External HeavyAtom Spin—Orbital Coupling Effect. V. Absorption Studies of Triplet States S. P. McGlynn, T. Azumi, and M. Kasha Citation: J. Chem. Phys. 40, 507 (1964); doi: 10.1063/1.1725145 View online: http://dx.doi.org/10.1063/1.1725145 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v40/i2 Published by the American Institute of Physics. Additional information on J. Chem. Phys. Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions THE JOURNAL OF CHEMICAL PHYSICS VOLUME 40, NUMBER 2 15 JANUARY 1964 External Heavy-Atom Spin-Orbital Coupling Effect. V. Absorption Studies of Triplet States * S. P. MCGLYNN AND T. AzUM! Coates Chemical Laboratories, Louisiana State University, Baton Rouge, Louisiana 70803 AND M. KASHA Department of Chemistry and Institute of Molecular Biophysics, The Florida State University, Tallahassee, Florida 32301 (Received 16 September 1963) The .lowest energy Tl<'-SO tra~sitions of the molecules chlorobenzene, 1-chloronaphthalene, anthracene, 9, 10-dlbromoanthracene, naphthacene, pyridine, and phenazine have been Investigated III a varIety of solvents, some of which contain atoms of large atomic number (henceforth called "heavy atoms:'). It is shown that in heavy-atom solvents an increase of the T,<.-So absorptivity occurs, and the technIque of external heavy-atom perturbation is demonstrated as a method of conveniently locat~n~ and validating T,<.-So. ~ransitions. Monitoring experiments in which nonspin-intercombination transltI?nS, such as S,<.-So tra.nsltIons of the molecule being investigated, and S,<.-So transitions of low-level contammants have been subjected to external heavy-atom perturbation, have permitted the conclusion ~hat t~e met~lOd is unique !or !'I<'-SO transitions. It is further verified that the external heavy-atom effect IS a spm-orbltal perturbatIon In nature. Much new data on triplet-state energies and lifetimes has been obtained and is contained herein. ?, 10-~lchloro:mthrac~ne, INTRODUCTION NE of the principal difficulties inherent in the investigation of T1<c-So absorption processes is that of unambiguously establishing the absorption regions. Because of the very low molar extinction coefficients (e= 10-4 to 1), extensive purification of the compound being investigated is necessary to assure that a suspected T1<c-So absorption band may not be a Sl<C-SO absorption of some impurity present in very small (10-3-10-5 %) quantity. It is also essential that one employ high concentrations of the absorbing species; however, if considerations of solubility and resolution are of prime importance, one is forced to use long path lengths of absorbing material, and to adapt one's instruments accordingly. These difficulties are well illustrated in many of the experiments to be described in this work. No absorption technique thus far used has been entirely satisfactory. The oxygen perturbation method described by Evans! while providing more significant enhancement effects than those to be discussed here, is certainly more cumbersome and is fraught with considerable danger. Nag-Chaudhuri and Basu2 have used acetylacetonates of copper and iron as "external magnetic perturbers," but the results obtained have been O * This work was supported by Research Grants from the U.S. Atomic Energy Commission (Biology Branch) and from The National Science Foundation to The Louisiana State University, and from a contract between The Office of Scientific Research, U.S. Air Force, and The Florida State University. 1 D. F. Evans, Nature 178, 534 (1956); J. Chern. Soc. (London) 1957, 1351, 3885. 2 J. Nag-Chaudhuri and S. Basu, Trans. Faraday Soc. 54, 1605 (1958). questioned. 3 It appears from some recent work4 that ferric acetylacetonate does indeed increase the TI<c-So oscillator strengths of codissolved aromatics, and that the effect is associated with the formation of weak molecular complexes of a charge-transfer nature. It is implied then that the origin of the ferric acetylacetonate perturbation is similar, if not identical, to that of the oxygen perturbation as discussed by Tsubomura and Mulliken. 5 However, this acetylacetonate effect, while certainly of considerable interest, is at best minor and will not be of much importance in establishing the T1<c-So absorption regions of aromatic donors. In short, it is still thought that incontrovertible proof of a TI<c-So absorption requires recourse either to comparisons with results of emission (phosphorescence) methods or the study of internally heavy-atom perturbed derivatives such as has been discussed by McClure,6 or both. It is expected that the technique of solvent heavy-atom perturbations, reported here, will make the characterization of T1<c-So absorption regions a more definite and a less time-consuming task. Kasha7 reported the first example of a solvent heavyatom perturbation: upon mixing two colorless liquids, 1-chloronaphthalene and ethyl iodide, a yellow color developed. Spectroscopic examination revealed that the color was due to an increase in the oscillator strength of the lowest energy T1<c-So intercombination of 13 4 F. J. Wright, J. Phys. Chern. 