Fluorescence from Pyridazine and Pyrimidine (n, π*) StatesBeverly J. Cohen, Hiroaki Baba, and Lionel Goodman Citation: The Journal of Chemical Physics 43, 2902 (1965); doi: 10.1063/1.1697227 View online: http://dx.doi.org/10.1063/1.1697227 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/43/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Solvent effects on molecular spectra. III. Absorption to and emission from the lowest singlet (n,π*) state ofdilute pyrimidine in water J. Chem. Phys. 99, 1508 (1993); 10.1063/1.465319 Fluorescence spectrum from nπ state of pyridine vapor J. Chem. Phys. 69, 935 (1978); 10.1063/1.436611 n,π* fluorescence from selected vibronic levels of pyrimidine vapor: Franck–Condon factors and excitedstate anharmonic coupling J. Chem. Phys. 63, 4336 (1975); 10.1063/1.431150 Temporary negative ions and electron affinities of benzene and Nheterocyclic molecules: pyridine,pyridazine, pyrimidine, pyrazine, and striazine J. Chem. Phys. 62, 1747 (1975); 10.1063/1.430700 Compoundnegativeion resonance states and thresholdelectron excitation spectra of Nheterocyclicmolecules: Pyridine, pyridazine, pyrimidine, pyrazine, and symtriazine J. Chem. Phys. 58, 2110 (1973); 10.1063/1.1679477

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2902 LETTERS TO THE EDITOR

from the n+1 b u,: .. MO's and contain n unpaired electrons. The bond energies are given in Table I, where the value of (31 has been determined by least-squares analysis.

t Supported by the U.S. Atomic Energy Commission and The Robert A. Welch Foundation.

I J.-H. Yang and D. C. Conway, J. Chem. Phys. 40, 1729 (1964).

2 D. C. Conway, J. Geophys. Res. 69, 3304 (1964). a R. N. Varney, J. Chem. Phys. 31, 1314 (1959); 33, 1709

(1960) . • (a) W. L. Fite, J. A. Rutherford, W. R. Snow, and V. A. J.

van Lint, Discussions Faraday Soc. 33, 264 (1962); (b) J. A. Rutherford (private communication).

6 H. Eyring, J. O. Hirschfelder, and H. S. Taylor, J. Chern. Phys. 4, 479 (1936).

6 E. Hiickel, Z. Physik 76, 628 (1932), and earlier papers. 7 R. G. Parr, Quantum Theory of Molecular Structure (W. A.

Benjamin, Inc., New York, 1963). 8 This assumption is made so the model "predicts" the fact that

D(02-02n) ",,0. It appears reasonable on energetic grounds since the singlet states of O2 with the same electron configuration as the a~g- ground state are excited by 23 kcal (l.1.g ) and 38 kcal (l2;g+), whereas D(02-02+) is only 10 kcal.

Fluorescence from Pyridazine and Pyrimidine (n, 11"*) States*

BEVERLY J. COHEN, HmOAKI BABA, AND LIONEL GOODMAN

Whitmore Chemical Laboratory, The Pennsylvania State University University Park, Pennsylvania

(Received 3 August 1965)

THE emission properties of the diazines have re­cieved much attention from both an experimental

and theoretical viewpoint, and it has been supposed that these compounds do not fluoresce.I,2 Recently, however, Borresen3 observed an emission from a liquid solution of pyrimidine and attributed this to fluores­cence. The purpose of this communication is to report the (n, 11"*) fluorescence from pyridazine and also, to give conclusive evidence for the fluorescence of pyrim-

>0

0 300 c .. c .. ~ 200

0:

100

FIG. 1. Fluorescence, absorption, and excitation spectra of pyridazine and iluorescence spectrum of pyridazine-d. in iso­pentane-methylcyclohexane (5: 1) at 298°K. hand d refer to undeuterated and deuterated compounds, respectively.

idine. The accompanying communication4 by Logan and Ross reports an (n,1I"*) fluorescence from pyrazine.

