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Emission Characteristics of some 8-(arylazo)-7-hydroxy-4-methylcoumarins
0584&X539,87 $3.00+ 0.00 0 1987Pergamon Journals L.td
SpecrrochlmlcoAcra. Vol 43A. No. 5. pp. 709%710.1987 Printed m Great Britam
RESEARCH
EMISSION
NOTE
CfiARACTERlSTICS OF SOME S-(ARYLAZO)-7-HYDROXY-4METHYLCOUMARINS (Received 3 July
1986)
Abstract-The emission characteristics of some new 8-(arylazo)-7-hydroxy-4-methylcoumarins have been studied. Both emission and excitation spectra are pH-dependent and in alkaline media the emission from anionic species becomes dominant with emission maxima at ca 450 nm. The fluorescence quantum yields (4,) have been measured. Unlike many other coumarins, the present compounds give no laser action upon pumping with nitrogen laser (A,, = 337.1 nm) presumably because of their low 4, values. The excited state acid-base behaviour of some derivatives has also been evaluated.
[IO]. After acidification, the resulting precipitate was crystallized from either ethanol or acetic acid. The p-SOjH derivative (Ig) was recrystallized by the method described by MEHATA [I 11. Fluorescence and excitation spectra were recorded using a Shimadzu RF-510 spectrofluorophotometer for concentrations ofca 10 -.‘M. Fluorescence quantum yields (4,) were measured taking 9,10-diphenylanthracene as a reference standard [12] for which 4J = 0.95 in ethanol (lex = 370 nm). Laser action of (I) has been checked using a nitrogen laser with a peak power of 100 kW and is described elsewhere [ 131.
INTRODUCTION Coumarm emission is currently used in biological systems to estimate enzymes such as p-glucosides [l]. They are also widely used as blue emitting laser dyes [2-71. Typical coumarin derivatives used as laser dyes are 7-hydroxy-4and 7-methylamino-4-methylcoumarin. methylcoumarin Like many other laser dyes, the efficiency of fluorescence differs significantly as a result of substitution on the parent compounds. This is due to the role of substituents in altering both emission and reabsorption behaviour [8]. The substituent effect on the fluorescence intensities of some 7hydroxycoumarins has been reported earlier [l] and the emission of 7-hydroxy-4-methylcoumarin has been further examined in relation to medium effect 197. In absolute ethanol, only fluorescence characteristic of the neutral compound is observable but in aqueous ethanol, emission from the excited anions is dominant giving a maximum at ca 450 nm. In the present communication, the emission and excitation of some new %(arylazo)-7-hydroxy-4-methylspectra coumarins (1) are reported.
RESULTS AND DISCUSSION The emission spectrum of 7-hydroxy-4-methylcoumarin (II) in ethanol shows a shoulder at ca 440 nm and the emission band is asymmetrical. In alkaline solutions the emission peak becomes more symmetrical and more intensified at 450 nm. This isassigned to emission from the phenolate anion whereas the emission shoulder at 440 nm originates from the neutral molecules. This assignment is further elaborated by investigating the corresponding excitation spectra. In ethanolic solutions there are two overlapping excitation bands of maxima at 370and 350 nm. In alkaline medium (pH = 10) the peak at 350 nm is much reduced in intensity and an excitation peak at 400nm develops. The excitation peaks at 370 and 400 nm are assigned to neutral and anionic species. The emission of (1) is considerably less intense than that of (II). This is due to the role of the azo group as a fluorescence quencher 1143. Table 1 summarizes the emission, excitation and absorption spectral data of (I) in ethanol and in alkahne buffer solutions. The emission at 450nm emanates from the phenolate anions and is considerably more intense compared with the neutral molecules. For the iodo-derivative (Ie) in ethanol, 4,- = 0.03 that incrcdses to 4, = 0.17 in ethanolic buffer (pH = 11). There is a good agreement between the excitation maxima and the lowest energy absorption bands indicating that emission originates from the lowest energy electronic states. Table 1 shows that 4, values are generally low and this is a potential reason that no laser action has been observed from compounds (1) upon pumping concentrated ethanolic solutions (ca 10 ‘M) using nitrogen laser (i,, = 337.1 nm and a peak power of 100 kW). The values of pK: of some derivatives have been determined using the Forster cycle [ 151.
