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Some aspects of the lucigenin light reaction
Journal of Photochemistry
SOME ASPECTS
and Photobiology,
Chemistry,
OF THE LUCIGENIN
A. E. MANTAKA-MARKETOU, J. NIKOKAVOURAS$ Institute of Physical Chemistry, Attikis, I53 10 (Greece) (Received
A:
F. S. VARVERI, National
Research
48 (1989)
LIGHT
337
REACTION+
G. VASSILOPOULOS Center
337 - 340
“Democritos”,
and Aghia Paraskeoi
January 31, 1989)
Summary Evidence is given for an energy transfer step from IV-methylacridone to lucigenin in the light reaction of lucigenin. This is accomplished by the selective quenching of the acceptor by the Cl- anion as reflected in the chemiluminescence quantum yields. 1. Introduction The classical lucigenin (N,N-dimethyl-9,9’-biacridinium nitrate (L)) light reaction has for many years been controversial regarding both the mechanism and the primary emitter. Eventually, A?-methylacridone (NMA) was established as the primary emitter, while the discrepancy arising from the dissimilarity between the NMA fluorescence at about 430 nm and the L chemiluminescence (CL) spectrum at about 500 nm was attributed to energy transfer from NMA to L which fluoresces in this region. Further evidence for this effect was presented when we carried out this reaction in organized media [ 1, 21. Bruce and coworkers [ 3, 41; however, have presented strong evidence that this is because of the inner filter effect and there is no energy transfer involved. Without disregarding the arguments for the inner filter effect, we now wish to show that energy transfer is also involved in this light reaction, as selective quenching of the fluorescence of the acceptor by the colourless Clanion greatly reduces the CL efficiency of the reaction. 2. Experimental 2.1. Chemiluminescence CL measurements were performed with the aid of a LKB 1250 luminometer equipped with a thermostatted cell holder. The light integrals tPart of this work was presented July 17 _ 22, 1988, Bologna, Italy. *Author to whom correspondence lOlO-6030/89/$3.50
at the XII IUPAC
Symposium
on Photochemistry,
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
338 were obtained with the aid of a suitable recorder and the instrument’s integrator at 25 “C by squirting NaOH (0.1 M, 30 ~1) and H,O, (396, 30 ~1) into L (4 X 10m4 M, 250 ~1) containing the appropriate amount of salt. The light integrals were then plotted us. the salt concentration. Chemiluminescence spectra were run on an Aminco-Bowman SPF spectrofluorometer employing wide slits and fast scanning. 2.2. Fluorescence Fluorescence measurements were performed on the Aminco-Bowman spectrofluorometer on 1 O- 5 M L aqueous solutions or saturated NMA aqueous solutions containing the appropriate amount of NaCl at 25 “C.
3. Results and discussion The effect of Cl- ions on the L light reaction is shown in Fig. 1. A dramatic drop in the efficiency of the reaction is observed on the progressive addition of Cl- ions to the reaction mixture; S042- ions have a less pronounced yet similar effect. We have attributed this effect to the quenching of the fluorescence of both NMA which is the primary emitter in this reaction and L which is probably the secondary emitter, by the abovementioned anions. This quenching is depicted in the Stern-Vohner plots in Figs. 2 and 3 for L and NMA respectively. In the case of L and for a fluorescence lifetime rL = 18.8 ns [53 the quenching constant kstLj becomes equal to 2 . 2 x 1o’O s-l In the case of NMA and for a fluorescence lifetime equal to 18.5 ns [5] k s[~~~~ ' becomes equal to 1.84 X 10’ s-l.
o
’
“1
0
I””
0.5
1
“““‘I
1_
’ I5
1
2.0
CSalt],M
Fig.
1. Chemiluminescence
l, NaCl; 0, NazS04.
intensity-time
integrals
as a function
of salt concentration:
339
0
20
LO ----[NaCl]
Fig. 2. Stern-Volmer
60 -4 M x 10
plot of the L fluorescence
60
100
quenching
by NaCl.
Q6-
0
45
1
1.5
2
2.5
-[NaCI].M
Fig. 3. Stern-Volmer
plot of the NMA
fluorescence
quenching
by NaCl.
