3522
J. Phys. Chem. 1980, 84,3522-3524
Phosphorescence Spectra and Lifetimes of Symmetrical Tetrachloronaphthalenes Gary G. Giachlno Department of Chemistty, State University College, Geneseo, New York 14454 (Received: July 7, 1980)
Phosphorescence spectra and lifetimes are presented for 1,2,3,4-, 1,3,5,7-, 1,4,5,8-,and 2,3,6,7-tetrachloronaphthalene in EPA. The unusual results obtained for 1,4,5,8-tetrachloronaphthaleneare rationalized on the basis of steric interactions between adjacent chlorine atoms. Attempts to find a simple relationship between the magnitude of the internal heavy-atom effect and the positions and number of heavy atoms are also briefly discussed.
Introduction The introduction of one or more halogens into a naphthalene ring results in several changes in the phosphorescence properties, including an internal heavy-atom effect, a bathochromic shift, and the appearance of new vibronic structure.l Previous work, however, has been concerned primarily with mono- or disubstituted compounds, where the reduced molecular symmetry could contribute to some of the observed changes. This paper examines the phosphorescence from several symmetrical tetrachloronaphthalenes, including the 2,3,6,7 isomer, which has the full DZhsymmetry of naphthalene. The lifetime data will also provide additional information on the position dependence and additivity of the internal heavy-atom effect recently discussed by Miller, Meek, and Strickler.2 In addition to filling in some important gaps in the literature on the phosphorescence of polyhalogenated naphthalenes, the paper is concerned with the effects of intramolecular strain on phosphorescence. Steric interactions introduced when naphthalene is substituted in adjacent a positions (peri) have been used to explain spectral shifts in ultraviolet adsorption spectra and the increased reactivity of such compound^.^ More recently Reiser and Wright used such interactions to rationalize the enhanced intersystem crossing found in 1,8-dimethylLittle naphthlene and 1,4,5,8-tetrarnethylna~hthalene.~ work, however, has been done on the triplet state of these systems. Steric hindrance is expected to affect the phosphorescence in two areas. First, changes in strain energy on excitation should result in spectral shifts similar to those found in ultraviolet absorption ~ p e c t r a .Second, ~~~ because the normal spin-orbit selection rules for aromatic systems assume the molecule to be planar,6 the characteristics of the phosphorescence could be altered significantly if the steric interactions resulted in a nonplanar system. A comparison of the data from 1,4,5,8-tetrachloronaphthalene with those from other (unstrained) isomers will provide information about the role of steric hindrance in the phosphorescence process. Experimental Section Chemicals. The EPA was obtained from Matheson Coleman and Bell and used as received; it was checked regularly and found to be free of extraneous emission at 77 K. 1,2,3,4-Tetrachloronaphthalene was obtained from Aldrich, recrystallized twice from ethanol, and then vacuum sublimed. 1,3,5,7- and 1,4,5,&Tetrachloronaphthalene were prepared by the method of Reimlinger and King.’ The 2,3,6,7-tetrachloronaphthalenehad been prepared by Reimlinger and King and was kindly donated by Union 0022-3654/80/2084-3522$0 1.OO/O
Carbide; it was purified by vacuum sublimation. Methods and Techniques. All data were obtained with the solution (-5 X M) immersed in liquid nitrogen. The exciting light was obtained from a 150-W Xe arc lamp and passed through an appropriate filter. The emitted light was passed through an Oriel grating monochromator having a reciprocal dispersion of 6.4 nm/mm to a RCA 6342-A photomultiplier tube. The geometry of the phosphorimeter was right angle, and for spectra a rotating can was used. For lifetime measurements the can was removed, and the exciting light was blocked with a shutter. The decay curve was displayed on a Tektronix 531A oscilloscope and photographed. Measurements were performed on at least two different samples of each compound, and at least two different time scales of the oscilloscope were used. As a further precaution against impurities, the decay was monitored at the 0-0 band and at least one other wavelength of the emission.