65, 381 (1961). J. Nag-Chaudhuri, L. Stoessell, and S. P. McGlynn, J. Chern. Phys. 38,2027 (1963). Ii H. Tsubomura and R. S. Mulliken, J. Am. Chern. Soc. 82, 5966 (1960). 6 D. S. McClure, J. Chern. Phys. 17, 905 (1949). 7 M. Kasha, J. Chern. Phys. 20,71 (1952). 507 Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 508 S. P. McGLYNN, T. AZUMI, chloronaphthalene. It has since been confirmed8 that this .enhancement is spin-orbital coupling in nature and IS larger the greater the extent of acid-base interaction between the aromatic donor and the alkyl halide acceptor. The development9 of low-temperature (77°K) glass matrices containing alkyl halides has made emission work possible, and it has been shown Io- I2 that heavy-atom perturbation in a solid matrix results in an increase of the relative phosphorescence to fluorescence quantum yield (cf>P/cPF) and a decrease of the phosphorescence lifetime (Tp). Analysis of the emission data has shown I2 that the process most sensitively affected by external heavy-atom perturbation is the radiationless intersystem crossing process from excited singlet to triplet states. These emission results are very much the same as those obtained by Robinson13 in which the heavy-atom matrices used were argon, krypton, and xenon at 4 oK. Finally, it has been noted 14 that the absorptivity of 1-chloronaphthalene in ethyl iodide is quite markedly affected by pressure. It would appear that it is necessary for significant singlet-ttriplet absorption intensity that a heavy atom be fairly near to the region of the molecule within which an electronic transition is localized but, on the other hand, it is not necessary that the heavy atom be permanently attached to the molecule undergoing transition. The transitory perturbation provided by collision with another molecule, such as a solvent molecule which contains a heavy atom, should be sufficient to enhance absorptivity, as should any perturbation which induces even a slight amount of charge transfer from any of the optically combining states to the heavy atom solvent or matrix. The present work is concerned with the study of a vari:ty of solutes !n various heavy-atom perturbing medIa. These studIes are not designed to develop a m:chani~m of the effect being measured, although its spm-orbital coupling nature will be made evident. Rather, primary emphasis will be placed on the utilization of absorption spectrophotometry in the detection and characterization of the lowest energy triplet states of aromatic molecules. It is shown that this method has significant advantages over those discussed above, and should prove itself valuable in triplet-state studies. 8 S. P. McGlynn, R. Sunseri, and N. Christodouleas J. Chern. Phys. 37,1818 (1962). ' t See F. Smith, J. Smith, and S. P. McGlynn, Rev. Sci. Instr. 33, 1367 (1962) for an extensive listing. 101. J. Graham-Bryce and J. M. Corkhill Nature 186 965 (1960). " 11 S. P. McGlynn, G. Daigre, and F. J. Smith, J. Chern Phys 39,675 (1963). . . 12 S. P. McGlynn, M. J. Reynolds, G. Daigre, and N. Christodouleas, J. Chern. Phys. 66, 2499 (1962). 13 G. W. Robinson, J. Mol. Spectry. 6, 58 (1961)' see also A Grabowska, Spectrochim. Acta 19, 307 (1963)' . 14 W. W. Robertson and R. E. Reynolds, Chern Phys 29 138 (1958). . ., J: AND M. KASHA TERMINOLOGY In this paper the term external heavy-atom spinorbital coupling effect is used to describe the spinorbital coupling induced in one molecule by high atomic number atoms of a second molecule, whether both mol.ec;ules exist as a stable molecular complex, or as a coll~slOn pair, or whether they are merely forced, by lattICe formation, into close proximity to each other. This effect is considered to be intermolecular in all cases. When an increase of spin-orbital coupling results from the covalent binding of an atom of high atomic number to an aromatic system, or from variation of the metallic component of an aromatic metallo chelate, this will be termed internal heavy-atom spinorbital coupling and will be considered to be intramolecular. When and if an increase of spin-orbital coupling results from substitution of a skeletal atom, or atoms, of a 11" system by another atom, or atoms, and as occurs, for example, in the series benzene, pyrolle, furan, thiophene, selenophene, etc., this will be designated homocyclic heavy-atom spin-orbital coupling. This effect is deserving of a specific name, if only because many effects are manifested in this last case which are not well understood; it is, of course intramolecular. EXPERIMENTAL Chemicals Since an impurity level of 10-6 parts may lead to significant error, all chemicals used were required to be extensively purified. Because of the importance of ~his. aspect of the experimental work, a detailed report IS gIven. Ethyl iodide. An Eastman-Kodak (EK) "White Label" grade. It was dried over CaCh and passed through an alumina column, whereby a reddish-brown band of iodine was adsorbed tightly at the top of the column; the eluent was of equal purity to that of Whiting. 