EXTERXAL SPIN-ORBITAL COUPLING
Dec., 1962
of molecular flu0 rescence and phosphorescence and groups of very weak lines characteristic of the Pr3+ ion. The latter are diffuse and ill-defined and are obtained only under prolonged exposures. For the benzoylacetonate chelate, four groups of lines are observed in the region of 6000-10,000 A. (see Fig. 7). The Pr3+ emission from the dibenzoylmethide chelate also consists of four groups of lines, but some prominent components present in the spectrum of the benzoylacetonate chelate are missing. The line emission from the o-hydroxybenzophenone chelate of Pr3+ is simpler still, with the highest energy group of lines disappearing entirely. I n Fig. 8 n e have plotted the known resonance levels of the Pr3+ ion and triplet state energies of the three chelates used in this study. The progressive simplification of the line spectra discussed above can be correlated to the positions of the resonance levels of the ion relative to the triplet state energies of the complexes. I n praseodymium trisbenzoylacetonate, all three resonance levels can be excited by energy transfer. In the dibenzoylmethide chelate, just the lower two levels can be excited, adequately accounting for the non-appearance of some of the Pr3+ lines in the spectrum of this compound. For the chelate derived from o-hydroxybenzophenone only the lowest resonance
2499
level of the Pr3+ ion can be excited by intramolecular energy transfer, resulting in the simplest ion spectrum observed from the three compounds. We wish to emphasize that the emission spectra observed from chelates of the Pr3+ ion are extremely weak and require many hours of exposure. Total luminescence yields from the compounds are low, indicating that the closely packed energy levels of the Pr3+ion provide an extremely efficient path for energy degradation. This fact along with the diffuseness of the observed lines shows that the Pr8+ion is coupled strongly to the ligands. Strong coupling for this ion also is reported for hydrated inorganic salts. Rare earth chelates comprise a class of compounds especially valuable for studying energy migration in complex molecules. Because of the intrinsic optical properties of these ions, they assume a unique role as internal indicators for these radiationless processes. In addition, the phenomenon of intramolecular energy transfer permits selective excitation of rare earth ions and provides useful information for locating the lowest triplet states of the complexes themselves. Acknowledgments.-The research presented in this communication was sponsored by Sandia Corporation under P.O. No. 51-0244 and by the National Science Foundation.
THE EXTERNAL HEAVY-ATOM SPIN-ORBITAL COUYLISG EFFECT. 111. PHOSPHORESCENCE SPECTRA AND LIFETIMES OF EXTERNALLY PERTURBED NAPHTHALENES1v2 BY S. P. MCGLYNN, M. J. REYNOLDS, G. W. DAIGRE, AND N. D. CHRISTODOYLEAS Coates Chemical Laboratories, Louisiana State University, Baton Rouge 3, Louisiana Received M a y 26, 1962
The phosphorescence spectra and decay times of naphthalene and all of its 1-monohalogenated derivatives have been measured a t -190" in EPA,l8 and in cracked glasses which consisted of the various combinations of halonaphthalene and propyl halide in 2: 5 mole ratio. The lifetimes were found to decrease as the spin-orbital coupling factor of either the internal or external halogen increased. It is concluded that weak complexes of a charge-transfer nature form and that there is a genuine heavy-atom effect. The phesphorescence decays, as expected, were found in all cases to be non-exponential, and to be reproducible analytically only aa the sum of a large number of first order decays of different rate constants. It is concluded from this behavior that complex geometry can vary considerably about some most probable conformation. It is found that the product r,(n be)* is roughly constant, deviations from constancy being interpretable aa due to increasing phosphorescence quenching and radial contributions to the perturbation integral H'T.sP. The results obtained a t f30' from absorption data and a t -195.8' from phosphorescence data are shown to be identical, and t o validate the spin-orbital coupling and com lexing premises. Shifts in the 0,O position o f t h e T S emission have been observed for the one emitter in various matrices. These shifts are of the order of 0.25-0.5 kcal./mole and are of the same relative behavior as the ratios of lifetimes in the various media. Shifts in various fundamental vibrational frequencies also have been noted, and a geometric specificity of interaction is derived tberefrom. An analysis of the T -.,S emission of naphthalene is possible in terms of four a, vibrational Ale nature of this transition. frequencies, in accord with the B1,
+
-
Introduction It was observed by Kasha3 in 1952 that a binary solution of two colorless components : l-chloronaphthalene and ethyl iodide, was of a yellow color. (1) This research was supported by a National Science Foundation Grant to The Louisiana State University, and by a Grantin-aid from the American Instrument Company of Silver Spring, Maryland. ( 2 ) Other papers in the present series are: (I) S. .'1 McGlynn, 11. Sunseri, a n d N . Christodoyleas, J . Chem. Phys., submitted for publication; and (11) ,J. Nag-Chaudhuri, L. Stoessell. and 9. P. RlcGlynn, J . Mol. Spectroscopy, submitted for publication. (3) M. Kasha, J . Cksm. Phys., 20, 71 (1952).
