Tkc., 1902
SPECTROSCOPIC STUDIES O F
RAREEA4RTIICHELATES
2493
SPFlCTROSCOPIC STUDIES OF RARE EL4RTH CHELATES RY G. A. CROSBY, R. E. WHAN,ANI) ,T. 5. FREEMAN Department of Chemistry, University of New Mexico, A lbuquerque, New Mexico Received Mag 86, 1.962
Intramolecular energy transfer in rare earth chelates is discussed and the role of the triplet states of the complexes in this type of energy migration is reviewed briefly. A spectroscopic study of the chelates of La3+, PrS+, Sm3+, Eu3+, and Gd3T with o-hydroxybenxophenone is reported. Luminescence spectra (at 77°K.) of these compounds are presented and the triplet state energies of the chelates and their dissociation products have been determined utilizing energy transfer as an aid in interpreting the complicated luminescences observed. Very weak “line” emission from the coordinated Pr3+ion is reported and selective excitation of the lDz, 3Po,and 3P1resonance levels of this ion via intramolecular energy transfer is demonstrated.
Introduction Luminescence spectra of rare earth chelates yield useful information about energy migration in complex molecules and furnish additional data on the ions themselves. The trivalent ions of the rare earth elements, lanthanum through lutetium, exhibit remarkable similarities in chemical properties. These ions with the exception of Ce3+ all form stable complexes, and a series of chelates of them derived from a specific eomplexing agent can be prepared by the same chemical method. Such a series of chelates possesses similar properties (solubility, melting point, solvate formation, etc.). The similarities are reflected also in the nearly identical absorption spectra of a given series. For example, the reported near ultraviolet absorption spectra of all the trisbenzoylaoetonate chelates M ) of the trivalent rare eazths (no data on Ce3+ and Pm3+) are identical within experimental error ; the same is true for the trisdibenzoylmethide chelates. Such similarities do not obtain in the luminescence spectra. Excitation of rare earth chelates to low excited singlet states results in complex luminescence spectra consisting of varying yields of molecular fluorescence and phosphorescence and of “line” emissions characteristic of the lanthanide ions. The quantum yields of total luminescence also vary widely, ranging from values approaching unity to values of less than The diversity in these observations must be explained in terms of competing radiative and radiationless processes for molecular de-excitation. The dependence of the luminescences of a series of chelates upon the central coordinated ions affords a sensitive means of studying these processes. Experimental Details for the preparation and analyses of the rare earth chelates of dibenzoylmethane and benzoylacetone are reported elsewhere.’ The rare earth chelates of o-hydroxybenzophenone were prepared in the following manner. Stoichiometric quantities of the rare earth chloride and ohydroxybenzophenone were dissolved in absolute ethanol, and anhydrous aminonia gas waa bubbled into this solution until no more precipitation occurred. Distilled water was added with stirring to the reaction mixture to complete the precipitation of the chelate, which waa quite soluble in absolute ethanol. The product was filtered, washed with distilled water, and dried under vacuum a t room temperature for 8 hr. The analyses for metal were performed by ignition in air, and the results were consistent with the expected theoretical values. (1) R. E. Whan and G.A. Crosby, J . Mo2. Spectry., 8, 315 (1962).
