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Photochromy and thermal stability of crystalline salicylidene-4-amino-antipyrene
361
Reactivity of Solids, 6 (1989) 361-368 Elsevier Science Publishers B.V., Amsterdam
- Printed
in The Netherlands
Photochromy and thermal stability of crystalline salicylidene-4-amino-antipyrene
El-Zeiny M. Ebeid, Department
of Chemistry, Faculty of Science, Tanta University, Tanta (Egypt)
Ahmed M. Donia and Fathy A. El-Saied Department (Received
of Chemistry, Faculty of Science, Menoufia University, Shebin El-Kom (Egypt) October
27th, 1987; accepted
September
26th, 1988)
Abstract The crystalline salicylidene-4-amino-antipyrene derivative (I) is photolabile and undergoes photochromism and an associated structural change. The compound shows a reasonable solid-state thermal stability but undergoes thermal decomposition in the fused phase.
Introduction
Antipyrenes (commercially known as 2,3-dimethyl-l-phenyl-5-pyrazolone, phenazones) have a diversity of applications including biological [l], clinical [2] and pharmacological [3,4] areas. Among the pharmaceutical applications, they are used as antipyretic, analgesic, antihistaminic, anti-rheumatic, and anti-inflammatory drugs [4]. Antipyrenes have also been used as analytical reagents in the estimation of some metal ions [5-91. Their complexing abilities have been modified by the condensation of their amino derivatives with aromatic aldehydes to give Schiff’s bases [6,10] which are important in liquid crystal technology [ll] and the fluorimetric estimation of metal ions [12]. Antipyrene derivatives have been subjected to several studies including chromatography [13], mass spectrometry [14], thermogravimetric analysis [15] and polarographic [16] studies. The presence of the salicylidene imine moiety may give rise to photochromism, namely, a change in colour under the influence of photoirradiation, which is thought to result from a hydrogen transfer in the excited state [17,18]. 016%7336/89/$03.50
0 1989 Elsevier
Science Publishers
B.V.
362
Photochromic materials have received increasingly more interest in view of their applications in various industrial processes including self-developing photography, Q-switching in pulsed lasers, data-storage technology and grainless micro-imaging [19,20]. Photochromism in several salicylideneaniline derivatives in the solid-state has been reported previously [21]. It has been shown [21] that proton transfer and molecular fluorescence are competing pathways that cause deactivation of the excited state. It follows that an increased rate of one of these pathways occurs at the expense of the other. Here we report the photochemical and thermal stability of I, a crucial aspect because drugs would then survive storage for long periods without undergoing chemical change [22].
Experimental Salicylidene4-aminoantipyrene (I) was prepared by condensation of 4aminoantipyrene (Aldrich; m.p. 107.55109 o C) and salicyl aldehyde by refluxing in ethanol for 10 min. The product that separated upon cooling was collected and purified by repeated re-crystallizations from ethanol (m.p. 210 o C). Differential scanning calorimetry (DSC) was carried out by use of a Du Pont 990 Thermal Analyzer under nitrogen at a heating rate of 10 OC mm’. X-ray powder diffraction patterns of the Cu-K, line were obtained by use of a Shimadzu XD-3 diffractometer. Both fluorescence and excitation spectra were recorded on a Shimadzu RF 510 spectrofluorophotometer. The solid samples in sealed Pyrex ampoules were subjected to prolonged photoirradiation by 180 W medium pressure mercury lamp (Emita VP-60). UVvisible absorption spectra were measured by Bausch&Lomb Spectronic 2000 spectrophotometer. IR spectra were measured using a Perkin-Elmer 598 spectrophotometer.