The effect was attributed to an increase of spinorbit coupling in the halonaphthalene. The supposition that just such a relaxation of spin-forbiddenness might occur preceded the ob~ervation,~ and this supposition apparently derived from an intuitive association of the known effectiveness of ethyl iodide as a fluorescence quencher5with the demonstrzttion of intrumolecular heavy-atom spin(4) M. Kasha, private discussion. ( 5 ) P. Pringsheim, "Fluorescence and Phosphoresceuce." Interscience Publishers, New York, N. Y . , 1989.
2500
S. P. MCGLYNN, M. J. REYNOLDS, G. W. DAIGRE, AND X. D. CHRISTODOYLEAS T’ol. 66
orbital coupling by McClure.6 It was thought that without any resulting significant stabilization or the ethyl iodide was functioning as a fluorescence destabilization of the donor-acceptor pair. In quencher in liquid solution by promoting inter- practice, however, it denotes the situation where system crossing from the fluorescent singlet level effects not attributable to energetic collisions are to the triplet manifold of levels. observed, but where definite complexing cannot be The generality of this intermolecular heavy- detected either. If some effect due to contacts atom enhancement of T +S transition probabilities (and not to complexes or collisions) were isolable now has been reasonably well established.*JJ then presumably such an effect mould be temHowever, all investigations, with a few exceptions perature independent. No such “clean” effect have been confined to measurement of absorption has thus far been unearthed, and in its absence the spectra. KO significant attention has been given present authors prefer to consider a contact as a to the equally relevant measurements of phos- complex, the stabilization energy of which is less phorescence lifetimes or the relative phosphores- than ambient thermal energy kT. This latter cence to fluorescence quantum yields of externally viewpoint is, of course, pragmatic in that it makes perturbed emitters. And yet these types of logical for us the situation in which at room temmeasurements would seem to afford a very direct perature one can measure for a binary solution by means of establishing whether the observed spin- Benesi-Hildebrand procedures’2 an equilibrium orbital coupling increases were brought about by constant K 0 and an extinction coefficient B energetic collisions as suggested by Kasha or by a m , 1 3 and for which a t - 190°, where collisions must weak complexation as suggested by M ~ G l y n n . ~be considered unimportant, one can detect large The reasons for this differentiation might be stated decreases of T~ for one of the components attributas follows: since emission experiments will be able to the presence of the second constituent. carried out a t -190’ in the conventional manner Recent work by Tsubomora and Mulliken14 in some sort of solid matrix, “energetic collision” on the oxygen enhancement of T + S transition will be impossible, and an observed decrease of probabilities indicates that this relaxation of spinphosphorescence lifetime, T ~ or , of the ratio fluoresforbiddenness is not connected with the magnetic cence ’phosphorescence, @pf @J~, will be compatible moment of oxygen. Rather, it would seem to inonly, to a good level of approximation, with the dicate that the charge-transfer state functions by idea of a complex between the emitter and the mediating the mixing of singlet and triplet states perturber. It is true that high local pressures of the donor, and that the phenomenon should be resulting from strains, defects, etc., in the solid quite general (at least for weak complexes) and matrix might produce the same effect as an independent of acceptor species ( i . e . , oxygen or “energetic collision.” However, in view of the ethyl iodide). There exists evidence that such is results of the high pressure experiments of Robert- true in some well defined complexes: it is son and (R. E.) Reynolds’O on the l-chloronaph- that in the complex of anthracene with sym-trithalene-thy1 iodide liquid system, where significant nitrobenzene, T, decreases and QI, increases relative changes of pressure produced only small changes in to uncomplexed anthracene, and Czekalla16 has optical density, major decrease of T~ or of @q/ shown that in a wide variety of complexes of nonaP due to local pressures (or “frozen collisional paramagnetic non-heavy-atom-containing comconformations”) in glasses mould not seem likely. ponents, decreases of rp by an order of magnitude The reasons behind the suggestion of a weak are not uncommon. An increase of the ratio complex between emitter (absorber) and perturber QIP/@f has been observed” for some of the same also must be elaborated. The interpenetration of complexes. Similar decreases of T~ of solutes disthe 7-electrons of, let us say, 1-chloronaphthalene solved in media with which no interactions of into the vicinity of the large field gradient of io- significance have ever been detected also have been dine which is necessitated by the results of Kasha3 observed (vide infra). ,4t least this latter case obviously implies charge transfer from the 1- accords with the predictions of Tsubomora chloronaphthalene to the ethyl iodide. The ques- and N ~ l l i k e n , ’ and ~ undoubtedly the case of tion then becomes one of degree: is the charge strong interaction could be similarly interpreted, transfer caused by complexing, contacting,” or although probably with lesser validity. collision? It seems appropriate to consider the Now that the situation to which the present concept of “contact.” A ‘‘contact’’ implies that work has reference has been sketched it is w-tll a t a distanvc of separation of a donor-acceptor to say what this work is and what it hopes to do. pair equal to the sum of their van der Waals radii, Briefly, the lifetimes ( T ~ ) and phosphoresccnw there is some donor-acceptor orbital mixing. It spectra of naphthalene, 1-fluoronaphthalene. I implies‘1a that mixing of wave functions can occur chloronaphthalene, 1-bromonaphthalene, and 1iodonaphthalene have been measured separately in (6) D. S. AIcClure, J. Chem. Phys., 17, 905 (1949).
