DAVID W. FINKAND WILLIAM E, OHNESORGE
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Luminescence of 2,2’,2”-Terpyridine by David W. Fink and William E. Ohnesorge Department of Chemistry, Lehigh UnivereQy, Bethlehem, Psnneylvanh 18016 (Received June 18, 1959)
2,2’,2’’-Terpyridine exhibits three luminescence emission bands in fluid aqueous and ethanol solutions: a violet band (maximum emission at 350 nm) in slightly acidic solution (b*-+ T ) , a blue band (maximum at approximately 440 nm) in neutral and alkaline solution (‘T -+ n), and a green band (520 nm) in very acidic solution (lr*+ r). The presence of these bands, their fine structure, and intensitiesare governed by the concentration of 2,2‘,2“-terpyridine, pH, and solvent. These bands are correlated with the acid forms of the solute species, and assignments of the electronic transitions involved are suggested on the basis of absorption, emission, and excitation spectra, solvent effects, and luminescence lifetimes. There is evidence for conformational change in the excited singlet state (IT*) of HzTer2+in fluid solutions.
Nakamoto has published the pH-dependent ultraviolet absorption spectra and assigned the 2,2’,2’’terpyridine species and their configurations.’ There has been no similar study of the pH-dependent luminescence properties of 2,2’,2’’-terpyridine. The spectral properties of Ter are complicated by (a) the acid-base properties of the molecule, which can accept one or two protons over the pH range 0 to 13, and (b) the possibilities of the three different cis-trans isomers. Nakamoto did discuss the possible cis-trans conformers of terpyridine and based upon spectrophotometric studies he assigned the conformation of the basic unprotonated terpyridine molecule (Ter) as trans-trans, the monocation (HTer+) as cis-trans, and the dication (HBTer f) as cis-cis. The nitrogen atoms in terpyridine introduce nonbonding electron pairs (n-electrons) into the molecular structure. These n-electrons may be promoted by absorption of ultraviolet-visible radiation to an antib o n d i n g d molecular orbital delocalized on the pyridine ring.2 Kasha3s4 has presented several experimental criteria to justify assigning absorption bands as A* t n. It had been thought previously that fluorescence from nitrogen heterocyclics was impossible and that only phosphorescence could be observed from these molecules. The (a*, n) excited singlet state, arising from an overlap-forbidden (absorption) transition, had 8 long lifetime, thus increasing the possibility Of deactivation processes which result in nonradiative dissipation of the excitation energy in fluid media. Moreover, the energy difference between singlet and triplet enhance further the (n*l n, states is small’ which intersystem crossing rate.6 However, fEuorescence from a ( ‘ A * , n) excited state has been reported for some nitrogen heterocyclics: sym-tetrazine (1,2,4,5-tetrazine) and its dimethyl derivative,6,7 from a series of diazines (pyrazine, pyrimidine, and pyridazine) 18 as well as from the N-heterocycle 9,10-diazaphenanthrene’g In Some of these molecules the absence of a ( A * , n) triplet be-I
The Journal of Physical Chemistry
tween the (T*, n) singlet and the ( T ” , n) triplet greatly reduces the probability of intersystem crossing and therefore fluorescence can compete favorably with the radiationless processes because there is very little direct spin-orbit coupling between (n”, n) singlet and (r”,n) triplet states.9 More recently, Cohen and Goodman have shown that rapid internal conversion between allowed and forbidden excited (T*, n) singlet states is responsible for the low fluorescence quantum yields of the diazines; for two of these (pyrazine and pyrimidine) the rate of internal conversion between the (a*,n) states exceeds that for intersystem crossing despite the presence of a triplet (a*,r) state between the excited singlet and triplet (a*,n).S 2,2‘,2’’-Terpyridine forms stable chelates with many metal ions and has been used in several spectrophotometric and fluorometric analytical methods. Recently the intense violet fluorescence of the monocation of 2,2‘,2’‘-terpyridine (HTer+) was employed in a sensitive analytical method for the determination of iron.Io
Experimental Section Chemicals. 2,2‘,2’’-Terpyridine was purchased from two suppliers to check and compare properties: G* F* Smith Chemical Company and Pierce Chemical Company. The Smith product was analyzed for carbon, hydrogen, and nitrogen. Anal. Calcd : C, 77,24; (1) K.Nakamoto, J . Phys. Chem., 64,1420 (1960). (2) D. R. Kearns and M. A. El-Bayoumi, J. Chem. Phus., 38, 1508 (1963). (3) M.Kasha, Discussions Faraday SOC.,9,14 (1960). (4) M. Kasha in “Light and Life,” W. D. McElroy and B. Glass, Ed., Johns Hopkins Press, Baltimore, Md., 1961,pp 31-68. (5) J. N. Murrell, “The Theory of The Electronic Spectra of Organic Molecules,” John Wiley and Sons, New York, N. Y., 1963,Ch. 14. (6) M. Chowdhury and L. Goodman, J. Chem. Phys., 36,648 (1962); 38*2979 (1963)* (7) M. A. El-Bayoumi and D. R. Kearns, ibid., 37,2516 (1962). (8) B.J. Cohen and L. Goodman, ibid., 46,713 (1967). (9) M.A. El-Sayed, ibid., 38, 2834 (1963). (10) D. W. Fink, J. V. Pivnichny, and W. E. Ohnesorge, Anal. Chem. 41,833 (1969).
