Edward M. Schulman and Cheves Walling
902
Fock energies of the molecule and molecule-ion plus a correlation c~rrection,~b agrees well with the experimental value, suggesting that the procedure for estimatihg the correlation correction is reliable. Moreover, the IP(SF) derived from Koopmans' theorem, 11.4 eV, is higher than the experimental value, and the uncorrelated difference between H-F energies of neutral and ion, 9.6 eV, is lower, the two values bracketing the true IP as pointed out by Richards.25 Information about the electron affinities of all the S-F molecular species but SF2 is available, although the data are of rather mixed accuracy. These values are EA(SF) = 2.5 f 0.5 eV;3a EA(SIF3) = 2.9 f 0.1 eV;20 EA(SF4) = 1.3 =k- 0.2 eV;20 EA(SF5) = 3.3 f 0.2 eV;20 and EA(SF6) = I .4 0 . 2 eV.21,26 Here again the even-odd electron varia-
*
tion manifests itself, with the odd-electron molecules naturally having the higher EA'S. Because of this regularity, one can estimate with some confidence EA(SF2) = 1.2 f 0.5 eV. It is then possible to use the electron affinities and the ionization potent,ials together to obtain a rather complete set of thermochemical data for the positive and negative ions in the S-F system.27
W. G. Richards, Int. J. Mass. Spectrom. /on Phys., 2, 419 (1969). J, Kay and F. M. Page, Trans. Faraday SOC.,6 0 , 1042 (1964). Note Added In Proof. After this manuscript was submitted, a value of 10.45 I 0.01 eV was reported for the adiabatic ionization potential of SCF2 from photoeiectron spectroscopy [H. W. Kroto and R. J. Suffolk, Chem. Phys. Lett., 17, 213 (1972)], as compared to the near-vertical lPof 10.53 f: O.lOeV reported here.
Triplet-State Phosphorescence of Adsorbed Ionic Organic Molecules at loom Temperature' Edwatd M. Schulman and Cheves Walling" Department of ChemWry, University of Utah, Salt Lake City, Utah 841 12 (Received June 2, 1972)
Strong phosphorescence (triplet-state emission) is observed at room temperature from salts of a wide variety of polynuclear carboxylic or sulfonic acids, phenols, and amines adsorbed on paper, silica, alumina, and other supports. This phosphorescence is 0 2 insensitive but requires thorough drying and is not observed1 with nonionic materials. Emission and excitation spectra are very similar to those observed from frozen solutions at - 196", and the technique provides a convenient means of demonstrating phosphorescence phenomena, measuring triplet-state and delayed fluorescence spectra, and identifying a variety of organic molecules including many of biological interest.
Introduction Although very weak. phosphorescence has been observed from rigorously deoxygenated solutions of dyestuffs, and in the classic studies of E-type delayed and recentLy a few examples of stronger emission involving particularly rigid molecules have been reported in solution a t room temperature,3 strong phosphorescence (efficient triplet-state emission) from organic molecules is normally observed only in the igas phase, in rigid media, or at very low temperatures.4 Accordingly, in the course of working up a reaction mixture from the decomposition of benzoyl peroxide in tetrachloroethylene5 by thiq layer chromatography on silica developed with isopropyl alcohol-ammonia, we were surprised to observe strong blue-green phosphorescence of dried spots corresponding to biphenylcarboxylic acids, present as their ammonium salts. We decided to pursue the matter further and this paper describes our results.
Results Although we have been unable to find any comparable observation in the literature,S further study showed that
similar phosphorescence is a general property of a wide variety of ionic organic molecules. Essentially every salt of a carboxylic acid, phenol, amine, or sulfonic acid investigated which might be expected to show visible phosphorescence did so, but none was observed with any nonionic species. Phosphorescence is observed from materials adsorbed on a variety of supports, silica, alumina, paper, asbestos, and (more weakly) glass fibers, although filter Report& in part at the 13th Annual Rocky Mountain Spectroscopy Cbnference, Society for Applied Spectroscopy, Denver, August, 1971. A preliminary account has also been published: E. M. Schulman and C. Walling, Science, 178, 53 (1972). C. A. Parker, "Photoluminescence of Solutions," Elsevier, New York, N. Y . , 1968, pp45-46. R. €3. Bonner, M, K. DeArmond, and G. H. Wahl. Jr., J. Amer. Chem. SOC.,94, 988 (1972). M . Zander, "Phosphorimetry," Academic Press, New York, N. Y., 1968, p 117. E. M. Schulman, R . D. Bertrand, D. M.Grant, A. R. Lepley, and C, Wailing, J. Amer. Chem. Soc., 94, 5972 (1972) (a) Polynuclear aromatics have been observed to phosphoresce at 77'K on paper chromatograms: A. Szent-GyQrgyi, Science, 126, 751 (1957). (b) Phosphorescence spectra of solutions frozen at 77'K on glass fiber paper, paper, and silica gel have been measured: M. Zander, Erdoel Kohle, 15, 362 (1962); E. Sawicki and J. D. Pfuff, Anal. Chim. Acta, 32, 527 (1965); E. Sawicki and P. Johnson, Microchem. J., 8, 85 (1964).