-
(7) S. P. McGlynn and &.I.Kasha, “Symposium on Molecular Structure and Spectra,” Ohio State University, Columbus, Ohio, June, 1954, to be published shortly: fluorine, chlorine, bromine, and iodine substituted in saturated hydrocarbons, and sulfur in carbon disulfide. (8) R. 9. Becker, private discussion. lead in tetraalkyllead deilvatives (9) 8. t’ McGly~in,Chem. Rev.,68, 1113 (1958). (10) W W. Robeitsun and R. E. Reynolds. J . Chem. P h y s . , 29, 138 (19581 (11) R . S. Xlulliken, Ree. Lmc. ciizm., 76, 845 (1956). (lla) J N Nurrell, Mol. Phys., 3, 310 (1960)
-
(12) 11. 4. Benesi and J H. Hddebrand, J 4m Chem Soc , 71, 2703 (1949) (13) S P. hIoGlyiin and R. Sunsell, to be published, systeiii 1chloronaphthalene and ethyl iodide. (14) H. Tsubomora and R . S. Mulliken, J. Am. Chem Soc , 82, 5966 (1960). and references contained therein. (la) S. P. McGlynri, J. D. Bopgus, and Ti: I liiri I . C item P ’ k y v , 32, 357 (1960). (16) J Czekalla, G. Briegleb, W H p i i e , aiid H T ’iahlensieck Z CIektiochem., 63, 713 (1959) (17) N Chnstudoyleas and S P. 11IcOl)nn to bL &)ublislled.
Dec., 1962
EXTERSAL SPIN-ORBITAL COUPLIWG
2501
each of the four solvent media: EPA,Is propyl did not adversely affect any of the work reported herein. 1-Iodonaphthalene, propyl bromide, and propyl iodide were chloride, propyl bromide, and propyl iodide. It is run thought that the results will show that the heavy use. through a 15-cm. column of activated alumina prior to atom enhancement effect of T t-+ S transition probApparatus and Methods.-( a ) Lifetimes: The phosphoresabilities is not collisional, and that there is a valid cence emission was excited by either a d.c. operated Hanovia high pressure mercury-xenon lamp or a d.c. General Electric heavy-atom effect, in contradistinction to the ap- AH-6 lamp. The phosphorescence emission wm parently insignificant paramagnetic field effects of purifiedmercury mechanically by a Becquerel phosphoroscope driven oxygen, nitric oxtde, and various transition metal by a Zeromax motor continuously variable from 0-1200 ions. This latter finding would require some modi- r.p.m. The phosphorescence decay was picked up by a lP21 photomultiplier, displayed on the screen of a Tektronix fication of the Mulliken-Tsubomora theory at least 545A oscilloscope, photographed, and analyzed on a microfor these systems. However, such is not the pur- film reader. pose of this note, ,and it will be deferred until presenThe photomultiplier voltage wm supplied by an Atomic Instrument Co. super-stable high voltage source to a stand.~' tation of the av3 ilable experimental e ~ i d e n c e ' ~ ~ ~ ard voltage dividing network of resistors which held the sucis complete. stages 100 v. apart. The 1megohm input impedance It is to be noted that some of these stated aims cessive of the oscilloscope was used as the anode load resistor. The previously have been achieved by others. It has entrance slit to the photomultiplier was adjusted so that the been observedIg that the phosphorescent lifetimes anode load current was always leas than 10 pa. for a various substituted phthalimides measured in some display on the oscilloscope screen and was t.hus within operalimits for maximum linearity recommended by the twenty-one different solvent media were lowest tional manufacturer. or among the Iom-est, in cracked glasses (?) of The oscilloscope was operated on d.c. coupled internal propyl bromide. Large decreases of T~ were, trigger with signal inverted. The sharp pulse given by the burst of light as each slot of the phosphoroscope opened however, also observed by these authors in non- initial was used as the t'rigger for the horizontal sweep. Single heavy-atom-contiiining media. The benzene trip- sweep pictures were taken only of the first two half-lives. let lifetime at liquid nitrogen temperatures is However, some visual observations were made on later