Rare earth oxides (99.9%) were obtained from Research Chemicals, Burbank, California, and were converted to the chlorides by treatment with hydrochloric acid and evaporation to dryness. The o-hydroxybenzophenone was obtained from K and K Laboratories, Jamaica 33, New York, and was purified by distillation under vacuum. For luminescence studies all chelates were studied a t 77°K. a t concentrations of 10-6 M in the following solvents which form rigid glasses at this temperature: (a) pure 3-methylpentane, (b) one part diethyl ether and one part 3methylpentane by volume (EP), and (e) two parts diethyl ether, two parts %methylpentane, and one part ethanol by volume (EPA) . The 3-methylpentane (Phillips pure grade) was passed through a column of silica gel and then distilled from sodium ribbon. The absolute ethanol (US1 Co. absolute pure ethanol, reagent grade) was distilled from Mg(0Et)z. The diethyl ether (Mallinckrodt anhydrous grade) was distilled from sodium ribbon. The apparatus and experimental details for the spectroscopic studies are given in reference 1. Filter combinations for all chelates excepting those of o-hydroxybenzophenone are reported in reference 2 . A combination of two Corning glass 9863 filters and a 5 em. path of CuSO4 5€$0 (100 g.11.) which transmitted light from 3000 to 4000 A. was used for excitation of the o-hydroxybenzophenone chelates.
-
-
Energy Migration in Chelates.-In order to delineate the possible paths of energy migration within a rare earth chelate molecule it is convenient to refer to an energy level diagram (Fig. 1). L4fter excitation of a chelate to a vibrational level of the first excited singlet state (So + SI), the molecule undergoes rapid internal conversion to lower vibrational levels through interaction with the solvent matrix. The excited singlet state may be deactivated by combining radiatively with the ground state (So c Sl), resulting in molecular fluorescence, or the molecule may undergo non-radiative intersystem crossing from the singlet to the triplet system. Again by internal conversion the molecule may reach the lowest triplet state, TI From this state it can then combine radiatively with the ground state by means of a spin-forbidden transition (SOc TI) giving rise to a typical long-lived molecular phosphorescence. Alternatively, the molecule may undergo a non-radiative transition from the triplet system to a low-lying rare earth ion state.2 The latter states are derived from the 4f electronic configuration of the coordinated trivalent rare earth ion. After this indirect excitation by energy transfer, the metal ion may undergo a radiative transition to a lower ion state resulting in characteristic line emission, or it may be deactivated via radiationless processes. Direct transfer of energy from the excited singlet state to the low-lying rare earth ion states has been shown t o (2) G.A. Crosby, R. E. Whan, and R. M. Mire, J . Chem. Phvs., 94, 743 (1961).
G. A. CROSBY,R. E. 'CVHAN, AND J. J. FREEM.4N
2494 Singlet
Triplet
Rare E a r t h Ion S t a t e s
/ I /
B3 D3
9
I
T
14t
Fig. 2.-Principal resonance levels of those rare earth ions exhibiting especially bright. line emission and the triplet state energies of some typical rare earth chelates. Chelates derived from benzoylacetone (MBs), dibenzoylmethane (MDJ, o-hydrox benzophenone (M( BP)s), and S-hydroxyquinolme
(~(8d~)~).
be unimportant.2 Likewise it has been shown that direct excitation of the ion is unimportant. Previous work on rare earth inorganic salts has shown that line emission characteristic of the ion originates from only a few specific states termed resonance levels. If the ion is excited to a nonemitting level, either directly or indirectly, the excitation energy is degraded via radiationless processes to lower states until a resonance level is reached. Radiative transitions then become competitive and characteristic ion emission is observed. These processes are indicated in Fig. 1 with d representing the resonance level. I n order to obtain the characteristic emission from a rare earth ion, it is necessary to excite a resonance level. For excitation of a rare earth ion by transfer of energy from an excited chelate molecule to the central coordinated metal ion, resulting in line emission from the ion, it is necessary that
Vol. 66
the lowest triplet state energy level of the complex be nearly equal to or lie above a resonance level of the ion. Otherwise sufficient energy is not available to excite the ion indirectly to its emitting level, and no line emission is observed. Thus the luminescence observed from a specific chelated rare earth ion is a sensitive function of the position of the lowest triplet energy level of the complex relative to a resonance level of the ion. Consequently, it is possible to control the emission from a given ion by varying the ligand and therefore the position of the triplet state of the complex. In other words the emission from an ion may be turned on or off by an appropriate choice of complexing agent. For example, the luminescence observed from dysprosium trisdibenzoylmethide consists of molecular fluorescence and phosphorescence, whereas dysprosium trisbenzoylacetonate exhibits primarily bright line emission characteristic of Dy3+. Here only a relatively minor change in the structure