221
PHOSPHORESCENCE SPECTRA OF SOMEHALONAPHTHALENES
-
Table 11: Temperature Dependence of the Optical Anisotropy of Water Assumptions'
7
Case
Bass
ysaaa
1 2 3
Pap8
'/a~aaaa l/a~rraaa
'IaPaa3
0
4
'IaPaaa
Yaa33
Datab
0
...
...
10"a'sc
,--t - 0
0.16 3.58 2.41 -0.92
1.00 1.00 1.00 1.00 1.00
...
b Assuming that a The assumptions are described in the text. at 25". curve in Figure 1. c Assuming that n'a = 0.013
7'8
--r'a($)/r'a(t = 0)-
----.---.---.---
t = 25
t = 62
t = 83
0.96
0.86 1.32 1.30 0.53 1.30
0.81 1.44 1.40 0.37 1.34
1.10 1.09 0.85 1.09
is proportional to P B , the values are estimated from the dashed
creases with increasing temperature and shows an inflection near 30". The plot of intensity vs. temperature is remarkably similar to the plot of Kerr constant vs. temperature. Efforts to uncover possible common molecular origins in these and other observations might well lead to further important insights in the three difficult areas involved in this problem.
tion must also be a factor in the temperature dependence, and quantum mechanical calculations of this effect would also be of interest. The need to simultaneously unravel problems involving molecular parameters, interactions, and liquid structure suggests the desirability of investigations covering many types of phenomena. For example, an infrared band of intermolecular origin has been observed a t 2100 cm-1.26 The intensity of this band de-
(25) C. Salama and D. A. I. Goring, J . Phys. Chem., 70,3838 (1966).
The External Heavy Atom Effect on the Phosphorescence Spectra of Some Halonaphthalenes by Linda G. Thompson1 and S. E. Webber* Department of Chemistry, The University of Texas at Austin, Austin, Texas 78718 (Received August 9, 1971) Publication costs assisted by the Robert A . Welch Foundation
Phosphorescence spectra are presented for 1- and 2-halonaphthalenes (halo = chloro, bromo, and iodo) in a series of matrices (ethanol and 1-propyl chloride, bromide, and iodide). The phosphorescence spectra of these molecules are found to be sensitive to external heavy atom perturbations, with 2-halonaphthalenes being more sensitive than 1-halonaphthalenes. It is suggested that the reason for this solvent effect is t h a t the vibrationally induced portion of the TI --c SOtransition intensity is not important in heavy atom solvents.
Introduction It has been known since the early work of McClure2 and Kasha8that internal ( i e . , chemically bonded) or external heavy atoms have a striking effect on processes involving the triplet state of aromatic molecules. It is currently supposed that aromatic molecules possess this extreme sensitivity to heavy atoms as a consequence of their planarity, which symmetry reduces the magnitude of the spin-orbit coupling between singlet and triplet T-T* excited state^.^ A number of reviews are available which discuss the so-called "heavy atom effect" in general.6-7
It is the purpose of this article to present some phosphorescence spectra of various 1- and 2-halonaphthalenes in various matrices (ethanol and 1-propyl chloride, (1) Present address: Department of Chemistry, Memorial University, St. Johns, Newfoundland. (2) D. 8. McClure, J . Chem. Phys., 17,905 (1949). ( 3 ) M. Kasha, ibid., 20, 71 (1952). (4) D.8. McClure, ibid., 17, 665 (1949). (5) M. A. El-Sayed, Accounts Chem. Res., 1, 8 (1968). (6) S. K. Lower and M . A. El-Sayed, Chem. Rev., 66, 199 (1966). (7) 8. P. McGlynn, T. Azumi, and M. Kinoshita, "Molecular Spectroscopy of the Triplet State," Pi-entice-Hall, Englewood Cliffs, N. J., 1969, Chapters 5-8. The Journal of Phyeical Chemistry, Vole76, No. 8, 1978
222
LINDAG. THOMPSON AND S. E. WEBBER
bromide, and iodide) a t 77°K. These spectra display a remarkable dependence on the degree of external spinorbit coupling which is unique to the halonaphthalene series, as far as the present authors are aware. We may summarize our results as follows. (1) 2-Halonaphthalenes are more sensitive to external heavy atoms than 1-halonaphthalenes. (2) I n all cases the phosphorescence spectrum of the halonaphthalene became more like that of naphthalene, indicating that the mixing scheme which gives rise to the phosphorescence in halonaphthalenes is simplified by the presence of an external heavy atom.8
