J. Phys. Chem. 1982, 86, 4281-4283
4281
Intramolecular Hydrogen Bonding. 2. Enormous Deuterium Isotope Effect on the Phosphorescence of 6-Hydroxyben~anthrone~ M. H. Van Benthem, G. D. Gllllsple," Department of Chemistry, State Universlty of New York at Albany, Albany, New York 12222
and R. C. Haddon Bell Laboratories, Murray Hill, New Jersey 07974 (Recelved: June 11, 1982; I n Final Form: September 7, 1982)
The laser-excitedfluorescence and phosphorescence spectra of the title compound and its hydroxy-deuterated analogue have been measured in an n-hexane Shpol'skii matrix at 10 K. The deuterium substitution has only a minor effect on the fluorescence intensity and the fluorescence and phosphorescence vibronic patterns but leads to a greater than order of magnitude increase in the phosphorescence quantum yield. The phosphorescence lifetimes have been measured to be 19 ms for the normal isotopic species and 300 ms for the deuterated form. This probably represents the largest deuterium isotope effect yet observed in aromatic molecule photophysics.
Substitution of aromatic carbonyls to form pseudoaromatic rings containing intramolecular hydrogen bonds (e.g., o-hydroxybenzophenone) often brings about drastic changes in photophysical properties. In particular, molecules with such a pseudo-ring arrangement can be especially photostable. It has been proposed that the source of this stability is an ultrafast internal conversion process, such that the photochemically active triplet state is effectively by-passed. Further, it is thought that the mechanism of the rapid internal conversion involves proton transfer across the intramolecular hydrogen bond in S1, with the internal conversion taking place from the proton transferred form to an unstable region of the So hypersurface.'+ The extreme rapidity of the internal conversion is attributed to the reduced Sl-So electronic energy gap in the proton transferred form. Recently we published high-resolution fluorescence and fluorescence excitation spectra of quinizarin (structure I)
I
II
Ill
IV
in an n-heptane Shpol'skii matrix a t 10 K.6 The room temperature fluorescence quantum yield of quinizarin is ca.0.2 so SIdeactivation via proton transfer, while perhaps a factor, is by no means totally dominant. In an as yet unpublished study of the isomeric anthrarufin (structure 11), we find a much different b e h a ~ i o r . ~The low-temFor paper 1, see ref 6.
*ALSO Department of Physics and author to whom correspondence should be addressed. Permanent address after J a n 1, 1983: Department of Chemistry, North Dakota State University, Fargo, ND 58105. 0022-3654/82/2086-4281$01.25/0
T A B L E I:
S, State Energies and Deuteration Shifts Esl, cm-'
molecule
normal
9-HPO 23199 2-Me-9-HPO 23 165 6-HBA site 1 22976 site 2 22944 quinizarin 19131
deuterated
AE, cm-'
ref
23301 23 275
102 110
10 11
23101 23073 19190 ( d , ) 19 249 ( d , )
125 129 59 118
this w o r k
6
perature fluorescence displays a dual emission. We assign the structured fluorescence between 490 and 540 nm to emission from the Franck-Coridon excited state and attribute the much stronger and relatively structureless fluorescence at wavelengths greater than 550 nm to emission from an excited state prdton transferred form. The highly divergent excited electronic state behavior of the isomeric quinizarin and anthrarufin sugkests that conjugation between the pseudo-rings of the former is associated with a stability which inhibits proton transfer in S1. In support of this idea, we might note that the influence of the pseudo-ring arrangement ori S,-St, oscillator strengths in aminoanthraquinones wds identified many years agoas In light of the above, we felt a spectroscopicinvestigation of 6-hydro~ybenzanthrone~ (structure 111, symbolized here as 6-HBA) to be of interest. To a first approximation one might relate 6-HBA to quinizarin with one of the pseudo-rings replaced by a true aromatic ring. Alternatively, 6-HBA can be viewed as a derivative of 9-hydroxy(1)J. R.Merrill and R. G. Bennett, J.Chem. Phys., 43, 1410 (1965). (2)R. Pater, J. Heterocycl. Chem., 7, 1113 (1970). (3)N.S.Allen, P. Bentley, and J. F. McKellar, J.Photochem., 5,225 (1976). (4) N. S. Allen and J. F. McKellar, J. Photocheh., 5, 317 (1976). ( 5 ) J. McVie, R. S. Sinclair, and T. G. Truscott, Photochem. Photobiol., 29,395 (1979). (6) T. P. Carter, G. D. Gillispie, and M. A. Connolly, J . Phys. Chem., 86,192 (1982). (7)M. H. Van Benthem and G. D. Gillispie, to be submitted. (8) G. S.Egerton and A. G. Roach, J. SOC.Dyers Colour., 74, 401 (1958). (9)The formal chemical name is 6-hydroxybenz[de]anthracen-7-one. In the older chemical literature one may also find the compound identified as 4-hydroxybenzanthone. See R. C. Haddon, Aust. J . Chem., in press.