Results and Discussion Phosphorescence Spectra. The uncorrected phosphorescence spectra of the tetrachloronaphthalenes in EPA are presented in Figure 1. With the exception of 1,4,5,84etrachloronaphthalene, the spectra are similar to those reported for other halogenated naphthalenes. The position of the 0-0 band is in good agreement with that calculated by assuming the shift caused by each chlorine is additive,2 and the vibronic structure is as expected. Pavlopoulus and El-Sayed8i9have shown how such spectra can be divided into two subspectra. The first subspectrum is the “normal” naphthalene spectrum and consists of totally symmetric C-C vibrations at ca. 510,1380, and 1580 cm-’. The second subspectrum is primarily developed from an out-of-plane bending mode at 220-280 cm-l. It is worth noting that earlier work had ascribed the appearance of the extra bands in such phosphorescence spectra to a relaxation of selection rules caused by the reduced symmetry of the halonaphthaleneslO The fact that all vibronic bands reported in ref 8 for p-chloronaphthalene (C, symmetry) can be accounted for in the phosphorescence spectrum of 2,3,6,7-tetrachloronaphthalene(D2h symmetry) supports the more recent view that the new peaks are not simply caused by a reduction in the molecular symmetry. The obvious exception to the above features is 1,4,5,8tetrachloronaphthalene. The phosphorescence from this molecule occurs at much longer wavelengths than expected, and the vibronic structure is broad and poorly resolved. Both features are presumably a consequence of the intramolecular strain caused by the interactions between adjacent chlorine atoms. The bathochromic shift observed can be rationalized by making the reasonable assumption that the strain energy in the triplet state is less than that 0 1980 American Chemical Soclety
Symmetrical Tetrachloronaphthalenes
The Journal of Physical Chemistry, Vol. 84, No. 26, 1980 3523
TABLE I : Phosphorescence Decay of Tetrachloronaphthalenes in EPA isomer
kelfptl,
lifetime? ms
s
-
hcal_5d,b S
k'ca$d,c S
11.36 10.21 8.93 11.36 10.21 20.4 12.12 10.97 87.7 10.60 9.46 13.2 a Uncertainties represent 95% confidence limits. See Experimental Section for conditions. Calculated as in Reported as ref 2. k' = k , t z k i H t chix. 107 f. 1(SD) ms iin ref 2. 1,2,3,4 133,597 1,4,5,8 2,3,6,7
112 f 3d 49* 2 11.4t 0.2 76 f. 3
+::A I '
spin-orbit coupling between T,T* singlets and triplets is vanishingly small;6 this selection rule accounts for the relatively long lifetime and out-of-plane polarization of the phosphorescence from T,T* triplets. However, distortion of the aromatic plane can result in a breakdown of this spin-orbit selection rule and lead to a shorter lifetime12 and in-plane p~larization'~ of the phosphorescence. X-ray measurements on 1,4,5,8-tetrachloronaphthalenecrystals have shown not only that the chlorine atoms are out of the aromatic plane but that the ring itself is twisted.14 Hence the short lifetime may be explained by assuming that some T,T* singlet character has been mixed into the emitting triplet state. It is unfortunate that the broad spectrum obtained does not permit a determination of the amount of in-plane polarization present in the 0-0 band, which could verify thiie hypothesis. It is interesting to compare these results with those of Miller et al.2 They examined the additivity of the internal heavy-atom effect by testing eq 1where kbt is the observed kbt
= Ck,
(1)
1
Flgure 1. Piiosphowscence spectra of several tetrachloronaphthalenes in EPA. Although the spectra are uncorrected, the experimental conditions were the same with the monochromator slits set at 0.3 mm.