Comparison of the two quenching constants reveals that Cl- ions quench the L fluorescence far more efficiently than the NMA fluorescence, in fact almost 1000 times more efficiently. It is therefore reasonable to assume that the reduced CL in the presence of NaCl (Fig. 1) is caused by the quenching of the fluorescence of L, acceptor of the NMA excitation energy. In this case, the levelling of the curve (Fig. 1) at higher NaCl concentrations should be attributed to almost complete quenching of L and emission from NMA, far less sensitive to quenching. If the arguments presented above are valid the CL spectrum of the reaction in the presence of NaCl should be blue shifted towards the NMA fluorescence maxima at about 430 nm. This is shown in Fig. 4 where the CL spectrum is depicted in the presence and in the absence of 0.5 M NaCl. It should be noted here that the two spectra were recorded under identical conditions immediately after mixing of the reagents. Despite the inherent low quality of CL spectra (fast scanning, wide slits, etc.), the spectrum recorded in the presence of NaCl is certainly associated with lower intensity
Wavelength
( nm 1
Fig. 4. Chemiluminescence spectrum of the light reaction. NaCl; curve b, in the presence of 0.5 M NaCl.
Curve
a, in the absence
of
and unmistakably an increase in the NMA contribution. A similar effect is observed in micellar media [ 1, 21, where isolation of NMA in the mice&r Stern region results in the de-masking of its emission in the CL spectrum. In conclusion, our results indicate that irrespectively of the inner filter effect shown to exist in the L light reaction, there is also energy transfer from NMA, the primary excited product, to unreacted L.
References 1 C. M. Paleos, G. Vassilopoulos and 2 J. Nikokavouras, G. Vassilopoulos (1981) 1082. 3 R. Maskiewicz, D. Sogah and T. C. 4 R. Maskiewicz, D. Sogah and T. C. 5 K. Legg and D. M. Hercules, J. Am.
J. Nikokavouras, J. Photo&em., 18 (1982) 327. and C. M. Paleos, J. Chem. Sot., Chem. Commun., Bruce, J. Am. Chem. Sot., 101 Bruce, J. Am. Chem. Sot., 101 Chem. Sot., 91 (1969) 1902.
(1979) (1979)
5347. 5355.
SOME ASPECTS
and Photobiology,
Chemistry,
OF THE LUCIGENIN
A. E. MANTAKA-MARKETOU, J. NIKOKAVOURAS$ Institute of Physical Chemistry, Attikis, I53 10 (Greece) (Received
A:
F. S. VARVERI, National
Research
48 (1989)
LIGHT
337
REACTION+
G. VASSILOPOULOS Center
337 - 340
“Democritos”,
and Aghia Paraskeoi
January 31, 1989)
Summary Evidence is given for an energy transfer step from IV-methylacridone to lucigenin in the light reaction of lucigenin. This is accomplished by the selective quenching of the acceptor by the Cl- anion as reflected in the chemiluminescence quantum yields. 1. Introduction The classical lucigenin (N,N-dimethyl-9,9’-biacridinium nitrate (L)) light reaction has for many years been controversial regarding both the mechanism and the primary emitter. Eventually, A?-methylacridone (NMA) was established as the primary emitter, while the discrepancy arising from the dissimilarity between the NMA fluorescence at about 430 nm and the L chemiluminescence (CL) spectrum at about 500 nm was attributed to energy transfer from NMA to L which fluoresces in this region. Further evidence for this effect was presented when we carried out this reaction in organized media [ 1, 21. Bruce and coworkers [ 3, 41; however, have presented strong evidence that this is because of the inner filter effect and there is no energy transfer involved. Without disregarding the arguments for the inner filter effect, we now wish to show that energy transfer is also involved in this light reaction, as selective quenching of the fluorescence of the acceptor by the colourless Clanion greatly reduces the CL efficiency of the reaction. 2. Experimental 2.1. Chemiluminescence CL measurements were performed with the aid of a LKB 1250 luminometer equipped with a thermostatted cell holder. The light integrals tPart of this work was presented July 17 _ 22, 1988, Bologna, Italy. *Author to whom correspondence lOlO-6030/89/$3.50
at the XII IUPAC
Symposium
on Photochemistry,
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
338 were obtained with the aid of a suitable recorder and the instrument’s integrator at 25 “C by squirting NaOH (0.1 M, 30 ~1) and H,O, (396, 30 ~1) into L (4 X 10m4 M, 250 ~1) containing the appropriate amount of salt. The light integrals were then plotted us. the salt concentration. Chemiluminescence spectra were run on an Aminco-Bowman SPF spectrofluorometer employing wide slits and fast scanning. 2.2. Fluorescence Fluorescence measurements were performed on the Aminco-Bowman spectrofluorometer on 1 O- 5 M L aqueous solutions or saturated NMA aqueous solutions containing the appropriate amount of NaCl at 25 “C.
