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J. Phys. Chem. 1983, 87,566-569
Fluorescence Quenching of Water-Soluble Porphyrins. A Novel Fluorescence Quenchlng of Anionic Porphyrin by Anionic Anthraquinone Kojl Kano,' Toshlnorl Sato, Sunao Yamada, and Tellchlro Ogawa Department of Molecular Science and Technology, Graduate School of Engineering Sclences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan (Received: August 18, 1982; In Final Form: October 14, 1982)
The fluorescence quenching of 5-phenyl-10,15,20-tris(p-sulfonatophenyl)porphine (TPPS3-) and 5,10,15,20tetrakis(4-N-methylpyridy1)porphine(TMPyP4+)has been studied in water (pH 8.0) by using 9,10-anthraquinone-2,6-disulfonate (AQDS2-)and methyl viologen (MV2+)as quenchers. While electrostatic repulsion is expected, AQDS2-quenched the TPPS3- fluorescence more efficiently than MV2+. The steady-state and time-resolved fluorescence measurements indicated that static quenching took place in the TPPS3--AQDS2system. Studies on the absorption spectra and the effects of ionic strength on the fluorescence quenching indicated the formation of the ground-state complex of TPPS3- and AQDS2-. The thermodynamic parameters (AHand AS) suggested that the ground-state complex was formed via van der Waals interaction.
Introduction Experimental Section The tetrahydrate of 5-phenyl-10,15,20-tris(p-sulfonatoIn view of solar energy conversion, photochemistry of pheny1)porphinedisulfonicacid salt and 5,10,15,20-tetratetraphenylporphine and its analogues has been focused kis(4-N-methylpyridy1)porphinetetra (p-toluenesulfonate) on photoredox reactions of porphyrins in triplet states.' were purchased from Dojindo Laboratories and used It is very important, however, to investigate the interaction without further purification. Disodium 9,lO-anthrabetween photoexcited singlet states of the porphyrins and quinone-2,6-disulfonate (Tokyo Kasei) was recrystallized electron acceptors in relation to the photochemistry of from water by use of active charcoal. Methyl viologen chlorophylls in biological systems. There are several chloride (Nakarai) was used as received. The purities of studies on fluorescence quenching of the porphyrins by these materials were checked by elementary analysis. quinones and nitroaromatics in organic solvents2where a Water was distilled and deionized by being passed through weak interaction between porphyrins and electron accepof ion-exchange resin. tors in their ground states has been d e m o n ~ t r a t e d . ~ ~ ~a ~column ,~ The fluorescence emission and excitation spectra were In this paper, we report the fluorescence quenching of measured on a Hitachi 650-10s spectrofluorimeter. The water-soluble porphyrins, such as 5-phenyl-10,15,20-trisabsorption spectra were taken on a Jasco UVIDEC 505 (p-sulfonatopheny1)porphine (TPPS3-) and 5,10,15,20spectrophotometer. The fluorescence decay curves were tetrakis(4-N-methylpyridy1)porphine(TMPyP4+) by obtained by using an Ortec single-photon counting appa9,10-anthraquinone-2,6-disulfonate (AQDS2-)and methyl ratus with an output displayed on a Norland IT-5300 viologen (MV2+). It can be expected that an electrostatic multichannel analyzer. The sample was excited by an attraction between the anionic porphyrin and the cationic Ortec nanosecond light pulser Model 9352 (ca. 2.5-11s duquencher (and vice versa) leads a static quenching of the ration). The emitted lights were passed through a Toshiba porphyrin fluorescence. Indeed, Schmehl and Whitten cutoff filter 0-55 and monochromated by a Nippon Jurhave reported the static quenching of the fluorescence rell-Ash JE-25 monochromator. A HTV-R1477 photoemission from 5,10,15,20-tetrakis(p-su1fonatophenyl)pormultiplier was used for detection of photons. All experphine (TPPS4-)by MV2+.4 In the present study, a novel iments were undertaken in aerobic phosphate buffer (0.05 fluorescence quenching was observed in the TPPS3-M) adjusted to pH 8.0 at 25 OC unless otherwise noted. AQDS2- system where the quenching efficiency was much Since the absorption spectra of TPPS3- and TMPyP4+ higher than that for the TPPS3--MV2+ system. On the were changed upon addition of MV2+and/or AQDS" exother hand, no appreciable quenching was observed in the cept for the TMPyP4+-MV2+ system, the excitation TMPyP4+-MV2+system. wavelength for measuring the fluorescence spectra of each system was set to the wavelength at which an isosbestic (1)(a) K. Kano, K. Takuma, T. Ikeda, D. Nakajima, Y. Teutsui, and point was observed. T. Matsuo, Photochem. Photobiol., 27,695 (1978). (b) M. Calvin, Acc.