Pyridazine and pyrimidine, obtained from Aldrich Chemical Company and Nutritional Biochemicals Corporation, respectively, were purified by vacuum distillation. Pyridazine-d4 from Merck, Sharp & Dohme of Canada, was generously donated to us by K. K. Innes. Pyrimidine-d4, obtained from the same company, was purified by vacuum distillation. 3, 6-Di­chloropyridazine, from the K & K Laboratories, was recrystallized twice from n-hexane. Samples consisted of a solution (concentration ""'1.5XlO-3M) in hydro­carbon. Emission measurements were obtained with a Baird Atomic fluorescence spectrophotometer, Model SFI. Radiation from a 150-W xenon source was dis­persed by a double-grating monochromator. The emis­sion, observed at right angles to the excitation beam, was dispersed by a second double-grating monochro-

400

>0

! 300

.. 200 >

.2 .. 0: 100

~~~-L-L-L~~~~~~~~~~~O 22 24 26 28 30 32

V,10- 3 (em-I)

FIG. 2. Fluorescence, absorption, and excitation spectra of pyrimidine in iso-octane at 298°K. Four vibrational components are found in the excitation spectrum; the intensity of the exciting light falls off rapidly with increasing frequency so that no vibra­tional peaks are observed in the region higher than 33000 em-I.

mator and detected with a 1P 28 phototube. Excita­tion spectra were corrected by the use of rhodamine B. The fluorescence lifetimes were kindly measured by R. Kellogg.

Pyridazine, pyrimidine, and some of their derivatives exhibit weak emission in liquid solutions at room tem­perature, and we have assigned this emission to fluores­cence from the singlet (n, 11"*) state on the basis of the following observations. The fluorescence and excitation spectra, together with the absorption spectrum in the n~1I"* region for pyridazine are shown in Fig. 1. Figure 2 shows the same spectra for pyrimidine. It is seen that the excitation and absorption spectra agree well, even with respect to fine structure in the case of pyrim­idine. It is also seen that there is a mirror-image rela­tionship between the fluorescence and absorption spec­tra. The spectra of pyridazine-d4 and pyrimidine-d4

are very similar to those of the respective undeuterated compounds, but the 0-0 absorption band is slightly shifted to the blue on deuteration. This blue shift ("",130 em-I) was clearly observed in the excitation spectrum of pyrimidine-d •. Preliminary measurements of the polarization of pyridazine-d. fluorescence indicate

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LETTERS TO THE EDITOR 2903

that it is positively polarized with respect to excitation into the n---+7r* band. The transition moment of the emission is, therefore, parallel to that of the n~* absorption. The fluorescence maximum of 3, 6-dichloro­pyridazine is blue shifted by 1800 cm-l from pyridazine, consistent with the blue shift of the n---+7r* absorption band. Lifetimes are given in Table I. The excitation spectra, mirror-image relationship, short lifetimes, polarization, and substituent effects all indicate that the emissions originate in the azines themselves, and are from the lowest (n, 1T*) singlet state.

Figure 1 shows the effect of deuteration on the fluorescence of pyridazine. The intensity is increased by a factor of 1.4 at 298°K and by "'3.1 at 77°K. The large heavy-atom effect indicates that there is significant internal conversion from the (n, 1T*) singlet

T ABLE I. Fluorescence data for diazines.

Fluorescence T

max. (em-I) (10-9 sec)·

Pyridazine 23 800b 2.6±0.5b

Pyridazine-d. 23 800b 3.2±0.5b

3,6-Dichloro- 25 600b 0.9±0.lb pyridazine

Pyrimidine 27 0000 1.5±0.2b

Pyrimidine-d. 27 0000 1.0±0.le

• Measured by Dr. Reid Kellogg from degassed samples. bAt 298°K in isopentane-methylcyclohexane (5:1). CAt 298°K in pentane-methylcyclohexane (1:2).

<Pn/~

l.4b

1.0e