in the ground state. If one assumes2 that the addition of each CY chlorine shifts the 0-0 band 600 cm-', then the additional shift observed corresponds to -2 kcal/mol less strain energ!{ in the triplet state than the ground state. Although the total strain energy present in the molecule has not been reported, this might be compared with the strain energy of :15 kcal/mol found in 1,4,5,8-tetramethylnaphthalene by calorimetric methods.'l The broad bands observed could be caused by the presence of moleculles having slightly different molecular distortions arid hence different strain energies. Efforts are currently being made to find a solvent (or crystal host) in which better resolution can be obtained. Lifetime Measuivments. The results of the phosphorescence lifetime measurements are summarized in Table I. The decay curves were found to be exponential over several lifetimes and independent of the wavelength monitored. The most significant observation is the relatively short lifetime obtained for 1,4,5,8-tetrachloronaphthalene. This may be explained by the distortion from planarity caused by steric interactions between the peri chlorine atoms. It might be noted here that 1,8-dichloronaphthalene had the shortest lifetime of the dichloronaphthalenes examined by Miller, Meek, and Strickler,2although the effect was much less pronounced. It is well known that for planar aromatic compounds
rate constant foy the decay of phosphorescence and the ki values represent the rate constants observed for the appropriate monolhalogenated molecules. Table I includes a comparison of the observed rate constants and those calculated by using 3.03 and 2.65 s-l for k , and k,, respectively.2 Agreement between the calculated and experimental values is poor, with the calculated value usually being too low. These results are similar to those reported in ref 2, where the only polyhalogenated naphthalenes for which the calculated value exceeded the observed rate were those substituted in both the 1and 2 positions. Although Miller et al. ascribe these low values to steric effects, this seems unlikely in light of the previous discussion and the fact that their d a h show the disagreement to be essentially the same for l,2-dibromonaphthalene and 1,2-dichloronaphthalene but less in 1,2,3,4-tetrachloronaphthalene. In spite of the fact that the calculated rates are generally lower than those observed, eq 1should be expected to yield rate constants which were too high. This is because each k, includes the inherent decay constant of naphthalene. One possible way to avoid adding this factor in several times would be to use an equation of the form
+
(2) = ko Ck[ where ko is the rate constant for naphthalene and the k[ values are again obtained from the data on the monohalogenated compounds. Alternatively, one can modify the equation sucessfully used by Watts and Strickler15to account for lifetime data of various deuterated naphthalenes. The modified equation becomes ktot
kbt' = ko
+ Ck,H + Ck
I
(3)
where ko is the rate constant observed for perdeuterionaphthalene, kZHaccounts for the increased (nonradiative)
3524
J: Phys. Chem. 1980, 84, 3524-3529
decay on substituting H, and k; accounts for the increased (radiative and nonradiative) decay on substituting species x. Watts and StricklerI5found ko = 0.0458 s-', kaH = 0.0523 s-l, and kBH = 0.0318 s?. Applying these values to the lifetime data of the monochloronaphthalenes reported in ref 2 yields k,C1 = 2.70 5-l and kaC1= 2.30d. These values may then be used to calculate the decay rate for the tetrachloronaphthalenes. These calculated rate constants are also included in Table I, and as expected they are slightly lower than those calculated previously. The failure of a linear relationship to account for the results should not be unexpected. Addition of a heavy atom increases the mixing between triplet and singlet states in the molecule. If one makes the reasonable assumption that this mixing (i.e., the spin-orbit coupling) is proportional to the number of heavy atoms, then the transition probability should be proportional to the square of the number of heavy atoms. Inspection of the data, however, clearly shows that this would vastly overestimate the rate constants (except for the 1,4,5,8isomer). Thus it would appear that there is no simple relationship between the triplet lifetime and the position and number of halogens in polyhalogenated naphthalenes. Although more information about the relative amount of radiative and nonradiative decay would be helpful, it is unlikely that there is any simply relationship between the internal heavy-atom effect and the number and position of the heavy atoms. Different geometries could be expected to mix in different singlet states, and transition moments
associated with these singlet states might add or cancel to different degrees depending on the g e ~ m e t r y .These ~ factors alone would make any simple correlation unlikely. Acknowledgment. I am grateful to Dr. Robert Eagar, Jr., and the Union Carbide Corp. for providing me with the 2,3,6,7-tetrachloronaphthaleneused in this study as well as authentic samples of 1,3,5,7and 1,4,5,8isomers. I am also grateful to Dr. H. Reimlinger for his help in locating these samples, which he had prepared.
