JAMES B. GALLIVAN
3070 the transition is in line with the effects observed in some biological membranes for which there is some evidence of a conformational t r a n ~ i t i o n . ~ JIn order to study the role of a conformational transition on the electrical properties of a membrane, we are now investigating the transport properties a t pH's where the protein is no longer isoelectric using salt concentrations considerably smaller than those considered here.
Acknowledgments. The authors are greatly indebted to Professor A. Borsellino for his interest in this work, to Dr. R. Fioravanti for designing the apparatus for thickness measurement, to Dr. R. Pierantoni for the electron micrographs of the membranes, and to Mr. G. Prestipino for help in the experimental work. This work has been supported by the "Gruppo Nazionale di Cibernetica," C.N.R., Genoa, Italy.
Luminescence Characteristics of Phenyl- and Halophenylnaphthalenes
by James B. Gallivan American Cyanamid Company, Central Research, Divdsion, Stamford Laboratories, Stamford, Connecticut (Received December $7, 1968)
The extent t o which phenyl substitution on naphthalene alter; its luminescence properties depends on the number and position of the phenyl groups. I n 3-methylpentane a t 77'K, successive e-phenyl substitution reduces both singlet- and triplet-state energies and considerably shortens triplet-state lifetimes. The ortho phenylated naphthalenes examined were characterized by higher excited-state energies and longer phosphorescence lifetimes than even less-substituted derivatives. The luminescence features of the 1-(halopheny1)naphthalenes are particularly susceptible t o the position of the halogen atom. I n the para position, the halogen produces effects which one normally associates with the presence of a heavy atom, whereas in the ortho position the heavy atom effect is partially obscured because of steric factors. The combined fluorescence and phosphorescence spectra, +P/+)F ratios, and triplet-state lifetime data were used t o assess the role of electron densities, steric factors, geometrical configurations, and heavy-atom perturbations in determining the excited-state properties of the systems examined.
Introduction The changes in luminescence characteristics of aromatic hydrocarbons induced by different substituents are many and varied. At the one extreme, simple alkyl substituents produce relatively minor changes in luminescence On the other hand, certain chromophores such as nitro or carbonyl groups result in major changes in the excited-state properties, due to the presence of nonbonding electrons and n, s* states of low en erg^.^^^ The effects of heavy atoms, particularly halogens, on the luminescence properties of aromatic hydrocarbons have been reported in numerous publication^.^-^ These latter studies have been of particular interest since no new types of states have been introduced, yet changes in spin-orbit coupling factors result in great changes in the rates of relevant radiationless and radiative rate prqcesses-and hence luminescence properties. The most extensively examined system has been naphthalene, with halogens serving as ~ ~ ~ ~atoms. '~ interna11p5Jjor external (in s o l ~ e n t ) heavy I n addition to noting changes in the relative yields of fluorescence and phosphorescence and changes in phosphorescence lifetimes, the perturbations produced by The Journal of Physical Chemistry
heavy atoms have been monitored by phosphorescence polarization, 11,12singlet-t'riplet absorption, 13,14and electron-spin resonance techniques.'6t16 (1) V. L. Ermolaev and K. K. Svitashev, Opt. Spectry., 7,399 (1959). (2) G. N. Lewis and NI. Kasha, J.Amer. Chem. Soc., 66, 2100 (1944). (3) M. Kasha, Radiation Res. Suppl., 2, 243 (1960). (4) M. Kasha, Discussions Faraday Soc., 9, 14 (1950). (5) D . S. McClure, J. Chem. Phys., 17, 905 (1949). (6) S. P. McGlynn, R. Sunseri, and N. Christodouleas, J . Chem. Phys., 37, 1818 (1962). (7) J. K. Roy and L. Goodman, J . Mol. Spectry., 19, 389 (1966). (8) T. Pavlopoulos and M. A. El Sayed, J . Chem. Phys., 41, 1082 (1964). (9) S,P. McGlynn, J. Daigre, and F. J. Smith, ibid., 39, 675 (1963). (10) S. P. McGlynn, M. J. Reynolds, G. W. Daigre, and N. D. Christodouleas, J. Phys. Chem., 66, 2499 (1962). (11) M. A. El Sayed and T. Pavlopoulos, J . Chem. Phys., 39, 1899 (1963). (12) N. K. Chaudhuri and M. A. El Sayed, ibid., 46, 1358 (1966). (13) D. S. McClure, N. W. Blake, and P. L. Hanst, ibid., 22, 225 (1954). (14) A. P. Marchetti and D. R. Kearns, J. Amer. Chem. Soc., 89,768 (1967). (15) S. Siege1 and H. S. Judeikis, J. Chem. Phys., 42, 3060 (1965). (16) M. S. de Groot and J. H. van der Waals, Mol. Phys., 4, 189 (1961).