Chem. Res., 11, 369 (1978). (c) W.Potter and G. Levin, Photochem. Photobiol., 30,225(1979). (d) A. Harriman, G. Porter, and N. Searle, J. Chem. Soc., Faraday Trans. 2,75,1515(1979). (e) M-P. Pileni and M.
GrPltzel, J. Phys. Chem., 84,1822(1980). (0K. Kalyan.asundaram and M. Griltzel, Helu. Chim. Acta, 63,478(1980). (9) A.Hamman,G. Porter, and M-C. Richoux, J. Chem. SOC.,Faraday Trans. 2,77,833(1981). (h) E. Borgarello, K. Kalyanasundaram, Y. Okuno, and M. Gritzel, Helu. Chim. Acta, 64, 1937 (1981). (2)(a) R. Livingston and C-L. Ke, J. Am. Chem. Soc., 72,909(1950). (b) R. Livingston, L. Thompson, and M. V. Ramarao, ibid., 74, 1073 (1952). (c) G. S. Beddard, S. Carlin, L. Harris, G. Porter and C. J. Tredwell, Photochem. Photobiol., 27,433(1978). (d) J. K. Baird and S. P. Escott, J. Chem. Phys., 74,6993 (1981). (e) A. Harriman and R. J. Hosie, J. Photochem., 15, 163 (1981). (3) (a) M. Gouterman and P. E. Stevenson, J . Chem. Phys.,37,2266 (1962).(b) T. K. Chandrasheker and V. Krishnan, Inorg. Chem., 20,2782 (1981). ( c ) S.Yamada, T. Sato, K. Kano, and T. Ogawa, submitted for publication in Photochem. Photobiol. (4)R. H. Schmehl and D. G. Whitten, J. Phys. Chem.,85,3473(1981).
Results Fluorescence Quenching of TPPS3-. As Schmehl and Whitten have shown, the fluorescence quenching of TPPS3- (2.5 x M) by MV2+did not obey a simple Stern-Volmer linear relationship, i.e., the inversed fluorescence intensities of TPPS3-at 645 nm (lo/l) vs. the quencher concentrations ([Q]) deviated upward from a straight line (Figure 1). This should be ascribed to static quenching because of the formation of an electrostatic association complex of TPPS3-and MV2+. On the other hand, it is expected that AQDS2- cannot associate with TPPS3- in the ground state because of an electrostatic repulsion and, hence, only dynamic quenching takes place in this system. The experimental results are contradictory
0022-365418312087-0566$0 1.5010 0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983
Quenching of Anionic Porphyrin by Anionic Anthraquinone
TABLE I: Fluorescence Lifetimes ( T ~ ) Rate , Constants for Dynamic Fluorescence Quenching (kq),and Binding and TPPS3--AQDS2Constants ( K ) for TPPS3--MVZ+ Systems at 25 "C
15 h
4
-r" 10
ns
system
T ~ ,
k,,
KfluoresP
M-
M'ls-I
Kabyb
M-
TPPS3 -MV2+ 9.2 1.0 X 10" 4300 4600 TPPS3--AQDS2- 10.2 3 X lo9 810 740 The Kfluoresvalues were calculated by using eq 6. Kabs values were obtained from the Benesi-Hildebrand
-
b h
4.