Results and discussion The room-temperature emission and excitation spectra of fresh and photo-irradiated I are shown in Fig. 1. The effects of the irradiation (A... = 405 nm) on the emission spectrum of I is also shown. The emission intensity at 515 nm decreases as a result of photo-irradiation (X,,, = 405 nm) and a shoulder at ca. 540 nm develops as the photo-irradiation proceeds. A well resolved emission peak at 540 nm grows with prolonged photo-irradiation (ca. 24 h, 180 W medium pressure mercury lamp), and the colour of the material changes from bright yellow to orange. The excitation spectra obtained by following the emission peaks at 545 and 510 nm possess different excitation maxima, at 430 and 380 nm respectively. The observed photochromism in crystalline I is thought to be as depicted in Scheme 1. The
363
II * #
11) $+ //
II
i; ii i; ii 'i 5
l-*
600
500
I
I
400
300
h (nm)
Fig. 1. Room-temperature emission and excitation spectra of crystalline I: ( -) keto (A,, = 545 nm) and (c) Excitation form; (a) Emission (A,, = 365 nm): (b) Excitation (A,, = 510 nm), (.-.-.) Emission (A,, = 365 nm) of the keto form left in the dark for 30 d, X) effect of the irradiation ageing (A,, = 405 nm) on the emission spectra of and (Xthe yellow enol form the irradiation times (in minutes) are shown on the corresponding spectra.
photoproduct (Ib) exists in the quinonoid form and absorbs at a longer wavelength owing to a well-developed ?T* + n transition that occurs in the visible region, whereas the first r* + 7~ band in the enol form (Ia) occurs at shorter wavelength [l&23]. This is consistent with both emission and excitation spectra (Fig. 1). Self-absorption plays a role in modifying the fluorescence properties of these systems. The change in colour from yellow to orange was not observed when the yellow enol form was heated at ca. 100” C for ca. 75 h, indicating the absence of thermo-chrornism. The orange photoproduct Ib, however, undergoes a very slow reverse reaction to Ia if stored in the dark for ca. 60 d. The phenomenon of reverse-reaction taking place in the dark has been reported previously for several photochromic systems [l&21]. The reverse dark
(Ib)
(Ic)
0-H
Scheme
1
reaction is inferred from the observed decrease in the emission band (.A,_..= 365 nm) at 545 nm (which is associated with the orange photoproduct) compared with the emission band at 510 nm (diagnostic for the yellow I’\ .i I i i ;
‘:
!
1.;”
i
i
i. \ i i i
i i
.;
i \
‘\
.f
.
L.’ ~
I 200
I 300
L
400
J 500
)I (nm) 600 Fig. 2. (a) Absorption and (b) emission (X,, = 36.5 nm) spectra keto forms in methanol (concentration 5 X low5 M dmm3).
500 400 X (nm) of ( ----) enol and (.-.-.)
365
A
Fig. 3. A schematic diagram showing the approximate energy levels and electronic transitions in crystalline I. Code: A, A*: ground and excited states of the enol form; B, B*: ground and excited states of the keto form.
enol form as shown in Fig. 1). The orange keto form is stabilized by additional constraints in the solid state. These constraints are released upon dissolution and the reverse reaction Ib --, Ia occurs as shown in Scheme 1. This is concluded since solutions of both the yellow and the orange samples give the typical UV-visible absorption and emission spectra shown in Fig. 2. Moreover, photo-irradiation (A,,, = 365 nm) of a methanolic solution of Ia gave no change in the emission spectrum. The fast reverse reaction Ib + Ia in solution prevented the build-up of Ib upon photo-irradiation. The role of the condensed phase in stabilizing the quinonoid form in photochromic salicylidene anilines (also known as anils) has been reported previously [19]. The photochromy, emission and excitation processes in I are outlined in Fig. 3. The IR spectral data for the enol and keto forms are listed in Table 1. The results are consistent with Scheme 1. The peaks at 900 and 910 cm-’ that are assigned to the C-H out-of-plane deformation are absent in the orange keto form as a result of the photoinduced structural change (vide infra) and the associated change in molecu-
Table 1 Assignment of IR peaks for orange and yellow forms in KBr discs V (cm-‘)
1600 930 900, 910 3400-3500 1030
Assignment
Ref.