307 1
LUMINESCENCE CHARACTERISTICS OF PHENYLAND HALOPHENYLNAPHTHALENES I n this communication, the luminescence features of some phenyl- and halophenylnaphthalenes are reported. I n an earlier paper,17 changes in excited singlet- and triplet-state energies resulting from successive a-phenyl substitution on naphthalene were reported. Here, the luminescence results of the a-phenyl derivatives are compared to p-phenylnaphthalene and some orthosubstituted derivatives. These results provide some insight into the way the extended .rr-orbital interaction is influenced by position of substitution (charge density) and by steric and geometric factors. The l-(halopheny1)naphthalenes introduce still another parameter -the heavy atom and its consequent effect on spinorbit coupling. I n the l-(o-halophenyl)naphthalenes, the heavy-atom effect has to compete with steric effects which minimize inter-ring r-orbital interaction.
Experimental Section Relevant information on the synthesis and purification of the naphthalene derivatives received from Drs. W. A. Henderson, Jr., and A. Zweig has been presented elsewhere. l* The commercial (Aldrich) 2-phenylnaphthalene contained anthracene which was removed by the method of Kooyman and F a r e n h 0 r ~ t . l ~The 1,2,4triphenyl and 1,2,3,4-tetraphenyl derivatives were synthesized by Dr. J. Innes following procedures given elsewhere. 2o The solvent used for the luminescence measurements, 3-methylpentane (3?tIP), was purified in a manner described previously.21 The spectrometer and low-temperature (77°K) cell used for the luminescence measurements have been referred to before,22while the method used for determining corrected C$p/& ratios has also been ~ u t l i n e d . ' ~ M in I n all cases, samples ranging from to 3RIP were cooled to 77°K and excited with uv light through a monochromator and appropriate filters. The 1000-W Xenon arc used for excitation purposes has a continuum output in the uv so that excitation wavelengths below 300 mp permitted fluorescence measurements without interference from scattered exciting light. Phosphorescence lifetimes were measured by photographing the decay of the phosphorescence signal (displayed on an oscilloscope screen) following extinction of the exciting light. For both +P/+F determinations and phosphorescence-lifetime measurements, the precision fell within the limits of &lo%. Results and Discussion Phenylnaphthalenes. The effects of monophenyl substitution on the fluorescence and phosphorescence spectra of naphthalene are shown in Figure 1. All spectra were measured using comparable slit widths (-50 cm-l) ; however, the intensities are arbitrary since the phosphorescence spectra were measured in the delayed-emission (chopped) mode. The naphthalene spectra show well-resolved vibrational structure so that the energy of the excited states can be defined
320
,
I
360
400
-. 440
I
I
480
520
560
600
a(mr)
Figure 1. Fluorescence and phosphorescence spectra in 3-methylpentane at 77°K: (top) naphthalene, (middle) l-phenylnaphthalene, (bottom) 2-phenylnaphthalene.
accurately by the maxima of the 0-0 bands. The monophenylnaphthalene spectra are much more diffuse, with the 0-0 bands of the fluorescence partially submerged in more intense, longer-wavelength bands, and the 0-0 bands of the phosphorescence completely lost in broad-band envelopes. Thus the energies of the lowest-lying excited singlet and triplet states of the phenyl derivatives cannot be determined as accurately as those of naphthalene. Since for our purposes intermolecule comparisons of excited-state energies are more important than the absolute energies for any single molecule, the energies reported for all molecules showing such spectra were taken a t 10% of peak intensity for reasons given by Zweig and Gallivan." The fluorescence and phosphorescence spectra of the 1,2,3,4- and 1,4,5,8-tetraphenyl naphthalenes shown in Figure 2 illustrate rather dramatically the importance of steric effects on these processes. The excited-state energies, mean phosphorescence lifetimes, and ~ P / c $ ratios for all the phenylnaphthalenes examined are (17) A. Zweig and J. B. Gallivan, J . Amer. Chem. SOC.,91, 260 (1969). (18) W.A. Henderson, Jr., and A. Zweig, ibid., in press. (19) E. C. Kooyman and E. Farenhorst, Trans. Faraday SOC.,49, 58 (1953). (20) (a) L. F. Fieser and M. J. Haddadin, Can. J . Chem., 43, 1604 (1965); (b) U.8. Patent 3,123,649(Sun Oil Co.). (21) J. B. Gallivan and W. H. Hamill, J . Chem. Phys., 44, 1279 (1966). (22) J. B.Gallivan, J. S. Brinen, and J. G. Koren, J . Mol. Spectrosc., 26, 24 (1968).