Triplet-State Phosphorescence of Organic Molecules
903 n
100,
80
----29S°K,
I
I I
PAPER
t
i
w
2 a i E i !
eo0
300
400
500
600
700
800
WAVELENGTH, nm
Figure 1. Comparison of emission (450-700 nm) and excitation (200--400 n m ) spectra of Na naphthalate adsorbed on paper and in frozen solution.
paper seems to give the best results. Phosphorescence appears to involve surface adsorbed molecules, since none could be detected from finely ground samples of pure salts, or from crystals grown from solutions of the organic acid in just sufficient NaOH to neutralize, whereas the same solutions gave excellent phosphorescence samples when impregnated onto paper and dried. 2-Naphthoic acid dissolved in excess aqueous NaOH and rigorously evaporated to dryness (giving material adsorbed on NaOIH-Na&03) phosphoresced strongly. Dryness is also essential; samples adsorbed on paper lose the ability to phosphoresce when exposed to a humid atmosphere, but regain it on heating or drying in a desiccator. The phosphorescence i b completely oxygen insensitive. No differ: ence is observed in phosphorescence intensity of samples stored for weeks under pure atmospheres of d r y oxygen, nitrogen, or air.*[ Finally, the intensity of emission is sometimes most striking. Phosphorescence of sodium, potassium, or ammonium salts of 2-naphthoic acid adsorbed on Whatman No I paper, irradiated intermittently by a 15-W Mineralight. short-wave uv source, is detectable under ordinary room lighting and may be observed for over 6 sec in the dark. Quantitative Meusurements. For more detailed study, materials adsorbed on paper were examined in an Aminco-Keirs spectrophotophosphorimeter, and emission and excitation speclra compared with those from frozen solutions at --196". Figure 1 shows an example and, in general, peak positions under the two conditions were almost identical although the adsorbed species exhibited some line broadening and loss of spectral detail. Our measurenaents are summarized in Table I which gives the location of maxima for both excitation and emission, together with relative intensities. In some cases (e.g., the naphthol sulfonates) it was possible to obtain spectra from both singly and douhlly ionized species, Figure 2. Although ionization of the naphthol group shifts the excitation spectrum ( i e . , singlet absorption) to longer wavelengths, emission is hardly changed, consistent with Jackson and Porter's results on 2-naphthol and its anion in rigid media a t low temperatuses.8 Phosphorescence lifetimes of several molecules were also examined by flash photolyqis with results listed in Table 11. characteristic decay times ( t = l / k ) lie for the mosk part in the 100-700-msec range. These were usually
W A V E L E N G T H ,nm
Figure 2. Comparison of emission and excitation spectra for Na
2-naphthol-6-sultonate adsorbed on paper from water solution with that adsorbed from 1 M sodium hydroxide solution (singly and doubly ionized species). somewhat shorter than those observed a t liquid nitrogen temperatures, although some lifetimes appeared longer on paper a t ambient temperatures. Finally, it seemed possible that the technique might permit observation of esr spectra of the triplets involved. However, irradiation of the sodium salt of 2-naphthoic acid adsorbed on paper produced only a broad, ill-resolved esr peak which increased in intensity with irradiation time and failed to decay in the dark, a result similar to that observed on irradiation of many solid organic compounds in bulk,
Discussion Evidently surface adsorption of the triplet states of organic molecules holds them rigidly so as to retard their usual nonradiative decay, and also somehow inhibits oxygen quenching. Why this should be the case is not at all clear,7b but nevertheless we believe that the technique should have a number of applications, Qualitatively it provides a simple means of demonstrating phosphorescence phenomena and identifying substances in chromatographic separations without resorting to oxygen exclusion or cryoscopic techniques. Combined with quantitative spectrophosphorimetry it yields quite well-resolved phosphorescent spectra, suitable for further product identification or the study of triplet states. Thus, for a number of the molecules we have investigated we have been able to approximate 0,O peaks and thus determine separation of singlet and triplet energy levels, Table 111. As might be anticipated, species showing small separations such as Eosin Y exhibit E-type delayed fluorescence as well as phosphorescen~e,~ Figure 3, so our technique may be used as well to examine this phenomenon which is not accessi(7) Similar insensitivity of phosphorescence to oxygen in solid solutions at normal temperatures have been observed. (a) P. Pringsheim and H. Vogeis, J. Chim. Phys., 33, 345 (1936), noted 02 insensitive delayed fluorescences in dyes dissolved in gelatine, sugar glasses, and cellophane but found color differences indicative of phosphorescence quenching by 02.(b) G. Oster and G . K. Oster in "Luminescence of Organic and Inorganic Materials," H. P. ilallmann and G. M. Spruch. Ed., Wiley, New York, N. Y., p 186, report that polymer molecules are effective in protecting included dyes from oxygen quenching. Our thanks to a referee for pointing out these references. (8) G. Jackson and G. Porter, Proc. Roy, Soc., Ssr. A . 260,,13(1961). (9) C . A. Parker and C. G. Hatchard, Trans. Faraday SOC., 57, 1894 (1961) .
