J. Phys. Chem. 1992,96, 1073-1082
of 50-90 cm-’. Using the potential energy surfaces and the sfi-orbit coupling elements, we estimated the probability of the A’AJB intersystemCroaping in terms of the Landau-Zener model. The transition pbability is fairly large when the trajectories cross over the A~A-~B scam near the exit channel. It was also c ” e d that most of the trajectories cross over the seam near the exit channel before dissociating by rough classical trajectory calculations. considering that the intersystem CfOBping from A’A to ’B affects the photodissociation dynamics appreciably, one can explain the preference of A-type components and its amelation to the product rotation. The product OH dissociated from the lowest ’B is thought to show the preference of A-type components, because the ’B state has strong uu* character, which correlates to the fragments with unpaired e l m orbitals pointingtoward the 0-0 axis in high J. Moreover, the correlation of the ratio P(A”)/P(A’)
1073
and the product rotation J is consistent with the experimental finding about u-J correlation that in low J $0(22) = 0, which indicates little v-J correlation, but that in high J $0(22) > 0, the distinct tendency of u 11 J. Now we are planning to define the potential energy surfaces on the general configuration by removing the restriction of C, symmetry and to perform the calculation of quantum mechanical dynamics.
Acknowledgment. We thank h f c s s o r Morokuma for valuable comments on this topic. We also thank Dr. Fujimura for stimulating discussion. Numerical calculation was performed at IMS Computer Center and Data Recessing Chter of Kyoto University. This work was supported by the Grant in Aid for Scientific Research from the Ministry of Education.
Effect of the Exciton Coupling on the Optical and Photophysical Properties of Face-to-Face Porphyrin Dimer and Trimer. A Treatment Including the Solvent Stabilization Effect T. H.Tran-Thi,* J. F. Lipskier, Centre #Etudes Nucltaires de Saclay, DSM/DRECAM/SCM, CEA-CNRS UA 331, Laboratoire de Photophysique et de Photochimie, 91 191 Gif sur Yvette Cedex, France
P. Maillard, M. Momenteau, Institut Curie, Section Biologie, URA 1387 CNRS, Universitt de Paris Sud, Orsay. France
J.-M. Lopez-Castillo, and J.-P. Jay-Grin DJpartement de Mgdecine Nucltaire et de Radiobiologie, Facultt de Mtdecine, Universitt de Sherbrooke, Sherbrooke, Qutbec, Canada JI H S ” (Received: December 28, 1990; In Final Form: August 12, 1991)
A treatment of the exciton coupling interaction including the solvent stabilization effect is given. The method is successfully applied to porphyrin dimer and trimer: the maxima of the Soret bands can be. predicted with a high accuracy. The effect of exciton coupling on the photophysical properties of basket handle cofacial, doubly linked porphyrin dimer and trimer is investigated and compared to that in the singly linked dimer. Exciton coupling is very weak for the latter, whose chromophores behave independently from each other: the quantum yields of the singlet decay pathways (fluorescence, intersystem crossing, internal conversion) remain approximately the same as in the monomer. For the faceto-face compounds, the drastic decrease of the fluorescenceand triplet yield could not be entirely explained in termsof the unique exciton coupling effect. The existence of a low virtual charge-transfer state lying near above the lowest singlet excited state is postulated and discussed. It allows us (i) to explain the increase of the internal conversion rate constant at the expense of the other radiative and radiationless processes and (ii) to account for the invariance of the singlet and triplet lifetimes of excited dimer and trimer with respect to the monomer.
Introduction Dimers and oligomers of porphyrins and phthalocyanines have been subjected to extensive investigationsin the past two decades, partiaUy in relation to the green plant and bacterial photosynthetic reaction Centers, and also because such assemblies can provide promising properties in the search of new molecular materials for electronic devices. Most of the dimers and oligomers are either linked via a metal’ or via semiflexible chains or rigid spacers2 with, in some cases, -(1) (a) Buchlcr, J. W.; de Cm,A.rFischcr,J.; Kihn-&tulinsld, M.; Weiss, R. Itwrg. Chem. 1988,27,339-345. (b) de Cian, A.; Moussavi, M.; Fischcr, J.; Webs, R. I w g . Chem. 1985,243162-3167. (c) Kadish, K. M.; Liu, Y. H.; Anderson, J. E.; Charpin, P.; Chcvrier, G.; Lance, M.; Nierlich, M.; Vignier, D.; Dotmond, A.; BcLalem, E.; G u i l d , R. J. Am. Chem. Soc. 1988, 110,6455-6462. (d) Markovitei,D.; Tran-Thi, T. H.; Even, R.; Simon, J. Chcm.Phys. Lett. 1987,137,107-1 12. (e) Lachlrar, M.; de Cian, A,; Fischer, J.; W e h , R. Nnv J . Chem. 1988,12,729-731. (f) Diel, E. N.; Inabc, T.; Jaggi, N. K.; Lyding, J. W.; Sheide, 0.;Hanack, M.; Kannewurf, C. R.; Marks, T. J.; Schwartz, L. H. J. Am. Chem. Soc. 1984,106. 3207-3214.