Volume 79, Number 9 September 1969
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JAMES B. GALLIVAN
3072 ~
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~
Table I : Luminescence Characteristics of Naphthalene.and Some of I t s Phenyl Derivatives in 3-Methylpentane a t 77°K Compound
Naphthalene 1-Phenylnaphthalene 2-Phenylnaphthalene ll4-13ipheny1naphthalene l15-Diphenylnaphthalene l,%Diphenylnaphthalene 1,2,4-Triphenylnaphthalene l14,5-Triphenylnaphthalene 1,2,3,4-Tetraphenylnaphthalene l14,5,8-Tetraphenylnaphthalene
31,645 31 ,055 30,120 29 ,585 30 ,485 30,395 29,760 28,735 29 ,585 26,525
21 ,185 20,660 20,745 19,840 20 ,240 20 ,080 19,960 19,155 20,285 17,700
0.12 0.11 0.10 0.06 0.08 0.05 0.07 0.03 0.18 SO0 cm-’) due to the broadband nature of the spectra and the absence of clearly defined electronic origins. given in Table I. Significant differences in these These features can be explained by Franck-Condon parameters are apparent for the various isomers. considerations if the transitions originate from a state Specific features which stand out include the differences having one equilibrium geometrical configuration and in phosphorescence lifetimes of the two monophenyl terminate in a state having a very different equilibrium derivatives and significant differences in all three configuration.14 Comparison of the singlet-triplet abparameters between the two triphenyl and two tetrasorption spectra and phosphorescence spectra of these phenyl isomers. two compounds led ,Marchetti*and Kearns to the conThe results in Figure 1 and Table I illustrate that clusion that the lowest triplet state is probably planar monophenyl substitution on naphthalene results in luand the ground state twisted in both systems. Reminescence characteristics that depend on the position of cently, Holloway, et aZ.,28 reached similar conclusions substitution. The a position has been found to have concerning the ground state and lowest excited singlet much higher electron-spin density than the ,R position in the lowest triplet statezsand in the naphthalene negative (23) C. A. Hutchison, Jr., and B. W. Mangum, J . Chem. Phys., 34, ion.24 In addition, the a position is more reactive 908 (1961). chemically toward electrophilic, nucleophilic, and rad(24) T. R. Tuttle, R. L. Ward, and 8. I. Weissman, ibid., 25, 189 ical reagentsz6 with the differences in chemical reactiv(1956). (25) K. Higasi, H. Baba, and A. Rembaum, “Quantum Organic ity between the two positions being adequately exChemistry,” Interscience Publishers, New York, N. Y., 1965,p 116. plained by the frontier-electron theory.26 The differ(26) K. Fukui, T. Yonezawa, and H. Shingu, J . Chem. Phys., 20, ences in luminescence properties of the two monophenyl 722 (1952). naphthalenes are thus consistent with various other ob(27) H.H. Jaffe ancl,M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy, John Wiley & Sons, Inc., New York, N. Y., servations indicating significant differences in electron 1965,p 307. density at the a and ,R positions in both ground and ex(28) H.E.Holloway, R. V. Nauman, and J. H. Wharton, J . Phye. cited states. Chem., 72, 4468 (1968). 1
The Journal of Physical Chemistry
LUMINESCENCE CHARACTERISTICS OF PHENYLAND HALOPHENYLNAPHTHALENES state of some 2-phenylnaphthalenes, based on absorption and fluorescence data under various experimental conditions. An empirical relationship between the measured phosphorescence lifetime ( T ~ )and the energy of the phosphorescent state (ET), has been proposed by Robinson and FroschZ9 and further refined by SieSince the triplet-state energies of 1- and 2phenylnaphthalene are very similar according to our estimates (Table I), the factor of 2 difference in T~ is rather surprising. Because of the uncertainty in determining ET and the geometry differences between the states involved, the validity of applying Siebrand’s formulation to these systems may be questioned. However, even if the high-energy extreme were used for 2phenylnaphthalene (20,920 cm-l) and the low-energy extreme for l-phenylnaphthalene (19,800 cm-’), the factor of 2 difference in lifetime would not be predicted. These results point to the difficulties encountered if present theories are used to explain absolute lifetime differences of the magnitude observed here. The striking similarities between the singlet-triplet absorption spectra of these isomers14indicate that the measured lifetimes primarily reflect differences in radiationless rates of depopulating the lowest triplet state. Brinen and Orloff3’ have noted in their studies of zero-field splitting (ZFS) of aromatic triplets that molecules with low D values have shorter triplet-state lifetimes than isomeric molecules with larger D’s. EXperimental values of 0.092 cm-’ for l-phenylnaphthalene and 0,099 cm-’ for 2-phenylnaphthalene yield from a of D os. log T~ values for T,,, of 1.1and 2.4 sec, respectively, both