lifetimes using the sweep delay feature of the Tektronix 545A. in EPA,2° '7 sec.: dioxane,20 5 see.; 3.3 sec.; water,21 0.93 sec.; and carbon tetra- This was effected by the simultaneous use of two time bases, -4and B. Time base B was used to provide an accurate time chloride, 0.66 sec. Graham-Bryce and Cork- delay while time base A presented a normal horizontal sweep hi1122 undertook an investigation of the phosphores- a t t,he end of the delay period. After determination of the cence of coumarin, acid fluorescein, N,N-dimethyl- magnitude of the first observable half-life, this value was aniline, and eight isomeric dinitronaphthalenes used on B as the delay. The value of T,, measured for each half-life was added to the delay time. The enin glassy solutions containing one part in twenty- successive trance slit on the phototube was adjusted each time to give one (by volume) of ethyl iodide; in all cases the full scale display at the beginning of the delayed sweep. In effect of the ethyl iodide was to increase the in- this manner aa many as eleven consecutive half-lives were tensity of phospliorescence and decrease T ~ . The measured. All measurements were made at 77°K. in quartz sample experimental work of these latter authors v a s holders immersed in quartz liquid nitrogen dewar vessels. superior to the present in at least one respect: The experimental arrangement was such that selective the use of a glassy medium at liquid nitrogen tem- filters, neutral density filters, or monochromators could be peratures. Homver, the effect of ethyl iodide interposed in the exciting beam or the purified phosphoresbeam. A variety of high intensity flash sources with probably was qomewhat obscured, in view of the cent sec. to sec. were available, flash times from 5 X results of Sveshnikox- and Petrov,?' by the large and were used, particularly, in conjunction with manual proportion of alcohols in the glass used; in addi- shuttering. In addition the Aminco-Keirs spectrophostion since only one heavy atom-containing solvent phorimeter became available toward the end of this project, most of the lifetimes quoted were measured or checked was used it is irnpossible to state unambiguously and with this instrument. that a heavy atom effect is operative. Apart (b) Phosphorescence Spectra: Essentially the same from these comments, the motives and results of physical set-up was used as for the lifetime measurements these authors22accord with those of the present except that)an a.c. operated AH-6 lamp was used, and that a Steinheil spectrograph replaced the multiplier-oscilloscope work. assembly. The linear recipro2al dispersion at $he employed Experimental spectrograph tetting was 80 A./mm. at 5800 A. and 20 1.l Chemicals.-Diei,hyl ether, ethyl alcohol, and isopentane nere purified in the previously described manner.I6 The other chemicals were all Eastman White Label grades. 1Fluoronaphthalene, 1-chloronaphthalene, and l-bromonaphthalene were frwtionated under vacuum. The fractions used were: 1-chloronaphthalene 142-144" (30 mm.), 1bromonaphthalene 05-96.5" ( 3 mm.), and l-fluoronaphthalrne 57-59" ( 3 mni.). The 1-bromonaphthalene was run through a column of activated alumina prior to use. Naphthalene and propyl chloride were used without further purification. Samples of naphthalene which had been extensively zone-purified and chromatogrammed were available and it was verified that direct use of the Eastman product (181 .i parts ether, 6 parts isopentane. and 2 parts alcohol, by volume. (19) E. N. Viktorovi, I. .I. Zhmyreva, V. P. Kolobkor, and .1.A . Bapanenko, O p t i k a i S w k t . . 9, 349 (1960). (20) Y. Kanda and R . Riiiiiia~la.Spertvochirn. i l r f a . 17, 7 ( I P t i l ) . (21) H. Y. Sveshnilrov and A . .I. P e t r u v , Dokladij A k a d . ,\.auk SSSZ?, 71, 46 11%0!. (22) 1. J. Craham-Brycv ;im1 .J. 31. Corkhill, .Vafure, 186, 96.: (1960).