Tautomer
C=N frequency (a&unsaturated) C-N stretching C-H out-of-plane deformation Hydrogen bond of the type 0- - -H-O OH stretching
24
yellow (enol) orange (keto) yellow (enol) yellow (enol) and orange (keto) yellow (enol)
25 25 26 24
366
L--b
40
20
30
10
28"
Fig. 4. X-ray powder
diffraction
patterns
of (a) enol and (b) keto forms.
6; *. II x. II
I
I
100
150
i I
i ;
I 200
250
I 300
x ) keto forms of I and (Fig. 5. DSC calorigrams of (. -. - .) enol-( X Runs were made m a nitrogen atmosphere at a scanning rate of 10” C min-‘.
) fresh II.
367
lar packing. The broad absorption band at 3400-3500 cm-’ that is assigned to a hydrogen bond of the type 0- - -H-O [26] is present in both yellow and orange forms, and indicates the presence of a small percentage of c&isomer (Ic shown in Scheme 1) in both the yellow and orange tautomers. The strongly basic character of the carbonyl group in the antipyrene moiety [27] facilitates such hydrogen-bond formation. The photochromism in I is associated with a structural change. The X-ray powder diffraction patterns of yellow and orange tautomers are different as shown in Fig. 4. The structural changes are caused by photo-induced molecular rearrangements which change the molecular interactions and the packing in the lattice. The thermal stability of the tautomers Ia and Ib has been studied using (DSC) and the calorigrams of both forms (shown in Fig. 5) indicate reasonable thermal stability in the solid state (i.e. prior to melting). Both forms show a sharp endothermic peak starting at ca. 210°C that indicates the onset of melting. The melting peaks are followed by broad exothermic peaks (in the range 250-320 o C) that are due to decomposition in the fused phase.
References 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18
D.G. Craciunescu, An. R. Acad. Farm., 43 (1977) 265. J. Hosler, C. Tschanz, C.E. Hignite and D.L. Azarnoff, J. Invest. Dermatol., 74 (1980) 51. P.J. Meffin, R.L. Williams, T.F. Blaschke and M. Rowland, J. Pharm. Sci., 66 (1977) 135. E.V. Schmidt, T.P. Prishchep and N.A. Chernova, Izv. Tomsk. Politekhn In-Ta, (1975) 156; M.M. Shoukry, A.Kh. Ghoneim, E.M. Shoukry and M.H. El-Nagdi, Synth. React. Inorg. Met. Org. Chem., 12 (1982) 15. N.L. Olenovich and L.I. Kovalchuk, Zh. Anal. Khim., 28 (1975) 2162. Sh.T. Talipov, A.T. Tashkhodzhaev, L.E. Zel’tser and Kh. Khikmatov, Iz. Vyssh. Ucheb. Zaved. Khim. Khim. Tekbnol., 15 (1975) 1109. A.T. Pihpenko, S.L. Lisichenok and A.I. Volkova, Ukr. Khim. Zh. (Russ. Ed.), 42 (1976) 76. V.V. Savant, P.P. Ramamurthy and C. Chunibhai, J. Less-Common Met., 22 (1970) 479. B. Janik and S. Sawicki, Mikrochim. Acta, (1970) 1050. V.I. Letunov, Zh. Org. Khim., 20 (1984) 162. G. Elliot, Chem. Brit., May (1973) pp. 213-220; P. Keller and L. Liebert in L. Liebert (Ed.), Liquid Crystals, Academic Press, New York, London, 1978, p. 23. K. Morishige, Anal. Chim. Acta, 121 (1980) 301. F. BuhI, U. Hachula and M. Chwistek, J. Chromatogr., 219 (1981) 189; P. Guinebault and M. Broouaire, J. Chromatogr., 217 (1981) 509. U. Bahr and H.R. Schulten, J. Labelled Compd. Radiopharm., 1X (1981) 571; B.K. Tang, H. Uchino, T. Inaba and W. KaIow, Biomed. Mass Spectrom., 9 (1982) 425. S. Lukkari, Farm. Aikak, 80 (1971) 402. S.V. Tatwawadi, A.P. Singh and K.K. Narang, J. Sci. Res. Banaras Hindu Univ., 3 (1980) 143. G.H. Brown (Ed.) Photochromism, Techniques of Chemistry, Vol. 3, Interscience, New York, 1971. W. KIopffer, Adv. Photochem., 10 (1977) 311.