LUMINESCENCE OF 2,2’,2’’-TERPYRIDINE
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H, 4.75; N, 18.01. Found: C, 77.49; H, 4.74; N, 17.87. Some yellow impurity was removed from each lot by a series of recrystallizations from petroleum ether (bp 30-60’). One recrystallization usually lightens the yellow color noticeably; two or more were required t o obtain the (pure) white powder from most batches; melting points 90-90.5” and 92-92.5”; lit. 88-89’. m-Terphenyl was purchased from Eastman Chemicals and used without further purification. Absorption Spectra. All absorption spectra were recorded on either a Beckman Model DK-2A or on a Cary Model 14R recording spectrophotometer. Quantitative absorption measurements were made exclusively on the Cary instrument; the low-intensity ?r* -+n absorptions were observed on this same instrument with the expanded-scale slide wire (0.0-0.1 absorbance full scale) installed. Luminescence Spectra. A Farrand Model 104242 spectrofluorometer was used for all luminescence measurements. It was equipped with a General Electric UA-2 250-Mi mercury lamp, an RCA 1P28 photomultiplier tube, and an RCA WV-S4C microammeter. (A xenon lamp was used to record the excitation spectra.) Emission spectra were recorded on a Hewlett-Packard Model 7035 B X-Y recorder. For all emission measurements both of the excitation monochromator slits gave effective band widths of 20 nm; the emission monochromator slits gave widths of 10 nm for the entrance beam and 5 nm for the exit beam. The sample container was a 10 X 10 X 4S-mm quartz cell. All emission intensities were recorded relative to the fluorescence intensity (450 nm) of a 1.0 ppm quinine sulfate in 0.1 N sulfuric acid reference solution; 366 nm excitation was used except where noted. Low-Temperature Luminescence Observations. Absolute ethyl alcohol was generally used as the rigid glass-forming solvent for observations a t ca. 80°K. The ethanolic solutions were placed in test tubes and immersed in a liquid nitrogen bath until the rigid glass was formed. Excitation was then provided by ultraviolet hand lamps (Ultra-Violet Products, San Gabriel, Calif. Models UVL-21 and UVS-11); color and duration of emission were detected visually. p H Measurements. pH was measured on a Corning Model 12 expanded scale pH meter. It was standardized against buffer solutions prepared from Coleman buffer tablets and also against Instrumentation Laboratory (Watertown, Mass.) phosphate and phthalate buffer solutions. The pKa values for HzTer2+have been determined as pKa1 = 2.64 0.07 and pKa2 = 4.33 0.03 by Martin and Lissfelt” and later redetermined by Nakamoto’ as pKa1 = 2.59 and pKa2 = 4.16. The absorption spectrum of terpyridine agreed completely with the spectra published by Nakamoto. The values pK,1 = 2.6 and pKa2 = 4.2 given by Nakamot0 were used for this work. Based on these values
*
PH Figure 1. 2,2’,2’’-Terpyridine (ground state) concentration-pH profile. f = fraction of terpyridine present; calcd from HzTerZfpK,1 = 2.6, pK,z = 4.2.
I
Figure 2. Fluorescence spectra of 2,2’,2”-terpyridine species. 1, 1.0 X lO+M HTer+ in lom4M HCI a t 313 nm; 2, 1.0 X lo-* M Ter in cyclohexane a t 366 nm; 3, 1.0 X H2Ter*+in 0.2 M HCl a t 313 nm.
the fractions of (ground state) 2,2’,2ff-terpyridine species in solution as a function of pH were calculated and plotted in Figure 1.
Results and Discussion An unequivocal assignment of the several luminescence bands to the emitting aoid-base species of terpyridine can be made from the comparison and correlation of absorption and luminescence spectra and their variation with pH. However, assignment of emitting states and electronic transitions is somewhat less certain. Acid-Base Species Assignments. Ultraviolet Emission Band. In slightly acidic aqueous solution (pH ca. 2 to 4), Ter emits very intense violet luminescence upon 313-nm excitation (Figure 2, Table I). The penetration of the long wavelength tail from the ultraviolet peak into the visible is responsible for its violet color. The sharp cut-off of luminescence intensity on the high(11) R. Martin and J. Lissfelt, J . Amer. C h e w Soc., 78, 938 (1966).