7 The liquid was colorless in a 50-cm path. Storage was effected at O°C, in the dark, and all operations using this chemical, because of its highly light-sensitive nature, were performed in subdued daylight, to avoid photochemical complications. 1-Chloronaphthalene. An EK "White Label" product purified by the method of Whiting. 7 Chlorobenzene. An EK "White Label" grade. It was purified, after drying over mangesium perchlorate, by fractional distillation through a 30-in. column packed with glass helices, at a reflux ratio of 20 to 1 and 0.1 mm pressure. The fractionation was repeated until constant optical density was achieved. Anthracene. EK scintillation grade was dissolved in petroleum ether and adsorbed on an alumina column (36 in.X 1t in.). The column was developed with petroleum ether, a very good zonal separation being effected. The lowest broad zone which fluoresced blue Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions EXTERNAL HEAVY-ATOM Was anthracene. Arranged above this band, and in the order in which they are named, were narrow strips of carbazole (light blue fluorescence) and naphthacene (green fluorescence) adsorbates. Two other bands with pink fluorescence appeared at the top of the column, and were probably due to higher polyacenes such as pentacene, etc. Elution and subsequent spectroscopic examination of the carbazole and naphthacene bands showed them to be present at a level of 10-4%. The anthracene obtained from various chromatographic runs was then combined and recrystallized from CC4 six times. The resulting product was chromatographically homogeneous. All of the operations described were performed in a nitrogen atmosphere to minimize danger of oxidation. 9,10-dichloroanthracene and 9, lO-dibromoanthracene. EK "White Label" grades. They were purified by repeated fractional crystallizations from ether. Ethylene bromide. A Matheson Company product. It was purified by distillation from mercury, and by static (H tube) distillation at 0.02-mm pressure and room temperature. N aphthacene. This compound is rather susceptible to oxidation on an alumina column, and all operations to which it was subjected were done in a nitrogen atmosphere. The starting material was the EK "White Label" grade, different lots of which were found to differ remarkably both in types and degrees of impurities which they contained. The eluting agent used was a 1% by volume solution of alcohol in petroleum ether. Use of this agent did not effect a very good separation of naphthacene and anthracene, but it was quite efficacious in separating out higher polyacenes whose Sl+--SO absorption spectra interfered in the region to be investigated (i.e., 6000-12000 A). The chromatogrammed product was finally recrystallized a number of times. Phenazine. An EK product. It was fractionally recrystallized a number of times, and finally chromatogramed in an oxygen-free atmosphere. Pyridine. An EK spectrograde chemical. It was statically distilled once at room temperature. A yellow residual liquid remained. Carbon tetrachloride, carbon disulfide, and hexane. These were cp grades, which were purified by methods described elsewhere. 16 Techniques All measurements of optical density were made on a Beckman DU spectrophotometer. All points on the included curves, whether they be denoted by circles, squares, etc., are experimental points, but only such are shown as are necessary to define a given absorption band. 15 G. J. Brealey and M. Kasha, (1955). J. Am. Chern. Soc. 77, 4462 509 SPIN-ORBITAL COUPLING Where the solutions used were dilute, the comparison blank was the pure solvent. If the solutions used were concentrated, such that one could not use the solvent as a blank, without introducing an error due to noncompensation for the amount of solvent displaced by the solute, it was found more facile to use a third liquid of similar refractive index as a blank. The optical density appropriate to the amount of solvent in the solution, as measured against the same blank, was then subtracted from that of the solution, leaving as a residue the optical density of the unperturbed solute plus the increment in same induced by the perturbing solvent. All solutions of a particular molecule in different solvents were run against the same blank, so that all absorption curves for anyone molecule have the same base line. In view of the interpretation of the enhancement of Tl+--SO transition probability to be given later, the light absorption of the perturbed molecule is presented in terms of a molar extinction coefficient, e, evaluated as follows: e= (l/el) logloll, where logloll is the residual optical density mentioned above, c is the concentration of solute in moles per liter, and 1 is the absorbing path length in centimeters. Use of the formula given by Kasha7 leads to a multiplicative increase of error. The abscissas of the figures are given in wavenumbers (cm- l ) in vacuum. The aromatic component of all solutions, irrespective of whether