mm. a t 4250 A . Kodak spectroscopic plates, Type 103a-F(3) were used throughout, and were traced on a Leeds and n'orthrup recording microphotometer. (c) Glass Matrix: It was found that a mixture of L: propyl halide and 1-halonaphthalene in a 6:2 mole ratio would form rigid cracked glasses a t 77°K. if extreme care were taken in preparation and cooling. Indeed glasses sometimes could he obtained but these cracked upon t'he slightest provocation (;.e., exposure to the exciting light). However, the transmittancy of these "cracked glasses'' was quite high and since significant crystallization seemed not to have occurred it is these glasses which were used in all cases of mixtures investigated. Resort t o these cracked glasses was had for a number of reasons. First, amounts of propyl halide which are capable of producing changes in T~ of the 1-halonaphthalene may not be added to the solution of 1-halonaphthalene in EPA with( J i l t the nccurrence of considerable cracking and crystallimt,ion upon cooling the resulting mixture; iri otlier words w e must have high concentrations of perturber. Second, even t,hough heavy-atom-containing glasses are :iv:dable ( e . . ! / . replacement, of alcohol of the EPA by ethyl iodide), these
2502
S. 1’. l f c G L r s s , XI. J. REYNOLDS, G . W. DAIG~W, AND S. D. C~~RISTODOYLEAS l7Ol. 66
glasses do not cover a representative range of heavy atoms. Thzrd, it was considered important to investigate all samples under as closely identical conditions as possible, and thus a series of cracked glasses was to be preferred to a series in which some were cracked and some were not. Fourth, it is significantly easier to work with propyl halides than with ethyl halides.
TABLE I1 MEAN LIFETIMETIMEST H E SUM O F I N T E R N A L AND EXTERNAL SPIN-ORBITALCOUPLINGFACTORS SQUARED (IN SECONDS CM.-* X 10-4) Solvent
Propyl
Propyl
Propyl
Results Emitter EPA chloride bromide iodide Phosphorescence Lifetimes.-The first lifetimes Naphthalene 50.2 39 104 219 (actually “first observable mean lifetimes,” a-Fluoronaph27.4 12.6 74.5 82.5 vide infra) of the pure halonaphthalenes in EP,4, thalene as well as those of the halonaphthalene-propyl a-Chloronaph12.6 10.3 55.3 73.7 halide cracked glasses, are given in Table I. thalene Marked changes of rp have occurred, and cover a a-Bromonaph9.4 6 8 16.7 35.8 thalene total range of 2.6 to 0.00095 sec. The remarkable fact emerges immediately that the decrease in rp a-Iodonaph6.2 4.16 6.85 11.4 occasioned by an external I-atom is greater than thalene that caused by an inlernal C1-atom, and roughly The inconstancies in Table I1 are quite regular: comparable to that due to an internal Br; an a decrease occurs in going down any column, and internal C1 is less effective than an external Br, and when the EPA and propyl chloride columns are roughly equal to an external C1. inverted an increase occurs on traversing any row TABLE I from left to right. Two explanations may be F I R S T OBSERVABLE h I E A N LIFETIMES( I N SECONDS) O F provided for these results. (1) An increase of {I XAPHTHALENE A N D ITS HALOGEN DERIVATIVES IN SEVERALis known to increase @ISC, the quantum yield of SOLVENTS radiationless intersystem crossing from the lowest Solvent--excited singlet state to the triplet state; by the Propyl Propyl Propyl same token it also must be expected to increase Lmitter EP.1 chloride bromide iodide @Qp, the quantum yield of the radiationless interS itph thalene 2.6 0 52 0.14 0 076 system crossing from the lowest triplet state to the a-Fluoronaph14 0.17 0.10 0.029 singlet ground state (i.e., increased quenching of thalene phosphorescence is expected). In other words, a-Chloronaph0.23 0.075 0.059 0 023 rp 7 l / ( k p ~QP), and the effect of internal spinthalene orbital coupling is to increase kQp a t a rate coma-Bromonaph0 014 0 0073 0 0069 0.0063 parable to if not greater than the rate of increase thalcne of k,. Some evidence may be adduced in favor or-Iodonaph0.0023 0.0014 0 0012 0.0009j of these assertions.” The trends within a given thalene row may be rationalized by assuming that {E I t is further to be noted that the heavier the increases kp a t a rate significantly faster than the halogen of the propyl halide the smaller is r p . rate of increase of kQp. Again, some experimental A sufficiently representative series of solutions have quantum yield data favor this conclusion. { E ) ~ = conbeen studied to conclude that there is indeed a ( 2 ) Use of the expression T~({I valid heavy-atom effect. This immediately im- stant implies neglect of, or assumes constancy of, plies extension of the Mulliken-Tsubomora model, l4 overlap factors in the integral (@oT/X’/@fp), since in its present form it does not encompass where aoTis the zeroth order triplet wave function, these results. 