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Investigation of photochromism of 2-(2′,4′-dinitrobenzyl)-pyridine by nanosecond laser absorption spectroscopy
Volume 55, number 1
INVESTIGATION BY NANOSECOND
CHEMICAI
OF PHOTOCHROMISM LASER ABSORPTION
E_ KLEMM, D_ KLEMM, A. GRANESS
1 April 1978
PHYSICS LETTERS
OF 2-(2’,4’-DINITROBENZYL)-PYRIDINE SPECTROSCOPY
and J. KLEINSCHMIDT
Sektion Chemie der Friedric~lSc~IilIer-~tliversitdt Jena. 69 Jena. DDR and Sekiion Physik der Friedrich-Schiller-CiversitZt Jena. 69 Jena. DDR
Received 4 October 1977
The photochromism of 2-(2’,4’dinitrobenzyl)-pyridine transients were observed in the spectral ranges of390410 first band (390-410
nm; lifetime
= 1~s) is the absorption
was investigated by means of laser tlash photolysis. Two coloured nm ad 510-580 nm in both ~0132 and nonpolar solvents. The band of the xi-mtro
form and the second band (5 IO-580
lifetime-z0.5 s) is the absorption band of the azamerocyanin form of 2-(2’,4’dinitrobenzyl)-pyridine. tion scheme of photochromism is supported by absorption measurements at different pH-values.
nm;
Our proposed reac-
1. Introduction
2.2. Materiais
Although as early as 1925 Tschitschibabin et al. [ I] synthesized 2<2’,4’-dinitrobenzyl)-pyridine and reported that crystals of this compound after irradiation with sunlight exhibited a colour change from pale yellow to dark blue followed by decoloration after storing in the dark, there still exist contradictory opinions in the literature about the constitution of the coloured species [2--51 and no information is available about the photoreaction. in order to obtain additional information about the photochromism, we investigated 2-(2’,4’-dinitrobenzyl) pyridine by means of laser flash photolysis.
The used photochromic 2(2’,4’-dinitrobenzyl)pyridine [ 1;I ] was pure by thin layer chromatography and by elementary analysis.
2. Experimental 2. I. Apparatus We used a laser flash spectrometer, which is described in ref. [6]. The time resolution of the experi-
mental arrangement was about 20 ns. The investigations were carried out in the spectral range from 400 to 700 run. The resolution in wavelength units was about ax = 5-l 5 nm.
3. Results and discussion We flashed solutions of 2-(2’,4’-dinitrobenzyl)pyridine (10 -3 mol Q-l) in different solvents (IZheptane, ethanol, rz-propanol) and observed immediately after excitation with a pulse of the second harmonic of a ruby laser (<20 ns) a short-lived transient characterized by an absorption in the region 390-410 nm and a long-lhed species characterized by an absorption in the region 550-570 nm (see table I). We obtaired the decay times of both species from photographs of the oscilloscope screen (see figs_ 1 and 2). The pictures show that the decay time of the shortlived species is identical with the rise time of the longlived species. The absorption behaviour was investigated in ethanol solution at different pH-values. We observed in acid solutions (ethanol with H,S04, pH l-6) only the longlived species in the region from 530 nm to 590 nm. The
113
I April 1978
CHEMICAL PHYSiCS LETTERS
Volume 55, number 1
Table 1 Absorption wavelength and decay times of flashed 2-(2’,4’-dinitrobenzyl)-pyridine
solutions
Absorption regions and decay times
Solvent
loi~g-lived species
short-lived species
n-heptane
390-410
ethanol n-propanol
400470
nm run
1.2x
400450
nm
1.5 x 10-7 s
8 x IO-’
s
104s
530-580
nm
5.8 x-LO4
550-590
run
0.5 s
510-580
nk
0.7 s
s
410 nm H
2-(2;C-din1~er1zyl)-pynti
5OOns
Inheptane 550nm -
2ps
S70nm w 2-C3%utrotxnzyl)-pynd1re )---I SOON
Fig. 1. Oscilloscope traces of flashed 2-(2’,4’&nitrobenzyl)pyridine in heptane and ethanol.