it is the major component of the solution, or not, is always designated as the solute. The absorption cells used ranged from 0.1 mm to 20 em in nominal length. With the exception of the 20 em cells, which were of Pyrex and stoppered, and which had been made to order by Aminco,16 all the others were stoppered fused silica cells. Oscillator strengths were evaluated by first drawing in the SI+--S0 tail, which was obtained in the usual manner by first extrapolating linearly a plot of loge vs ii, from shorter wavelengths, where the contribution of the T 1+--S0 absorption process is assumed negligible. The area contained between the actual absorption curve and the exponential tail gives fedii, and was measured with an Ott planimeter. Tr-~So phosphorescence lifetimes were evaluated by the equation T= 3.47X lOS (iiA 2n2)-1 (g,./gz) (fedii rl, where e is the extinction coefficient in a nonperturbing medium, iiA, the maximum of the absorption band, and n is the refractive index of the medium. gu( = 3) and gl( = 1) are the multiplicities of upper and lower states, respectively. 16 American Instrument Company, 8030 Georgia Avenue, Silver Spring, Maryland. Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 510 S. P. McGLYNN, T. AZUMI, AND M. 1- IN HEXANE, I: I (vjv) 2-IN C<4,I:1 Mv) :!i-IN ~H..Brz,I:1 (V/V) KASHA 1 • PURE LIQUID 1'2. (V/V)IN CCI, 3-1'2.(V/V) IN C,H.Sr, 4-1 '2.(V/V) IN C,H5 1 5-1'4(V/V) IN C.HsI (ALL IN 20 ""' PATH) ~- oj ~ '" 0 0 d I/ ~ c:. 0 is 0 d 26000 1)0000 54000 WAVENUM8ERS, Vern"' FIG. 1. The lowest energy T 1<-So absorption spectrum of chlorobenzene in hexane (Curve 1), in carbon tetrachloride (Curve 2), and in ethylene bromide (Curve 3), at room temperature (22°C). 25000 22000 19000 WAVENUMBERS RESULTS In order to establish that an absorption process being investigated was indeed a Tl+-SO intercombination, all, or most of the following criteria were used: (1) Comparison with the phosphorescence of the molecule in a rigid glass at nOK, and use of the spectral band mirror-image relationship, (2) comparison with the absorption spectrum of the species in a nonperturbing medium, in these cases in which it was possible to pick up the Tl+-SO absorption of the unperturbed species, (3) comparison with the absorption spectrum of intramolecularly perturbed derivatives, and (4) the correspondence of the lifetime estimated from the oscillator strength of the species when in a nonperturbing medium, with that measured by phosphorescence decay means. FIG. 2. The lowest energy Tl<-SO absorption spectrum of at room te~perature .(22°C). The. arrows indicate the ordmate scale to which a partlcular curve IS to be referred. 1-chloronaphtha~ene in this region since it itself absorbs too strongly (see Fig. 8). The lifetime of the Tl-?SO phosphorescence, calculated as described, is 0.006 sec and is to be compared with the experimental value l7 of 0.004 sec. PA was taken as 30950 cm-1 in:the calculation of the phosphorescence lifetime. The absorption obtained in hexane solution is to be compared with these for bromobenzene and iodoben- H;H.r. DECOMPQS£O <-l,IN C.H..l,f.05x/tr'M. '" d Chlorobenzene The spectra obtained are shown in Fig. 1. It was not possible to use ethyl iodide as a perturbing solvent TABLE 1. Effect of different external perturbers on Tl<-SO transition of 1-chloronaphthalene. 8 Solvent [IlJWii] / [IlJWii]eel( CCI. 350 9.7 97.1 17.5 73.5 Solv 8 The data in this table are of different origin to those in Table II of Ref. 9 of text, and are supplementary to it. b lsolv is the atomic spin-orbit coupling factor for the heaviest atom in the particular solvent. 450 550 WlVELENCTli IN m)J. MO FIG. 3. The 11;.+->"110+" absorption spectrum of iodine in ethyl iodide solution (Curve 2), at room temperature (22°C), and of ethyl iodide which has been decomposed by exposure to weak sunlight (Curve 1). The arrows indicate the ordinate scale to which the individual curves are to be referred. 17 M. Kasha, Discussions Faraday Soc. No.9, 14 (1950), quoting D. S. McClure, Ref. 6 of text. Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions EXTERNAL HEAVY-ATOM SPIN-ORBITAL zene given by McClure and co-workers. IS The ratio of the integrated T 1(-So oscillator strengths of chlorobenzene and bromobenzene is 1/25, while the square of the ratio of the atomic spin-orbit coupling factors is 1/17.5. This result coupled with McClure and coworkers' result of 1/4.2 for the ratio of the integrated T 1(-So oscillator strengths of bromobenzene and iodobenzene, the expected ratio being 1/5, seems to point to the fact that electronegativities have but a small effect, if any, in the relaxation of spin forbiddenness, at least in the monohalogenated benzenes. The above quoted agreement in the case of internal heavy-atom perturbation leads us to inquire if similar regularities exist in external perturbations. For this purpose we have evaluated the increment in optical density in going from hexane to the other solvents. The ratio of the increments is 1-IN C~,O.0515m. and is to be compared