-.
2 5 I
plots. 151
1
0
587
5
[Quencherl lo3,M
I
I
I
I
I
1
10
Flgure 1. Plots of I,lI and T,/T vs. [quencher] for the TPPSs-MV2+ ( 0 , O )and TPPSs-AQDS2- systems (A& In pH 8.0 phosphate buffer (0.05 M) at 25 O C . TPPS" (2.5 X M) was excited at 563 and 548 nm for the TPPS"-MV2+ and TPPS3--AQDS2- systems, respectively. The emission maxima of TPPS" were 645 and 700 nm and the fluorescence intensities at 645 nm were followed to obtain the plots of I,lI vs. [quencher].
of this expectation. As Figure 1shows, AQDS2-quenches the fluorescence emission from TPPS3-much more efficiently than MV2+and the plot of Io/I vs. [Q] deviates upward similarly to the case of the quenching by MV2+. The fluorescence lifetime of TPPS3-in the phosphate buffer (pH 8.0) in the absence of quencher (r0) was 9.7 f 0.5 ns. The fluorescence lifetime (7)was not as sensitive as the fluorescence intensity to the addition of MV2+ and/or AQDS2- (Figure 1). The linear relationship between 7 0 / 7 and [&] gave a rate constant for dynamic fluorescence quenching (k ), the k, values for the TPPS3--MV2+ and TPPS3--iQDS2- systems being 1.0 X 1O1O and ca. 3 X lo9 M-' s-', respectively. For each system, the value of Io/I is much larger than that of T ~ / at T a given quencher concentration, suggesting that the static quenching takes place:
P+QYPQ
(1)
P
(2)
+ hv
+
P*
P*+Q-P+Q PQ + hv PQ*
-
+
PQ*
PQ
(3) (4)
(5)
where P, Q, and PQ represent TPPS3-, quencher, and ground-state complex of P and Q, respectively, and K = [PQ]/([P][Q]). The observed I o / I can be correlated with [Q] by the following e q ~ a t i o n : ~ (Io/l)obsd
= I1 + kq~o[QIm.d(1+ K(e~~/e~)[Qltotall(6)
where ew and tp are the excitation coefficients of PQ and P, respectively, at the excitation wavelength. Since cPB = ep under the present conditions (see Experimental Section) and kq70 is determined from the plot of T ~ / Tvs. [Q], the binding constant ( K ) for the ground-state complex can be calculated by eq 6. The obtained K values are given in Table I. Interestingly, the K value for the TPPS3-AQDS2- system is much larger than that for the TPPS3--MV2+ system. Absorption Spectral Change of TPPS". The absorption spectrum of TPPS3-was changed upon addition of MV2+ and/or AQDS2-. Figure 2 shows the absorption spectra (5)T.L. Nemzek and W. R. Ware, J. Chem. Phys., 62,477 (1975).
Wavelength, nm Flgure 2. Absorption spectral change of TPPS" upon addition of AQDS2- in pH 8.0 phosphate buffer (0.05 M) at 25 O C . c represents the extinction coefficient. [AQDS*-] = 0, 0.7 X lo4, 2.9 X lo4, 4.8 X lo-', and 8.0 X lo4 M. The insert is the Benesi-Hildebrand plot measured at 540 nm.