References and Notes (1) S. P. MoGlynn, T. Azumi, and M. Kinoshita, "Molecular Spectroscopy of the Triplet State", Pretice Hall, Englewood Cliffs, NJ, 1969 (especially Chapter 7 and references therein). (2) J. C. Miller, J. S. Meek, and S. J. Strlckler, J . Am. Chem. Soc., 99, 8175 (1977). (3) See, for example, V. A. Koptyug and V. A. Plakhov, Zh. Obshch. Khim., 32, 256 (1962); J. Gen. Chem. USR(€ng/. Trans/.),32, 249 (1962). (4) A Reiser and T. R. Wright, J . Chem. Phys., 59, 3433 (1973). (5) R. I. T. Cromartle and J. N. Murrell, J . Chem. Soc., 2063 (1961). (6) D. M. McClure, J. Chem. Phys., 20, 682 (1952). (7) H. Relmllnger and 0. Klng, Chem. Ber., 95, 1043 (1962). (8) T. Pavlopoulusand M. A. ECSayed, J. Chem. phys., 41, 1082 (1964). (9) M. A. El-Sayed, J . Chem. Phys., 43, 2864 (1965). (10) J. Ferguson, T. Iredale, and J. Taylor, J. Chem. Soc., 3160 (1954). (11) M. Mansson, Acta Chem. Scand., Ser. B , 28, 677 (1974). (12) One example is the decreased phosphorescence llfetlme of the nonplanar helicenes. M. Sapir and E. Van der Donckt, Chem. phys. Lett., 36, 108 (1975). (13) See, for example, N. K. Chaudhuri and M. A. El-Sayed, J . Chem. Phys., 47, 2566 (1967). (14) G. Gafner and F. H. Herbstein, Acta Clysta//ogr., 15, 1081 (1962). (15) R. J. Watts and S. J. Strickler, J. Chem. Phys., 40, 3867 (1968).
Pulse Radiolysis Studies Concerning the Reactions of Hydrogen Abstraction from Tetraalkylammonium Cations' K. Bobrowski* Radiaflon Laboratory, Unlversyl of Notre Dame, Notre Dame, Indlana 46558 (Received: June 25, 1980)
The rate constants for reactions of hydroxyl (OH), sulfate (SO4-), phosphate (HPO,), and chloride radicals (Clf) with the first four homologues of tetraalkylammonium cations were determined by the spectrophotometric pulse radiolysis method. The reactivity of tetraalkylammoniumcations toward these radicals increases with increasing number of C-H bonds in the molecule. Intercomparisonof the rate constants for the different radicals shows a trend characteristic for a mechanism involving hydrogen abstraction, e.g., kOH+R,N+ > ks04-+F,N+> k H p 0 4 - +>~kc12-+Rfl+. ~ The ratios of the reactivities are in good agreement with those reported for other ahphatic compounds which undergo hydrogen abstraction. The reactivities of long-chain tetraalkylammonium cations are the sum of the partial reactivities of C-H bonds at different positions in the molecules, indicating that the reaction of OH radicals with these cations involves the abstraction of H atoms in the rate-determining step. The partial reactivity for hydrogen abstraction from carbon atoms adjacent to the nitrogen center is nearly two orders of magnitude less than for comparable groups in aliphatic hydrocarbons. Influence of the nitrogen center extends even to the /3 position where partial rate constants are a factor -5 less.
Introduction In the present study the reactions of hydroxyl (OH), sulfate (So4-),phosphate (HPo4-),and dichlorides (C12-) radicals with the homologues of tetraalkylammonium cations (from tetramethyl to tetrabutyl) were examined in order to determine and compare their rate constants and to elucidate the reaction mechanism. The reactions of these radicals with organic compounds in aqueous solutions have been extensively studied by the spectrophotometric pulse radiolysis technique.a4 In general, two types of 0022-3654/80/2084-3524$0 1.OO/O
reaction were found to take place, i.e., hydrogen abstraction and electron transfer. When electron transfer reaction is considered the OH radical is a weaker oxidant than SO4and H2P04. Hydroxide ion can be, in fact, oxidized by the latter radicals while oxidation of sulfate and phosphate The conclusion that ions by OH radicals is very the OH radical is a weaker oxidant can be also drawn from a comparison of the reactions of these various radicals with carboxylic acids.' However, the oxidizing capability of C12-scan be placed between those of SO4-and HP04-. In 0 1980 American Chemical Society