with the ratio of the squares of the spin-orbit coupling factors appropriate to the heaviest atoms in the solvent, which is 17.5. No allowance is made for the number of heavy atoms contained in the different solvents, the different concentrations, viscosities, etc. Despite this, it may be said that the increments in the T 1(-So absorption which occur are of the correct order of magnitude to be explained on a spin-orbit coupling enhancement basis. l-Chloronaphthalene The results obtained are given in Fig. 2. The ratio of the increments in oscillator strengths in going from the pure liquid 1-chloronaphthalene to various perturbing solvents are given in Table 1. The increments are again of the correct orders of magnitude for a spinorbital coupling effect. The phosphorescence lifetime calculated is 0.36 sec, while that measured experimentally is 0.23 sec. VA was taken to be 20600 cm-1 in the calculation of phosphorescence lifetime. II 14 20 511 COUPLING I --- -I~ ... "" 15 ",," " / W4.VENUMBERS, ~xKillcm-1 I I I '/ / IT J9 FIG. 5. The lowest energy T,<--SO absorption spectrum of 9,10-dichloroanthracene in carbon disulfide (Curve 1), and in ethyl iodide (Curve 2) at room temperature. The sharply defined band at 11 500 cm-1 is a vibrational overtone of 9,10-dichloroanthracene. The broken lines are the extrapolated tail of the lowest energy S,<--SO absorption band of 9,10-dichloroanthracene. It was observed that a change in the optical density of the ethyl iodide solution occurred with time. However, this change coincided with the appearance of a new band at 4750 A, which is just the spectral position of the l~a+--+3IIo+u transition of I2 in dilute solution in ethyl iodide 19 at 20 0 e (see Fig. 3). A further increase in the optical density sets in below 4000 A and is to be attributed to molecular complexing of the iodine (formed by decomposition of the ethyl iodide) with ethyl iodide20 and 1-chloronaphthalene,21 by which means charge-transfer absorption processes become operative. The above interpretation explains the time 19 This value is corroborated by D. E. Schuler and R. H. Schuler, J. Am. Chern. Soc. 76, 3092 (1954). FIG. 4. The lowest energy T,<--SO absorption spectrum of anthracene in carbon disulfide (Curve 2), and in ethyl iodide (Curve 1), at 22°C. The sharply defined band at 11 500 cm-1 is a vibrational overtone of anthracene. The dotted lines are the extrapolated tail of the lowest energy S,<--SO absorption band of anthracene. 18 D. S. McClure, N. W. Blake, and P. L. Hanst, Phys. 22, 255 (1954). J. Chern. 20 The calculated position of this charge transfer peak is 2800 A: S. H. Hastings, J. L. Franklin, J. C. Schiller, and F. A. Matsen, J. Am. Chern. Soc. 75, 2900 (1953). 21 The N -> E charge transfer absorption for the naphthalene iodine molecular complex is at 3600 A: N. W. Blake, H. Winston, and J. A. Patterson, J. Am. Chern. Soc. 73, 4437 (1951). The N-> E absorption for the l-chloronaphthalene iodine complex would be expected to lie at somewhat longer wavelengths, because of the lower vertical ionization potential of the chlorinated derivative. Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 512 S. P. McGLYNN, T. AZUMI, AND M. KASHA Anthracene and 9,IO-Dihaloanthracenes IN c,H,1 •aom M. _.......... '" ~.~. / / / I / / "/ // -- - - ---=-'-=-"'-;;..-------------l '-- 2:0 1 IN C5;. ,0.0874 M. / / I I / The results for anthracene, 9, lO-dichloroanthracene and 9, lO-dibromoanthracene are given in Figs. 4, 5, and 6, respectively. The peak which in each case occurs at ",11 500 cm- 1 is a vibrational overtone, and is seen to be but little affected either in position or intensity, by either internal or external intercombinational perturbations. Since only two solvents were used throughout this series of experiments, it is impossible to compare ratios of enhancements of optical density with the aforementioned ratios of squares of spin-orbit coupling factors. However, the ratios of the oscillator strengths of the enhanced absorption in ethyl iodide to that in carbon disulfide is expected, on the basis of our theory, to be / ---14000 - / / ~ ..... .... / / / 0.00091 fA., IN C,H,! Sot.N . 20 cmPATH. 2 FIG. 7. The lowest energy T 1..... S 0 absorption spectrum of naphthacene in chloroform (Curve 1), and in ethyl iodide (Curve 2), at 22°C. The dotted lines are the extrapolated tail of the lowest energy S1 ..... S0 absorption band of naphthacene. 16000 WAV£NUM&RS FIG. 6. The lowest energy T1 ..... S o absorption spectrum of 9,10dibromoanthracene in carbon disulfide (Curve 1), and in ethyl iodide (Curve 2), at 22°C. The sharply defined band at 11500 cm-I is a vibrational overtone of 9,1O-dibromoanthracene. The dotted lines are the extrapolated tail of the lowest energy SI ..... SO absorption band of 9,10-dibromoanthracene. increase of optical density fully and accords with the observation that the rate of increase is light sensitive. The increase in optical density which takes place in CCl4 solution as compared to the pure liquid might seem somewhat surprising since one l-chloronaphthalene molecule can collision ally perturb others of the same species. However, considerations of liquid packing, viscosities, number of perturbing CI atoms, and steric factors do not render the result unexpected. TABLE II. Effect of external and internal heavy-atom perturbations on lowest energy T1 ..... S o transitions of anthracene and 9,1O-dihaloanthracenes. Species [feap T, Lr / [feap ls, secB,b p(0, 0), cm- I Anthracene 9,109,10Dichloro- Dibromoanthraanthracene cene 1.77 1.64 1. 