Edward M. Schulman University of South Carolina Columbia, 29208
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Room Temperature Phosphorescence An experiment for the undergraduate physical or analytical laboratory
While it is quite easy to demonstrate the fluorescence phenomenon to a n assemhlv of students bv simplv exposine any of a wide variety of inorganic or organic maiekals to u c traviolet irradiation and allowing students to observe the visible emission of radiation during exposure to the source, corresponding demonstration of organic phosphorescence has not been so facile. Phosphorescence is inherently a n exciting phenomenon for the undergraduate to observe, since the visible emission continues for a suhstantial time after the end of exposure to the ultraviolet source. However. most examoles of the nhosnhorescence nhenomenoo in organic materials require low temperatures or the riaorous exclusion of oxveen ( 1 ).These restrictions do not lend thkmselves well to lectlre hall demonstration. We have recentlv discovered a techniaue . bv" which room temoerature phosphorescence of simple organic molecules can hedemonstrated in free communication with the atmosphere of the lecture hall or classroom. We find that when aqueous solutions of ionic organic materials are adsorbed onto surfaces bearine free hydroxyl groups and the support is rigorously dried, intense 5,isihle phoiphorescenres of long lifetirnc can he ensilv observed ( 2 ) . - ~ h e s i m ~ l emethod st orsample preparation & a s follows
phors a t room temperature, in air by preparing samples a s described above and then cutting the filter paper into 5 X 0.5-mm strips (or by simply impregnating and drying the strips). These strips are then held by means of a notched stick, such as those normally found in molecular model sets, in the Dewar assembly of an Aminco-Keirs spectrophosphorimeter.
A circle of Whatman No. 1filter paper is saturated with a 5 mM solution of a polynuclear aromatic acid in 1 M sodium hydroxide (4-biphenylcarboxylicacid yielding brilliant turquoise emission o1-naphthoicacid yielding green, for example) and is then rigorously dried with a heat gun. The sample can be prepared directly in front of the class, or prernade and stored in a dry atmosphere.' When the class is ready for the demonstration,'thedried filter paper is simply placed under a short wavelength uvsqurce (e.g., Mineralight) on the table. The rwm is darkened, and after an appropriate pause for dark acclimation, the sample is pulled from under the lamp and held up for all tosee. The emission from the sodium salt of 2-naphthoic acid is so hrieht and lone lived ( T l i . = 740 ms (2h)) that it can easilv , he s e n fr& all pornt%ka darkened lecture hail. when mail gruups can be gnthewd mound B tal,.t, ur when one nlloar "hands.onMdemunstraucms, the hght is hright rnuugh to rend hv.
Figure 1. Comparison of emission and excitation specha of sadium ~phmalate adsorbed on paper and frozen in solution.
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Undergraduate Laboratory Experiment We have easily and routinely measured both the excitation and emission spectra of a wide variety of ionic organic phos-
A desiccator is, of course, useful for the storage of prepared samples, although we have found that simply placing the sample in a plastic bag containing some Drierite allows one to travel about the country for several weeks without notable loss of phosphorescence intensity.
522 / Journal of Chemical Education
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i 200
300
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500
600
7-
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Figure 2. Emission spectra of 0.5 mMEosin Y in phosphate buffer, adsorbed Jn paper at mom temperature,and in frozen solution at 77'K. The band at 555 nm is the classic E-type delayed fiuorescence.
Room Temperature Phosphorescence Spectra on Paper SupportY
A excitation nmb
A emission nmb
Relative intensity
Corboxylic A d d s 2%iphenyIcarboxylic acid 2-Biphenyicarboxylic acidc
12801 (3001
485
20
445 47 5 49-515
65 80 50
Figure 3. E-type delayed fluwescence. (A)The phosphorescenceof Eorin Y is explained by (?)absorptionexciting lhe molecule to its first excited singlet. (2) intersystem crossingto the first excited triplet, and (3) emission from triplet to ground state. (8)The delayed flucxescsnce can mmpete wilh phcsptwascence only at elevated temperature. (1) a b m p t i i . 12) intmystem crossing. (3) mermal reexcitationhorn Tt to St, requiring 9.5 kcallmole, and (4)delayedfluxescence.