‘;1 N
(I) Where X = m-OH(a), p-OH(b), m-Cl(c), p-Cl(d), p-COOH(f), p-SO,H(g) and p-NO,(h).
p-l(e),
The phenolate anions generated throughout this study are obtained in alkaline buffers instead of simply adding water as previously reported [9]. EXPERIMENTAL Compounds corresponding methylcoumarin
pKZ= pK,-2.1
x 10 -3(Ca-YtlJ,
where G,and Vsare the wavenumbers (incm ‘)oftransitions from A to A* and B to B*, respectively. A and B denote acid and basic forms, respectively. To a first approximation VAand
(I) were prepared by coupling the diazotized amines (0.01 mol.) with 7-hydroxy-4(0.01 mol.) in sodium hydroxide solution 709
Research Note
710
Table 1. Summary of spectral data of compounds (I) Emission max.* EtOH pH = 10
X m-OH p-OH m-Cl p-c1 P-I
p-COOH p-SOaH P-NO,
420 445 430 450 420 435 445 430
Excitation max.? Absorb EtOH pH = 10 max.$
445 435 450 445 445 450
380 390 360 375 340 365 380 370
370 365 370 370 380 375
390 395 355 355 350 365 390 365
&§ 0.03 0.006 0.01 0.03 0.036 0.04
*I,, = 365 nm. tFollowing corresponding emission maxima. $Values for the lowest energy absorption bands in EtOH from Ref. [lo]. §n,, = 365 nm in EtOH.
va were taken from the fluorescence bands of acid and conjugate base in case where both members of the conjugatk pair are fluorescent. For p-NO2 derivative (Ih), pK: = pK, -4.30 = 5.10 -4.30 = 0.80 and for p-1 (Ie), pK: = pK, - 4.20 = 6.38 - 4.20 = 2.18. This is in agreement with the general behaviour of phenolic -OH groups that are stronger acids in the excited’ state than the ground state [ 151. Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt * Tanta University, Tanta
A. L. EL-ANSARY E. M. EBEID* M. M. OMAR
REFERENCES [l] W. R. SHERMANand E. ROBIN& Anal. Chem. 40, 803 (1968). [2] C. V. SHANK, A. DIENES M. A. TRAZZOLO and J. A. MYER, Appl. Phys. Lett. 16, 405 (1970). [3] A. DIENES C. V. SHANK and A. M. TRAZZOLO, Appl. Phys. Lett. 17, 189 (1970).
[4] J.
KUHL, H. TELLE, R. SCHIEDERand U. BRINKMANN,
Opt. Commun. 24, 251 (1978). [S] T. E. BUSH and G. W. Scorr, J. phys. Chem. 85, 144
(1981). [6] B. H. WINTERS, H. I. MANDELBERGand W. B. MOHR, Appl. Phys. Lett. 25, 723 (1974). [7] I. V. KOMLEV, M. A. TAVRIZOVA,0. R. KHROLovAand T. A. MIKHAILOVA,Zh. Obshch. Khim. 55, 888 (1985). [8] E. L. WEHRY (Ed.) Modern Fluorescence Spectroscopy,
Vol. 1, Chap. 4. Plenum Press, New York (1976). [9] G. S. BEDDARD,M. S. CARLINand R. S. DAVIDSON,J. them. Sot. Perkin II, 262 (1977). [lo] A. L. EL-ANsARuand M. M. OMAR,Indian Text. J. Nov. 95 (1985). [ll] H. MEHATA, M. RAVIKRISHNANandA. CHITALE,J. Sot. Dyers Co/. 78, 552 (1962). [12] J. V. MORRIS M. A. MAHANEYand J. R. HUBER,J. phys. Chem. So, 969 (1976). [13] M. M.F. SABRY,A. HAssANand M. EWAIDA,J. Phys. E. scient. Jnstrum. 17. 103 (1984). [14] J. A. BARLTROPand J. D. COYLE, Excited Srates in Organic Chemistry. John Wiley, London (1975). [is] S. G. SCHULMAN, Modern Fluorescence Spectroscopy, Vol. 2, Chap. 6, (edited by E. L. WEHRY) Plenum Press, New York (1976).