76 0.09 0.06 0.03 14 820 14 080 14 080 • Calculated from the oscillator strengths of the TI<-So absorptions of the 9 ,1O-dihaloanthracenes in nonperturbing media. b The effect of 9,lO-disubstitution on lifetime is seen to be quite small. It is this fact which prompts us to suggest in the text that the magnitudes of external perturbation should be about the same for all three 9,IO-dihaloanthracenes. It is shown in Ref. 8 of the text that [f eap] EtI/[f EdP]cs,= (a+fJ)'/a', where Ol and {3 are descriptive of internal and external perturbations, respectively. Since a must be roughly constant in this series of molecules, the ratio (a+~)'/a', which is the ratio of external plus internal effects to internal effect, must be roughly constant also. roughly of the same order for both anthracene and its derivatives. That this is so is readily seen from Table II. P(0,0) and the lifetimes of the phosphorescent emissions, as calculated by the previously described method, are also tabulated in Table II. These values for the lowest energy (0,0) absorption positions have been confirmed by subsequent phosphorescence measurements. 22 The limits of the phosphorescence lifetime of anthracene have been fixed experimentally as lying between 0.1 and 0.01 sec. 23 The agreement of the value calculated with these limits is good. Naphthacene The lowest triplet level of naphthacene has been supposed to lie 24 at 18500 cm- 1 and to have a lifetime24 of 2 sec. Both the position and lifetime of this emission are inconsistent with the smooth correlation23 of the lowest triplet levels of aromatic hydrocarbons when one uses the values known22 for the position and lifetime of the lowest anthracene triplet state. Theoretical 22 M. R. Padhye, S. P. McGlynn, and M. Kasha, J. Chern. Phys. 24, 588 (1956). 23 See discussion by S. P. McGlynn, M. R. Padhye, and M. Kasha, J. Chern. Phys. 23, 593 (1955). 24 C. Reid, J. Chern. Phys. 20, 1214 (1952). Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions EXTERNAL HEAVY-ATOM ~ PUR£ LIQUID, IZ.?>9Z M. 50CM. ~TH BlANK: AIR ~ 8 ..... 0 10 12.:59'zM. r;j 2 :z SPIN-ORBITAL COUPLING 513 01202 M.)N HftANE 20CM.~TH ~TH:OIO:3mm. BlANK:CCL4 BlANK: HEXANE d !oJ ~i 80 'Q r;j ~~ l£. w S Q r;j 1=0 >< ~~ ~o ~ r;j ::!; --------- 0 6 I?> 9 15 16 V'IO~(CM-') o 27 Z5 29 40 FIG. 8. The absorption spectrum of ethyl iodide in the range 8000-45 000 cm-I, at 22°C. This curve defines the useful working ranges of ethyl iodide as a spectrophotometric perturbing solvent, calculations25 ,26 on polyacenes have shown that such a break in correlation is llllreasonable, and a triplet level at ,......,10 000 cm- I is expected in naphthacene. 25 ,26 From Fig. 7 it is evident that an increase in absorption with onset, and thus also ii(O, 0), at 10 250 cm-I has occurred. The lifetime calculated from the unperturbed absorption is of the order of 10-2 sec. These values for both lifetime and the 0,0 vibrational band position again lead to smooth correlation of the lowest energy triplet states. The intensity error in this experiment may amount to 20%, this estimate being derived from the results obtained by repeated repetition of the experiment. Efforts to reduce this error by using reflection type cells of 50 cm length were unsuccessful since the error, introduced by the extra operational procedures required, increased more than proportionately to the advantage obtained from the added path length. Ef. . o.2m.-PH[NAZ1N[ IN Ell - 14 9 0.04 m.- PHENAZIN[ IN ETH[R 15 16 17 15 19 20 WAVENUMBERS, '1-0" FIG. 9. The lowest energy T,t-So absorption of phenazine. - - - at any time in a stable ether solution, - - - - at 1=0, -,-,-,-. at 1=3 h, -, ,-, ,-., at 1=3 days. 25 26 J. Chern. Phys. 24, 250 (1956). G. G. Hall, Proc. Roy. Soc. (London) A213, 112 (1952). R. Pariser, forts to obtain higher solution concentrations of naphthacene by use of higher temperatures were llllsuccessful due to decomposition of the ethyl iodide at higher tempera tures. The two cells used in this experiment were the 20 cm pair already mentioned. They differed in length only by 1 part in 105• The amount of ethyl iodide displaced from the solution cell by the naphthacene was only 1 part in 104• Neither of these factors, whether they act conjointly or contrarily, can lead to the observed increase in optical density as is evident from the appended absorption curve for ethyl iodide. (Fig. 8). In view of the latter considerations, and the agreement of both calculated lifetime and spectral position with expectancy, we accept the existence