the two subunits facing each other.3 In mixed doubly linked dimers, intramolecular electron-transfer reactions in the singlet excited states were shown to be very fast processes giving rise to very short lived biradical species? while in the singly linked ones, energy transfer can significantly compete with a charge-transfer proms, depending on the nature of the two subunits, the chain length, and the s o l ~ e n t . ~ In singly linked pure dimers, bearing (2)(a) Boxer, S. G.; Bucks, R. R. J. Am. Chem. Soc. 1979, 101, 1883-1885. (b) Hunter, C. A.; Nafees Meah, M.; Sanders, J. K. M. J. Chcm. Soc., Chcm. Commun. 1988,692-696. (3) (a) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A. Tetrahedron Lerr. 1982,23,3215-3218. (b) Neumann, K.H.; Vogtle, F. J. Chem. Soc., Chem. Commun. 1988,520-522. (c) Abdalmudhi, I.; Chang, C. K. J . Org. Chem. IWU,50,411-413. (d) A n d e m , H.; Sanders, J. K. M. J. Chcm.Soc.. Chcm. Commun. 1989. 1714-1715. (e) Nagata, T.;Osuka, A.; Maruyama, K. J . Am. Chem. Soc. 1990, 112,3054. (4) Fujita, I.; Fajer, J.; Chang, C. K.; Wang, C. E.; Bergkamp, M. A.; Nctzel, T. L. J. Phys. Chcm. 1982,86, 3754-3159.
0022-365419212096-1073$03.00/00 1992 American Chemical Society
1074 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
identical chromophores, it appeared from the work of Kaizu et al? and Gurinovich et al.’ that the singlet lifetime and fluorescence quantum yields are the same for the monomer and the corresponding singly linked dimer. Moreover, no drastic changes were observed for the triplet decay process from the kinetics of phosphorescence quenching? The same trend is observed for restrained porphyrin dimer and tetramer bridged by a rigid spacer! In the above systems, neither singlet to singlet nor triplet to triplet charge-transfer processes occur. The issue is less clear for the face-to-face compounds. In most of the cases, exciton interactions, based on the blue shift of the &ret band, were reported. This effect is often accompanied with an appreciable quenching of the fluorescence yield. Pure exciton coupling effect or chargetransfer states were altemately invoked to account for the fluorescence quantum yield decrease. However, in all the abovecited cases, neither singlet-singlet nor triplet-triplet absorption spectra were reported for these compounds, which could directly argue for the existence of charge-transfer states. Moreover, no attempt to predict the fluorescence quantum yield from the exciton coupling theory was found in the literature. The present work aims at elucidating whether a pure exciton coupling effect or (and) a charge-transfer process or other procesw can account for the change in the photophysical properties of a series of metal-free pure dimers and trimers with respect to the monomer properties. An important part of the work is devoted to the treatment of the exciton coupling interaction, including the solvent effect, whose objective is to predict (a) the spectral changes of the ground-state dimers and trimers and (b) the fluorescence quantum yield of the excited dimers and trimers from the monomer properties. The other part concerns the determination of the quantum yield of the radiative and radiationless deactivation pathways of the singlet and triplet excited states of these compounds. It also includes the analysis of their decay kinetics in order to clearly discriminate between the different possibilities.