SpecrrochlmlcoAcra. Vol 43A. No. 5. pp. 709%710.1987 Printed m Great Britam
RESEARCH
EMISSION
NOTE
CfiARACTERlSTICS OF SOME S-(ARYLAZO)-7-HYDROXY-4METHYLCOUMARINS (Received 3 July
1986)
Abstract-The emission characteristics of some new 8-(arylazo)-7-hydroxy-4-methylcoumarins have been studied. Both emission and excitation spectra are pH-dependent and in alkaline media the emission from anionic species becomes dominant with emission maxima at ca 450 nm. The fluorescence quantum yields (4,) have been measured. Unlike many other coumarins, the present compounds give no laser action upon pumping with nitrogen laser (A,, = 337.1 nm) presumably because of their low 4, values. The excited state acid-base behaviour of some derivatives has also been evaluated.
[IO]. After acidification, the resulting precipitate was crystallized from either ethanol or acetic acid. The p-SOjH derivative (Ig) was recrystallized by the method described by MEHATA [I 11. Fluorescence and excitation spectra were recorded using a Shimadzu RF-510 spectrofluorophotometer for concentrations ofca 10 -.‘M. Fluorescence quantum yields (4,) were measured taking 9,10-diphenylanthracene as a reference standard [12] for which 4J = 0.95 in ethanol (lex = 370 nm). Laser action of (I) has been checked using a nitrogen laser with a peak power of 100 kW and is described elsewhere [ 131.
INTRODUCTION Coumarm emission is currently used in biological systems to estimate enzymes such as p-glucosides [l]. They are also widely used as blue emitting laser dyes [2-71. Typical coumarin derivatives used as laser dyes are 7-hydroxy-4and 7-methylamino-4-methylcoumarin. methylcoumarin Like many other laser dyes, the efficiency of fluorescence differs significantly as a result of substitution on the parent compounds. This is due to the role of substituents in altering both emission and reabsorption behaviour [8]. The substituent effect on the fluorescence intensities of some 7hydroxycoumarins has been reported earlier [l] and the emission of 7-hydroxy-4-methylcoumarin has been further examined in relation to medium effect 197. In absolute ethanol, only fluorescence characteristic of the neutral compound is observable but in aqueous ethanol, emission from the excited anions is dominant giving a maximum at ca 450 nm. In the present communication, the emission and excitation of some new %(arylazo)-7-hydroxy-4-methylspectra coumarins (1) are reported.
RESULTS AND DISCUSSION The emission spectrum of 7-hydroxy-4-methylcoumarin (II) in ethanol shows a shoulder at ca 440 nm and the emission band is asymmetrical. In alkaline solutions the emission peak becomes more symmetrical and more intensified at 450 nm. This isassigned to emission from the phenolate anion whereas the emission shoulder at 440 nm originates from the neutral molecules. This assignment is further elaborated by investigating the corresponding excitation spectra. In ethanolic solutions there are two overlapping excitation bands of maxima at 370and 350 nm. In alkaline medium (pH = 10) the peak at 350 nm is much reduced in intensity and an excitation peak at 400nm develops. The excitation peaks at 370 and 400 nm are assigned to neutral and anionic species. The emission of (1) is considerably less intense than that of (II). This is due to the role of the azo group as a fluorescence quencher 1143. Table 1 summarizes the emission, excitation and absorption spectral data of (I) in ethanol and in alkahne buffer solutions. The emission at 450nm emanates from the phenolate anions and is considerably more intense compared with the neutral molecules. For the iodo-derivative (Ie) in ethanol, 4,- = 0.03 that incrcdses to 4, = 0.17 in ethanolic buffer (pH = 11). There is a good agreement between the excitation maxima and the lowest energy absorption bands indicating that emission originates from the lowest energy electronic states. Table 1 shows that 4, values are generally low and this is a potential reason that no laser action has been observed from compounds (1) upon pumping concentrated ethanolic solutions (ca 10 ‘M) using nitrogen laser (i,, = 337.1 nm and a peak power of 100 kW). The values of pK: of some derivatives have been determined using the Forster cycle [ 151.