The Journal of Physical Chemistry, Voi. 77, NO. 7, 7973
Edward M. Schulman and Cheves Walling
904 TABLE I: Ambient Temperature Phosphorescence Spectra on Paper Supporta
Compound
X excitation,
X emission,
nmb
nmb
Relative intensity
excitation, nmb
Compound
Xemission, nmb
Relative intensity
I
l _ l -
Carboxylic Acids 2-Biphenylcarboxyliis acid 2-Biphenylcarboxylic acidC
4-Biphenylcarboxylii:
acid
4-Biphenylcarboxylic acidc
20 65 80
(475) 450 482 500-51 5
95 75 84 56-53 84 08
(480) 350 41 2 488 523 535-575
(530)
10
497 535 570-585
Napblthalic acid
(535) 490 530 575 61 0-640
NaDhthalic acidC
1-Naphthoic acid
490 525 550-570
2-Naphtiioic acid
(525) 490 523 Sulfonic Acids 1-Aminonaphthalene-4(345) sulfonic acid 260 350
50-25
(530) 500 530 560-575
(490)
1 -Naphthol-5-sulfonate sodium salt 2-Naphthol-6-sulfonate sodium saltd
15 79 70
69 69 75 38-04 69 48 42 45
505-555 (535)
90 19 86
(310) 235 290 -320
50
69 80 75 30-1 0 80 80 39-34 15 84 100 89 35 9-5 87 68 74 40-37
(520) 490
Biphenic acid
Naphthalene-@-sulfonic acid
485 445 475 490-51 5
(385) 255 372 (350) 260 315 367
2-Naphthol-6-sulfonate sodium salte
(300) 240 295 330
2-Naphthol-7-sulfonate sodium saltd
(360) 298 368
2-Naphthol-7-sulfonate sodium salte
(335) 235 277 335
69 78 23
70 37
100 19 96 92 19 64 99 87 18 72 78 97 39 97 88
19 37 80
Miscellaneous Auramine O f
Coproporphyrin I I Ig,h
(460) 375 460 (410)
85 24 89 20 54 16
55 10 Eosin y h 495 530 Eosin Y c , i
(520) 305 345
8 89 66 40 67 75
15 17
Ethyl violetj
a All samples prepared 3-5 mM in 1 M NaOH and dried on Whatman No. 1 paper unless otherwise noted. Numbers in parentheses indicate fixed waveiengths for scanning of spectrum presented in other column. Frozen solution, 77'K. d Sodium sulfonate in 1 M NaOH to yield disodium salt. e Sodium SUIfonate in water to yield monosodium salt. ,' 0.4 mM hydrochloride in water. g -0.3 mM in pH 8 buffer. 0.5 mM in pH 8 phosphate buffer. 0.1 mM in pH 9 borate buffer. 10.4m M chloride in water. kThe emissions at 625 and 682 nm correspond to fluorescences reported in pH 7.4 aqueous buffer at 61 1 and -672 i-m (P.Sayer, private communication).