368 19 F.P.A. Zweegers, Ph.D. Thesis, State University, Gorlaeus Laboratories, The Netherlands, 1978. 20 P.P. Hanson, Photogr. Sci. Eng., 14 (1970) 438; R.J. Anderson, Proc. Sot. Inform. Display, 11 (1970) 151. 21 M.D. Cohen and S. Flavian, J. Chem. Sot. (B), (1967) 334; G.M.J. Schmidt, in Reactivity of the Photoexcited Organic Molecule, Interscience Publishers, London, New York, 1976, pp. 227-288. 22 J. Haleblian and W. McCrone, J. Pharm. Sci., 58 (1969) 911. 23 J. Griffiths, Colour and Constitution of Organic Molecules, Academic Press, London, New York, 1976, p. 221. 24 D.H. Williams and I. Fleming, Spectroscopic Methods in Organic Chemistry, McGraw Hill, London, New York, 1966, p. 49. 25 J.B. Lambert, H.F. Shurvell, L. Verbit, R.G. Cooks and G.H. Stout, Organic Structural Analysis, Macmillan Publishing Co. Inc., New York, 1976. 26 R.E. Evans and W. Kynaston, J. Chem. Sot., (1962) 1005. 27 S. Viebel, E. Eggersen and S.L. Linholt, Acta Chem. Stand., 8 (1954) 763.
Reactivity of Solids, 6 (1989) 361-368 Elsevier Science Publishers B.V., Amsterdam
- Printed
in The Netherlands
Photochromy and thermal stability of crystalline salicylidene-4-amino-antipyrene
El-Zeiny M. Ebeid, Department
of Chemistry, Faculty of Science, Tanta University, Tanta (Egypt)
Ahmed M. Donia and Fathy A. El-Saied Department (Received
of Chemistry, Faculty of Science, Menoufia University, Shebin El-Kom (Egypt) October
27th, 1987; accepted
September
26th, 1988)
Abstract The crystalline salicylidene-4-amino-antipyrene derivative (I) is photolabile and undergoes photochromism and an associated structural change. The compound shows a reasonable solid-state thermal stability but undergoes thermal decomposition in the fused phase.
Introduction
Antipyrenes (commercially known as 2,3-dimethyl-l-phenyl-5-pyrazolone, phenazones) have a diversity of applications including biological [l], clinical [2] and pharmacological [3,4] areas. Among the pharmaceutical applications, they are used as antipyretic, analgesic, antihistaminic, anti-rheumatic, and anti-inflammatory drugs [4]. Antipyrenes have also been used as analytical reagents in the estimation of some metal ions [5-91. Their complexing abilities have been modified by the condensation of their amino derivatives with aromatic aldehydes to give Schiff’s bases [6,10] which are important in liquid crystal technology [ll] and the fluorimetric estimation of metal ions [12]. Antipyrene derivatives have been subjected to several studies including chromatography [13], mass spectrometry [14], thermogravimetric analysis [15] and polarographic [16] studies. The presence of the salicylidene imine moiety may give rise to photochromism, namely, a change in colour under the influence of photoirradiation, which is thought to result from a hydrogen transfer in the excited state [17,18]. 016%7336/89/$03.50
0 1989 Elsevier
Science Publishers
B.V.