LlOnm 430 nm LSOMI L70 nm 510 nm 530 nm
-
200 PS
2-(7~.-~Ntmbenzyl)-pyr~
tn n-propanote
Fig. 2. Oscilloscope traces of flashed 2-(2’,4’+iinitrobenzyl)pyridine in n-propanol.
114
2ps
in ethono!
Fig. 3. Oscilloscope traces of flashed 2C?‘,4’-dinitrobenzyl)pyridine in ethanol + KOH (pH 7.2).
results obtained in a basic solution (ethanol with KOH, pH 7.2) are shown in fig. 3. The short-lived species at 410 nm decays within about 200 ns. The lifetime of
the long-lived species at 530-590 nm decreased rapidly (see fig. 3, trace 2) to 2 X low6 s and in addition, we measured a new absorption band at 450-490 nm. The rise time of this absorption is identical with the decay time of the transient at 530-590 nm. In basic solutions with pH > 7.5 the absorption band of the 550 run species was not observed. The appearance and decay of the observed species were not changed by variation of the oxygen concentration (1O-5--5 X 10e3 mol Q-r), solvent viscosity (n-propanol, 20-6O”C), and concentration of the photochromic substance_ Hence, we think that our experiments can be explained with the results of Klemm and Klemm [S] by reaction scheme-f. The coloured azamerocyanin of the 2(2’,4’dinitro= 550 run) is photochemicaily benzyl)-byridine (A,, formed in polar and nonpolar solvents. We assume, that
Volume 5.5. number 1
CHEMICAL PHYSICS LETTERS
the transient at 4iO run is the aci-nitro form of 2-(2’, 4’-dinitrobenzyl)-pyridine, which is photochemicaliy produced from 2(2’,4’dinitrobenzyl)-pyridine (<20 ns). The aci-nitro form undergoes a thermal reaction in the ground state to ffie azamerocyanin. This explains, that the decay time of the 410 nm absorption band and the rise time of the 550 nm absorption band are identical_ The appearance of the 550 nm species in nonpolar solvents shows that intramolecular proton transfer occurs to the pyridin nitrogen. This concept is supported by previously reported results [6 ] , which showed that in nonpolar solutions of tetranitrodiphenylmethanes this reaction is not possible_ We measured the lifetime of azamerocyanin solutions (ethanol with H2S04) in dependence on pHvalue (pH = l-7) and observed a decrease of this lifetune from 0.50 s (pH = 7) to 2 X 1O-6 s (pH = 1) 181. We assume that the chemical equilibrium between the azamerocyanin form and the cation of the azamerocya+n is shifted to the side of the latter (see scheme). The above shown reaction scheme also explains our results-obtained in a basic solution. After excitation (s20 ns) three species are formed: the aci-nitro form
I April 1978
(h = 410 nm), the azamerocyanin (X = 550 run) and a species at 450 MI. We assume, that this new species is an anion of the azamerocyanin. This assumption&o explains the facts, that the lifetime of the azamerocyanin decreased rapidly from OS s (pH = 7) to 1 X 10e6 s (pH = 7.2), and that the rise time of the 490 nm absorption band is equal the decay time of the azamerocyanin.
References ] l] A.E. Tschitschibabin, B-M. Kuindshi and SW. Benewolensk.rja, Ber. Chem. Ges. 58 (1925) 1580. [2] B.r and C. Decoret, Tetrahedron Letters (1971) (31 C. Wettermark, Nature 194 (1962) 6777. [4] H.S. Masher, E.R. Hardwick and D. Ben-Hur, J. Chem. Phys. 37 (1962) 908. [S] E. Klemm, Dissertation B, Jena (1977); D. Klemm. Dissertation B, Jena (1977). [6] D. Ktemm, E. Klemm, A_ Gnness and J. Kleinschmidt, Chem. Phys. Letters, to be publiihed. [7] D. Klemm and E. Klemm, Z. Chem. 15 (1975) 47. [S] D. Klemm, E. Klemm, A. Graness and J. Kleinschmidt, Z. Physik. Chem. (Leipzig), to be published.