‘;1 N
(I) Where X = m-OH(a), p-OH(b), m-Cl(c), p-Cl(d), p-COOH(f), p-SO,H(g) and p-NO,(h).
p-l(e),
The phenolate anions generated throughout this study are obtained in alkaline buffers instead of simply adding water as previously reported [9]. EXPERIMENTAL Compounds corresponding methylcoumarin
pKZ= pK,-2.1
x 10 -3(Ca-YtlJ,
where G,and Vsare the wavenumbers (incm ‘)oftransitions from A to A* and B to B*, respectively. A and B denote acid and basic forms, respectively. To a first approximation VAand
(I) were prepared by coupling the diazotized amines (0.01 mol.) with 7-hydroxy-4(0.01 mol.) in sodium hydroxide solution 709
Research Note
710
Table 1. Summary of spectral data of compounds (I) Emission max.* EtOH pH = 10
X m-OH p-OH m-Cl p-c1 P-I
p-COOH p-SOaH P-NO,
420 445 430 450 420 435 445 430
Excitation max.? Absorb EtOH pH = 10 max.$
445 435 450 445 445 450
380 390 360 375 340 365 380 370
370 365 370 370 380 375
390 395 355 355 350 365 390 365
&§ 0.03 0.006 0.01 0.03 0.036 0.04
*I,, = 365 nm. tFollowing corresponding emission maxima. $Values for the lowest energy absorption bands in EtOH from Ref. [lo]. §n,, = 365 nm in EtOH.
va were taken from the fluorescence bands of acid and conjugate base in case where both members of the conjugatk pair are fluorescent. For p-NO2 derivative (Ih), pK: = pK, -4.30 = 5.10 -4.30 = 0.80 and for p-1 (Ie), pK: = pK, - 4.20 = 6.38 - 4.20 = 2.18. This is in agreement with the general behaviour of phenolic -OH groups that are stronger acids in the excited’ state than the ground state [ 151. Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt * Tanta University, Tanta
A. L. EL-ANSARY E. M. EBEID* M. M. OMAR
REFERENCES [l] W. R. SHERMANand E. ROBIN& Anal. Chem. 40, 803 (1968). [2] C. V. SHANK, A. DIENES M. A. TRAZZOLO and J. A. MYER, Appl. Phys. Lett. 16, 405 (1970). [3] A. DIENES C. V. SHANK and A. M. TRAZZOLO, Appl. Phys. Lett. 17, 189 (1970).
[4] J.
KUHL, H. TELLE, R. SCHIEDERand U. BRINKMANN,
Opt. Commun. 24, 251 (1978). [S] T. E. BUSH and G. W. Scorr, J. phys. Chem. 85, 144
(1981). [6] B. H. WINTERS, H. I. MANDELBERGand W. B. MOHR, Appl. Phys. Lett. 25, 723 (1974). [7] I. V. KOMLEV, M. A. TAVRIZOVA,0. R. KHROLovAand T. A. MIKHAILOVA,Zh. Obshch. Khim. 55, 888 (1985). [8] E. L. WEHRY (Ed.) Modern Fluorescence Spectroscopy,
Vol. 1, Chap. 4. Plenum Press, New York (1976). [9] G. S. BEDDARD,M. S. CARLINand R. S. DAVIDSON,J. them. Sot. Perkin II, 262 (1977). [lo] A. L. EL-ANsARuand M. M. OMAR,Indian Text. J. Nov. 95 (1985). [ll] H. MEHATA, M. RAVIKRISHNANandA. CHITALE,J. Sot. Dyers Co/. 78, 552 (1962). [12] J. V. MORRIS M. A. MAHANEYand J. R. HUBER,J. phys. Chem. So, 969 (1976). [13] M. M.F. SABRY,A. HAssANand M. EWAIDA,J. Phys. E. scient. Jnstrum. 17. 103 (1984). [14] J. A. BARLTROPand J. D. COYLE, Excited Srates in Organic Chemistry. John Wiley, London (1975). [is] S. G. SCHULMAN, Modern Fluorescence Spectroscopy, Vol. 2, Chap. 6, (edited by E. L. WEHRY) Plenum Press, New York (1976).