1lcClure studied the effect on life- @$p is the zeroth order perturbing singlet wave times of changing like atoms in a substitutional function, and X’ is the spin-orbital coupling Hamilseries, as for example in going from l-fluoronaph- tonian. The perturbation of spin-orbital coupling thalene to 1-chloronaphthalene, etc. The product in a-electron systems depends on two main factors: rptr2,where {I is the atomic spin-orbital coupling (a) the degree of penetration of the a-electrons into factor of the internal halogen, was found to be a the field gradient of the nucleus of the perturbing reasonable constant. If the perturbation by an atom, and (b) the magnitude of the field gradient. external halogen is considered to be also of spin- An overlap integral of the form (7-electron orbital orbital coupling origin, and to be independent of the of emitter/halogen orbital) will be a limiting paraminternal perturbation, then one may conclude that etw for (a), and { will be a limiting parameter for ~~(j-1 where {E is the atomic spin-orbital (b). I t is thus to be appreciated that Table I1 coupling factor of the external halogen, should be neglects contribution (a). In the case of the a constant. Such a tabulation is given in Table 11, external heavy atom effect n-e may say? that the is the quanwherein it is seen that the constancy of the tabu- donor-acceptor overlap integral,14SDA, lated numbers is better by a factor of 102than the tity of importance, in which case we conclude from numbers of Table I, and is sufficiently impressive Table I1 that propyl chloride is a better acceptor to conclude the approximate truth of the assump- species than propyl iodide, a result already deduced? tions made. The ?I considered characteristic of from room temperature absorption data. It seems reasonable that the trends of Table I1 naphthalene as that for the H-atom, and oF EPX \ \ a s considered to be that for the OIT- nil1 find cq~larintionin terms of a cnombination of group It would seem that these assumptions, (1) and ( 2 ) above, or by introduction of exchange howertir nnir-e, are justified by the numbers of coupling constants into the external and interrial heavy nuclei. Table 11. r_________
+
+
+ r~)*,
r~
EXTFRKAL SPIX-0RRITAL
Dec., 1962
A COMPARISON OF
THE
TABLE I11 PERTURBATION INDUCED BY PROPYL IODIDE IN HALONAPHTHALENES AT 33 A N D -195.8'
Naphthalenic speciea
A P
P
Af/f
AkPC
bd
Akp/kp
Naphthalene ... ... ... 12.815 0.385 33.3 22.81 0.42 54.31 l-Fluoronaphthalene 33.7 0.73 46.4 l-Chloronaphthalene 2.86 11.27 30.21 39.15 4.35 9.0 53.08 42.11 1.261 1-Bromonaphthalene 95 73 1.3 l-Iodonaphthalene 172.30 386.60 0.44 525 435 1.21 a Increase in the integrated extinction of T + S absorption of the halonaphthalene when in binary solution with propyl iodide. b Integrated extinction, JtdQ, of T + S transition of pure liquid halonaphthalene. c Ak, = I / T ~' 1 / ~where ~ , T ~ is ' the phosphorescence lifetime of the binary cracked-glass system of naphthalenic species and propyl iodide and T,, is the phosphorescence lifetime of the naphthalenic species in EPA. d kp l/rp.
TABLE IV COMPARISON OF MEASURED PHOSPHORESCENT LIFETIMESIN EPA WITH PREVIOUSLY REPORTED VALUES(IN SEC.) This researcli
Emitter
DeGroot and van der Waalsa
McClureb
II~itchison~
Ermolaevd
Czekalla'
Kaphthalene 2.6 30 2.6 2.1 2.3 2.5 a-Fluoronaphthalene 1.4 .. 1.5 .. .. .. a-Chloronaphthalene 0.23 .. 0.30 .. 0.29 .. a-Bromonaphthalene 0.014 .. 0.018 .. .. a-Iodonaphthalene 0.0023 .. 0.0025 .. 0.002 .. a M. S. DeGroot and J. H. van der Waals, Mol. Phys., 3, 190 (1960). b See ref. 6 of text. c C. A. Hutchison, Jr., and B. W. Mangum, J. Chem. Phys., 29, 952 (1958). d V. L. Ermolaev, Optiku i Speklroskopiya, 6 , 417 (trans1,ztion) (1959). e See ref. 16 of text
A Comparison With Room-Temperature Absorption Data.-The ratio for external to internal perturbation effects as measured by absorption a t room temperature in binary liquid solutions and by T~ a t W'K. in cracked glasses are compared in Table 111, the external perturber being propyl iodide in all cases. It is seen that as {I increases the external effect increases much as could be expected because of the occurrence of a crossterm {I{E in the product ({I { E ) ~ . It is seen that as {I increases the ratio of external effect to internal effect decreases, again as expected, since this ratio is given by ({E/{I)~ ~{E/{I. It is further seen that a remarkable parallelism exists between the relative pertubations Af!f and Ak,/kp a t 33' and -195.8', respectively. The one case where significant discrepancy exists is 1-iodonaphthalene kg,), where ALP. which actually equals A(kP is increased by a large i n ~ r e a s eof ' ~ the quenching rate constant, kQp. The data of Table I11 are of importance in two regards. First the two sets of data mutually validate each other, and second it is implied that whatever the origin of the effect a t room temperature, it is the same a t low temperature. It is to he realized in this regard that the observation of a decreased lifetime a t lorn temperatures is indicative of a certain preponderance of perturber-perturbed pairs, but that the value of this lifetime is itself independent of the concentration of such pairs once a certain minimal value of their concentration is exceeded. Such is the case in the present instance, and the good correlation of the two sets of data a t 33' and -195.8' is hardly in accord with a collisional mechanism which should exhibit a TI/? dependence in fluid media. The precision in these measurements was calculated to be 2% in measuring a given frozen sample repeatedly. When a cracked glass sample was