Diphenic acid Naphthalic acid
Naphthaiic acidc
I-Naphthoic acid
. ~ 2-Naphthoic acid
12901 290
.. 490 525 55-570 (5251
69 75 38-34 69 dR
l-Aminonaphthalene-4IUifonic acid Naphthalene-0-rulfonic acid
I-Naphthol-5-sulfonate sodium ralt 2-Naphthol-6-Suifonate sodium W t d
F gLre 4 Comparison at em rr on and exclmt on svecna for sodobm 2-napnUm -B-sulfonateadsorbsa on paper from water s o l ~gn t w lhthat aasorbea hom 1M s m l ~ m hyorox de solutmn (smgly and doubly lonlzed speces)
soon become one of outstanding analyrical utility ( 3 , 4 J The . exritation and emission spertra of a wide variety of simple ionic polynuclear aromatics can be measured as shown in the table (5).Another very interesting experiment is the measurement of 0.5 mM Eosin Y in pH 8 phosphate buffer both by R T P and then a t 77OK in frozen solution. Figure 2 shows the emission results of that experiment. The temperature dependence of E-type delayed fluorescence along with the temoerature indeoendence of ohosnhorescence is thus easilv shown. The delayed fluorescence is, of course, simply ex~ l a i n e dbv the thermal reexcitation of molecules in the first - . excited triplet to the flrst excited singlet prior to emission (fi,. Such delaved fluorescence is onlv" oossible when the ambient . temperature is sufficient to provide reexcitation e n e r n and the triplet state is still lonr lived. Figure R shows the oathwa\,s for both phosphorescenc~and delayed fluorescence in EO&
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OAli sampler prepared 3-5 m M in 1M N a O H and dried on Whatman No. 1 paper unless otherwise noted. b ~ u m b e r irn parenthere$ indicated fixed wavelengths for scanning of spectrum presented in other column. C~rozen solution, 77'~. d ~ o d i u mruifonate in 1M NaOH t o yield dirodium ralt. eS~dium ruifonate in water t o yield monorodium salt.
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The paper is positioned so as to he a t 45' with respect to incident and emitted light (the phosphoroscope effectively removes the problem of reflection) and both the emission and excitation spectra are recorded. Figure 1-shows a n example of the excellent agreement one obtains for spectra measured bv the room temperature techniaue with those obtained bv the conventional low temperature method. The spectra shown were ohmined with a I jO-CI' xenon I m o . hut we have obtained excellent results with a 350-W high pressure mercury lamp as well. Several experiments can be carried out using the room temperature phosphorimetry (RTP) technique which will
Yet another interesting experiment that can be done by R T P is the measurement of singlet-triplet splitting in singly and doubly ionized species, showing the effect of ionization on the energies of singlet and triplet states (or the relative variation in pKa of the singlet and triplet states (7)).The experiment is simply carried out by preparing R T P samples of commercially available sodium 2-naphthol-6-sulfonate dissolved in both water and 1M sodium hydroxide. As can easily be seen in Figure 4, the triplet energy is almost unaffected by the ionization of the phenol while the singlet is substantially lowered in energy. Simple linear extrapolation of the trailing edge of the excitation and the leading edge of the emission to approximate 0,O bands yields the result that Volume 53. Number 8, August 1976 / 523
the singlet-triplet splitting is reduced from 19.7 kcallmole to 11.6 kcallmole a t room temperature by the ionization of the phenolic hydrogen. This phenomenon can be explained as follows (7).
S ,ofthe parent compound. The same argument can be used to explain the re1ark.e stabilization of the 7'1 srate; however, in tr$ets cannonical form (11) is relatively less important than form (111). This is simply a restatement of Hund's rule in that two electrons in sin& nccu~iedorbitals are more likelv to be together (as in form-61)) when the spins are paired, hui more likelv to be senarated in mace (as in form (111))when the soins are parallel. ~
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Acknowledgment
If we consider three of the contributing resonance structures to the hybrid constituting 2-naphthol, we can readily see that forms (I) and (11) are the major contributors to the ground state, So, and also to the first excited singlet, SI.However, in the SI state an electron has been removed from the highest occupied molecular orbital and placed in the lowest unoccupied molecular orbital: this constitutes a formal separation of charge and can hest be expressed in resonance terms as an increase in (11) character in the hybrid. The greater importance of (11) causes the first excited singlet to he a stronger acid, i.e., have a lower pKa. Thus, the SI of the anion of 2naphthol lies at lower energy and is stabilized with respect to
524 / Journal of Chemical Education
The author is pleased to acknowledge helpful and stimulating conversations with Professor R. L. Cargill of this Department and Professor Joel F. Liebman of the Department of Chemistry, University of Maryland, Baltimore County. Literature Cited (1) Zander, M., "Phosphorimetry." Aesdemie Press. New York, 1968. p. 117. (21 (a) khulman, E. M., and Walling, C., Science. 178.53 (1972):(b) Schuhan.E. M., and Wsl1ing.C..J.Phya Chrm., 77,902 (19731. (3) Psynfer,R.A., Wellons, S.L..and Winefordner, J.D..Anol Chem.. 46.736(1974). (41 Sevbold.P.G..endWhite. W. A n d Chem.. 47.1199 (1975).
