bonded or "free". However, we only find spectroscopic evidence for a single deuterium carrying form, similar to our observations for 1-aminoanthraquin~ne.~ We have suggested previously that deuterium substitution of the non-hydrogen-bonded amino proton has no effect on the position of the 0-0band because the stretching frequency of the free N-H bond is virtually the same in both So and in SI;this band is also likely uncoupled from the rest of the vibrational manifold. Henceforth in this paper we refer to 6-ABA-d to indicate either of the forms in which the proton of the intramolecular hydrogen bond has been isotopically substituted. The fluorescence origin of 6-ABA-d is blue-shifted 16 cm-' from that of the normal isotopic species. This is less than half the shift in 1-aminoanthraquinone and only about 20% of the shift in 6-hydroxybenzanthrone. Blue shifts in the fluorescence origin are generally observed for molecules with A-H-.O=C intramolecular hydrogen bonds. Other workers have ascribed the blue shift entirely to the A-H stretching vibration. With this interpretation, the blue shift requires a reduction in the A-H stretching frequency in the SIstate, and hence a stronger intramolecular hydrogen bond in SI than in the ground electronic state. Although the hydrogen bond probably is stronger in SI than in So, the justification for ascribing the entire isotope shift to a single vibration is dubious. Regarding the phosphorescence origin shifts, we can only compare with our own previous work on 6-hydro~ybenzanthrone'-~ and 9-hydro~yphenalenone.~ This comparison is made in Table 11. Since there is no precedent for phosphorescencepf an NH-.O=C hydrogen-bonded system, we can only note here that 6-ABA does not show the unusual red shift of the O-H-O=C systems. D. Vibronic Distributions. 1 . Fluorescence. The fluorescence quantum yield, although not determined quantitatively in our work, is definitely well below that for 9-HPO or 6-HBA. The reported fluorescence quantum yields at room temperature in alkane solvent are 0.05 for 6-HBA and 0.006 for 6-ABA.9 We suspect the quantum yield ratio for the two compounds is even greater in the low-temperature matrix. Consequently, the signal-to-noise ratio of the 6-ABA and 6-ABA-d fluorescence spectra in Figure 1 is not as good as in some of our previous work. Nonetheless, the similarities with the 1-aminoanthraquinone fluorescence spectrum are readily apparent. The origin band is several times stronger than the most intense fundamental, and the bulk of vibronic activity is carried by fundamentals in the 300-550-cm-' range. Deuterium substitution of the hydrogen-bonded proton has more of an effect on the intensity distribution than it does on the frequencies of the active vibrations. 2. Excitation (SI So). The luminescence excitation spectra of 6-ABA and 6-ABA-d are shown in Figure 2. Little comment is necessary here since the two spectra are very similar. As in the fluorescence spectra, the deuterium substitution causes small changes in the fundamental frequencies and more marked changes in the relative intensities of the active vibrations. Combination bands are more easily assigned for the normal isotopic species, and the vibrational analysis is straightforward.

-

(9) Bentley, P.; McKellar, J. F. J . Chew. SOC.,Perkin Tram. 2 1976, 1850.

Figure 1. Site-selected fluorescence spectra of 6 - A B A and 6-ABA-d. Excitation is into the respective S1-So origin bands (444.84 nm for 6ABA, 444.51 nm for 6-ABA-d). Wavenumber displacements from the origin band are given. The broad background beneath the 6 - A B A bands from 448 to 541 cm-I is apparently an artifact associated with the large amount of scattered exciting light entering the emission monochromator; its position shifts with the wavelength of excitation. The intensity of the 0-0 band is approximately 3 times that of the strongest fundamental.

EXCITATION SPECTRUM

A

6-ABA

I

6-ABA-d

Figure 2. Luminescence excitation spectra of 6 - A B A and 6 - A B A - d for excitation into S , between 445 and 430 nm. The respective spectra were monitored at the phosphorescence 0-0 bands. These spectra are not corrected for wavelength variation of laser power. TABLE 111: Comparison of Ground- and Excited-State (SI) Vibrational Frequencies (in cm-I) 6-ABA

6-ABA-d

SO

SI

335 381 394 448 490 541 64 1 664

336 357 389 453 484 529 624 649

SO 332 372 385 437 47 1 536 639 660

SI 326 355 384 446 467 528 629 646

In Table I11 we compare the frequencies of active So fundamentals from the fluorescence spectra and active SI fundamentals from the excitation spectra. 3. Phosphorescence. Phosphorescence spectra of 6-ABA and 6-ABA-d are shown in Figure 3. Very little intensity is found

Gillispie et al.

2336 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 PHOSPHORESCENCE

TABLE IV: Vibrational Frequencies for N-H Involved in Intramolecular Hvdroeen Bonding

m 0)

compound 2-aminoacetophenone methyl anthranilate 2-aminobenzophenone 1-aminoanthraquinone 6-aminobenzanthrone

cm-‘ 3347 3379 3351 3332

Au,b cm-‘

96 64 88

3320

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“The lower frequency of the two N-H stretches. Corresponds to the symmetric stretch in aniline. *Frequency reduction attributable to hydrogen bonding; see ref 1 1.