in very good agreement with experiment (Table I). Although these relationships between D and log r m and D and ET are extremely interesting and yield generally good linear plots, the basis for these relationships is not understood a t this point. The introduction of a second phenyl group into 1phenyl naphthalene results in a lowering of excited singlet- and triplet-state energies and reduced + P / ~ F ratios. Two of the isomers show significantly shorter triplet-state lifetimes than the l-phenyl derivative, while the 1,5 derivative actually has a slightly longer lifetime, The lower excited-state energies of the diphenyl derivatives have been accounted in terms of extended r-orbital delocalization due to p r p n overlap of nonperpendicular adjacent orbitals on the phenyl and naphthyl rings. Energy and lifetime differences of the magnitude observed among these isomers cannot be explained since the electronic interactions and steric factors involved are not that well understood. The importance of steric factors in the excited states becomes more evident in the tri- and tetraphenylnaphthalenes. Whereas the excited-state energies of the aphenylated derivatives continue to fall with the introduction of each additional phenyl group, little change in energy is observed when a phenyl is added ortho to
3073
another phenyl in the same ring (Table I), In addition, the C$P/~#JF ratios and phosphorescence lifetimes continue to fall in the polyphenylated a derivatives, while these trends are reversed for the o-phenylnaphthalenes. These results indicate decreased inter-ring interaction in the ortho-substituted compounds due to severe steric factors, with no such restrictions applying t o the m-subst itu ted derivatives The results reported here demonstrate that in the lowest excited electronic states significant interaction occurs between the aryl rings of all but the most sterically hindered phenyl naphthalenes. The concept of a planar triplet-state configuration for biphenyl was first proposed by Lewis and Kasha,2 and experimental evidence consistent with such a proposal was presented by McClure.6 Recent experimental eviden~e~~v34 supports the idea of a planar triplet-state geometry for biphenyl and related compounds, and such a configuration has been proposed for phenylated naphthal e n e ~ . ’ Based ~ ~ ~ ~on ground-state bond lengths and bond angles,a6as well as atomic radii, there are obvious steric barriers to coplanarity, even in the monophenyl derivatives. However, it is worthwhile noting that in the racemization of optically active biphenyl derivatives, increased resonance interaction is achieved when the transition state assumes a coplanar configuration by bending carbon-carbon-halogen bonds.37 In related reactions, the increased resonance interaction may result from the stretching of interannular bonds, the compression of ortho bonds, or by the warping of aromatic rings.38 These observations point to the danger of drawing conclusions about excited-state configurations based on information that pertains solely to the ground state. Energy barriers of a few kilocalories which may determine ground state configurations may be of little significance in excited states of considerably greater energy * ~-(HuZophenyl)nuphthalenes. In Table 11, the excited-state energies, mean phosphorescence lifetimes, and $ P / ~ F ratios are given for some halogenated phenyl-
.
(29) G. W. Robinson and R. P. Frosch, f. Chem. Phya., 38, 1187 (1963). (30) W. Siebrand, ibid., 44, 4055 (1966); 47, 2411 (1967). (31) J. 8. Brinen and M. K. Orloff, Chem. Phys. Letters, 1, 276 (1967). (32) J. S. Brinen and M. K. Orloff, private communication. The plot used consisted of experimental values of D vs. experimental values of log T for the compounds reported in ref 31. (33) M.K.Orloff and J. 9. Brinen, J . Chem. Phys., 47, 3999 (1967). (34) P. J. Wagner, J. Amer. Chem. SOC.,89, 2820 (1967). (35) J. S. Brinen and M. K. Orloff, Int. J . Quantum Chem., 3, 225 (1969). (36) L. Pauling, “The Nature of the Chemical Bond,” 3rd ed, Cornell University Press, Ithacrt, N. Y., 1961,Chapter 7,p 221. (37) F. H. Westheimer in “Steric Effects in Organic Chemistry,” M. 8. Newman, Ed., John Wiley & Sons, Inc., New York, N. Y., 1956,p 549. (38) E. Eliel, “Stereochemistry of Carbon Compounds,” McGrawHill Book Co., Inc., New York, N. Y., 1962,p 161.
Volume 73, Number 0 September 1000
3074
JAMES B. GALLIVAN
Table I1 : Comparison of the Emission Properties of l-Halonaphthalenes"'b and 1-( Ha1ophenyl)naphthalenes' ET,om-1
Compound
&/+F
rm, 8ec
a-Chloronaphthalene 20,700 5 0.30 1-(0-Chloropheny1)naphthalene 20 ,900 3.7 1. 7 0.58 0.78 1-(p-Chloropheny1)naphthalene 20,530 a-Bromonaphthalene 20,650 270 0.02 1-(0-Bromopheny1)naphthalene 21 ,000 30 0.50 a-Iodonaphthalene 20,500 800 0.002 1-(0-1odophenyl)naphthalene 21,000 300d 0.050 1-Phenyl-8-iodonaphthalene 20,080 500d