0 1982 American Chemical Society
4282
The Journal of Physical Chemistry, Vol. 86, No. 22, 7982
I 0'11
Ht'I
1 312
1
h
?
nr -
@ & $,
L WAVELENGTHNM 558 -
500
562
-
Flgure 1. Nitrogen laser excited fluorescence and phosphorescence spectra of 6-hydroxybenzanthrone in an n-hexane Shpol'skii matrix at 10 K. Only the origin band regions are shown. The sample is a mixture of the normal (H) and hydroxydeuterated (D) species. There are two main sites, labeled as 1 and 2.
phenalenone (structure IV, abbreviated as 9-HPO). From laser fluorescence studies of 9-HPO in rare gas matrices, similar in approach to the work we report here, Rosetti et demonstrated that the hydroxy proton rapidly tunnels along a symmetric double minimum in both So and S1. It was then further shown" that 2-methyl substitution of 9-HPO removes the symmetry of the double minimum potential, localizing the u = 0 level of So in one of the wells; the vibrationless level in S1is still substantially delocalized over both wells owing to the low barrier between them. From our initial study of the 10 K fluorescence of 6-HBA in an n-hexane Shpol'skii matrix, we observed its S1energy to be nearly the same as those of 9-HPO and 2-Me-9-HPO with a similar blue shift of the 0-0 band upon hydroxy deuteration. Clearly, then, 6-HBA resembles 9-HPO more closely than it does quinizarin. The extra benzene ring in 6-HBA has not substantially changed the a-electron delocalization (or at least the differential S1-So delocalization) from that in 9-HPO. The relevant data are given in Table I. Note there are two major sites for 6-HBA in hexane; the population of the site with higher SI energy is approximately 1.5 times that of the other. While performing the experiments on 6-HBA-dl (the hydroxy-deuterated species), a striking orange delayed emission was observed. The excitation spectrum of this luminescence exactly matches that of the fluorescence; accordingly, we assign the delayed emission as T1-So phosphorescence. Phosphorescence was not observed in 9-HPO or 2-Me-9-HP0loJ' nor have we found it in any of a large number of hydroxy- and aminoanthraquinones investigated to date. We then reexamined the normal isotopic species of 6-HBA for phosphorescence. Phosphorescence of 6-HBA-dodoes, in fact, occur but the intensity is much weaker than or 6-HBA-dl. The enormous deuterium isotope effect on the phosphorescence is convincingly illustrated in Figure 1, where we give portions of the nitrogen laser excited spectra of a sample containing both 6-HBA-doand 6-HBA-dl. As in our previous work,6the partial deuteration was achieved by adding a few drops of D 2 0 to ca. 3 mL of an n-hexane solution of 6-HBA. The 337-nm output of the nitrogen laser almost exactly matches the Sz So origin of 6-HBA. Since the S2 So room temperature absorbance is shifted negligibly by the deuteration (not true for the S, So
-
-
-
(10) R. Rosetti, R. C. Haddon, and L. E. BNS, J . Am. Chem. SOC.,102, 6913 (1980). (11) R. Rosetti, R. Rayford, R. C. Haddon, and L. E. Brus, J . Am. Chem. SOC.,103, 4303 (1981).