Experimental Section Materials. The trans linked “basket handle” porphyrin H2 monomer, H4 dimer, and H6trimer (Figure 1) were synthesized using Collman’s methodtb modified by Momenteau et al. 12b~12c The singly linked dimer (PoC3o’P2) (Figure 1) was synthesized as described earlier,l3 The free base tetraphenylporphine (H2TPP) comes from Strem Chemicals. All the solvents from Merck, dimethyl sulfoxide (DMSO), dichloromethane (DCM), dichloroethane (DCE), toluene, and methanol (MeOH), were of spectroscopic grade. (5) (a) Schwarz, F. P.; Gouterman, M.; Muljiani, Z.; Dolphin, D. H. Eioinorg. Chem. 1972, 2, 1-32. (b) Anton, J. A,; Loach, P. A.; Govindjee. Photochem.Phorobiol. 1978,28,235-242. (c) Mialocq, J. C.; Giannotti, C.; Maillard, P.; Momenteau, M. Chem. Phys. Lett. 1984, 112, 87-93. (d) Brookfield, R. L.; Ellul, H.; Harriman, A. J . Chem. Soc., Faraday Trans. 2 1985, 81, 1837-1848. (e) Heiler, D.; McLendon, G.; Rogalskyj, P. J. Am. Chem. Soc. 1987,109,604-606. (f) Levanon, H.; Regev, A.; Das, P. K. J . Phys. Chem.1987,91,14-16. (g) Ohno, 0.;Ogasawara,Y.; Asano, M.; Kajii, Y.; Kaizu, Y.; Obi, K.; Kobayashi, H. J . Phys. Chem. 1987,91,4269-4273. (h) Osuka, A.; Maruyama, K.; Yamasaki, I.; Tamai, N. J. Chem. Soc., Chem. Commun. 1988, 1243-1245. (i) Tran-Thi, T. H.; Thiec, C.; Desforge, C.; Gaspard, S.J . Phys. Chem. 1989, 93, 1226-1233. (6) Kaizu, Y.; Maekawa, H.; Kobayashi, H. J . Phys. Chem. 1986, 90, 4234-4238. (7) Zen’kevich, E. I.; Shul’ga, A. M.; Sagun, E. I.; Chemook, A. V.; Gurinovich. G. P. J. Appl. Specrrosc. USSR.1985, 1023-1028. (8) Osuka,A.; Ida, K.; Maruyama, K. Chem. Len. 1989, 741-744. (9) (a) Chang, C. K. J . Hererocycl. Chem. 1977, 14, 1285-1288. (b) Chang, C. K.; Kuo, C. K.; Wang, C. K. J . Hererocycl. Chem. 1977, 14, 943-945. (10) (a) Dubowchik, G.M.; Hamilton, A. D. J . Chem. Soc., Chem. Commun. 1986,665-666. (b) Hunter, C. A,; Leighton, P.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1989, 541-552. (11) Kagan. N. E.; Mauzerall, D.; Marifield, R. 9. J . Am. Chem. Soc. 1977, 99, 5484-5486. (12) (a) Collman,J. P.; Elliot, C. M.; Halbert, C. M.; Tovrog, B. S.Prof. Narl. Acad. Sci. U.S.A. 1977, 74, 18-22. (b) Momemteau, M.; Mispelter, J.; Loock, B.; Lhoste, 9. J . Chem. Soc., Perkin Trans. 1 1985,221-231. (c) Seta, P.; Bienvenue, E.; Maillard, P.; Momenteau, M. Photochem. Phorobiol. 1989,49, 537-543. (13) Little, R. G.J . Hererocycl. Chem. 1978, 15, 203-208.
Tran-Thi et al.
Figwe 1. Schematic representationsof the basket handle porphyrins: (a) H2monomer; (b) H4dimer; (c) H6trimer; (d) P&3o’P.
Metbods. The UV and visible electronic absorption spectra of the porphyrins in various solvents were recorded with a Perkin Elmer Lambda 5 spectrometer. The concentrations of the solutions were varied from lo4 to 10” M. Fluorescence spectra recorded at room temperature with a Spex fluorimeter were automatically corrected for the lamp intensity fluctuation and for the monochromator-photomultiplier response. The porphyrin solutions were excited at 550 nm, and their fluorescence quantum yield was determined by the comparative method using H2TPP as the standard ~0mpound.l~ The fluorescence d a y times were measured by using the Edinburgh instrument 199 F time-correlated single photon counting system and a dye laser pumped by a frequency doubled Nd:YAG (Quantronix 76 MHz) as the excitation source. The excitation wavelength was 580 nm and the full width at half-maximum (fwhm) of the pulse was 700 ps. The decay times were determined using a nonlinear least-squares method by convoluting the instrumental profile with an assumed single or biexponential decay model. The x2 criterion of the weighted deviations and the autocorrelation function of the residuals were used as an indication of the quality of the fit. The nanosecond absorption spectroscopy set up comprising a Nd:YAG laser (6 ns fwhm pulse radiation) and a xenon arc as probing light has been described in detail elsewhere.15 All the samples were excited at 532 nm with variable laser intensities in order to obtain the required condition of a complete photobleaching of the chromophores. The transient differential absorption spectra were recorded as a function of time over the 350-900-nm domain. Results and Discussion Spectroscopic Properties. The electronic absorption spectra of the basket handle H2 monomer, H4 dimer, Hbtrimer, and singly (14) (a) Ohno, 0.;Kaizu, Y.; Kobayashi, H. J . Chem. Phys. 1985, 82. 1779-1787. (b) Seybold, P. G.;Gouterman, M. J . Mol. Specrrosc. 1%9,31, 1-13. (1 5 ) Tran-Thi, T. H.; Markovitsi, D.; Even, R.; Simon, J. Chem. Phys. Leu. 1987, 139, 207-211.