+
+
+
allowed to warm up and then refrozen between runs reproducibility was of the order of 20%. This was not true of EPA glassy solutions. Some idea of the errors involved in these latter measurements may be gained from Table IT.'. In the worst comparison there is a 23%, difference between the present rP and that of h!tcClure,6 and in the best, 0%. However, it is interesting to note that the results of this study were equal to or consistently lower than those of hfcClure (vide i n j h ) . The Decay of Phosphorescence.-The phosphorescence of naphthalene in EPA was found to be exponential over a time interval of seven halflives. Indeed, the extent of exponentiality exhibited by a given material is an extremely good criterion of purity. Kone of the binary cracked glass systems investigated in this work had exponential decays of phosphorescence; this nonexponentiality is to be expected. Let us suppose, for instance, that there exist only complexed emitter species and uncomplexed emitter species, and that all the species within one of the two types specified are identical; in this case we find where I is the phosphorescence intensity, nTC is the number of complezed species in the triplet state and ~ T their C decay constant, and nT is the number of icncomplexed species in the triplet state and k T their decay constant. The decay will be nonexponential, but will be resolvab!e into two esponential processes. Indeed it is possible that such decay curves may be used to determine equilibrium constants; such is presently being investigated, and will be reported17 elsewhere. The use of the term "lifetime" in conjunction with a non-exponential process might seem odd. However, in the sense used here the first half-life is the time required for the phosphorescence intensity to
____
__
TABLE V THEPHOSPHORESCENCE SPECTRA OF NAPHTHALENE IN VARIOUS MATRICES AT 77°K. -Ether- isopentane gIass--
-Propyl
7
Y,
om. - 1
21335 20825 20550 20290 20170 19935
hromiile---.
Y,
1n t erpre t a t i on
A,,
0 510 785? 1045 1165 1400
19745 19435 19260 18960 18780 18535 18465 18100
1590 1900 2075 2375 2555 2800 2970 3235
17780 17610 17350 17130 17000 16825
3555 3725 3985 4205 4355 4510
16550 16310 15980 15790 15470 15040 14600
4685 5025 5355 5545 5865 5993 6735
512 763 2(512) = 1024 1146 1380 1575 512 1380 = 1892 512 1575 = 2087 1380 = 2404 2(512) 21512) 1575 = 2597 2(1380) = 2760 1380 1575 = 2955 2( 1380) 512 = 3272 or 2(1575) = 3150 1380 1575 512 = 3467 2(512) 2(1380) = 3784 1575 1380 2(512) = 3979 3(1380) = 4140 2( 1380) 1575 = 4335 1575 1380 3(512) = 4491 or 2(1575) 1380 = 4530 3( 1380) 512 = 4652 2(1575) 1380 512 = 5042 2(1380) 1575 2(512) = 5359 4(1380) = 5520 2( 1380) 2( 1575) = 5910 4( 1380) 512 = 6032 3(1380) 2(1575) = 6715
+ +
+
+ + + + + + + + +
+
+ + + + + + + + + +
drop to one half its original intensity at zero time after cut-off of excitation, the second half-life is the time required to drop from one half to one fourth its original intensity, etc. All the lifetimes of Table I are first obseroable lifetimes; it is necessary to make this distinction because there is a finite time required for the phosphoroscope to cut off the exciting light and open the photomultiplier to the phosphorescent light. Thus, with slow phosphoroscope speeds the measured lifetime was too long due to the deviation of the decay curve from exponential. I t was found that the measured apparent lifetime decreased as phosphoroscope motor speed increased, but become constant above some minimal rate of rex-oliition. With motor speeds above this point one was investigating an approximate exponential region of decay (first two or three halflives), and constancy of rP was to be expected. Still there might be a small difference between the first observable lifetime and the first lifetime. For this reason the phosphoroscope was run as fast as possible while still maintaining a display of one or two half-lives on the oscilloscope screen. 4 , good approximation to the first lifetime should be obtained by plotting log (intensity) versus time and using the limiting slope at zero time to determine the limiting slope lifetime. ,411 such limiting slope lifetimes measured corresponded within experimental error to the first observable lifetime measured at rotor speeds above the previous noted minimal r.p.m.; either one or both of these life-
em.
1
21280 20770
510
...
...
20290 20120 19880
990 1160 1400
0
Y,
r m . -1
A”
500
21010 20510
0 500
...
...
... ...
... ...
... ...