-
Letters
transition) and owing to the expected broad S2 So vibronic bands even a t low temperature, the nitrogen laser excitation should not give rise to any site or isotopic selectivity. On the left hand side of the figure is the 0-0band region of the fluorescence as measured with a boxcar averager. The two major site origin bands of 6-HBA-doare labeled H(1) and H(2); these are separated by 32 cm-l. To the blue are the corresponding 6-HBA-dl features. On the right hand side of the figure is the phosphorescence origin region of the same sample with the boxcar now operated in the continuous gate mode. Two facts are immediately evident: (a) The phosphorescence intensity of 6-HBA-d1 is more than an order-of-magnitude greater than that of - 6-HBA-do. (b) Deuteration of the hydroxy proton has led to a 28-cm-' red shift of the Tl-So electronic energy gap. Full vibrational analyses of the site-selected fluorescence, phosphorescence, and excitation spectra of both isotopic species are in progress and will be published in the future. For the present, let us focus on some of the more qualitative features. Again, the data summarized in Table I indicate that neither methyl nor phenyl substitution of 9-HPO represents a substantial perturbation of the SI or So electronic structures. On the other hand, these substituents can exert a profound effect on the vibrational character, particularly that associated with the proton tunneling. If the substitution distorts the previously symmetric double minimum potential function, transitions such as O+-O-, forbidden in the symmetric case, become allowed." Rossetti et al." have proposed the notation T to represent the tunneling vibration; they assign a strong band 197 cm-' above the origin in the excitation spectrum of 2-Me-9-HPO-d1as Ti. A much weaker band at 172 cm-' in 9-HPO-dl was similarly assigned and assumed to arise from a slight matrix perturbation of the otherwise symmetric double minimum potentia1.l' The excitation spectrum of 6-HBA-dl is analogous for there is a strong excitation band, nearly as intense as the origin, at 178 cm-'. Correlations between the fluorescence spectra of 6-HBA-doand 6-HBA-dl are readily made. The vibrational spectra are rich with fundamentals in the region 200-700 cm-' from the origin. The majority of these have frequencies identical to within 10 cm-l for the two isotopic variants. The same is true for the phosphorescence spectra. On the other hand, the vibronic patterns in fluorescence and phosphorescence do not match up well for either do or d,. A few vibrations are recognizable in both spectra, but for the most part strong vibrations in the fluorescence are inactive in the phosphorescence and vice-versa. We conclude from the above that the vibrational properties of the S1and T, states must be considerably different as well as more sensitive to deuteration than is the ground electronic state. Certainly the deuteration red shift of the T, state is highly unusual and worthy of further investigation. A key experiment would be to probe the T, vibrational manifold and its isotopic variation via the T, So phosphorescence excitation spectrum; we intend to attempt this experiment. The photophysical process responsible for the enormous deuterium isotope effect, probably the largest yet observed So intersystem for an aromatic molecule,12 is the T, crossing. The room temperature fluorescence quantum yield of ca. 0.0513is unchanged by the deuteration, as then
-
-+
-
(12) A contender is 2,6-dimethylpyrazine in which deuteration of the two ring protons slows the TI So intersystem crossing by a factor of 14 (personal communication from E. C. Lim). (13) P. Bentley and J. F. McKellar, J. Chem. SOC.,Perkin Trans. 2, 1850 (1976).
Letters
must surely be the case for the quantum yield of triplet formation. To further demonstrate this point we have measured the 77 K phosphorescence lifetimes in n-hexane and we find these to be 19 ms for 6-HBA-doand 300 ms for 6-HBA-dl. The factor of 16 difference is in good agreement with the phosphorescence intensity ratios shown in Figure 1. In the future this work will be extended to 6-aminobenzanthrone, reported to phosphorescence with a 77 K
The Journal of Physical Chemistty, Vol. 86,No. 22, 1982 4283
lifetime of 120 ms,I4 and the S-hydroxy- and S-aminobenzanthrones. Acknowledgment. The work at SUNY-Albany was supported by a grant (CHE-8111960) from the National Science Foundation. (14)P. Bentley, J. F. McKellar, and G. 0. Phillips, J. Chem. SOC., Perkin Trans. 2, 523 (1974).