of the lowest triplet state in naphthacene to be at 10 250 cm-I • Nitrogen Heterocyclics Of these, only two examples have been studied here: pyridine and phenazine. Reid27 reported that upon dissolving pyridine in ethyl iodide an enhancement occurred with ~=9, and maximum at 28000 cm- I • Investigation of this enhancement shows, however, that it is due to reaction with formation of ethyl pyridiniul11 iodide. Similar reaction occurred with ethylene bromide and pyridine. Phenazine reacts with ethyl iodide to form ethylphenazinium iodide, and the reaction is complicated by a simultaneous decomposition of the ethyl iodide to form 12, even when in the dark. Fortunately, the reaction is fairly slow (see Fig. 9). Spectra obtained at different times during the reaction are shown in Fig. 9. Three peaks are seen to develop upon introduction of phenazine into ethyl iodide, and these peaks are in very good mirror-image relation to the beautiful cherry-red phosphorescence with qv......,1 observed by 27 C. Reid, Conference on Molecular Structure and Spectroscopy, Ohio State University, Columbus Ohio, June 1953. Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 514 S. P. McGLYNN, T. AZU~ll, TABLE III. Vibrational bands of the lowest energy T1-S O transition of phenazine. Tl<-SO cm-1 in ether, !J.ii, cm-1 Tl<-SO cm-1 in EtI, T1---->So' cm-1 in EPA, !J.ii cm-1 !J.ii, cm-1 15 576b 0 15 198 378 AND M. KASHA for phenazine. There seems no doubt then that the external heavy-atom technique is fully applicable to nitrogen heterocyclics. Some difficulty is experienced with reaction which causes product absorption to interfere spectrally; for example, ethylphenazinium iodide has an absorption maximum at 17200 cm-1• Nonetheless, fast mixing of perturber and aza-aromatic solutions and fast measurement of absorption spectra will generally provide useful data. 15 000 0 15 600 0 16 200 700 16300 700 17 000 1500 17 000 1400 FI 265b 1301 MONITORING EXPERIMENTS 17 600 2000 13 736 1840 18 300 2700 12 8871> 2689 18 900 3300 In order that the external heavy-atom effect be considered characteristic of triplet states, it is necessary to investigate the effects exerted by such perturbation on vibrational overtones and on SI+-S0 absorptions; it is also necessary to investigate the ability of such 18 300 2800 19 500 4000 a From R. W. Harrell, Ref. 29 of text. b The three strong bands observed by Lewis and Kasha, Ref. 28 of text; the spertrallocations specified here are from Ref. 29 of text. ... :g d z; Lewis and Kasha28 for phenazine. It is consequently tempting to attribute these three peaks to the 11", 11"* transition 3B 2u+-lA g of phenazine. That this is appropriate is further evidenced by the fact that the lifetime of the phosphorescence is observed29 to be 0.023 sec, and is calculated from the absorption in ethyl ether to be 0.045 sec. The value calculated in ethyl iodide solution at zero time is O.OlO sec. Further vindication of the attitudes displayed here are contained in Table III, where the vibrational peaks observed in absorption are matched with those found in the phosphorescence by Harrel1. 29 The heavy-atom effect on the lowest T 1+-S absorption of acridine has also been investigated by Harrell,29 with results fully comparable to those reported above TABLE IV. Results of monitoring experiments. Vibrational overtone lLa<-lA Transition 11 360 0.0095 26 640" 26 308 332 9,10-Dichloroanthracene 11 420 0.0092 b 24 684 246 9,10-Dibromoanthracene 11 490 Anthracene Naphthacene 24 930 24 563 0.0088 21 225 b 20 990 ~:g Fd li1 b~ ~ ;;jl: ::." d 20 21 II 2' ZO WAVENUNBERS, "'0" 21 II W FIG. 10. The absorption spectrum of anthracene, and of anthracene with a known added contamination of naphthacene in the region 20000-24000 cm-1 at 22°C. Curve 1, 0.062M-anthracene in ethyl iodide; Curve 2, 0.062M-anthracene containing an added 3.14XI0-4% contamination of naphthacene, in ethyl iodide; Curve 3, 0.0840M-anthracene, in carbon disulfide; Curve 4, 0.0840M-anthracene containing an added 6.82X 10-4% contamination of naphthacene, in carbon disulfide. The dotted lines are the extrapolated tail of the lowest energy Sl<-SO absorption band of anthracene for Curves 1 and 3 only. Vibrational Overtones ii(O, 0) cm-1 Alcohol Compound ~ u SOlVENT CSa perturbation to discriminate weak impurity SI+-S0 transitions. Such experiments are reported below. ii(O, 0) cm-1 Ethyl iodide ii cm-1 ~g 1::0 sO!.V£NT ;C,H,1 235 In Figs. 4, 5, and 6 are shown the vibrational overtone C",,11200 cm-1 ) of anthracene, 9, lO-dichloroanthracene and 9, 10-dibromoanthracene, respectively; their wavenumbers and intensities are tabulated in Table IV. Each of these vibrational overtones was investigated in carbon disulfide and in ethyl iodide solution. In no case was there any detectable difference found, in either solvent, as regards molar extinction coefficients or spectral locations of the overtones. S I+-S0 Absorption Bands • R. N. Jones, Chern. Rev. 41,353 (1947). bE. Clar and Ch. Marschalk, Bull. Soc. Chim. France 17,434 (1950). 