of the ligand results in a striking difference in the luminescences observed from the two compounds, when they are irradiated with near ultraviolet light, Another example of the phenomenon and an indication of the selectivity of this method of indirect ion excitation is illustrated by chelates of trivalent europium. This ion, which usually emits brightly from two well established resonance levels, can be limited to emit radiation only from the lower level by choosing appropriate complexing agents.2 If the triplet state energies of the complexes are known with certainty, one can use energy transfer to simplify the emission spectrum of a given ion by selectively exciting resonance levels. This technique has been used to aid in the assignment of a new group of lines in the emission spectrum of Tm3+.4 Bracketing Triplet State Energies.-Conversely, whenever the luminescence spectra observed from a series of lanthanide chelates are too weak or too complex to ensure accurate measurements of the phosphorescing (triplet) states, one can employ energy transfer as an aid in bracketing the energies of these states. I n Fig. 2 we have plotted the principal resonance levels of those rare earth ions exhibiting especially bright line emission and the triplet state energies of some typical rare earth chelates. Both resonance levels of trivalent europium have been included. This manifold of resonance levels spans an energy range of -3700 cm.-'. The energies of the lowest triplet states of most of the rare earth complexes studied thus far fall within the same region. This happy circumstance has enabled us to determine the positions of the triplet state energies within certain limits whenever direct measurements of the phosphorescences led to uncertain results. Benzoylacetonate chelates of the ions indicated in Fig. 2 all yield bright line emission, thus locating the lowest triplet level above 20,958 em.-' (Dy3+ resonance level). For the corresponding dibenzoylmethide chelates, line emission originating from (3) G. A. Crosby and R. E. Whan, J . Chem. Phve., 32, 614 (1960). (4) G. A. Croeby and R. E. Whan, ibad., 36, 863 (1963).
Dee., 1962
SPECTROSCOPIC STUDIESOF RAREEARTHCHELATES
all these trivalent ions is observed with the single exception of DyS+. This brackets the triplet levels of these. complexes between 20,430 and 20,958 cm.-1.2 For many of the benzoylacetonate and dibenzoylmethide chelates (trivalent La, Sm, Gd, Dy, Tm, Yb, Lu) the phosphorescenceswere sufficiently intense so that measurements of the highest energy phosphorescence bands were readily obtained. The averages of these measurements place the triplet state energies of the benzoylacetonate and dibenzoylmethide chelates at 21,480 and 20,520 cm.-l, respectively. These measured values of the triplet state energies fall within the limits determined by the line emission properties of the chelates as given above. For the rare earth chelates derived from 8-hydroxyquinoline the problem is more complicated. The phosphorescences from these complexes are very weak and the measurements of the weak emissions observed are subject to much uncertainty. The dEiculty is compounded further by the fact that dissociation and photodecomposition of the complexes occur, which casts considerable doubt concerningthe origins of the emissions photographed experimentally. A very weak and diffuse band is found at -17,760 & 100 ern.-' for the gadolinium complex. That this is indeed the emission from the triplet state of the chelate is rendered more certain by the fact that europium forms the only rare earth complex derived from 8-hydroxyquinoline that exhibits bright line emission, and this emission originates only from the lower resonance level (17,250 cm.-l). This brackets the triplet energy level of the rare earth complexes derived from 8-hydroxyquinoline between 17,250 and 17,800 cm.-', lending credence to the previously measured value of 17,760 cm.-'. The utility of employing intramolecular energy transfer as an aid for assigning triplet state energies is well illustrated by this series of compounds. An even morie complicated set of luminescence spectra is obtained from the series of chelates derived from o-hydroxybenzophenone. A solution of Gd(BP)S in pure 3-methylpentane exhibits an intense but diffuse phosphorescence with the first band maximum rappearing a t 17,400 cm.-l (Fig. 3a). I n an EP glass5 this compound also phosphoresces brightly, but the phosphorescence extends to somewhat shorter wave lengths with an additional prominent peak appearing a t -18,200 cm.-'. In a hydroxylic EPA glass a t least two phosphorescences are observed; an $tense but diffuse band in the range of 5000-7000 A. and a less intense but well d@ined phosphorescence be tween 4000 and 5000 A. There is no observable fluorescence in any of the solvents. Analogous behavior is exhibited by the corresponding La3+ complex in EP and EPA (Fig. 3b). The change of the observed luminescences with solvent change and the appearance of more than one molecular phosphorescence suggest that appreciable dissociation is occurring. Increased dissociation of the complex upon addition of a hydroxylic component is expected on the basis of increased polarity and the possibility of solvation through hydrogen bonding. (5) See Experimental section.