,
,
I
I
1
I
r
(O.D.)
- 1.0
J
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1075 J
I
l
I
I
l
I
I
l
I
I
TABLE I: Spectroscopic Properties of H2,H,,and H(
n
tMX,
,I
compd solvent H 2 DMSO
I1 ::::
- 0.8
:: ::
I
I I
: : : :
: : : :
I
: :
1
:
:
1 I
: : . .
1
- 0.6
1
I
in
DMSO
H4
Figure 2. Electronicabsorption spectra of the basket handle compounds. Optical path length I = 1 cm.
linked PoC3o’P are typical of Dlh symmetry free base porphyrins with an intense Soret or B band in the UV and four small Q bands in the visible whose intensities vary with the solvent. In Stern’s classification, based on the variation of intensity among the four Q bands, depending on the external substituent, the basket handle compounds are of the “phy1lo”-typewith the Q,(O,l) band more intense than the Q (0,O)one, whereas H2TPPis of the “eti0”-type with its four bands decreasing monotonously in intensity. The presence of substituents on the phenyl group of the macrocycle induces not only a red shift of both the Soret and Q bands for the free base monomer and PoC3o’P but also a decrease of the extinction coefficient value as compared to H2TPP. This effect, which is pronounced when DMSO is the solvent, was known to occur in substituted porphyrins. Longo et a1.I6 have previously noted that the strongest effect is obtained for ortho substituents on the phenyl group of TPP. In Figure 2 are superimposed the spectra of the basket handle mmpounds. A blue shift of the Soret band relative to the monomer is observed for the dimer (24.3 nm) and the trimer (8.7-8.9 nm) in both DCM and DMSO (Table I), whereas a very slight red shift takes place for the Q bands. This behavior has been observed for a large number of face to face compounds and predicted by the molecular exciton theory. The cases of dimers with transition dipoles in different arrangements have been examined by Kasha’’ and applied to porphyrins or porphyrin-like dimers.18 In the present work, we used the formalisms developed by Scherz et a1.,18 which were quite successful in reproducing the main spectroscopicproperties of dimers of bacteriochlorophyll and related molecules. The expressions explicitly consider the mixing of the four main excited states (Bx,By,Q,, Qy) into the excited states of the dimer. In order to account for the hyperchroism of the Q,, band of the bacteriochlorophyll dimer and the nonconservative circular dichroism, these authors have also considered the mixing of doubly excited states into the dimer ground state. As no experimental hyperchroism was found for the basket handle dimer and trimer, we neglect, in our calculations, the (16)(a) Kim,J. B.; Lconard, J. J.; Longo, F. R. J. Am. Chem. Soc. 1972, 94, 39863992. (b) Quimby, D.J.; Longo, F. R. J. Am. Chem. SOC.1975, 97, 5111-5117. (17) Kasha, M.Pure Appl. Chem. 1%5,11, 317-393. (18)Gouterman, M.;Holten, D.; Lieberman, E. Chem. Phys. 1977, 25, 139-153. Scherz, A.; Parson, W. W. Biochim. Biophys. Acra 1984, 766,
666-618.
H6
A, nm M-I cm-l
424.1 519.1 554.0 593.0 652.0 DCM 419.3 513.4 547.0 588.2 651.2 420.0 DCE 514.3 546.4 588.6 651.8 DMSO 419.8 519.6 554.6 594.0 653.2 414.4 DCM 515.1 549.3 588.0 651.0 414.9 DCE 514.8 549.5 587.3 644.3 DMSO 415.4 520.1 555.0 594.7 653.9 DCM 407.3 515.0 550.0 587.8 651 .O DCE 408.4 515.7 55 1 587.7 653
E, cm-l
290000 23579 16 200 19264 5 500 18051 5 700 16 863 3 900 15 337 279 600 23 849 18 500 19478 5 0 0 0 18 282 5 700 17001 3 200 15 356 28 1 000 23 810 17 200 19 444 4 600 18 302 5 920 16 989 4 200 15342 446 000 23 821 28 700 19 246 1 1 200 18031 11200 16 835 5 0 0 0 15309 393 000 24 131 28 000 19414 6 800 18 205 8 900 17007 2 700 15 361 418900 24 102 30 100 19425 7 200 18 198 9 600 17027 2 950 15 521 474 000 24 073 30 200 19 227 1 1 600 18018 10 500 16815 6 700 15 293 452OOO 24 552 32 400 19417 1 1 800 18 182 13 100 17013 7 500 15361 466 000 24 486 28 300 19 391 7 400 18 149 10900 17015 4 900 15314
&p”
Dit:
nm
14.4 26.8 24.9 29.9 26.8 16.0 23.6 21.0 22.8 20.0 17.0 24.2 28.0 23.5 22.0 21.8 25.0 28 24 27 24 23.5 22 22 20 23.4 23.8 26.0 22.5 24.0 27.0 26.0 30.5 29.0 25.0 29.0 26.5 26.0 33.7 29.0 26.2 24.0 27.0 24.0 27.0
1.966 0.333 0.099 0.115 0.064 2.125 0.339 0.076 0.088 0.039 2.266 0.322 0.094 0.094 0.056 9.227 0.550 0.225 0.180 0.082 9.068 0.509 0.109 0.133 0.033 9.413 0.554 0.136 0.147 0.044 12.274 0.601 0.254 0.204 0.102 12.822 0.664 0.222 0.299 0.133 11.910 0.525 0.144 0.177 0.081
f 1.11 0.07 0.02 0.02 0.01 1.10 0.07 0.02 0.02 0.01 1.17 0.07 0.02 0.02 0.01 2.38 0.11 0.04 0.03 0.01 2.37 0.11 0.02 0.02 0.01 2.46 0.12 0.03 0.03 0.01 3.20 0.13 0.05 0.04 0.02 3.41 0.14 0.04 0.06 0.02 3.16 0.11 0.03 0.03 0.01
“Fwhm. bDipolle strength. cOscillator strength.
mixing of the doubly excited states into the dimer ground state, but include the solvation effect which has not been treated in previous works. The solvation effect was often invoked to account for the discrepancies observed between the experimental and predicted values of the dimer absorption maxima. Although it has been admitted that the solvation term could be important enough to balance the blue shift due to the exciton coupling energy, the solvent shift term was often neglected because it is generally hard to estimate it. Our procedure to include the solvation energy term to the monomer and oligomers will be discussed in the “solvent effect” section. According to the exciton coupling theory, for weakly interacting chromophores, the ground and excited states of the dimer can be exptessed on the basis of those of the isolated monomer, using the perturbation method to take into account the interchromophorc Coulombic interaction. Briefly, the wave function and energy of the ground state of a dimer of identical, nonoverlapping, uncharged, and nonpolar molecule (A and B) are
EGO
2E0
and the first-order wave function of the Jth excited state of the dimer is
1076 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
where the basis set of wave function q q are ql = aA1@m,\k2 = @Ao@Bl, 93= @Al@BI, ..., qI2 = aAo@W The transition energies, Doo = Ej - Eoo and coefficients (CqJ)are obtained as the eigenvalues and eigenvectors of the secular equation (3) [ H p , q - ~ p , q ~ J l * [ c q J=l 0 where (H,,J is an element of the matrix hvl
VAI,BL
‘A1,Bl h 2 ‘AZ,Bl
‘A,,,,
0
‘Al,B2 VAZ,B~ h3
‘UB2
‘A232
hv4
v.&i
..*
0
*”
*”
. .*.
0
‘Al.B6
0 ‘A2,BtS
(4)
0 : h i 2
-
with u1 = vAl, v2 = YB1, u3 =: YA2, etc., p, is the electric transition Zero elements dipole for the molecular transition 9, aA0am indicate that the singly excited states of an individual molecule are not mixed if the other molecule remains in its ground state (in absence of any permanent dipole). Because regular porphyrin has zero permanent dipole moment in the ground and excited states, the interaction energies depend mainly on the transition dipole moment. The coupling energy between two excited states j and k of chromophores A and B, VAj,Bk, is expressed in the dipoledipole approximation by
where wMj is the electric transition dipole for the transition aW aMO and R the vector connecting the centers of molecules A and B. We have used a program written in Fortran to solve the coefficients and energy associated with each excited state of the dimer. The interaction energies (VNBk)are first obtained for the face to face geometry, using eq 5 . The Hamiltonian is then diagonalized. This procedure generates a 12 X 12 matrix of ooeffcients which are scaled to normalize the wave functions. The energies associated with each excited state of the dimer are obtained. The electric transition dipole, pJ, corresponding to the transition @ J aG0of the dimer can be calculated as a vector sum using the equation +
+
12
PJ
(*&A
+ P B I ~ O ) = CC q j k q
(6)
9- 1
The dipole length, D,,,, corresponding to a given transition can be calculated and compared to the experimental values obtained from the absorption spectrum
G being the degeneracy of the corresponding state. The Solvent Effect. The energies of the absorption maxima of cofacial dimers and trimers predicted from the exciton coupling theory are always higher than the experimental values. For the which %ret band, typical D values are around 1000-2000 an-’, are overestimated by a factor 5-10 compared to experimental values. The solvent effect is important enough to balance the exciton coupling effect and therefore should not be treated s e p arately. To include the solvent effect, one should (i) know the optical properties of the monomer molecule in the gas phase, (ii) be able to determine the nature of the solvent-solute interactions and quantify the stabilization energy. If one neglects specific interactions such as hydrogen bonding, the two molecular properties, multipole moments and polarizability, of the solute @MI CYM) and solvent &,as)yield four major interactions: (1) the dipoledipole (pM-&); (2) the solute dipolesolvent polarizability ( p M - a s ) ; (3) the solute polarizabili-
Tran-Thi et al. ty-solvent dipole (aM-&);(4) the polarizability-polarizability (arras). A fifth term is related to the transition dipole moment of the solute molecules, which behave like instantaneous dipoles and produce induced dipoles in the solvent. These interactions can be expressed in exact analytical forms using the simple model of a point dipole immerged in a sphere of radius a (solute), surrounded by a dielectric continuum (solvent).19 In the present case, since the permanent dipole moment of symmetrical porphyrins, either in the ground or in the excited state, are zero, the dipoledipole and solute dipole-mlvent polarization terms can be neglected. The random motion of the solvent dipoles around a nonpolar but polarizable solute can yield small, fluctuating electric fields which produce an induced dipole moment in the solute molecule. This interaction, which is also called ‘the solvent Stark effect” can be expressed as
Es - L Y M U - ~ ~ C-An2)) ~ ~ ~ ) (8) where L is a fluctuation factor, (I the radius of the solute cavity, Gut the static dielectric constant withf(Gut) = 2(c, - 1)/(2esut + 1) and fin2) is a function of the ‘optical” dielectric constant and equals 2(n2 - 1)/(2nZ + 1). The Stark effect is usually rather small and can be neglected.19 The interaction of two nonpolar but polarizable particles is the “dispersion interaction”which results from the fluctuations of their instantaneousdipoles. The corresponding stabilization energy is Es = - r ~ ~ ( ~ - ~ C f ( n ~ )
(9)
and the solvatochromic shift which results from the change in the solute polarizability between the ground and excited state is
= -(ac - ao)a-3CAf(n2)1-2 (10) Here Cis a fluctuating factor which is different from L and the subscript 1-2 correspond to solvents 1 and 2. The stabilization energy due to the interaction of the transition moment (pJ)of the solute with the induced dipoles of the solvent is given by Es
-pj2/2a3f(n2)
(1 1)
and the solvatochromic shift can be expressed as (&)1-2
= -~?/2a~Af(n~)~-~
(12)
The polarizability of porphyrins, either in the ground or in the excited state, is not given in the literature. The value of (a,aG) is thus hard to estimate. However, ab initio SCF-MO-CI calculations of both So and SIfor porphine and chlorophyll-like compounds show that the *-electron populations at each atom in the T system do not differ significantly between S, and SI.” One can therefore reasonably assume that their polarizability must not be very Werent and that the value of (a,- .o)must be small. On the contrary, the transition moments of porphyrins are high, especially for the &ret band. One can therefore assume that the stabilization energy will be mainly due to this last effect. The validity of this assumption is checked for the free base te~phylporphine,(H2TPP), whose ga~-phasespectrum ha9 been The solvatochromic shift of the &ret band of the H2TPP,from the gas to the liquid phase, was calculated for seven solvents. For the H2TPP monomer which has a flat shape, the solvent cavity can fit with an ellipsoidal cavity. Lippert demonstrated that the solvation energy of a point dipole placed in the center of an ellipsoidal solvent cavity, whose large axis is 6, is identical with that of the same dipole in the center of a s herical cavity of radius u = 0.46. Here, (I is taken equal to 7.6 which corresponds to 40% of the long axis of H2TPP (19 A).22
[
(19)Suppan, P.J . Photochem. Phorobiol. A: Chem. 1990,50,293-330. Onager, L.J . Am. Chem.Soc. 1936.58, 14861493. (20)Petkc, J. D.;Maggiora, G. M.; Shipman, L. L.;Christoffcrncn, R.E. J. Mol. Specfmc.1978,71,64-84;Photochem. Photobiol. 1979,30,203-223; 1980,32,399-414.
(21) Edwards, L.; Dolphin, D. H.; Gouterman. M.; Adler, A. D. J. Mol. Sprcfrosc. 1W1,38, 16-32. (22) Lippert, E. 2.Elektrochem. 1957,61,962-915.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1077
Photophysical Properties of Porphyrin
TABLE II: Sohatachro~mShift of the H2TPP &ret Band with Respect to the Cas Phase M-l cm-' lpJ2, u.a. , A, nm I, cm-' , ,c nm far solvent vapP benzene CHCl, DCM DMOE DMF DMSO pyridine
24845 23895 23964 24027 24096 23998 23952 23923
402.5 418.5' 417.3 416.2 415' 416.7c 417.5 418.p
166000 395ooob 380000 400000 410000
27 12.3b 12.6 11.5 11.1'
370000 3900006
12 12.1b
1.1952 1.1984 1.1878 1.1472 1.1415 1.1120 1.1004 1.1668
7.9194 8.2565 8.1601 7.8604 7.7992 7.6284 7.5634 8.0290
n
An2) O.oo00
AEclk, cm-' 0.0
A,!?,, cm-l
1 1.5011 1.4459 1.4242 1.381 1.4305 1.477 1.5095
0.4552 0.4210 0.4067 0.3769 0.4109 0.4406 0.4602
954.7 872.6 812.1 746.6 796.2 846.5 938.5
949.9 881.1 817.8 748.3 846.6 892.6 921.3
0.0
'Reference 21. bReference 30. cReference 31. Note that& = 1.712 given by these authors seem excessive. dFwhm. Abbreviations: DCM, dichloromethane; DMOE, ethylene glycol dimethyl ether; DMF, dimethylformamide; DMSO, dimethyl sulfoxide.
a DlmaI
23800 23700
0.35
0.40
0.45
0.50
flnzl
Figure 3. Solvatochromism of the HITPP Q,(l,O) absorption band.
Y
wllan
Y
Y
"sn
m m Figure 5. Schematic representation of the tautomers of H, dimer and H6 trimer. lthlXl.1
TABLE III: Calculated Transitiooa for Dimers I and II in DCM
Figure 4. Solvatochromism of'the H2 monomer Soret band.
The data are shown in Table 11. The ( L L Z T ) values ~ ~ ~ agree quite well with mob, (within 5%) for the Soret band. For the Q bands, with the exception of the most intense Qy(l,O) transition (see Figure 3), there is no clear correlation between Es andf(n2). This discrepancy could be due to the fact that the transition moments corresponding to the Q bands are much more weaker than the %ret one. One of the possible consequences is that, for the Q bands, the dispersion interaction term could become relatively important. However, the discrepancy could also be due to the lack of precision in the determination of the values, as the Q bands are very weak, broad, and overlap each other. The results obtained for H2TPPshow that (1) the solvent does not affect the transition moment of the monomer solute; (2) the solvatochromic shift of the B band is correctly given by the analytical expression (1 1); and (3) the solvatochromic shift of the Q bands, except for the most intense QY(1,O), poorly fits the same expression, but remains very weak. Figure 4 shows the application of the method to the B band of the basket handle monomer. A straight line is obtained from eq 12, taking a = 7.6 A and pJ as the average value of the transition moments measured for different solvents. As for H2TPP, the main solutesolvent interaction for the monomer remains the interaction of the transition dipole moment of the solute with the induced dipoles of the solvent. We can therefore apply the exciton coupling theory, including the stabilization effect of the solvent, to calculate the exciton energies for the dimer and trimer. Application to the Dimer. Due to the relative position of the N-H groups on each chromophore, the dimer exists in two forms (see Figure 5). The isomer I has a D2,,symmetry and dimer I1 a C, symmetry. Therefore, the exciton coupling treatment is applied separately to each monomer. VP values are calculated using the transition moments of the monomer (Table I). Note that, for the Soret band, B, and B are close to being accidentally degenerate and we assume that d e y have equal energy and intensity in the monomer. The interaction matrix is constructed. The transition energies and coefficients obtained, after resolving the secular equation, cor-
Dimer I 15 320 15 360 16940 17030 18220 18310 19 260 19460 23 520 23 560 24 234 24 186
0 0.2427 0 0.3686 0 0 0 0 0 0 -2.0769 0
15 340 15 340 16980 16980 18 270 18 270 19 348 19 355 24216 24216
0.1666 0 -0.0707 0.4832 0.25
0 0 0 0 0
0 2.1286
0 0.0589 0 0.1359 0 0.0854 0 0.4688 0 0 4.3134 4.5307
0 0.1666 0.2178 0.0145 0 0.25 -0.0502 0.4832 0 2.1022
0.0277 0.0277 0.0524 0.2337 0.0625 0.0625 0.2522 0.2337 4.4194 4.4194
0.2923
0 0.6847 0
0