ble at l o w t e m p e r a t u r e s . Delayed fluorescences a r e also n o t e d in A u r a m i n e 0 and Ethyl Violet.10
TABLE 1 1 1 Phosphcre$;conce Lifetimes for Sodium Salts of Organic Acids o n Paper Supports at 298" _______(ll_l._ls_---Compound -
T, msec
2-Naphthotc iscid 4-Biphenyicarboxylic acid 2-Naphthalleriesulfonic acid 1-Naphtholc acid Naphthalic acid 2-Naphthol;-6-sulfonic acid
740 71 0 655 469
Diphenic acid The Journal of Physical Chemistry, Vol. 77, No. 7, 1973
356 177 80
-
Finally, t h e v a r i e t y of c o m p o u n d s l i s t e d in T a b l e I, inc l u d i n g p o r p h y r i n s , suggests that the t e c h n i q u e s h o u l d h a v e a p p l i c a t i o n for t h e q u a l i t a t i v e analysis and i d e n t i f i c a t i o n of n u m e r o u s t y p e s of i o n i c o r g a n i c m o l e c u l e s inc l u d i n g many of b i o l o g i c a l i n t e r e s t . SI 0,O of Auramine 0, for example, lies at approximately 505 nm, thus a 9-12-kcal activation energy would lead us to expect the TI 0.0 band somewhere between 600 and 650 nm; no such band is observed. The failure to observe these bands, however, does not rule out E-type delayed fluorescence.
(10) The
Triplet-State Phosphorescence of Organic Molecules
905
TABLE iI I: Singset-Triplet Energy Splittings from Extrapolated (0,Q) Bands of Sodium Salts
cence and phosphorescence. In general, nonionic compounds showed only fluorescence. The papers were then -spotted with 1 M NaOH, redried, and reexamined under & uv irradiation. Those samples which phosphoresced were Compound XZ nm XI, n m kcal/mol Conditions - examined quantitatively. Quantitative. Reagent grade chemicals were in the gen466 23.7 77"K, frozen solution 4-Biphenylcarbox- 336 eral used as received. Solutions of the compounds were 24.7 ylic acid 315 433 298"K, paper made up to be approximately 4 X 10-8 M i.i 1 M aqueous 20.0 77'K, frozen solution 2-Biphenylcarbox- 331 43 1 sodium hydroxide. Whatman No. 1 filter paiber, cut in 5.3 ylic acid 323 41 5 19.6 298"K, paper X 0.5 cm strips, was then saturated with the solution and 9.5 298"K, paper Eosin Y 521 630 slowly dried under an infrared lamp. Solutions of com11.6 298"K, paper, 2-,Naphth01-7408 489 disodium salt mercially available sodium naphthol sulfonates were made sulfonate 472 19.7 monosodium salt 356 both in water and in 1 M NaOH to observe singly and 4tQ4 11.2 298"K, paper, 2-Naphthol-6480 doubly ionized species. disodium salt sulfonate Excitation and emission spectra were obtained on an 18.9 monosodium salt 464 3:55 Aminco-Bowman spectrophotofluorometer equipped with accessory phosphoroscope, ellipsoidal condensing assembly, Hanovia 901-C-11 xenon lamp, and an R 446 S flat response photomultiplier. Paper strips were placed in the standard quartz dewar used for low-temperature phosphorescence measurements. The dewar was rotated to the optimum angle for maximum response, slits were chosen for 1 1 1 1 maximum sensitivity and both emission and excitation I 1 spectra recorded. Frozen aqueous solutions were measured I I in the same instrument with the dewar filled with liauid I 1, I \ nitrogen and dry air being passed over the window. The phosphorescence blank from paper treated only with NaOH and dried was negligible at 298"K, although substantial at 77°K. Phosphorescence lifetimes were measured by flash photolysis using the apparatus described by Wadley.11 Rate i \ \ / constants and lifetimes were determined from first-order 0 --,------.7 zoo 300 400 500 600 700 800 rectified least-squares plots. WAVELENGTH, n m _I__-
i';
1
---.I
~
J
Figure 3. Emission spectra of 0.5 mM Eosin Y in phosphate buffer, adsorbed on paper at room temperature, and in frozen solution at 7 7 O K The band at 555 nm is the classic Gtype delayed fluorescence.
Experimental ~ e ~ t ~ ~ ~ Qualitative. Solutions of compounds to be tested in any convenient solvent (usually tetrachloroethylene) were spotted onto cit.ck?s of Whatman No. 1 filter paper. The papers were then rigorously dried either by means of a heat gun or more gently with an infrared lamp (I2R Hotspot), The spots were then examined with both the shortand long-wave tuber; of a 15-W Mineralight for fluores-
Acknowledgments. This work was supported by a grant from the National Science Foundation. We wish to express our thanks to Professor Art Lepley of the Department of Chemistry, Marshall University, for many helpful discussions and assistance in the spectral experiments. ~ thanks also go to Professor John D. Spikes of our Our biology department for the kind use of his spectrophotofluorometer and the donation of several biological samples and to Mr. Vernon Alvarez and Professor Stephen 6. Hadley of this department for the flash photolysis measurements. ( 1 1 ) S. G. Hadley, J. Phys. Chem., 74, 3551 (1970).
The Journalof Physicai Chemistry, Voi. 77, No. 7, 1973