Experimental Section The experimental arrangement used to obtain the phosphorescence spectra was straightforward and has been described p r e v i o ~ s l y . ~Excitation was by means of a low pressure Hg lamp and a 100-8 band-pass, 31308 interference filter (Oriel Optics Gorp.). No phosphorimeter was used as there was no scattered light or impurity fluorescence present. All spectra presented are uncorrected for photomultiplier response (EM1 9514 S). Typical slit widths on the McPherson Model 218 Ob3-m monochromator were 1000 p, which results in a 25-A band pass. Ethanol was freshly distilled from an ethanol-magnesium ethoxide mixture to ensure dryness. All propyl halides were passed through an activated alumina column before use, although propyl iodide was the only compound to show evidence of decomposition. No phosphorescence was detected from these solvents. Solvents were not outgassed before use as outgassing has not been found to affect phosphorescence spectra in solid matrices. The concentration of the naphthalenic M . The ethanol maspecies was approximately trix was glassy but the propyl halides yielded white, opaque polycrystalline matrices. The present results are presented for pure solvent materials. Essentially the same effects are found when 1:1 ethanol-propyl halide glassy matrices are used. Apparently Eisenthal10 has observed the same effect with even more dilute heavy atom solvents. With the exception of 2-iodonaphthalene (Pfaltz and Bauer) all materials could be used without further purification (Eastman Organic Chemicals). The former material was passed through an alumina column and then recrystallized from ethanol. All phosphorescence spectra in ethanol agreed with those reported by Pavlopoulos and El-Sayed."
Results I n Figures 1 and 2 are presented the phosphorescence spectra of 1- and 2-chloronaphthalene, respectively. For 1-chloronaphthalene the phosphorescence spectra in ethanol and propyl chloride are essentially the same, with the latter spectrum being broadened slightly. The spectra in propyl bromide and propyl iodide are The Journal of Physical Chemistry, Vol. 76, No. 2, 1972
460
480
500
520
540
560
580
W A V E L E N G T H (nm)
Figure 1. The phosphorescence spectrum of 1-chloronaphthalene in various matrices a t 77°K. The following notation is used to denote the solvents: EtOH = ethanol, PX = 1-propyl halide where X = C1, Br, I. The topmost spectrum in this and all succeeding figures is that of naphthalene in ethanol.
definitely perturbed and resemble the phosphorescence spectrum of naphthalene (upper spectrum). A similar but much more striking change occurs for 2-chloronaphthalene. For this molecule the presence of propyl chloride serves to perturb the spectrum. This observation is illustrative of the generally greater sensitivity to external heavy atoms of the 2-halonaphthalenes. Similar sets of spectra are presented in Figures 3 and 4 for 1- and 2-bromonaphthalene. For the former molecule there is no significant perturbation except for the propyl iodide matrix while for the latter molecule a significant perturbation is present for the propyl bromide matrix. For the 2-bromonaphthalene case the matrices propyl chloride, bromide, and iodide form a clear progression of perturbative effects on the phosphorescence spectrum. In Figure 5 we have combined the phosphorescenco spectra for 1- and 2-iodonaphthalene. As expected, the perturbation on the phosphorescence spectrum is less important for these cases because the internal heavy atom effect is quite strong. I n particular, the l-iodonaphthalene spectrum is primarily broadened in going from an ethanol to propyl iodide matrix, although the (8) I n the notation of ref 6, our results demonstrate the external heavy atom enhancement of subspectrum I relative t o subspectrum 11. (9) 8. E. Webber, J . Phys. Chem., 7 5 , 1921 (1971). (10) K. B. Eisenthal, J. Chem. Phys., 45, 1850 (1966). (11) T. Pavlopoulos and M. A . El-Sayed, ibid., 41, 1082 (1904).
223
PHOSPHORESCENCE SPECTRA OF SOMEHALONAPHTHALENES
>
>
crn
crn
z w
f
w
+ E
c
-z
h 460
480
EtOH
EtoH
5 0 0 5 2 0 540 560 WAVELENGTH (nm)
Figure 2. Phosphorescence spectra of 2-chloronaphthalene; see Figure 1 for notation.
>
E rn z W c
E
-
480
0
580
500 5 2 0 540 560 WAVELENGTH(nm1
580
Figure 4. Phosphorescence spectra of 2-bromonaphthalene; see Figure 1 for notation.
L
>
c rn
z I-
z
2-1N/ EtOH
\ l-IN/PI
1-IN/EtOH
EtoH
)
480
5 0 0 520 540 560 WAVELENGTH (nm)
580
Figure 3. Phosphorescence spectra of 1-bromonaphthalene; see Figure 1 for notation.