362
Photochromic materials have received increasingly more interest in view of their applications in various industrial processes including self-developing photography, Q-switching in pulsed lasers, data-storage technology and grainless micro-imaging [19,20]. Photochromism in several salicylideneaniline derivatives in the solid-state has been reported previously [21]. It has been shown [21] that proton transfer and molecular fluorescence are competing pathways that cause deactivation of the excited state. It follows that an increased rate of one of these pathways occurs at the expense of the other. Here we report the photochemical and thermal stability of I, a crucial aspect because drugs would then survive storage for long periods without undergoing chemical change [22].
Experimental Salicylidene4-aminoantipyrene (I) was prepared by condensation of 4aminoantipyrene (Aldrich; m.p. 107.55109 o C) and salicyl aldehyde by refluxing in ethanol for 10 min. The product that separated upon cooling was collected and purified by repeated re-crystallizations from ethanol (m.p. 210 o C). Differential scanning calorimetry (DSC) was carried out by use of a Du Pont 990 Thermal Analyzer under nitrogen at a heating rate of 10 OC mm’. X-ray powder diffraction patterns of the Cu-K, line were obtained by use of a Shimadzu XD-3 diffractometer. Both fluorescence and excitation spectra were recorded on a Shimadzu RF 510 spectrofluorophotometer. The solid samples in sealed Pyrex ampoules were subjected to prolonged photoirradiation by 180 W medium pressure mercury lamp (Emita VP-60). UVvisible absorption spectra were measured by Bausch&Lomb Spectronic 2000 spectrophotometer. IR spectra were measured using a Perkin-Elmer 598 spectrophotometer.
Results and discussion The room-temperature emission and excitation spectra of fresh and photo-irradiated I are shown in Fig. 1. The effects of the irradiation (A... = 405 nm) on the emission spectrum of I is also shown. The emission intensity at 515 nm decreases as a result of photo-irradiation (X,,, = 405 nm) and a shoulder at ca. 540 nm develops as the photo-irradiation proceeds. A well resolved emission peak at 540 nm grows with prolonged photo-irradiation (ca. 24 h, 180 W medium pressure mercury lamp), and the colour of the material changes from bright yellow to orange. The excitation spectra obtained by following the emission peaks at 545 and 510 nm possess different excitation maxima, at 430 and 380 nm respectively. The observed photochromism in crystalline I is thought to be as depicted in Scheme 1. The
363
II * #
11) $+ //
II
i; ii i; ii 'i 5
l-*
600
500
I
I
400
300
h (nm)
Fig. 1. Room-temperature emission and excitation spectra of crystalline I: ( -) keto (A,, = 545 nm) and (c) Excitation form; (a) Emission (A,, = 365 nm): (b) Excitation (A,, = 510 nm), (.-.-.) Emission (A,, = 365 nm) of the keto form left in the dark for 30 d, X) effect of the irradiation ageing (A,, = 405 nm) on the emission spectra of and (Xthe yellow enol form the irradiation times (in minutes) are shown on the corresponding spectra.
photoproduct (Ib) exists in the quinonoid form and absorbs at a longer wavelength owing to a well-developed ?T* + n transition that occurs in the visible region, whereas the first r* + 7~ band in the enol form (Ia) occurs at shorter wavelength [l&23]. This is consistent with both emission and excitation spectra (Fig. 1). Self-absorption plays a role in modifying the fluorescence properties of these systems. The change in colour from yellow to orange was not observed when the yellow enol form was heated at ca. 100” C for ca. 75 h, indicating the absence of thermo-chrornism. The orange photoproduct Ib, however, undergoes a very slow reverse reaction to Ia if stored in the dark for ca. 60 d. The phenomenon of reverse-reaction taking place in the dark has been reported previously for several photochromic systems [l&21]. The reverse dark
(Ib)
(Ic)
0-H
Scheme
1
reaction is inferred from the observed decrease in the emission band (.A,_..= 365 nm) at 545 nm (which is associated with the orange photoproduct) compared with the emission band at 510 nm (diagnostic for the yellow I’\ .i I i i ;
‘:
!