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DAVID W. FINKAND WILLIAME, OHNESORGE
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Table I : Comparison of the Relative Intensities of the Emission Bands of Terpyridine
a
Band
Terpyridine concn
Excitation wavelength
Blue Green Violet
1 . 0 x 10-M 1 . 0 x 10-8M 1 . 0 x 10-bM
366 nm 366 nm 313 nm
Relative source intensity at excitation wavelength
Absorbancea at excitation wavelength
Relative deteotor sensitivity at emission wavelength
64 64 15
0.01 0.02 0.11
78 48 100
IL meter reading
2.3 4.0 41.0
1.00-cm cells.
71
0
I 1
I 2
I
PH
3
I 4
I 5
I 6
I 7
Figure 3. Luminescence-pH profile for the three emission bands of 2,2’,2”-terpyridine. 1, excitation, 365 nm; emission, 520 nm; 2, excitation, 313 nm; emission, 350 nm; 3, excitation, 365 nm; emission, 440 nm.
energy side of this emission band is due to self-absorption: 2,2’,2”-terpyridine absorbs very strongly at wavelengths less than 350 nm.‘ Comparing the pH-IL (350 nm) profile (Figure 3) and its correlation with the (ground state) concentration-pH profile (Figure 1)confirms that monoprotonated terpyridine, HTer +, is the species emitting the violet luminescence.lo The excitation spectrum for this violet emission recorded with a xenon lamp excitation source exhibits a peak at ca. 335 nm and a shoulder at ca. 280 nm. These correlate, as expected, with the absorption spectrum of monoprotonated terpyridine, HTer f , which has absorption peaks a t approximately 330 and 280 nm.’ Low-Energy (Green) Emission. A green emission (Figure 2) appears in very acidic aqueous solutions (pH below ca. 2) of 2,2/,2”-terpyridine excited a t 365 or 313 nm. The spectrum is a symmetrical Gaussianshaped curve; a single electronic transition is suggested. Variation of the intensity of this band a t 520 nm is plotted us. pH in Figure 3 and comparison with Figure 1 indicates that diprotonated terpyridine, HzTer2+,is the emitting species. Blue Band. In alkaline aqueous solution 2,2’,2“The Journal of Physical Chemistry
Figure 4. Solvent effects on band of 2,2’,2”-terpyridine.
?F* +
n absorption
terpyridine exhibits a blue luminescence. The pH-dependence of this band is also plotted in Figure 3. Comparing this pH-IL profile and the Ter species concentration-pH profile (Figure 1) indicates that unprotonated terpyridine, Ter, is the emitting species. This same blue emission is observed also in organic solvents of low polarity e.g., n-hexane, cyclohexane, and benzene (Figure 2). Assignment of Excited States and Radiative Transitions. Ter is the species emitting the blue band. In this form the terpyridine has three unprotonated basic sites and six loosely held, nonbonding (n) electrons available for excitation by low energy radiation; thus, there is strong probability for A* en transitions. Low-Energy, Low-Intensity Absorption Band of Ter near 360 nm as T* +n. In organic solvents, a low-intensity absorption band appears (at ea. 360 nm) on long wavelength tail of the intense A* + A terpyridine absorption bands (Figure 4). The singlet A* + n transition is often the longest wavelength transition of the molecule containing nonbonding (n) electrons and is almost always lower in energy than singlet A* + ?r
LUMINESCENCE OF 2,2’,2’’-TERPYRIDINE transitions in N-heterocyclics. Kasha3v4has presented a series of qualitative tests for assigning such low intensity absorptions as a* c n. The criteria were applied to this terpyridine absorption band and indicate that it results from a (spin-allowed) ?r* t n. The molar absorptivity for the long wavelength band of terpyridine is small (e n-hexane > benzene > n-hexanol > absolute ethanol > methanol > chloroform > 1:1 ethanol-water. The absorption band exhibits the same solvent-structure relationships : structure is lost as the solvent becomes both more polar and more hydrogen-bonding. The decreased vibrational structure in the absorption spectrum indicates more solvation in the ground state, or in this case, also more hydrogen bonding to the lone electron pair on nitrogen. (12) G. J. Grealey and M. Kasha, J . Amer. Chem. rSoc., 7 7 , 4462 (1966). (13) R.M.Hochstrasser, Accts. Chem. Res. 1,266 (1968).
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January 8, 1070
DAVIDW. FINKAND WILLIAME, OHNESORGE
76 2.2:2:-Terpyricine iC8: Z,?!
c,:,c-c