of TPPS3-in phosphate buffer in the presence of various amounts of AQDS2-. Up to an AQDS2-concentration of 8X M, isosbestic points were observed. The Benesi-Hildebrand plot gave a K value of 4600 M-' which is in good agreement with that (4300 M-') obtained from fluorescence quenching (Table I). Apparently a similar spectral change of TPPS" was observed upon addition of MV2+but higher concentrations of MV2+are needed to observe an appreciable spectral change ( K = 740 M-'). Effects of Ionic Strength on Fluorescence Quenching of TPPS*. Pasternack et al. have found that the addition of KN03 causes an absorption spectral change of TPPS3in waterae They assumed that the KN03-dependent change in the absorption spectra is ascribed to the formation of TPPS3- dimer! We also found a red shift in the absorption bands of TPPS3- in phosphate buffer upon addition of NaC1. The absorption maxima of TPPS3-at 516, 553, 578, and 634 nm shifted in pH 8.0 phosphate buffer (0.05 M) to 520, 558, -590 (sh), and 648 nm, respectively, upon addition of 1.2 M NaC1. Isosbestic points were observed at 526,555, 572, 591,618, and 641 nm in the absorption spectra of TPPS3-in the presence of various amounts of NaC1. Figure 3 shows plots of Io/I w. [Q] for the TPPS*-MV2+ and TPPS3--AQDS2- systems in phosphate buffer in the absence and presence of 1.2 M NaC1. NaCl caused acceleration of the fluorescence quenching by AQDS2-while it inhibited quenching by MV+. The K values calculated by using eq 6 were 170 and 7800 M-' for the TPPS3--MF and TPPS3--AQDS2- systems, respectively, in pH 8.0 phosphate buffer (0.05M) containing 1.2 M NaC1. (6) R. F. Pasternack, P. R. Huber, P. Boyd, G. Engasser, L. Francesconi, E. Gibbs, P. Fasella, G. C. Verturo, and L. deC. Hinds, J.Am. Chem. SOC.,93,4511 (1972).
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The Journal of Physical Chemistry, Vol. 87, No. 4, 1983
Kano et al. TABLE 11: Temperature Dependence of Binding for Constants ( K ) and Free-Energy Changes ( A G ) Complexation of TPPS3- with A Q D S 2 -
T, K 288
298 308 316
10
-.
K ~ o r e sM" ,
-AG,kJ/mol
5500 4300
20.6
2700 1800
20.2
20.7 19.7
- 5
1
0
10
5
CMV2+11O3,M ( 0 4 * )
Flgure 3. Plots of I,lI vs. [quencher] for the fluorescence quenching of TPPS3- in pH 8.0 phosphate buffer (0.05 M) in the absence (0, A) and the presence (0,A) of 1.2 M NaCl at 25 OC. The data for fluorescence quenching In the absence of NaCl are the same as those shown in Figure 1. TPPS3- in phosphate buffer Containing 1.2 M NaCl was excited at 579 and 575 nm for the TPPS3--MV2+ and TPPS3-AQDS2- systems, respectively, and the emission maxima were observed at 660 and 700 nm (sh) in both cases. The fluorescence intensities at 660 nm were followed to obtaine the plots of I,lI vs. [quencher].
0
5 C AQDS2-l
10
lo4, M
Flgure 4. Plots of I,lI vs. [AaDS*-] for the TPPSs-AQDS2- system in pH 8.0 phosphate buffer (0.05 M) at various temperatures.
Effect of Temperature on the Fluorescence Quenching of TPPS3- by AQDS2-. The efficiency of the fluorescence quenching of TPPS* by AQDS2- decreased with increasing temperature (Figure 4). Since the 70 value of TPPS3-did not change upon alteration of the temperature (288-316 K), the observed temperature dependency should be ascribed to the formation of a thermally dissociable molecular complex of TPPS3-and AQDS2- which deactivates instantaneously after photoexcitation. Over the temperature range 288-316 K, the fluorescence lifetime of TPPS3was scarecely altered by the addition of AQDS2-, suggesting a negligibly small contribution of the dynamic quenching. The temperature dependence of K is shown in Table I1 together with free-energy change (AG) calculated by eq 7. AG = -RT In K (7) and T (298-316 K) was observed (Figure 51, enthalpic (AH) and entropic changes (AS) of the binding of TPPS3-and AQDS2- are calculated by eq 8, AH and AS being -38.2 AG = AH - TAS (8)
*-2o.oL d
159-.