to the ground state. The increase in T due to deuteration supports this conclusion. Johnson, Logan, and Ross5 found a similar effect in the fluorescence of azulene. In contrast to this, deuteration caused no appreciable change in the fluorescence intensity of pyrimidine. It might be noted that no phosphorescence could be detected in any of the pyridazines investigated.

In the light of the present observations, the energy­transfer mechanism in the azines is being reinvesti­gated. Work along these lines will be reported in the near future.

We wish to thank Dr. Reid Kellogg of E. I. Du Pont de Nemours and Company, Inc., for measuring the fluorescence lifetimes, and Mr. Peter C. Falenti for his assistance with the experimental work.

* Supported by a grant from the Petroleum Research Fund of the American Chemical Society.

I M. Kasha, Radiation Res. Suppl. 2,243 (1960). 2 M. A. El-Sayed, J. Chern. Phys. 36, 573 (1962); 38, 2834

(1963) . 3 H. C. Bi:irresen, Acta Chern. Scand. 17,921 (1963). • L. M. Logan and I. G. Ross, J. Chern. Phys. 43, 2903 (1965). 'G. D. Johnson, L. M. Logan, and r. G. Ross, J. Mol. Spectry.

14,198 (1964).

Fluorescence and Phosphorescence of Pyrazine, as Vapor and in Solution *

L. M. LOGAN AND I. G. Ross Chemistry Department, University of Sydney, Sydney, Australia

(Received 3 August 1965)

STUDIES of the luminescence of nitrogen hetero­cycles have led to the generalization that radiation­

less processes between excited states are particularly efficient in these molecules. l Thus, while phosphores­cence is readily observed in suitable condensed phases, fluorescence has been deemed hardly ever to occur. It has also been stated2 that compounds of this class do not emit radiation in the vapor phase.

We now report, in contrast to the above, that pyrazine (1: 4-diazine, C6ILN 2) shows fluorescence as well as phosphorescence, both in solid solution and as a vapor. In an accompanying paper, Cohen, Baba, and Goodman3 report fluorescence from 1: 2- and 1: 3-dia­zines as well. Current views on luminescence mecha­nisms in compounds of this class thus need reappraisal.

Pyrazine (Fluka, "purum" grade) was zone refined and vacuum sublimed. Pyrazine-d4 (Merck, Sharp & Dohme; a sample kindly given us by K. K. Innes) was vacuum sublimed. Luminescence was excited by irradi­ation into the first singlet (1T*-n) absorption system, using the 3130-A Hg line isolated with K2Cr04 solution. Spectra were photographed with a Hilger medium quartz spectrograph.

The vapor luminescence (Fig. 1) was excited with a l-kW Toronto arc at vapor pressures of pyrazine from 1 to 80 mm Hg. No emission was observable at 200 mm Hg. Vapor pressures were calculated from the temperature of a cold reservoir and the following meas­ured relations: loglOP(millimeters) = 10.9967 - 2971/T (T<326°K), or 7.9166-1969/T (T>326°K). The vapor temperature was 80°, and exposure times with Ilford Zenith plates were of the order of hours. Varying the pressure over the range quoted affected the intensity of the spectrum, but had no marked effect on its appearance.

The vapor emission spectrum consists of two regions. The onset of the longer-wavelength region coincides exactly with the origin of the weak lowest triplet­ground-state absorption,4 and is clearly phosphores­cence. The shorter-wavelength region occurs in the same region as the hot bands of the lowest singlet -ground-state absorption, and has roughly the same Franck-Condon contour. It is natural, therefore to identify it with fluorescence, though unambiguous cdrre­lation of emission bands in this region with known absorption bands was not possible, for two reasons. First, at all practical pressures, the origin and the most prominent hot bands are self-absorbed. Secondly, though the absorption spectrum is exceedingly sharp the emission spectrum is somewhat diffuse. There ar~ precedents in naphthalenes and azulene6 for such a qualitative difference in spectral definition.

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