20020 19740
1160 1400
19910
1100
cm. --I
A 2’
,-Propyl iodidp-
21180 20700
A;
0
...
...
19550
1460
...
...
...
... 2010
19780 19380 19260
1500 1900 2020
19650 19330
1530 1850
...
19100
...
...
...
18500 18390 18140
2780 2890 3140
...
... ...
...
...
... ...
...
...
...
...
18330
2850 ...
18210 18100 ...
2800 2910
17860 17620 17300 17180 17010 ...
3420 3660 3980 4100 4270
...
... ...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
16850
4330
... 7 6900
...
...
...
...
4310 ...
... ... ...
...
...
... ...
...
...
...
... ...
15720 15410 15200
5560 5870 6080 ...
... ...
...
...
... ...
... ...
... ...
...
15320
5860 ...
15180
5830
...
...
... ...
... times may then be assumed to correspond to the first lifetime, and it is such mean-lives that are reported in Table I. It is thought that this is also the reason for the consistently smaller values of T~ than those of McClure6 obtained in the present work. Phosphorescence Spectra.-The phosphorescence spectra of naphthalene in various media are analyzed vibrationally in Table V. As is evident, there is no significant self-absorption of the luminescence, and the widespread similarity of the spectra in Table V indicates that the measured lifetimes are indeed those of the naphthalene. X similar conclusion is appropriate to all halonaphthalene spectra. For example, despite the considerable selfabsorption of the cracked glasses of l-iodonaphthalene shown in Fig. 1. it is still readily apparent that the phosphorescence spectra are identical. This identity was further confirmed by front-face illumination experiments wherein the self-absorbed high energy bands of the phosphorescences of Fig. 1 were recovered. It is not generally realized that self-absorption of phosphorescence may be important in emission experiments ; for example, in the work of Cxekalla and co-workers16 it was noted that large shifts of the phosphorescence spectra of many aromatics occurred 1-3000 ern.-’) when they were studied as crystalline complexes with tetrachlorophthalic anhydride. A comparison of these authors’ Fig. 1, curve 3, XTith Fig. 1 of the present work mill indicate that at least for the com...
ncc., 1962
EXTERSAL SPIX-ORDITAL COUPLING
2505
Wave number, cm.-I x 10-3. Fig. 1.-The phosphorescence emissions of 1-iodonaphthalene in several solvents a t 77°K: (a) in EPA glass; (b) in propyl bromide cracked glass; (c) in propyl chloride cracked glass; and (d) in propyl iodide cracked glase,.
plex tetrachlorophthalic anhydride-naphthalene these shifts were spurious (ie., they are not shift,s of the 0,O vibrational band) and entirely due to selfabsorption, ,4 similar conclusion seems reevlant to the other crystalline complexes studied by Czekalla, et ~ 1 . ' ~ The order of accuracy in Table V is h l 0 cm.-'. Consequently, a number of conclusions are pertinent. The triplet state experiences a red-shift relative to the ground singlet state as the spinorbital coupling nature of the matrix is increased. This is entirely to be expected if the perturbing singlet which mixes with the triplet of the aromatic lies higher in energy than the triplet states. This accords with the Tsubomora-Mulliken idea that this singlet is the charge-transfer singlet state, although as far as the present results are concerned it could be any higher energy singlet. The 0,O shift in going from propyl bromide to propyl iodide matrices is approximately twice as large as that experienced in going from propyl chloride to propyl bromide. This indicates that there are factors other than spin-orbital coupling operative, since if this coupling were solely responsible a reverse order of shift would be expected. These conclusions accord exactly with those from the lifetime data. It is also to he noted that decreases
of some vibrational fr9quencies (the 1575 and 512 ag modes) have occurred, indicative of a geometric specificity of interaction. The lowest triplet state of naphthalene is of 3B2,(D2h) species. The analysis of Table V agrees with this assignment since the only active vibrations are the 512, 1146, 1380, and 1575 cm.-' frequencies, and these are all of ag specie^.?^ This agrees with the analysis of Ferguson, Iredale, and Taylor,Z4 except that the difficulties these authors experienced vith the 1146 and 1575 cm.-' fundamentals which were thought to be of b,, species are no longer relevant. Furthermore the present analysis covers a phosphorescence spectral width of 6735 cm.-' as compared to a width of 2987 in the work of Ferguson, el al., because of the limitations of photomultiplier detection. 24 Acknowledgment.-The authors acknowledge their indebtedness to the National Science Foundation and The American Instrument Company for having sponsored this work. They are also grateful to the Misses D. Chapman and K. Caliian who photographed many of the emission spectra run in conjunction with this research. (23) E. W.Schmid, Z. Elektrochem., 62, 1005 (1988) (24) J. Ferguson, T Iredale, and J 1 Ta>lot, .I ? h e m S o / , 7160 (1984).