310
320
Temperature , K
Flgure 5. Plot of
-AG vs. T for complexation of TPPS3- with AQDS2-.
kJ/mol and -58.7 J/(mol.K), respectively. AG at 288 K is larger than that expected from the straight line shown in Figure 5. The absorption spectrum of TPPS3-at 288 K is represented by a superposition of those of monomeric and dimeric TPPS*. The coexisting dimer of TPPS* may lead to a deviation from the linear relationship between AG and T at lower temperature. Fluorescence Quenching of TMPyP+. The fluorescence lifetime of TMPyP4+in the absence of quencher was 5.2 f 0.4 ns which is about half that of TPPS3-. No appreciable quenching of TMPyP4+fluorescence was observed when MV2+was used as the quencher. The absorption spectrum of TMPyP4+ did not change even if a large amount of MV2+(6.3 X M) was added. On the other hand, AQDS2- markedly quenched the fluorescence of TMPyP4+. When TMPyP4+was excited at 590 nm, the plot of Zo/Z vs. [AQDS2-] deviated upward from a straight line. The Io/Z value was 4.3 for the equimolar solution of TMPyP4+and AQDS2- (2.5 X M). The analysis of the correlation between Zo/Z and [AQDS2-] by using eq 6 did not give a constant K value at each quencher concentration, i.e., the apparent K value increased with increasing [AQDS2-]. This indicates that the stoichiometry of the association complex of TMPyP4+and AQDS2- varies with increasing [AQDS2-].
Discussion An anionic porphyrin, TPPS*, associates with a cationic quencher, MV2+,to form a ground-state complex. By the same manner, TMPyP4+ binds with AQDS2-. In these cases, effective fluorescence quenching takes place. The steady-state and time-resolved fluorescence measurements clearly indicate that static quenching is predominant in these systems. Ground-state complex formation was also supported by means of absorption spectroscopy. For the TPPS3--MV2+system, isosbestic points were observed in the spectra of TPPS3- in phosphate buffer containing various amounts of MV2+([MV2+]I1 X M). The Benesi-Hildebrand plot gave a binding constant for the complex of TPPS3-and MV2+( K = 740 M-I) which is in good agreement with that obtained by analysis of the plot of Io/Z vs. [Q] using eq 6 ( K = 810 M-I). This suggests a 1:l stoichiometry for this complex. By the way, the K value for the TMPyP4+-AQDS2-system calculated by eq
J. Phys. Chem. 1983, 87, 569-572
6 increased with increasing [AQDS2-],suggesting the formation of a ground-state complex (or complexes) with a stoichiometry other than 1:l. A novel fluorescence quenching was observed when the fluorescence of TPPS3- was quenched by AQDS2-. Although it is expected that electrostatic repulsion prohibits the association of TPPS" with AQDS2-, static quenching took place more efficiently than the case of the TPPS3--MV2+ system (Figure 1). The absorption and fluorescence spectral measurements gave almost identical K values (4600and 4300 M-l) for the complex of TPPS3and AQDS2- which are much larger than those for the complex of TPPS3-and MV2+(740and 810 M-l). Electrostatic association seems to be interfered by the addition of inorganic salt. This is the case for the complex of TPPS" and M P . The K value was reduced to 170 M-' by the addition of 1.2 M NaC1. An opposite phenomenon was observed for the TPPS3--AQDS2- system, i.e., the K value increased to 7800 M-l upon addition of 1.2 M NaC1. Both TPPS3- and AQDS2- are assumed to be amphiphilic molecules because the negative charges of the sulfonate anions of these compounds are localized. It seems, therefore, that the driving force for the complexation of TPPS3- with AQDS2- is van der Waals or hydrophobic interaction. In an aqueous solution of concentrated inorganic salt, the negative charges of the sulfonate anions may be neutralized by the cation of the added inorganic salt to result in an increase of the van der Waals or hy-
569
drophobic interaction. The hydrophobic interaction is characterized by positive and large AS.' The complexation of TPPS3- with AQDS2-, however, is enthalpically favorable (AH = -38.2 kJ/mol) and entropically unfavorable (AS = -58.7 J/(mol.K)). This thermodynamic character is similar to that for inclusion complexes of cyclodextrins.* It may be reasonable to assume that the van der Waals interaction is the preferential force for complex formation. In contrast with the TPPS3--AQDS2- system, the fluorescence of TMPyP4+ was scarecely quenched by MV2+. Delocalization of positive charges of the pyridinium cations of TMPyP4+and MV2+may enhance the electrostatic repulsion of these compounds. The present study suggests a strong ability of porphyrins for formation of molecular complexes in aqueous media. Further studies are now in progress to reveal the characteristic functions of the porphyrins in water.