2495
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-
--
Further evidence for dissociation is presented in Fig. 3c and 3d. The total emission spectra of ohydroxybenzophenone and its sodium salt should be compared with the spectra of the rare earth complexes in EPA. The well defined 4000-5000 8. phosphorescences in the latter spectra correlate in detail with the prominent emissions observed from the chelating agent and its sodium salt. A conspicuous -1550 ern.-' progression is common to them all. The addition of trichloroacetic acid
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G. A. CROSBY, It. E. WHAS,ANI J. J. FEEEMAN
2490
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the corresponding rare earth complexes in 3-methylpentane, where little or no dissociation is expected, show two broad peaks in this region; ?ne with A,, 3425 A.,and another with A,, 3850 A. Both these bands must be attributed to transitions within the fully chelated rare earth complexes. The absorption spectra in EP are very similar to those obtained in 3-methylpentane with the long wave length band having a greater absorbance than the short wave length one (Fig. 4b). In EPA, where more extensive dissociation is expected, however, the relative absorbances are reversed. The absorption spectrum of the sodium salt of o-hydroxybenzophenone also shows solvent effects. This compound exhibits two absorption maxima at about the same wave lengths as those present in the rare earth complex (no figure included). These two bands have about equal intensities when the sodium salt is dissolved in EPA but the long wavc length band almost disappears when the solvent is changed to water. (The compound is not soluble in carefully dried EP.) Since the absorbance of band of the sodium salt and the rare the 3400 earth complexes always increases on going to more hydroxylic solvents, then part of the absorbance must be attributed to a dissociation product. probably the parent chelating agent. The corrgsponding decrease in the intensity of the 3850 A. peak accompanying the same changes in solvent identifies it as belonging to the complexes. Thus the absorption spectra of these compounds establish the presence of a dissociation species possessing an electronic transition similar to that of the parent chelating agent, supporting our previous assig;ment of the blue phosphorescence (4000-5000 A.) to the parent chelating agent. The low egergy and long life (> lo-* see.) of the 5000-7000 A. band appearing prominently in the luminesceiices of the chelates establish it to be triplet-singlet emission. The enhancement of the intensity of this band upon changing the coordinated metal ion from Pr3+ to Gd3+ shows that the emitting species in these cases contain the metal ions. This enhancement is in accord with the general observation that for a series of chelates the G d 3 +compound has the highest yield of luminescence.' The lack of any well defined structure on this long wave length phosphorescence band and its intensity changes upon addition of metal chloride to a solution of chelate suggest that it originates from more than one species containing a metal ion. That the 5000-7000 A. phosphorescence band does originate from more than one species is clearly demonstrated by the emission properties of solutions of the samarium chelate of o-hydroxybenzophenone. When the samarium complex is dissolved in 3-methylpentane and excited by ultraviolet light, only extremely weak lines are observed from the complexed Sm3+ ion. In an EP solvent the lines are slightly enhanced. Addition of ethanol to the solution results in a definite increase of intensity of these lines. -1 marked enhancement of them is observed upon addition of exrcss SmCln to the chelate solutio11 (see Fig. 3 ) . SmCl, alone, at this mnccntration, yiclds negligihlt. l i i r n i w w m w . Siiicc : i < l d i i i o i i of : ~ l c ~ ~ l favor.: ~ol
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Fig. 4.-( a) Absorption spectrum of o-hydroxybenzophenone a t 1.4 X 10-4M in EPA or EP at 25" in 1 cm. path. (b) Absorption spectra of the trivalent lanthanum chelate of o-hydroxybenzophenone a t 5.5 X 10-5 ;M at 25": -, EP; - - - -, EPA.