Figure 3. Site-selected phosphorescence spectra of 6-ABA and 6-ABA-d in a 10 K n-hexane Shpol’skii matrix. The origin bands lie near 552 nm. The lower spectrum has been shifted slightly to bring the 0bands into coincidence. The intensity of the 6-ABA-d 0band is slightly truncated (ca. 25%). Wavenumber displacements of major bands are noted. There are no significant features greater than 800 cm-I from the origin bands.

farther than 800 cm-I from the origin band in either case. Note the two bands in the 6-ABA spectrum at 384 and 689 cm-I, respectively, which have intensities comparable to that of the origin band. It is therefore somewhat surprising that comparably strong overtones or combination bands involving these two modes are not found. In fact, there is an overall dearth of combination bands. Since the phosphorescence zero phonon lines are reasonably narrow and uncomplicated by phonon sidebands, such combination bands should be easy to find if present. The effect of isotopic substitution on the vibronic pattern is much more pronounced for the phosphorescence than for either the fluorescence or excitation spectrum. Overall, there is a definite shift of the intensity moment closer to the origin band. It seems likely that the features at 384 cm-’ in 6-ABA and 324 cm-I in 6-ABA-d arise from the same ground-state fundamental; possibly the strong 689-cm-’ band in the normal isotopic species and the pair at 490, 522 cm-’ for the deuterated form are similarly related.

Discussion The singlet manifold vibronic spectra (fluorescence and luminescence excitation) of 6-ABA rule out a major role for the intramolecular hydrogen bond. As is the case for l-aminoanthraquinone, there is a readily apparent resemblance between the fluorescence and excitation spectra. A plausible correlation between So and SI fundamentals can be drawn on the basis of frequencies (Table 111). However, our recent free jet fluorescence study” of 1-NH2-AQhas shown that the matrix spectra can be misleading in this regard. Dispersed fluorescence spectra from various SI single vibronic levels reveal a surprising degree of Duschinsky rotation despite the seemingly obvious correlation. The variation in the intensity pattern when the hydrogen-bonded proton is isotopically replaced also hints at the Duschinsky mixing. Nevertheless, there does not seem to be any reason to invoke excited-state proton transfer in either l-NH,-AQ or 6-ABA. In fact, it is more to the point to ask why the intramolecular hydrogen (10) Balakrishnan, N.; Gillispie, G . D. J . Phys. Chem., following paper in this issue.

bonds in these and other aromatic amines are so weak. Intramolecular hydrogen bonds of the type N-H-O=C in a six-membered chelate ring are not very strong. For a “free” amino group the symmetric and antisymmetric N-H stretching frequencies are about 3400 and 3500 cm-’, respectively. If one of the amino protons participates in an intramolecular hydrogen bond, the lower frequency vibration is reduced in frequency and the magnitude of the reduction is a measure of the hydrogen bond strength. Bellamy and Pace” have described how partial deuteration studies can be used to separate the effects of coupling between the N-H stretches from the effects of the hydrogen bond. A qualitative, simpler method is to just use the lower N-H stretching frequency as a measure of hydrogen bond strength. The data shown in Table IV indicate the N-H stretching frequency is only reduced to the extent of 100 cm-’ or so. This is to be contrasted with the 0-H stretching frequency reduction of 500-700 cm-’ in analogous compounds with O-H-O=C hydrogen bonds. The correlations of A-H stretching frequency reduction with A-B distance in A-H-B hydrogen-bonded systems are well-known. These suggest that for R(A-B) = 0.265 nm, a reasonable estimate of the heavy atom separation in molecules of the type under consideration here, the N-H.-O=C and O - H 4 = C hydrogen bonds ought to be of similar strength. The inability of the hydrogen-bonded proton to achieve coplanarity with the remainder of the chelate ring is a possible explanation of the weakness of the N-H.-O=C hydrogen bonds, but an unproven one. Further work in this area is desirable. For now we simply note that the absence of dramatic effects of the intramolecular hydrogen bond in the singlet manifold of 6-ABA is consistent with the weakness of the interaction. The phosphorescence spectra are not easy to interpret. In 6-HBA we have noted slight frequency differences (up to 10 cm-I) of fundamental vibrations depending on whether they were determined from the fluorescence or the phosphorescence spectraS2 Such discrepancies are more pronounced in 9-hydroxyp h e n a l e n ~ n e . ~Now, in 6-ABA the fluorescence and phosphorescence spectra are almost orthogonal in the sense that any vibration active in fluorescence is silent in phosphorescence, and vice versa. The number of coincidences is probably not greater than one would expect purely on statistical grounds with so many active low-frequency vibrations. Another puzzle is the sensitivity of the 6-ABA phosphorescence vibronic pattern to the deuterium substitution. That observation in itself would likely be interpreted in terms of strong coupling of the hydrogen bond to the rest of the molecule. The absence of similar effects in the singlet manifold is noteworthy. The situation is unclear now since so few molecules with intramolecular hydrogen bonds exhibit phosphorescence. Further examples are desirable since unusual spectroscopic and photophysical behavior has been found in each case. Acknowledgment. This work was supported by a grant from the National Science Foundation. Registry No. 6-Aminobenzanthrone,43099-12-3; deuterium, 778239-0. ( 1 1 ) Bellamy, L. J.; Pace, R. J. Spectrochim. Acta 1972, 28A, 1869.