28 G. N. Lewis and M. Kasha, J. Am. Chem. Soc. 66, 2100 (1944). 29 R. W. Harrell, Ph.D. dissertation, The Florida State University, January 1959. 1 The lLa+- A transitions in anthracene, 9, 10-dichloroanthracene and 9,10-dibromoanthracene were measured in ethyl iodide solution, using ethyl iodide as a comparison blank in the region 4300 A-33oo A. In no case was there any difference in molar extinction Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions EXTERNAL HEAVY-ATOM SPIN-ORBITAL COUPLING for a T2~SO transition (to the second triplet of anthracene). The oscillator strengths of these bands were detectably the same in both CS 2 and.in ethyl iodide solution, as would be expected if the heavy-atom effect were discriminatory. The results of these experiments, as well as the further absorption work necessary to verify that these bands were indeed due to naphthacene impurity are diagrammed in Fig. 10. It is concluded that the method of external perturbation affords a means of uniquely distinguishing between low-level impurity SI~SO transitions and Tl~SO absorptions proper to the compound being investigated. coefficient noted. There was, however, in each case, a red shift in going from alcohol to ethyl iodide as solvent. These are also tabulated in Table IV. Impurity SI~SO Transitions A sample of unchromatogrammed anthracene was investigated, and as expected, two weak absorption maxima whose extinction corresponded to 10--4 % naphthacene impurity were located on the rising tail of the SI~SO absorption of anthracene. These maxima were at 20990 and 22460 cm-1, respectively, and were of such intensity that they could very well be mistaken THE JOURNAL OF CHEMICAL PHYSICS 515 VOLUME 40, NUMBER 2 15 JANUARY 1964 Hole Mobility in Organic Molecular Crystals R. RAMAN* AND S. P. MCGLYNN Coates Chemical Laboratories, Louisiana State University, Baton Rouge, Louisiana 70803 (Received 4 June 1963) Adoption of a simple crystal counter technique has made possible a study of carrier mobilities in organic molecular crystals. The results give a mean value of 0.48 cm'lV -sec for the mobility of holes in anthracene in the low-field region. The data indicate that study of pulse rise times at high electric field strengths will provide information on charge-carrier scattering mechanisms in these materials. INTRODUCTION HE basic difficulties which limit understanding of the mechanism of photoconduction in organic molecular crystals are electrode-crystal contact and barrier effects, development of space-charge regions in the crystal, and production of secondary photo and electrical effects; all of these are inherent in normal dc or ac photo current measurements. In common practice it is hoped to avert these difficulties by the use of pulse techniques. Even here there exist a few variations: pulsed illumination of a crystal having a dc bias voltage, synchronized pulse illumination and pulsed voltage onto a crystal, superimposition of pulsed voltage on a crystal having dc bias voltage, etc. However, most work on organics has been confined to thick crystals which required excitation by very-high-intensity light flashes while under the influence of an arbitrary electric field. Secondly, considerable difficulty is experienced in achieving the bandwidth and signal-to-noise ratio required for observation of true characteristics. The present work has been done with a view to obtain optimum experimental conditions for the study of thin crystals (10-100 J.I.) using low-intensity light pulses. T ,. Post Doctoral Research Fellow supported by contract between U.S. Atomic Energy Commission-Biology Branch and The Louisiana State University. Secondary photoeffects may thus be minimized and materials which may only be grown as large single crystals with considerable effort may be studied with ease in the form of thin layers. In the pulse measurements to be described the crystal acts essentially as a counter and almost all the secondary effects, such as space charge, etc., may be minimized by using thin crystals, high fields, and low intensity of illumination. The theory of operation of the crystal counter has been thoroughly discussed by Yamakawa,1 Brown,2 Van Heerden,3 Williams,4 and Hartke.s The crystal forms the dielectric of a capacitor subject to an arbitrary electric field, and current pulses are produced when either of the electrodes are bombarded with photon or particle radiation. The time required for the generated charge pulse to rise to its maximum value in a field sufficiently large to ensure that the range of the carriers is larger than the electrode separation is then measured. Under these conditions, Qt= (noeJ.l.E/d) t, (1) where Qt is the charge induced at time t, no the number 1 K. A. Yamakawa, Phys. Rev. 82,522 (1951). 'F. C. Brown, Phys. Rev. 97, 355 (1955). 3 P. J. van Heerden, Phys. Rev. 106; 468 (1957). 4 R. L. Williams, Can. J. Phys. 35, 134 (1957). 5 J. L. Hartke, Phys. Rev. 125, 1177 (1962). Downloaded 22 Feb 2013 to 128.118.88.48. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions