J . Phys. Chem. 1989, 93, 1226-1233
1226
Singlet-Singlet and Triplet-Triplet Intramolecular Transfer Processes in a Covalently Linked Porphyrin-Phthalocyanine Heterodimer T. H. Tran-Thi,* C. Desforge, C. Thiec, Centre d'Etudes NuclPaires de Saclay, IRDI/DESICP/DLPC/SCM. CEA-CNRS UA331, Iaboratoire de photophysique et de photochimie, 91 191 Gif sur Yvette Cedex, France
and S . Gaspard CNRS, Institut de Chimie des Substances Naturelles, 91 190 Gif sur Yvette, France (Received: March 12, 1988; In Final Form: August 2, 1988)
Intramolecular transfer processes in a covalently linked zinc porphyrin-zinc phthalocyanine heterodimer are investigated as a function of the solvent polarity. In toluol, selectiveexcitation of the porphyrin chromophore is followed by a very efficient energy-transfer process to the phthalocyanine moiety. This reaction takes place in both the singlet and triplet porphyrin excited states in competition with the other radiative and nonradiative decay pathways. In contrast to this phenomenon, a charge-transfer reaction occurs in dimethyl sulfoxide. The differenceof behavior has been explained in terms of conformational change due to the nature of solvent interaction.
Introduction The effect of mutual orientation of the chromophore subunits in dimers or heterodimers is one of the most important factors influencing the intramolecular electron- and energy-transfer efficiencies. Mast of the studies devoted to this subject involve dyes' or porphyrin and porphyrin-like compounds linked together in a rigid manner or via flexible chains,2 as they can serve as model systems for the reaction center of photosynthesis. In most systems, the donor and the acceptor groups were attached either to rigid systems such as steroids, so that their spatial relationship was known but could be varied only by using different spacers, or to a flexible chain so that their spatial relationship could not be determined. More recently, Hassoon et aL3 carried out a partial intramolecular energy transfer in systems containing phenanthrene and a-diketone moieties linked in close proximity by two -(CH2)n- bridges with limited flexibility and suggested a Dexter-type exchange transfer mechanism. More generally, the approach likely to yield such information would imply having a bichromophoric molecule consisting of a donor and an acceptor (D-A) held in proximity, with specific properties: (i) D and A ( I ) Bourson, J.; Mugnier, J.; Valeur, B. Chem. Phys. Lett. 1982, 92, 430. Speiser, S.; Hassoon, S.;Rubin, M. B. J. Phys. Chem. 1986, 90, 5085. (2) Schwartz, F. P.; Gouterman, M.; Mulijiani, 2.;Dolphin, D. H. Eioorg. Chem. 1972, 2, I . Anton, J. A.; Loach, P. A.; Govindjee Photochem. photobiol. 1978, 28, 235. Netzel, T. L.; Kroger, P.; Chang, C. K.; Fujita, I.; Fajer, J. Chem. Phys. Lett. 1979, 67, 223. Tabushi, I.; Koga, N.; Yanagita, M. Tetrahedron Lett. 1979, 3, 257. Ho, T. F.; McIntosch, A . R.; Bolton, J. R. Nuture 1980, 286, 254. Magita, M.; Okada, T.; Magata, N.; Nisbitani, S.; Kurata, N.; Sakata, Y.; Misumi, S. Chem. Phys. Lett. 1981, 2, 263. Kong, J. L. Y.; Spears, K. G.; Loach, P. A. Photochem. Photobiol. 1982, 35, 345. Berkamp, M. A.; Dalton, J.; Netzel, T. L. J . Am. Chem. Sor. 1982, 104, 253. Nishitani, S.;Kurata, N.; Sakata, Y.; Misumi, S.; Karen, A.; Okada, T.; Mataga, N . J . Am. Chem. Sor. 1983,105, 7771. Siemiarzuk, A,; McIntosch, A. R.; Ho, T. F.; Stillman, M. J.; Roach, R.; Weedon, A. C.; Bolton, J. R.; Connolly, J. S.J. Am. Chem. Sor. 1983, 105, 7224. Lindsey, J. S.;Mauzerall, D. C. J. Am. Chem. SOC.1983, 105, 6528. Wasielewski, M. R.; Niemczyk, M . P. J . Am. Chem. SOC.1984, 106, 5043. Becker, J. Y.; Dolphin, D.; Paine, J . B.; Wijesekera, T. J. Electroanul. Chem. 1984, 164, 335. Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J. C.; Chachaty, C.; Bensasson, R.; Doizi, D.; Lidell, P.A.; Lehman, W. R.; Nemeth, G . A,; Moore, A. L. Nuture 1984, 307, 630. Mialocq, J. C.; Giannotti, C.; Maillard, P.;Momenteau, M. Chem. Phys. Lett. 1984, 112, 87. Blondeel, G.; De Keukeleire, D.; Harriman, A.; Milgrom, L. R. Chem. Phys. Lett. 1985, 118,77. Benthem, L.; Koehorst, R. B. M.; Schaafsma, T. J. Mugn. Reson. Chem. 1985, 23, 9. Leland, B. A,; Joran, A. D.; Felker, P. M.; Hopfield, J . J.; Zewail, A. H.; Dervan, P. B. J. Phys. Chem. 1985.89, 5571. Kanda, Y.; Sato, H.; Okada, T.; Mataga, N . Chem. Phys. Letf 1986, 129, 306. Gonen, 0.;Levanon, H. J . Chem. Phys. 1986,84, 4132. Levanon, H.; Regev, A,; Das, P. K. J. Phys. Chem. 1987, 91, 14. Harrison, R. J.; Pearce, B.; Beddard, G . S.; Cowan, J. A.; Sanders, J . K. M. Chem. Phys. 1987, 116, 429. Heiler, D.; McLendon, G.; Rogalskyj, P. J . Am. Chem. Sor. 1987, 109, 604. ( 3 ) Hassoon. S.: Lustig, H.; Rubin, M. B.;Speiser, S. J. Phys. Chem. 1984, 88, 6361
0022-36S4/89/2093-1226!$01 .SO/O
should exhibit large differences in the ground-state absorption spectrum so that each moiety could be selectively excited. (ii) D and A should be attached in such a manner so that conformation and spatial relationship could be varied and controlled. In that regard, our present interest is focused on a system consisting of a porphyrin linked together with a phthalocyanine compound via an oxygen atom, in which conformational changes due to solvent interaction are more restricted (Figure 1). Moreover, the porphyrin-phthalocyanine heterodimer presents the peculiarity of having a ground-state absorption spectrum that covers the entire visible part of the solar energy spectrum, while allowing selective excitation of each individual chromophore. In the present paper, we report the study of the photophysical properties of the heterodimer in comparison with the corresponding monomers and the effect of the solvent polarity on the intramolecular transfer processes.
Experimental Section Materials. All the solvents, toluol, dimethyl sulfoxide (DMSO), ethanol, from Merck, are of spectroscopic grade. The zinc tetraphenylporphyrin (ZnTPP) comes from Strem Chemicals and the zinc phthalocyanine (ZnPc) from Kodak. The zinc tetratolylporphyrin (ZnTTP), the zinc tetra-tert-butylphthalocyanine (ZnPctBu), and the porphyrin-phthalocyanine heterodimer were synthesized as previously d e ~ c r i b e d . ~ Methods. The UV and visible electronic spectra of porphyrin and phthalocyanine monomers and heterodimer in various solvents were recorded with a Beckman spectrophotometer (UV 5240). The concentration of the solutions were varied from 5 X to 1.5
x 10-5 M.
Fluorescence spectra were recorded at room temperature with a LS5 Perkin-Elmer fluorimeter equipped with a red-sensitive R928 photomultiplier. Very dilute air-saturated solutions were used to avoid spectral distortions due to the inner-filter effect and emission reabsorption. The optical density at the excitation wavelength of each solution was less than 0.05 for a 1-cm pathlength cell. The porphyrin solutions were excited at 424 and 550 nm and those containing phthalocyanines at 424, 610, and 650 nm. The fluorescence spectra were recorded over the 580-800-nm wavelength domain. The fluorescence quantum yields were determined from integrated emission band intensities and by comparative method using ZnTPP and ZnPc as standards, in various solvents. (4) Gaspard, S.;Giannotti, C.; Maillard, P.; Schaeffer, C.; Tran-Thi, T. H. J. Chem. Sor., Chem. Commun. 1984, 856. Gaspard, S.; Maillard, P. Tetrahedron, 1987, 43, 1083.
0 1989 American Chemical Society
Covalent Porphyrin-Phthalocyanine Hetercdimer
The Journal of Physical Chemistry. Vol. 93, No. 4, 1989 1227 500
600
....
I
ZnTTP ... Z"Pt1B"
c
-
ZnTTP-0-ZnPclBu
Electronic absorption spectra of ZnlTP, ZnPctBu, and ZnTTPUZnPnBu in toluol. Figure 2.
analyzer and analyzed by using a PDP 11-23 computer. The decay times were determined, using a non-least-squares method, by convoluting the instrument profile with an assumed single-, bi-, or triexpmential decay model. On iteration, the decay time that gave the best fit to the measured decay curve could be determined. The xz criterion of the weighted deviations and the autocorrelation function were used as an indication of the quality of the fit. The nanosecond absorption spectroscopy setup comprised of a Nd:YAG laser (6-11s fwhm pulse duration) and a pulsed xenon lamp as probing light has becn dexribed in detail elsewhere? The frequency doubling (or tripling) system coupled with a stimulated Raman scattering apparatus allows selective excitations at 355, 532, and 630 nm when needed. The maximum laser energy used a t these wavelengths were respectively 20, 40, and IO mJ. The samples were degassed by continuously bubbling argon through the solutions. Optical densities of the transient absorbing species formed during irradiation of the different solutions were measured as a function of time at each wavelength studied from 350 to 720 nm. The total depletion method6 was used, when the laser energy was available (only a t X,. = 355 and 532 nm), to determine the triplet quantum yield of excited prophyrin, phthalocyanine, and hetercdimer.
Figure 1.
dimer:
Schematicrepresentation of the ZnTTPOZnPctBu hetercconformation in toluol: (b) conformation in DMSO.
(a)
The fluorescence decay times were measured by using the Edinburg instrument 199F time-correlated single-photoncounting system, comprised of a coaxial flash lamp (as the excitation source) operating at 50 kHz and filled with hydrogen. The fwhm of the lamp profile through the scattered solution (Ludox Du Pont) and the overall instrumentation was 1 ns at the excitation wavelength. More recent experiments were performed with the laboratory setup with, as the excitation source, a dye laser pumped by a frequency doubled N d Y A G (Quantronix, 76 MHz). In the last case, the excitation wavelength was 580 nm, and the fwhm of the excitation source was 700 ps. The emission from the sample a t right angle to the excitation source was collected and stored in a multichannel
Results and Discussion Spectroscopic Properties. Figure 2 shows the electronic absorption spectra of ZnTTP, ZnPctBu, and the corresponding covalently linked hetercdimer in toluol. The dimer Q band can be described as the sum of the monomers Q bands with the exception of a very slight red shift ( I nm) and intensity decrease of the phthalocyanine Q band. On the contrary, the intensity of the most sensitive Soret or B band of the porphyrin moiety is significantly decreased the molar extinction coefficient of the dimer a t 425 nm is only 72% of the corresponding band in the mixture of monomers. A more precise and more fundamental quantity than c is the oscillator strength, or fvalue, which is proportional to the integrated intensity of the absorption band
f = 4.32
X
I 0 4 s c du
( 5 ) Tmn-Thi, T. H.;Markovitsi. D.; Even. R.; Simon,J. Chem. Phyz Letl. 1987, 139,207.
( 6 ) Carmichael. L.; Hug. G.L. 1.Phys. Chem. Ref. Dot. 1986, I S , 1. B.;Bensasson. R. Chem. Phys. Lett. 1975.34.44.
Amand,
1228 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
lp.
L
/
[
A
I 1 s,”
t -””” i S,”C
ACCEPTOR
DONOR MOIETY
MOIETY
Figure 3. Deactivation processes of ZnTTP-0-ZnPctBu excited states in toluol. Asterisks designate values from ref 11 and 12.
Assuming a Gaussian shape for the absorption band corresponding to a given transition, the fvalues were calculated for the porphyrin Soret band of the monomer (f) and heterodimer (f(D)) and for the phthalocyanine Q band of the monomer and dimer p(D)):
p)
p = 1.59 y(D) = 1.29
p” = 0.36 p”(D) = 0.38
The oscillator strength corresponding to the Q band remains unchanged within the approximation used for the band shape, whereasp(D)decreases like cP(D). This result is not surprising since no significant broadening of the bands was observed when going from the monomer to the dimer. Both these effects have been observed earlier on face-to-face or folded covalently linked porphyrin dimers.’ In the case of the face-to-face dimer, a red shift of the Soret band was observed and explained in terms of charge-transfer interaction between the two rings. The magnitude of such an interaction depends markedly upon the relative orientation of the two macrocycles. In many cases, a blue shift of the Soret band was observed and explained in terms of a strong exciton coupling between the two chromophores.8 In the present case, the fact that no blue shift was observed indicates the lack of strong intramolecular ground-state phthalocyanine-porphyrin interaction in toluol. The same behavior, with a stronger decrease of the oscillator strength of the Soret band, is observed when dimethyl sulfoxide is the solvent:
p
= 1.94
p” = 0.39
This indicates that a stronger interaction exists between the two chromophores, due to conformational changes when going from a nonpolar to a polar solvent. The absorption spectrum of the dimer obeys the Beer-Lambert law up to 9 X IO4 M in both toluol and DMSO. Above this value, deviation occurs more significantly for the phthalocyanine moiety Q band absorption. Aggregation or dimerization seems to occur via phthalocyanine chromophores. This result is consistent with the known tendency of these compounds to easily aggregate in water and organic solvent^.^^'^ Photophysical Properties. Two very different behaviors, depending on the nature of the solvent, are found upon excitation of the heterodimer. In toluol, the excitation processes, emission processes, and different radiationless decay pathways of the bichromophoric ( 7 ) Mauzerall, D. Biochemistry 1965, 4, 1801. Kagan, N. E.; Mauzerall, D.; Merrifield, R. B. J. Am. Chem. SOC.1977, 99, 5484. (8) Collman, J. P.; Elliot, C. M.; Halbert, T. R.; Tovrog, B. S . Proc. Natl. Acad. Sci. U . S . A . 1977, 74, 18. Chang, C. K.; Kuo, M. S . ; Wang, C. B. J. Heterocycl. Chem. 1977, 14, 943. Dubowchik, G. M.; Hamilton, A. D. J. Chem. Soc., Chem. Commun. 1986, 665. (9) Gaspard, S.; Viovy, R. J. Chim. Phys. 1979, 76, 751. (IO) Harriman, A,; Richoux, M. C. J. Chem. Soc., Faraday Trans. 2 1980,
76, 1618.
Tran-Thi et al. molecule can be summarized by the energetic diagram of Figure 3. Selective excitation at 424 nm (or 532 nm) induces the formation of a m*singlet state of the porphyrin moiety, SIp,which undergoes either an intersystem-crossing process (khp) to the TT* triplet state (TIp) or an energy-transfer process (kEt”) to the phthalocyanine moiety, in competition with the radiative decay ( k y ) and the internal conversion process (ki:). As the energy * lie below the levels of the l m * and 3 ~ of~ phthalocyanines porphyrin SIpand TIPlevels, one could expect the energy-transfer processes to occur in both excited states. Moreover, there is a considerable overlap between the room-temperature fluorescence spectrum of ZnTTP and the ground-state absorption spectrum of ZnPctBu, indicating that the energy transfer should be extremely efficient. The Forster equation can be used to determine the average donor-acceptor distance (R,)at which the rate constant for energy transfer ( k E t Sequals ) the rate constant for nonradiative decay of the donor:13
$Jr(ZnTTP)is the fluorescence quantum yield of ZnTTP in the absence of the acceptor, n is the refractive index of the solvent ( n = 1.494 for toluol), N Avogardro’s number, @ the orientation factor, with f? being equal to 2/3 for a random di~tributi0n.l~ For a mixture of ZnTTP and ZnPctBu monomers in toluol, the numerical value of the overlap, 0, is
R = S F D ( A ) tpc(X)A4 dA = 5.2
X
mol-’ cm6
whereas
fi = @(A)
€ p c ( ~ ) ( A ) A 4dX
= 6.1 x lo-’’ mol-’ Cm6
for the heterodimx. Both values are one order of magnitude higher than those found for porphyrin dimers by Brookfield et al. and by Anton et al. ( Q = 4.97 X lo-” mol-l cm6 (ref 14) and fl = 2.2 X lo-” mol-’ cm6 (ref 2)) for H2TTP-O-(CH2).-O-ZnTPP. This result is consistent with the much higher extinction coefficient values of ZnPctBu in comparison with those of H,TTP over the 500-700-nm wavelength domain. Substituting these values into eq I results in Ro values of 35.7 and 36.7 A, respectively, for the couple of monomers and for the heterodimer, showing that the singlet energy transfer can occur over quite large distances. Excitation of a 1/1 molar mixture of ZnTTP and ZnPctBu at 424 nm gives rise to the unique fluorescence of ZnTTP* (A, = 600 and 650 nm), while excitation at 650 nm gives the = 682 and 755 nm). Both fluorescence of ZnPctBu (A, fluorescence yields are identical with those found for the corresponding monomers. Despite the high values of the overlap integral, this result is not surprising if one takes into account the very low concentration of the reactants (9 X lo-* M). With such a concentration, the average distance between two molecules is given by
The maximum diffusion length ( L ) of the reactants in liquid can be calculated by knowing the diffusion rate constant D (D= 10” A2 s-’) and the singlet lifetime ( T ~ of) the donor molecule: L (2Drs)’/’ = 20 8, Therefore, neither fluorescence quenching by dipole-dipole interaction nor quenching by collision can occur in dilute mixtures ( 1 1) Vincett, 55, 4131.
P. S . ; Voigt, E. M.; Rieckhoff, K. E. J. Chem. Phys. 1971,
(12) Kalyanasundaram, K.; Neumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163, and references therein cited.
(13) Forster, T.Discuss. Faraday Soc. 1959, 27, 7. (14) Brookfield, R. L.; Ellul, H.; Harriman, A,; Porter, G. J. Chem. SOC., Faraday Trans 2 1986, 82, 219.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1229
Covalent Porphyrin-Phthalocyanine Heterodimer I
6 00
---
c, cn
O.D.
ZnTTP m o n o m e r
- ZnTTP
>
I
8 00
700
- +0.1
in t h e dimer
..... ZnPctBu
i n t h e dimer
f x l
300
..
. . I
200
.
100
A nm WAVELENGTH
(nm)
Corrected fluorescence spectra of ZnTTP monomer and ZnTTP and ZnPctBu in the heterodimer; &, = 424 nm, solvent = toluol. Figure 4.
I
I
500
600
I
I
I 1
Figure 6. Transient differential spectra at t = 20 ns of excited (-) ZnTTP (3.1 X lod M) in toluol, ;L, = 532 nm, and (---) ZnPctBu (1.75 X lod M) in toluol, A,, = 355 nm.
with
being the singlet lifetime of ZnTTP* monomer. @(D) measured from the normalized and corrected fluorescence spectra by using the known value (#f = 0.033) of the standard compound (ZnTPP in t ~ l u o l ) 'are ~ found to be respectively equal to 0.034 f 0.002 and 0.0045 f 0.0005 in toluol. The singlet lifetime was obtained from single-photon-counting data, and the 7,' value, which is equal to 2.00 f 0.05 ns in toluol, is identical with that found for ZnTPP (2.02 ns in benzene and 1.94 ns in dimethylf~rmamide).'~Therefore 7:
@ and
kE~S(tolUO1)= (3.3 f 0.6) x logs-' &tss(tolUO1) = 0.87 f 0.03 One can also calculate k p and estimate the singlet lifetime value of excited ZnTTP in the dimer:
(7:(D))
WAVELENGTH , A , (nm) Figure 5. ZnTTP-0-ZnPctBu in toluol: (- .-) fluorescence spectrum (A,, = 424 nm); (A)electronic absorption spectrum; (-) fluorescence excitation spectrum (Aern = 685 nm).
of monomers. In contrast to this phenomenon, with selective excitation at 424 nm of the heterodimer, a drastic decrease of the ZnTTP fluorescence is observed concomitant with a slight increase of the ZnPctBu fluorescence, which indicates a very efficient intramolecular energy-transfer reaction (Figure 4). The excitation spectrum (Figure 5 ) and the ground-state absorption spectrum are nearly identical: this indicates that the energy-transfer process is nearly quantitative. From the fluorescence quantum yield values of the ZnTTP* chromophore in the monomer ('#'') and heterodimer ('#'p(D)), one can determine kEtSand dE? by the following equations derived from the kinetic scheme of Figure 3:
7:(D)
=
((T:)-'
+ k ~ t ~ ) =- ' (2.6 f 0.5) X lo-''
S-'
= 260
PS
The lifetime of the resulting ZnPctBu singlet state in the dimer with that obtained in the monomer, while the fluorescence quantum yield is nearly the same ('#'$(D) = 0.24). These results suggest that the energy-dissipative pathways of ZnPctBu in the dimer are not very different from those in the monomer. Energy-transfer processes can also occur via the triplet states of excited porphyrin. The T-T energy-transfer quantum yield can be determined if one knows the triplet quantum yield of each chromophore in the dimer. To this end, nanosecond absorption spectroscopy was used to establish the time-resolved spectra of transient species generated during laser excitation of the heterodimer and the corresponding monomers. From the differential spectrum that results from ground-state bleaching and the T-T triplet absorption (Figure 6), the triplet excited-state concentration can be deduced by using the different following procedures.
(7p(D) = 3.3 f 0.1 ns) is identical
(15) Egorova, G. D.; Knyukshto, V.N.; Solovev, K. N.; Tsvirko, M. P.Opt. Spekrrosk. 1980, 48, 1101.
1230 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1
1
Tran-Thi et al. TABLE I: Spectroscopic Data
- A -
-A
'(ZnTTP! in Toluol
- '(ZnTTPr
in DMSO
compd ZnTTP
toluol
solvt
ZnTTP
DMSO
ZnPctBu
toluol
ZnPctBu
DMSO
ZnTTP-0-ZnPctBu
toluol
ZnTTP-0-ZnPctBu
DMSO
Y.
- 0 - '(ZnPctBufm Toluol - r - 3 ( ~ n ~ c t ~ u ) *DMSO m
-
-
A rm&
I
I
I
I
Figure 7. Triplet-triplet absorption spectra of excited (A)ZnTTP in toluol, (A)ZnTTP in DMSO, ( 0 )ZnPctBu in toluol, and (+) ZnPctBu in DMSO.
ZnTTP Monomer in Toluol. The molar extinction coefficients of the ZnTTP triplet state can be obtained at the isosbestic points of the ZnTTP* differential absorption spectrum: tT406
=
tSo406
= (4.4 f 0.3) x io4 M-I Cm-l
tT436
=
tSo436
= (5.4 f 0.4) X lo4 M-I cm-'
tT543= t s ~ 5 4 = 3 tT563
=
ES0563
tSoSS0
(1.9 f 0.2) X IO4 M-' cm-I
= (1.4 f 0.1) X lo4 M-l cm-I
= (2.6 f 0.2) X IO4 M-I cm-I
Assuming that the triplet molar extinction coefficient changes linearly over the 543-563-nm domain, its value at 550 nm can be calculated by linear interpolation6 tTSSO =
( 1 . 7 f 0.2)
X
lo4 M-I cm-I
The concentration of triplet states and the triplet quantum yield can therefore be deduced from the differential optical density at 550 nm: AOD550 =
(16) Le Roux, D.; Mialocq, J. C.; Anitoff, 0.;Folcher, G. J . Chem. Soc., Faraday Trans 2 1984, 80. (17) Pekkarinen, L.; Linschitz, H. J . Am. Chem. SOC.1960, 82, 2407. Hurley, J. K.; Sinai, N.; Linschitz, H. Phofochem. Phorobiol. 1983, 38, 9. Gurinovich, G . P.; Jagarov, B. M. Luminescence of Crystals, Molecules and Solurions; Plenum Press: New York, 1973. (18) Harriman, A.; Porter, G.; Richoux, M. C. J . Chem. Soc., Faraday Trans. 2 1981. 77, 833.
e. M-' cm-I 5.4 i 0.2 x 105 2.6 i 0.2 x 104 6.9 i 0.2 x 105 2.7 i 0.2 x 104 1.7 i 0.1 x 104 5.2 i 0.2 x 104 3.1 i 0.2 x 104 2.0 i 0.1 x 105 6.2 i 0.2 x 104 3.1 f 0.2 x 104 1.94 i 0.1 x 105 7.4 i 0.3 x 104 4.0 i 0.2 x 105 2.4 f 0.2 x 104 3.2 f 0.2 x 104 1.8 f 0.1 x 105 7.1 f 0.3 X IO4 4.3 i 0.2 x 105 2.1 i 0.2 x 104 3.5 i 0.2 x 104 1.7 i 0.1 x 105
TABLE II: Fluorescence Data
compd ZnTPP ZnTPP ZnTTP ZnTTP ZnPc" ZnPc ZnPctBu ZnPctBu ZnTTP + ZnPctBu ( 1/ 1)
solvt toluol
ZnTTP-OZnPctBu ZnTTP + ZnPctBu
ZnTTP-0-
bf or 6hD) ~ s ns , 550 600 0.033 (ref 15) 2.7 (ref 10)
Xem
XI,
nm
nm
650 DMSO 550 608 661 toluol 550 600 650 424 600 DMSO 550 610 663 580 610 EtOH 610 674 743 DMSO 610 680 748 toluol 610 682 755 350 682 DMSO 610 685 755 350 685 toluol 424 600
CTPl(tT550 - t'SS0)
CTpwas determined for two different solutions (C, = 3.1 X 10" and 1.5 X M). The ZnTTP triplet-triplet true absorption spectrum was reconstructed6 at t = 20 ns over the 350-650-nm wavelength domain (Figure 7 ) . At the maximum, the extinction coefficient value was found to be equal to 6.9 f 0.4 IO4 M-I cm-I, which is in agreement with those given in the literature for ZnTPP and its derivative^'^,^' (see Table 111). The relative actinometry method6 with ZnTPP as the reference compound was used to determine the quantum yield of ZnTTP. Taking 4 = 0.88 for ZnTPP and the corresponding extinction coefficient values at 470 nm of 3ZnTPP* and 3ZnTTP* (Table 11), the quantum yield of ZnTTP in toluol was found to be 0.88 f 0.03, in agreement with the values given in the literature for ZnTTP in toluolIs and for substituted zinc porphyrins in various
nm
424 550 428 560.5 600.5 350 610 677 350 610 678 350 425 550 612 679 350 428 560 615 680
toluol
0.039 0.034 2.0 f 0.05 0.035 1.75 f 0.05 O.2Ob 0.225 0.23 3.3 i 0.1 0.18 3.1 f 0.1 0.034
650 650 685 0.23 755 424 600 0.0045
685 0.24 756 DMSO 424 610 0.035 663 650 685 0.18 755 580 663
0.26 f 0.04c 3.3 i 0.1 0.23 i 0.04c
2.05 f 0.05
ZnPctBu
686 755
0.33 i 0.03 2.1 f 0.05 0.33 f 0.03 2.1 += 0.05
"Standard Compound. bValduga, G.; Reddi, E.; Jori, G. J . Inorg. Biochem. 1987, 29, 59. 'Calculated from the fluorescence yields ($I and $F(D)and the monomer singlet lifetime.
solvent^'^-^^ (see Table 111). The transient triplet excited state disappeared within 40 & 5 ps with a first-order kinetics law. ( 1 9) Dzhagarov, B. M. Bull. Acad. Sci. URSS, Phys. Ser. 1972,315,984. Translated from Ira. Akad. Nauk. SSSR, Ser. Fir. 1972, 36, 1093.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1231
Covalent Porphyrin-Phthalocyanine Heterodimer TABLE 111: Triplet-State Properties A,,
nm
nm
toluol
532
ZnTTP
DMSO
532
ZnTPP
t0lUOl”J~
530
470 480 408 470 470
104emarT, M-l cm-l 6.9 T 0.4 6.9 T 0.4 4.0 F 0.1 4.1 F 0.1 7.3
ZnTPyrP+ ZnPc
water16 DMSO
532 355
460 480 490
6.6 3.0 f 0.1 3.0 f 0.1
ZnPc ZnPc ZnPctBu
chloronaphthaIeneZo water pyridine12 toluol
480
3.4 f 0.4
480 490
2.8 f 0.2 2.8 f 0.2
ZnPctBu ZnTTP-0-ZnPctBu
DMSO toluol
490
2.8 f 0.2
compd ZnTTP
solvt
AmarT,
TT.
9h
W
0.88 f 0.03
40 f 5
0.88 f 0.03
1500 f 200
0.83 f 0.4 0.88 (ref 18) 0.89 0.65 f 0.02
1400 1800 330
* 25 330 * 25
630
+
ZnTTP-0-ZnPctBu
DMSO
355 630 355 630 532
245 80 f 5 80 f 5 350 f 20 75 f 5 12f2 75 5 620 f 60
0.65 f 0.01 0.12 f 0.02p 0.68 f O.OIR 0.078” 0.008p
532
It is possible to deduce the rate constant of the intersystem conversion process for ZnTTP* from the triplet yield value: Pisc
0.65 0.56 0.66 f 0.02
I
I
400
500
1
1
6 0 0 nm + 0.1
= $ i s c / T s P = (4.4 f 0.2) x 108 s-I
ZnPctBu Monomer in Toluol. The transient differential absorption spectrum of excited ZnPctBu in toluol is shown in Figure 7. In the present case, the assumption that &a77 = 2 X 10’ M-I cm-’ >> tT677 can be made, and the triplet quantum yield can directly be deduced from the differential optical density at 677 nm:
W
- c ~ ~ ~ l € ~ ~ 6 7 7 AOD677 = CTPCl(tT677- 6‘677) The triplet true absorption spectrum was reconstructed (Figure 7) over the UV and visible domain. The extinction coefficient value at the absorption maximum, 6480 = 6490 = (2.8 f 0.2) X lo4 M-l cm-l, is not very different from those obtained for 3ZnPc* in various media (see Table 111). The relative actinometry method with ZnPc as the reference compound (@T = 0.65) gave $TPC
=
$is?
= 0.66 f 0.02
in agreement with the literature values found for ZnPcZ0and its derivatives (see Table 111). The intersystem crossing rate constant kPCiscis found to be (2.10 f 0.12) X lo8 s-l. The ZnPctBu triplet excited states disappear in toluol within 80 f 5 p s with a first-order kinetics law. ZnTPP-GZnPctBu in Toluol. The transient differential absorption spectrum of the excited heterodimer is shown in Figure 8. It appears as the sum of both ZnTTP and ZnPctBu triplet states as confirmed by the kinetics data. Two monoexponential decays can be deconvoluted: the shorter lifetime obtained (1 2 f 2 ps) is attributed to the ZnTTP* triplet state partially quenched by ZnPctBu, whereas the longer lived species (75 f 5 ps) is attributed to ZnPctBu* triplet state. The proportion of each transient species has to be determined. To this end, the assumption that tSo679(ZnPCtBU) >> €T679(ZnPCtBU) + was made. Therefore, from the differential optical density at 679 nm, the total ZnPctBu triplet yield in the heterodimer was deduced: @TPc(D)
= 0.68 f 0.02
1=680nrn lo= 5 8 mv 50mvi 20ps
Figure 8. Time-evolved differential absorption spectrum of excited ZnTTP-0-ZnPctBu (7.9 X lod M ) in toluol obtained at low laser energy (A,, = 532 nm, 10 mJ) and transient kinetics decay at different wavelength: (-) t = 20 ns; (---) t = 40 ks; t = 80 ps. ( e . . )
density AODx at X = 424 nm, by using the eTP(424 nm) and tTpc(424nm) determined for each monomer (Figure 7):
= /CTP(tTP(424) - ~“(424))
AOD424
with
6 ~ ‘ ~ ‘=~(@Etss$iscPc) ) + (discP(D)$EtTT) (11) The porphyrin triplet quantum yield in the heterodimer, &P(D), was determined by two methods. In the first case, the porphyrin triplet concentration was calculated from the differential optical (20) Jacques, P.;Braun, A. M . Hefu. Chim. Acta 1981, 64,1800.
+ ICTPC(tTPC(424)- tsp(424))
The ZnTTP triplet quantum yield in the heterodimer is $TP(D)
= 0.1 1 f 0.02
In the second method, one assumes that kiscP(D)
and therefore
=
= 4.4 x 108 s-1
1232 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 @(D)
Tran-Thi et al.
= k.1SCP(D)TSP(D) = 0.12 f 0.02
I
in agreement with the previous value extracted from the differential spectrum of excited ZnTTP-O-ZnPctBu. One is now able to calculate the quantum yield of the triplet-triplet energy-transfer process using eq 11:
i
1
1
I
1
I
I
OaD.
4Etrr = 0.88 f 0.26 The 4 ~value , can ~ also be obtained from the triplet decay lifetime values of the excited ZnTTP chromophore in the dimer (7TP(D) and in the monomer ( T ~ ~ Taking ) . into account the different decay pathways of 3ZnTTP*
EFT = ( ~ T P ( D ) ) - ' - (7r~1-1= (5.8
2.0) x 104 s-1
(111)
4 ~ := ~kEtrrTTP(D) = 0.70 f 0.40 which is in agreement with the previous found value of 0.88 f 0.26. Both singletsinglet and triplet-triplet energy-transfer processes from the excited porphyrin moiety to the phthalocyanine chromophore are found to occur efficiently in toluol. They appear as the main route of disappearance of the ZnTTP excited states in the heterodimer. Once the energy is transferred, the energy dissipative pathways of the singlet and triplet states of the ZnPctBu* moiety remain the same as those of ZnPctBu* monomer as indicated by the unchanged singlet and triplet lifetimes of ZnPctBu* in the heterodimer. Effect of a Polar Solvent. Dimethyl Sulfoxide. As in toluol, a drastic decrease of the ZnTTP* fluorescence quantum yield was observed when the porphyrin moiety was selectively excited at 424 nm. However, in the present case, the energy-transfer process as the main route of the porphyrin singlet excited-state decay can be ruled out: in contrast to the enhanced fluorescence observed for the acceptor chromophore in toluol, the fluorescence quantum yield of the phthalocyanine moiety in DMSO dramatically dropped to 0.038. Moreover, the excitation spectrum and the ground-state absorption spectrum are in poor agreement, indicating that the singlet-singlet energy-transfer process is inefficient. Two other mechanisms could be responsible for the drastic decrease of the fluorescence quantum yield of both the donor and acceptor: (i) the spin-orbit coupling of ' ( x , x * ) and 3(7r,s*) states of porphyrin due to the phthalocyanine attachment and vice versa; (ii) the electron-transfer process and the formation of short-lived biradical intermediates. A spin-orbit coupling effect'' should decrease the fluorescence quantum yield and the triplet lifetime while increasing the triplet yield of the porphyrin and phthalocyanine in the heterodimer. Nanosecond absorption spectroscopy experiments were performed, with DMSO as the solvent, to measure the triplet lifetime and quantum yield of porphyrin and phthalocyanine triplet excited states of the corresponding monomers, in comparison with that of the heterodimer. The singlet So depletion method was used, as for toluol, and from the differential absorption spectra of ZnTTP* and ZnPctBu* monomers in DMSO (see Figure 9), we found
SOLVENT: DMSO
1
1
x nm
..
,
400
,
500
600 I
I
I
Figure 9. Transient differential absorption spectra at 20 ns of (-) ZnTTP (2.4 X IO" M ) in DMSO, &, = 532 nm, and (---) ZnPctBu (4.1 X 10" M ) in DMSO, Lc= 355 nm.
-
(XD. *.,
. . . .. . .
....
.*..e.
:
..e.
*
I
....a-.
;
mol *
2.
.
.. .. .
'
'.. .: .. .,.
I
I.
**
e .
*
-0.02
-0.04 Solvent I
~
.. ... .. .. .. . ._ ... ... ... .. .. .. .. .. .. ... ..... -
DMSO
ZnTTP-o-ZnPctBu
-0.06
+,,P(DMSO) = 0.88 f 0.03 &,PC(DMSO) = 0.65 f 0.02
&,seems to remain solvent independent. On the contrary, both triplet states are longer lived in a polar medium: T~~(DMSO = )1.5 -+ 0.1 ms T~"(DMSO) = 350 f 20 ws These values are in agreement with those obtained for positively or negatively charged substituted ZnTPP in aqueous and for ZnPc and substituted compounds in polar organic solvent and aqueous media.I0 The differential absorption spectrum of excited ZnTTP-OZnPctBu in DMSO established at nanosecond time scale (Figure 10) resembles the one obtained in toluol (Figure 8). However, the differential absorbances of the transient species generated in
400
500
600
A nm I
Figure 10. Transient differentia1 absorption spectrum of 2nTTP-OZnPctBu (3.7 X 10" M) in DMSO, A,, = 532 nm.
DMSO are an order of magnitude lower than those found in toluol for the same heterodimer concentration. In the present case, the total ZnPctBu* triplet quantum yield would be 0.078. On the basis of the very low triplet quantum yield of both ZnTTP* and ZnPctBu*, the hypothesis of the existence of a spin-orbit coupling effect in the heterodimer can be ruled out. Moreover, the ZnPctBu* singlet-state decay curves at 686 and 755 nm obtained from the time-resolved fluorescence experiments present two components, a short lifetime (0.33 f 0.03 ns) and a long one (2.1 0.05 ns).
*
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1233
Covalent Porphyrin-Phthalocyanine Heterodimer SCHEME I
hv
P-0-Pc
Et
I +
P -0-Pc
Direct excitation of P gives 1
*
P-0-Pc 1
P-0-Pc
*
9s
+Et
lisc +isc
1 *
-
I *
-
et
P -0-Pc
ISC
P -0-Pc
3
P-0-
+T 9s
p+-o-pcor P--o-Pc+ 3
Pc(D)
PC(0)
*
Pc
*
=O12fO06 = 0.65f0.02
=0
= O 8 4 f 0 30
+TP(D) = 0
P -0-Pc
078 f 0 020
008 f 0 002
IEf 3
P-0-
*et hv
P-0-Pc 1
Pc
*
P-0Pc Direct excitation of Pc gives P-o--IPC* P+-o-Pc-
+et
ss
= 0.84 f 0 30
or
p--op-o-Ipc*
!% p-0-
PC+ 3 pc*
Pc(D) =
+lSC
0 65 f 0 02
The short lifetime would result from the charge-transfer reaction occurring upon excitation of the ZnTTP chromophore leading either to the ZnTTP+-0-ZnPctBu- or ZnTTF-0-ZnPctBu’ biradical species. As both chromophores are excited at 580 nm, the long-lived species can be attributed (i) to the ZnPctBu* singlet state obtained upon direct excitation of the ZnPctBu chromophore, (ii) to the ZnPctBu* singlet state resulting from the energy-transfer process from ZnTTP* to ZnPctBu, or (iii) to both processes i and 11.
From the lifetime values, one is able to determine k,,SS(ZnPctBu*) = 2.6 f 0.4
X
lo9 s-’
+,ts(ZnPctBu*) = 0.84 f 0.3 which are consistent with those calculated from the ZnTTP* singlet lifetime and fluorescence quantum yield values: k(et+Ety(ZnTTP*)= 3.8 f 0.6
X
lo9 s-’
4(et+Et)SS(ZnTTP*) = 0.87 f 0.3 The existence of conformers in DMSO is suggested since it seems that energy transfer from ZnTTP* to the ZnPctBu moiety, despite being inefficient, can also occur in competition with the electron-transfer process. The differential spectrum of Figure 10 (to be compared with Figure 8) corresponds to the sum of the ground-state bleaching of ZnTTP and ZnPctBu and of the T-T triplet absorptions of the residual ZnTTP* and final ZnPctBu* resulting from the S-S and T-T energy-transfer processes, as found in toluol. The single observed lifetime 620 f 20 ps would correspond not only to the ZnTTP* triplet-state lifetime shortened by intramolecular quenching by ZnPctBu, but also to the ZnPctBu triplet-states lifetime becoming longer because of the very low triplet yield of ZnPctBu* in the dimer (see Table 111). The ZnPctBu* triplet self-quenching by a ZnPctBu ground-state
molecule is one of the possible decay pathways of ZnPctBu* triplet states, since the rate constant of disappearance, measured in the case of the corresponding monomer, is ZnPctBu-concentration dependent. The efficiency of the T-T energy transfer is (59 f 12)% according to eq 111, whereas for the singlet excited state, dEtS (calculated from eq 11) is found to be low, (12 f 6)%. The different decay pathways of the excited ZnTTP-0-ZnPctBu in DMSO are summarized in kinetic Scheme I. The difference of behavior observed when going from toluol to a polar solvent could be explained in terms of conformational change of the heterodimer due to the nature of solvent interactions. The use of space-filled molecular model allows one to show that due to free motion of the ZnPctBu chromophore around the ZnTTP-O- and ZnPctBu+ axes, the heterodimer conformation can vary from an extended conformation with the two chromophores well separated in parallel planes to a folded conformation which allows the overlapping of the orbitals of one of the phenyl group of each moiety (Figure 1). The first boundary conformation (a) would correspond to that in toluol and the second (b), which favors the intramolecular charge-transfer process, would prevail in DMSO. Such solvent-dependent behavior has been found for the copper(I1)-free-base porphyrin dimer covalently linked by a flexible chain.21 A drastic shortening of both the free-base singlet- and triplet-state lifetimes in the dimer, concomitant with an enhanced triplet yield of the free-base moiety, have been reported. Despite the fact that the electron-transfer process has not been completely rejected, these results have been explained in terms of energytransfer process from the Cu moiety to the H2 moiety with an enhanced electronic relaxation process for the final free-base triplet state due to the proximity of the Cu porphyrin. In contrast to these results, when energy transfer occurs in the ZnTTP-0-ZnPctBu heterodimer, only the donor singlet- and triplet-state lifetimes are shortened, whereas both the singlet- and triplet-state lifetimes of the acceptor chromophore remain unchanged. The difference of behavior might result from the existence of conformers due to the flexibility of the - 0 - ( C H 2 ) 4 chain. A partially face-to-face conformer would favor the charge-transfer decay pathway. Further experiments using a femtosecond absorption spectroscopy setup are foreseen to directly detect the transient biradical species ZnTTP+-0-ZnPctBu- or ZnTTP--0-ZnPctBu’ in DMSO. In comparison with the covalently linked heterodimer, studies of fact-to-face heterodimers obtained either by pairing cationic (or anionic) porphyrins with anionic (or cationic) phthalocyaninesZ2 or from amphiphilic porphyrin and phthalocyanine mixtures in the Langmuir-Blodgett thin films are in progress. Acknowledgment. We are indebted to Dr. M. Rougee and Dr. T. Montenay-Garestier, heads of the laboratoire de Biophysique, Museum National d”istoire Naturelle, for allowing us to perform our first single-photon-counting experiments in their laboratory. Registry No. ZnTPP, 14074-80-7; ZnPc, 14320-04-8; ZnTTP, 19414-67-6; ZnPctBu, 113530-55-5; Z n T T P U Z n P c t B u , 106704-16-9. (21) Ohno, 0.; Ogasawara, Y.; Asono,M.; Kajii, Y.; Kaizu, Y.; Obi, K.; Kobayashi, H. J . Phys. Chem. 1987, 91,4269. (22) Gaspard, S.;Tran-Thi, T. H. J. Chem. Soc., Perkin Tram., submitted. Tran-Thi, T. H.; Gaspard, S. Chem. Phys. Leu. 1988, 148, 327.