Acknowledgment. K.K. is grateful for financial support by a Grant-in-Aid from the Japanese Ministry of Education, Science and Culture. Registry No. TPPS3-, 84057-68-1; TMPyP4+,38673-65-3; AQDS2-, 84-50-4; methyl viologen, 1910-42-5. (7) A. Wishnia and T. W. Pinder, Jr., Biochemistry, 5, 1534 (1966). (8) M. L. Bender and M. Komiyama, "Cyclodextrin Chemistry", Springer-Verlag, Berlin, 1978, Chapter 3.
Excitation Energy Dependence of Intramolecular Excimer Formation in N,N,N/,N/-Tetramethylpropanedlamine Vapor Mlchlya Itoh,'
Yuklo Hanashlma,
Faculty of Pharmaceutlcal Sciences, Kanazawa Unlverslty, Takara-machi, Kanazawa 920, Japan
and Ichlro Hanarakl Institute for Molecular Science, Okazaki 444, Japan (Received: June 2, 1982; In Final Form: October 7, 1982)
-
The excitation energy dependence of intramolecular excimer formation in NJV,iVJV'-tetramethylpropanediamine (TMPD) in collision-freevapor was observed for the excitation into the lowest-energy absorption band (n 3s Rydberg band). The time-resolved fluorescence study demonstrates that the association rate constant of the intramolecular excimer formation increases remarkably with increasing excitation energy within the SI absorption band of the dimethylamino moiety.
The excitation energy dependence of the electronic relaxation of excited states has been extensively studied in the collision-free vapor of polyatomic molecules.'S2 The nonradiative decay process of such molecules has been reported to be exponentially dependent on the respective vibronic energy levels. Okajima and Lim have3suggested that the rate constant of the excited state molecular complex formation (exciplex) from the vibrationally hot S1 state of tetracyanobenzene and the ground state of p xylene is 1order of magnitude greater than that from the vibrationally relaxed S1state. Recently, Itoh and Hana(1) s. A. Rice in 'Excited States", Vol. 2., E. C. Lim, Ed., Academic Press, New York, 1975, p 111. (2) K. F. Freed in "Topica in Applied Physca",Vol. 15, F. K. Fong, Ed., Springer, West Berlin, 1976, p 23, and references therein. (3) S. Okajima and E. C. Lim, Chem. Phys. Lett., 70, 283 (1980). 0022-3654/83/2087-0569$01.50/0
shima4 and Itoh et al.5 reported that the association and dissociation rate constants for the intramolecular exciplex formation in 1-(9,10-dicyan0-2-anthryl)-3-(2-naphthyl)propane and 1-(9,10-dicyan0-2-anthryl)-3-(pentamethylpheny1)propane in collision-free vapor show a remarkable excitation energy dependence, which was suggested to be attributable mostly to the density of the vibrational levels of the final state of the exciplex. Halpern and Chan6 reported intramolecular excimer formation and electronic relaxation in two saturated diamines, N,N,N',N'-tetramethylethylenediamine and N,N,N',N'-tetramethylpropanediamine (TMPD), in the va(4) M.Itoh and Y. Hanashima, Chem. Phys. Lett., 83, 445 (1981). (5) M.Itoh, Y.Hanashima, N. Wada, and I. Hanazaki, Bull. Chem. SOC.Jpn., submitted for publication. (6) A. M.Halpem and P. P. Chan, J.Am. Chen. SOC.,97,2971 (1975).
0 1983 American Chemical Society