to the EPA solution of the Gd(BP)dcomplex, which dissociates extensively in an acidic medium, results in a solution which exhibits only this blue phosphorescence. From this we conclude that the dissociation product giving rise to this phosphorescence is the parent chelating agent. Material balance requires the existence of other species such as complexes of a rare earth ion with two or possibly only one ligand. Emission from these species is expected to be a t longer wave lengths than the emission from the parent chelating agent but a t shorter wave lengths than the phosphorescence from the fully chelated species. The appearance of the additional peak a t 18,200 em.-' in the phosphorescence of Gd(BP)3in EP and EPA glasses, where dissociation does occur, suggests that this peak originates from the partially chelated species. The addition of excess GdC13 to an EPX solution of Gd(BP)J results in an increase in the intensity of this 18,200 em.-* peak. Reasoning from chemical equilibrium this addition is expected to increase the concentration of partially chelated species and to diminish that of the fully chelated one. Absorption spectra of the compounds also support these conclusions. The absorption spectrum of o-hydroxybenzophenone in either EP or EPA possesses a !)road band in the near ultraviolet with A,, 337.5 A. ( E ~ , , 4300) ~ ~ (Fig. 4a). Poliitions of
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more dissociation of the trichelated species and the subsequent addition of excess SmCL also promotes an increase in concentration of the mono- and dichelated Sm3+ ion, we conclude that the species giving rise to the Sm3+ lines is either the Sm(BP)21+or Sm(I3P)I2+but is certainly not Sm(BP)3. (An attempt to suppress the dissociation of the Sm(BP)3by addition of the corresponding sodium salt failed because of precipitation at low temperatures.) Further evidence in support of this conclusion is the unexpected low intensity of the Sm3+ line emission observed from this chelate in 3methylpentane and E P ; for when line emission from chelates of Sm3+ is observed, it is generally much stronger (by a factor of lo3). We interpret these results to mean that the dissociation products giving rise to the sensitized line emission of dm3+are present only a t low concentrations. Even though accurate measurements of the triplet energy levels of the various dissociation products present in solutions of these rare earth complexes are precluded by the complexity of the phosphorescences measured from an equilibrium mixture, it is still possible to bracket the energy levels within narrow limits by a study of the luminescence spectra obtained from the Sm3+ and Eu3+ complexes. Europium trisdibenzoglmethide exhibits bright line emission originating from both the Flu3+ resonance levels (Fig. Ga) but the europium complex of o-hydroxybenzophenone exhibits bright line emission originating only from the lower resoiinnc~l r w l of tlw inn (17,250 cm.-l) (Fig. GI)), in-
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WAVENUMBERS x
Fig. 6.-Eu3+ line emission from: (a) chelate of dibenzoylmethane in EPA glass at 77'K.; (b) chelate of ohydroxybenzophenone in EPA glass a t 77°K. The arrows in (a) indicate prominent lines originating from the upper resonance level of the europium ion; these lines do not appear in (b).