0-0 band is somewhat strengthened in the latter matrix. The 2-iodonaphthalene phosphorescence spectrum is more sensitive to a propyl iodide matrix than the 1-iodonaphthalene spectrum, consistent with the previously mentioned trend. 12 An interesting empirical rule emerges from a study of these spectra. For 2-halonaphthalenes the external
460
480
500 520 540 560 WAVELENGTH(nm)
580
Figure 5. Phosphorescence spectra of 1-iodonaphthalene (1-IN) and 2-iodonaphthalene (2-IN). Other notation follows Figure 1.
heavy atom must be the same as (or exceed in atomic number) the internal heavy atom for an appreciable ef(12) The present results are in conflict with 8. part of the earlier work of S. P. McGlynn, M. J. Reynolds, G. W. Daigre, and N. D. Christodoyleas, J . Phys. Chem., 66, 2499 (1962). These workers state that the phosphorescence spectrum of 1-iodonaphthalene is unchanged in cracked propyl halide glasses. The Journal of Physical Chemistry, Vol. 78, No. 3, 1978
224
fect to be observed on the phosphorescence spectrum. For the 1-halonaphthalenes it is necessary for the atomic number of the external atom to exceed that of the internal atom for a spectral perturbation to occur. We are not aware of any reason why these different sensitivities should have been anticipated. The phosphorescent lifetimes of the 1- and 2-halonaphthalenes are similar for a given halogen substitution.2v12 This implies that the 1 or 2 positions are roughly equivalent as far as the efficiency of the internal heavy atom effect is concerned. If this were not the case one could argue that our observations reflect the relative magnitude of internal singlet-triplet coupling. l a Another possible explanation of the difference between 1- and 2-halonaphthalenes is that the latter steal intensity from the propyl halides triplet-singlet emission more efficiently than the We note that the triplet energy of the 2-halonaphthalenes is always higher than that of 1-halonaphthalene. Presumably the energy gap between the states described by 3+(T1 X PO) (triplet state of aromatic and ground state of “atomiclike” perturber) and ”(So X 3P1)(ground state of aromatic and excited triplet of perturber) is less for the former molecule than for the latter. It follows from the so-called “exchange mechanism”14,161*7that there will be a larger contribution of the perturber transition strength to the TI --t SO transition for the 2-halonaphthalenes than for the 1halonaphthalenes. We may note one further trend displayed by our spectra. The appearance of the phosphorescence spectra for all 1-halonaphthalenes in propyl iodide are quite similar to each other and likewise for the 2-halonaphthalenes. However, the phosphorescence spectra in propyl iodide for the 1- and 2-halonaphthalenes are dif-
The Journal of Physical Chemistry, Vol. 76, N o . I, 1972
LINDAG. THOMPSON AND S.E. WEBBER ferent from each other. If our interpretation of the present results as intensity stealing from solvent transitions is correct, then the phosphorescence spectra in propyl iodide no longer reflect the vibronically induced transitions observed in the unperturbed halonaphthalenes.’* The similarity of the propyl iodide spectra reflects the similarity of the potential energy surfaces in the TI and So electronic state for all naphthalenes substituted a t a particular position but regardless of the halogen substituent. This observation should be of interest to theoreticians concerned with the effect of substituent groups on the energy levels of aromatic molecules.
Acknowledgments. We wish to acknowledge the generous support of this work by the Robert A. Welch Foundation. We have also benefited from NSF Grant GP-10021. We wish to thank Mrs. Rayna Kolb for her help in preparing the figures. (13) The differences in the observed triplet lifetime for 1- or 2halonaphthalenes do imply that the 1 position is slightly more efficient in spin-orbit coupling than the 2 position, i.e., the lifetime of a 1-halonaphthalene is always shorter than that of a 2-halonaphthalene. The largest difference between positions is found for chloronaphthalenes ( T ~= 0.30 and 0.47 sec for 1- and 2-chloronaphthalene, respectively). (14) See ref 7, Chapter 8, section 6. (15) Recent work by G . C. Giachino and D. R. Kearns, J . Chem. Phys., 52, 2964 (1970), has shown that the electronic states of the external heavy atom are mixed with those of the perturbed aromatic molecule. (16) G. W. Robinson, J . Mol. Spectrosc., 6 , 58 (1961). (17) M. A. El-Sayed, J . Chem. Phys., 47, 2200 (1967). (18) The presence of a vibronically induced pathway for TI 4 SO in halonaphthalenes has been discussed in ref 5 and 11. For a discussion of halophenanthrenes see J. L. Ginsburg and L. Goodman, J . Chem. Phys., 52, 2369 (1970).