115
INVESTIGATION BY NANOSECOND
CHEMICAI
OF PHOTOCHROMISM LASER ABSORPTION
E_ KLEMM, D_ KLEMM, A. GRANESS
1 April 1978
PHYSICS LETTERS
OF 2-(2’,4’-DINITROBENZYL)-PYRIDINE SPECTROSCOPY
and J. KLEINSCHMIDT
Sektion Chemie der Friedric~lSc~IilIer-~tliversitdt Jena. 69 Jena. DDR and Sekiion Physik der Friedrich-Schiller-CiversitZt Jena. 69 Jena. DDR
Received 4 October 1977
The photochromism of 2-(2’,4’dinitrobenzyl)-pyridine transients were observed in the spectral ranges of390410 first band (390-410
nm; lifetime
= 1~s) is the absorption
was investigated by means of laser tlash photolysis. Two coloured nm ad 510-580 nm in both ~0132 and nonpolar solvents. The band of the xi-mtro
form and the second band (5 IO-580
lifetime-z0.5 s) is the absorption band of the azamerocyanin form of 2-(2’,4’dinitrobenzyl)-pyridine. tion scheme of photochromism is supported by absorption measurements at different pH-values.
nm;
Our proposed reac-
1. Introduction
2.2. Materiais
Although as early as 1925 Tschitschibabin et al. [ I] synthesized 2<2’,4’-dinitrobenzyl)-pyridine and reported that crystals of this compound after irradiation with sunlight exhibited a colour change from pale yellow to dark blue followed by decoloration after storing in the dark, there still exist contradictory opinions in the literature about the constitution of the coloured species [2--51 and no information is available about the photoreaction. in order to obtain additional information about the photochromism, we investigated 2-(2’,4’-dinitrobenzyl) pyridine by means of laser flash photolysis.
The used photochromic 2(2’,4’-dinitrobenzyl)pyridine [ 1;I ] was pure by thin layer chromatography and by elementary analysis.
2. Experimental 2. I. Apparatus We used a laser flash spectrometer, which is described in ref. [6]. The time resolution of the experi-
mental arrangement was about 20 ns. The investigations were carried out in the spectral range from 400 to 700 run. The resolution in wavelength units was about ax = 5-l 5 nm.
3. Results and discussion We flashed solutions of 2-(2’,4’-dinitrobenzyl)pyridine (10 -3 mol Q-l) in different solvents (IZheptane, ethanol, rz-propanol) and observed immediately after excitation with a pulse of the second harmonic of a ruby laser (<20 ns) a short-lived transient characterized by an absorption in the region 390-410 nm and a long-lhed species characterized by an absorption in the region 550-570 nm (see table I). We obtaired the decay times of both species from photographs of the oscilloscope screen (see figs_ 1 and 2). The pictures show that the decay time of the shortlived species is identical with the rise time of the longlived species. The absorption behaviour was investigated in ethanol solution at different pH-values. We observed in acid solutions (ethanol with H,S04, pH l-6) only the longlived species in the region from 530 nm to 590 nm. The
113
I April 1978
CHEMICAL PHYSiCS LETTERS
Volume 55, number 1
Table 1 Absorption wavelength and decay times of flashed 2-(2’,4’-dinitrobenzyl)-pyridine
solutions
Absorption regions and decay times
Solvent
loi~g-lived species
short-lived species
n-heptane
390-410
ethanol n-propanol
400470
nm run
1.2x
400450
nm
1.5 x 10-7 s
8 x IO-’
s
104s
530-580
nm
5.8 x-LO4
550-590
run
0.5 s
510-580
nk
0.7 s
s
410 nm H
2-(2;C-din1~er1zyl)-pynti
5OOns
Inheptane 550nm -
2ps
S70nm w 2-C3%utrotxnzyl)-pynd1re )---I SOON
Fig. 1. Oscilloscope traces of flashed 2-(2’,4’&nitrobenzyl)pyridine in heptane and ethanol.