dicating that all the species in solution containing coordinated Eu3+ have triplet levels below 19,020 cm.-' and that the fully chelated species has a triplet level between 17,250 and 19,020 cm.-l (the two Eu3+ resonance levels; see Fig. 2). For solutions of the chelate of samarium with o-hydroxybenzophenone the weak line emission observed does not originate from the fully chelated ion, placing the triplet of this species below 17,800 em.-' (samarium resonance level) but above 17,250 cm.-' (the lower Eu3+level). The value of li,400 em.-' measured from Gd(BP)3 in 3-methylpentane falls within this range. Since Sm3+line emission is observed from a dissociation product, this locates the triplet level of the product above 17,800 cm.-' but below 19,020 cm.-'. The 18,200 cm.-' peak (mentioned above), which was attributed to a partially chelated species, lies in an energy region consistent with this analysis. It is assumed for the above arguments that the absence of fluorescence from the second resonance level of Eu3+ is proof that the triplet level of thc complex lies below this level. It might be argued that interactions with the ligand are quenching this higher level so efficiently that no fluorescence can be observed from it. Because the local environment of the metal ion does not differ appreciably from one chelate to the other, one mould not expect the quenching to differ much eithrr. I:or chelates of lCi13+ witli hrnmylnrctniir n i i d dilwnxoyl-
G. A. CROSBY, R. E. WHAN,AND J. J. FREEMAN
2498
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Fig. &-Energy level diagram showing the energy levels of Pra+ (after Dieke and Sarup, ref. 6) and triplet state energies of Pr8+ chelates. For abbreviations see Fig. 2. indicates resonance level.
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line emission spectra of Pra+ chelates of: EPA glass at 77'K.; (b) (c) dibenzoylmethane (10-6M) in EPA glass a t 77'K.; o-hydroxybenzophenone ( M ) in EP glass a t 77°K. Fig. 7.-The
( a ) bensoylacetone ( 1 0 + M ) in
methane, the upper resonance level is not quenched out (see Fig. 6). This lends strong support to our thesis that the position of the triplet level of the complex is the variable which is responsible for the presence or absence of luminescence from ion resonance levels. Another interesting feature of the emission spectra of the rare earth chelates of o-hydroxybenzophenone is the virtual disappearance of the composite 5000-7000 A. phosphorescence band upon changing the coordinated metal ion from La3+ or Gd3+to Sm3+, Pr3+, or Eu3+. This behavior is in agreement with the evidence from other series of rare earth chelates that low-lying 4f electronic energy levels provide an eficient path for the
quenching of the phosphorescence state of the complex molecules. In summary, it has been shown that (a) rare earth chelates of o-hydroxybenzophenone dissociate appreciably in EPA; (b) the triplet state energy of the fully chelated metal ion complex lies definitely between 17,250 and 17,800 cm.-' with a measured value of approximately 17,400 cm.-'; (c) at least one dissociation product containing the metal ion has a triplet state energy above 17,800 cm.-' but below 19,000 cm.-', with a measured value of approximately 18,200 cm. -'; (d) intramolecular energy transfer to the metal ion is an efficient process even in partially dissociated complexes; and (e) low-lying 4f electronic levels of the rare earth ions provide an efficient means for quenching the phosphorescence of the complex molecules. Selective Excitation of Coordinate Pr3+.Having established the triplet state energies of the lanthanide chelates of benzoylacetone, dibenzoylmethane, and o-hydroxybenzophenone, this information can be used to study the luminescences of other rare earth ions whose spectra are more difficult to interpret. These data are useful in the study of the emission spectrum of the coordinated trivalent praseodymium ion. This ion is reported to fluoresce brightly from the anhydrous chloride under direct excitation and the prominent resonance levels have been established.6 No emission from the coordinated ion has been reported. By using energy transfer in the above chelates to excite this ion indirectly, we have succeeded in photographing the emission of the coiirdinated Prs+ ion. For all the complexes the luminescences consist (8)
G. H. Dieke and R. Ssrup, 1. Chcm. Phya., 29, 741 (1958).
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 Kasha3in 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.