3. Results and discussion The effect of Cl- ions on the L light reaction is shown in Fig. 1. A dramatic drop in the efficiency of the reaction is observed on the progressive addition of Cl- ions to the reaction mixture; S042- ions have a less pronounced yet similar effect. We have attributed this effect to the quenching of the fluorescence of both NMA which is the primary emitter in this reaction and L which is probably the secondary emitter, by the abovementioned anions. This quenching is depicted in the Stern-Vohner plots in Figs. 2 and 3 for L and NMA respectively. In the case of L and for a fluorescence lifetime rL = 18.8 ns [53 the quenching constant kstLj becomes equal to 2 . 2 x 1o’O s-l In the case of NMA and for a fluorescence lifetime equal to 18.5 ns [5] k s[~~~~ ' becomes equal to 1.84 X 10’ s-l.
o
’
“1
0
I””
0.5
1
“““‘I
1_
’ I5
1
2.0
CSalt],M
Fig.
1. Chemiluminescence
l, NaCl; 0, NazS04.
intensity-time
integrals
as a function
of salt concentration:
339
0
20
LO ----[NaCl]
Fig. 2. Stern-Volmer
60 -4 M x 10
plot of the L fluorescence
60
100
quenching
by NaCl.
Q6-
0
45
1
1.5
2
2.5
-[NaCI].M
Fig. 3. Stern-Volmer
plot of the NMA
fluorescence
quenching
by NaCl.
Comparison of the two quenching constants reveals that Cl- ions quench the L fluorescence far more efficiently than the NMA fluorescence, in fact almost 1000 times more efficiently. It is therefore reasonable to assume that the reduced CL in the presence of NaCl (Fig. 1) is caused by the quenching of the fluorescence of L, acceptor of the NMA excitation energy. In this case, the levelling of the curve (Fig. 1) at higher NaCl concentrations should be attributed to almost complete quenching of L and emission from NMA, far less sensitive to quenching. If the arguments presented above are valid the CL spectrum of the reaction in the presence of NaCl should be blue shifted towards the NMA fluorescence maxima at about 430 nm. This is shown in Fig. 4 where the CL spectrum is depicted in the presence and in the absence of 0.5 M NaCl. It should be noted here that the two spectra were recorded under identical conditions immediately after mixing of the reagents. Despite the inherent low quality of CL spectra (fast scanning, wide slits, etc.), the spectrum recorded in the presence of NaCl is certainly associated with lower intensity
Wavelength
( nm 1
Fig. 4. Chemiluminescence spectrum of the light reaction. NaCl; curve b, in the presence of 0.5 M NaCl.
Curve
a, in the absence
of
and unmistakably an increase in the NMA contribution. A similar effect is observed in micellar media [ 1, 21, where isolation of NMA in the mice&r Stern region results in the de-masking of its emission in the CL spectrum. In conclusion, our results indicate that irrespectively of the inner filter effect shown to exist in the L light reaction, there is also energy transfer from NMA, the primary excited product, to unreacted L.
References 1 C. M. Paleos, G. Vassilopoulos and 2 J. Nikokavouras, G. Vassilopoulos (1981) 1082. 3 R. Maskiewicz, D. Sogah and T. C. 4 R. Maskiewicz, D. Sogah and T. C. 5 K. Legg and D. M. Hercules, J. Am.
J. Nikokavouras, J. Photo&em., 18 (1982) 327. and C. M. Paleos, J. Chem. Sot., Chem. Commun., Bruce, J. Am. Chem. Sot., 101 Bruce, J. Am. Chem. Sot., 101 Chem. Sot., 91 (1969) 1902.
(1979) (1979)
5347. 5355.