LlOnm 430 nm LSOMI L70 nm 510 nm 530 nm
-
200 PS
2-(7~.-~Ntmbenzyl)-pyr~
tn n-propanote
Fig. 2. Oscilloscope traces of flashed 2-(2’,4’+iinitrobenzyl)pyridine in n-propanol.
114
2ps
in ethono!
Fig. 3. Oscilloscope traces of flashed 2C?‘,4’-dinitrobenzyl)pyridine in ethanol + KOH (pH 7.2).
results obtained in a basic solution (ethanol with KOH, pH 7.2) are shown in fig. 3. The short-lived species at 410 nm decays within about 200 ns. The lifetime of
the long-lived species at 530-590 nm decreased rapidly (see fig. 3, trace 2) to 2 X low6 s and in addition, we measured a new absorption band at 450-490 nm. The rise time of this absorption is identical with the decay time of the transient at 530-590 nm. In basic solutions with pH > 7.5 the absorption band of the 550 run species was not observed. The appearance and decay of the observed species were not changed by variation of the oxygen concentration (1O-5--5 X 10e3 mol Q-r), solvent viscosity (n-propanol, 20-6O”C), and concentration of the photochromic substance_ Hence, we think that our experiments can be explained with the results of Klemm and Klemm [S] by reaction scheme-f. The coloured azamerocyanin of the 2(2’,4’dinitro= 550 run) is photochemicaily benzyl)-byridine (A,, formed in polar and nonpolar solvents. We assume, that
Volume 5.5. number 1
CHEMICAL PHYSICS LETTERS
the transient at 4iO run is the aci-nitro form of 2-(2’, 4’-dinitrobenzyl)-pyridine, which is photochemicaliy produced from 2(2’,4’dinitrobenzyl)-pyridine (<20 ns). The aci-nitro form undergoes a thermal reaction in the ground state to ffie azamerocyanin. This explains, that the decay time of the 410 nm absorption band and the rise time of the 550 nm absorption band are identical_ The appearance of the 550 nm species in nonpolar solvents shows that intramolecular proton transfer occurs to the pyridin nitrogen. This concept is supported by previously reported results [6 ] , which showed that in nonpolar solutions of tetranitrodiphenylmethanes this reaction is not possible_ We measured the lifetime of azamerocyanin solutions (ethanol with H2S04) in dependence on pHvalue (pH = l-7) and observed a decrease of this lifetune from 0.50 s (pH = 7) to 2 X 1O-6 s (pH = 1) 181. We assume that the chemical equilibrium between the azamerocyanin form and the cation of the azamerocya+n is shifted to the side of the latter (see scheme). The above shown reaction scheme also explains our results-obtained in a basic solution. After excitation (s20 ns) three species are formed: the aci-nitro form
I April 1978
(h = 410 nm), the azamerocyanin (X = 550 run) and a species at 450 MI. We assume, that this new species is an anion of the azamerocyanin. This assumption&o explains the facts, that the lifetime of the azamerocyanin decreased rapidly from OS s (pH = 7) to 1 X 10e6 s (pH = 7.2), and that the rise time of the 490 nm absorption band is equal the decay time of the azamerocyanin.
References ] l] A.E. Tschitschibabin, B-M. Kuindshi and SW. Benewolensk.rja, Ber. Chem. Ges. 58 (1925) 1580. [2] B.r and C. Decoret, Tetrahedron Letters (1971) (31 C. Wettermark, Nature 194 (1962) 6777. [4] H.S. Masher, E.R. Hardwick and D. Ben-Hur, J. Chem. Phys. 37 (1962) 908. [S] E. Klemm, Dissertation B, Jena (1977); D. Klemm. Dissertation B, Jena (1977). [6] D. Ktemm, E. Klemm, A_ Gnness and J. Kleinschmidt, Chem. Phys. Letters, to be publiihed. [7] D. Klemm and E. Klemm, Z. Chem. 15 (1975) 47. [S] D. Klemm, E. Klemm, A. Graness and J. Kleinschmidt, Z. Physik. Chem. (Leipzig), to be published.
115