1.;”
i
i
i. \ i i i
i i
.;
i \
‘\
.f
.
L.’ ~
I 200
I 300
L
400
J 500
)I (nm) 600 Fig. 2. (a) Absorption and (b) emission (X,, = 36.5 nm) spectra keto forms in methanol (concentration 5 X low5 M dmm3).
500 400 X (nm) of ( ----) enol and (.-.-.)
365
A
Fig. 3. A schematic diagram showing the approximate energy levels and electronic transitions in crystalline I. Code: A, A*: ground and excited states of the enol form; B, B*: ground and excited states of the keto form.
enol form as shown in Fig. 1). The orange keto form is stabilized by additional constraints in the solid state. These constraints are released upon dissolution and the reverse reaction Ib --, Ia occurs as shown in Scheme 1. This is concluded since solutions of both the yellow and the orange samples give the typical UV-visible absorption and emission spectra shown in Fig. 2. Moreover, photo-irradiation (A,,, = 365 nm) of a methanolic solution of Ia gave no change in the emission spectrum. The fast reverse reaction Ib + Ia in solution prevented the build-up of Ib upon photo-irradiation. The role of the condensed phase in stabilizing the quinonoid form in photochromic salicylidene anilines (also known as anils) has been reported previously [19]. The photochromy, emission and excitation processes in I are outlined in Fig. 3. The IR spectral data for the enol and keto forms are listed in Table 1. The results are consistent with Scheme 1. The peaks at 900 and 910 cm-’ that are assigned to the C-H out-of-plane deformation are absent in the orange keto form as a result of the photoinduced structural change (vide infra) and the associated change in molecu-
Table 1 Assignment of IR peaks for orange and yellow forms in KBr discs V (cm-‘)
1600 930 900, 910 3400-3500 1030
Assignment
Ref.
Tautomer
C=N frequency (a&unsaturated) C-N stretching C-H out-of-plane deformation Hydrogen bond of the type 0- - -H-O OH stretching
24
yellow (enol) orange (keto) yellow (enol) yellow (enol) and orange (keto) yellow (enol)
25 25 26 24
366
L--b
40
20
30
10
28"
Fig. 4. X-ray powder
diffraction
patterns
of (a) enol and (b) keto forms.
6; *. II x. II
I
I
100
150
i I
i ;
I 200
250
I 300
x ) keto forms of I and (Fig. 5. DSC calorigrams of (. -. - .) enol-( X Runs were made m a nitrogen atmosphere at a scanning rate of 10” C min-‘.
) fresh II.
367
lar packing. The broad absorption band at 3400-3500 cm-’ that is assigned to a hydrogen bond of the type 0- - -H-O [26] is present in both yellow and orange forms, and indicates the presence of a small percentage of c&isomer (Ic shown in Scheme 1) in both the yellow and orange tautomers. The strongly basic character of the carbonyl group in the antipyrene moiety [27] facilitates such hydrogen-bond formation. The photochromism in I is associated with a structural change. The X-ray powder diffraction patterns of yellow and orange tautomers are different as shown in Fig. 4. The structural changes are caused by photo-induced molecular rearrangements which change the molecular interactions and the packing in the lattice. The thermal stability of the tautomers Ia and Ib has been studied using (DSC) and the calorigrams of both forms (shown in Fig. 5) indicate reasonable thermal stability in the solid state (i.e. prior to melting). Both forms show a sharp endothermic peak starting at ca. 210°C that indicates the onset of melting. The melting peaks are followed by broad exothermic peaks (in the range 250-320 o C) that are due to decomposition in the fused phase.
References 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18
D.G. Craciunescu, An. R. Acad. Farm., 43 (1977) 265. J. Hosler, C. Tschanz, C.E. Hignite and D.L. Azarnoff, J. Invest. Dermatol., 74 (1980) 51. P.J. Meffin, R.L. Williams, T.F. Blaschke and M. Rowland, J. Pharm. Sci., 66 (1977) 135. E.V. Schmidt, T.P. Prishchep and N.A. Chernova, Izv. Tomsk. Politekhn In-Ta, (1975) 156; M.M. Shoukry, A.Kh. Ghoneim, E.M. Shoukry and M.H. El-Nagdi, Synth. React. Inorg. Met. Org. Chem., 12 (1982) 15. N.L. Olenovich and L.I. Kovalchuk, Zh. Anal. Khim., 28 (1975) 2162. Sh.T. Talipov, A.T. Tashkhodzhaev, L.E. Zel’tser and Kh. Khikmatov, Iz. Vyssh. Ucheb. Zaved. Khim. Khim. Tekbnol., 15 (1975) 1109. A.T. Pihpenko, S.L. Lisichenok and A.I. Volkova, Ukr. Khim. Zh. (Russ. Ed.), 42 (1976) 76. V.V. Savant, P.P. Ramamurthy and C. Chunibhai, J. Less-Common Met., 22 (1970) 479. B. Janik and S. Sawicki, Mikrochim. Acta, (1970) 1050. V.I. Letunov, Zh. Org. Khim., 20 (1984) 162. G. Elliot, Chem. Brit., May (1973) pp. 213-220; P. Keller and L. Liebert in L. Liebert (Ed.), Liquid Crystals, Academic Press, New York, London, 1978, p. 23. K. Morishige, Anal. Chim. Acta, 121 (1980) 301. F. BuhI, U. Hachula and M. Chwistek, J. Chromatogr., 219 (1981) 189; P. Guinebault and M. Broouaire, J. Chromatogr., 217 (1981) 509. U. Bahr and H.R. Schulten, J. Labelled Compd. Radiopharm., 1X (1981) 571; B.K. Tang, H. Uchino, T. Inaba and W. KaIow, Biomed. Mass Spectrom., 9 (1982) 425. S. Lukkari, Farm. Aikak, 80 (1971) 402. S.V. Tatwawadi, A.P. Singh and K.K. Narang, J. Sci. Res. Banaras Hindu Univ., 3 (1980) 143. G.H. Brown (Ed.) Photochromism, Techniques of Chemistry, Vol. 3, Interscience, New York, 1971. W. KIopffer, Adv. Photochem., 10 (1977) 311.
368 19 F.P.A. Zweegers, Ph.D. Thesis, State University, Gorlaeus Laboratories, The Netherlands, 1978. 20 P.P. Hanson, Photogr. Sci. Eng., 14 (1970) 438; R.J. Anderson, Proc. Sot. Inform. Display, 11 (1970) 151. 21 M.D. Cohen and S. Flavian, J. Chem. Sot. (B), (1967) 334; G.M.J. Schmidt, in Reactivity of the Photoexcited Organic Molecule, Interscience Publishers, London, New York, 1976, pp. 227-288. 22 J. Haleblian and W. McCrone, J. Pharm. Sci., 58 (1969) 911. 23 J. Griffiths, Colour and Constitution of Organic Molecules, Academic Press, London, New York, 1976, p. 221. 24 D.H. Williams and I. Fleming, Spectroscopic Methods in Organic Chemistry, McGraw Hill, London, New York, 1966, p. 49. 25 J.B. Lambert, H.F. Shurvell, L. Verbit, R.G. Cooks and G.H. Stout, Organic Structural Analysis, Macmillan Publishing Co. Inc., New York, 1976. 26 R.E. Evans and W. Kynaston, J. Chem. Sot., (1962) 1005. 27 S. Viebel, E. Eggersen and S.L. Linholt, Acta Chem. Stand., 8 (1954) 763.