Dimer I1
0 -0.0069 -0.0145 2.1022
0
respond to those of the gas-phase dimer. To include the solvent effect, one has to add to the interaction matrix a diagonal matrix whose nonzero elements correspond to the solvent stabilization energy for the transitions of the Soret bands B, and Byand are given by Here, because of the low values of p J for the Q transitions, we neglect the corresponding solvent stabilization effect. On Table I11 are shown the data obtained for the two dimers. If one considers a 1 / 1 mixture of the two dimers, the calculated energies agree quite well with the experimental values (see Table I), especially for the Soret band at 414 nm: the difference of energy is less than 100 cm-'. The Soret band of the dimer corresponds to the sum of three transitions, two for dimer I (24234 and 24 186 cm-I) and a doubly degenerate at 24216 cm-' for dimer 11. The
1078 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
total dipole strength for the B band of the dimer in DCM is 8.84
A2. The calculated bathochromic shift of the Qy(l,O) and the
averaged dipole strength (0.48 A2 at 19400 cm-') for the 1/1 dimer mixture are consistent with the experimental data, even if one neglected the solvent effect for the Q transitions: the most probable Qy(l,O) transition occurs at 19 478 cm-'and the dipole strength is 0.51 A* for the dimer in DCM. The optical properties of the basket handle compounds have also been studied in various solvents such as methanol, chloroform, toluene, pyridine, dimethylformamide,and DMSO. The calculations are successful in reproducing the experimental values of the energies (within 250 cm-')and dipole strengths (within 5%), for chlorinated solvents, methanol, and toluene. On the contrary, large discrepanciesare found for solvents which show either a high donating character or an ability to form hydrogen bonds. For these last solvents, the solvatochromism of the B band of the H2 monomer shows new trends. It clearly indicates that the model based only on the electrostatic interactions between the solute and the solvent molecules is no longer valid. One should note that the dipole strengths are given for each transition polarized along the x and y axes. However, it can be seen that the B bands of free base porphyrins are not rigorously degenerated. A well-resolved spectrum of the free base porphine has been obtained at low temperature (-180 "C) in ethanol? B, and By are separated by 240 cm-', which is much less than the inhomogeneous broadening of the band. In methanol and chloroform, the experimental spectrum of the H4 dimer does show two components, approximately separated from each other by 400 cm-'.The calculated energies for the polarized transitions predict only 150 cm-'.This discrepancy must be due to the fact that the B, and Bycomponents of the H2monomer could not be separated, and that for our calculations we assumed an equiprobabilityfor the two polarized transitions. Exciton Coupling and Solvation Effect in the Trimer. Three isomers coexist with a 1:2:1 statistical distribution of I/II/III trimers, predetermined by the synthesis method (see Figure 5). We have calculated for each isomer the exciton coupling energy, including the stabilization effect of the solvent. The data obtained in DCM are reported in Table IV. Here, the distance R = 6 A remains the same as for the dimer and the cavity radius of the solute is 8.8 A. For a statistical distribution, our calculations predict a series of transitions centered around 24 850 cm-l, another intense one at 24 090 cm-' plus a tail at 23 140 cm-' composed of much less intense transitions. The corresponding averaged dipole strengths are 9.9, 4.9, and 0.425 A2.If one attributes to each transition a width equal to that of the monomer (800 cm-'), the total band width of the trimer B band will be 1550 cm-'. Its shape will be asymmetrical with a red shoulder and a red tail. The experimental absorption band of the trimer in DCM effectively presents an asymmetrical shape, with an intense broad band centmd at 24 550 cm-',a red shoulder, and a red tail. The bandwidth is 1710 cm-' (29 nm). Because of the asymmetry of the B band, the experimental dipole strength is obtained from eq 7, with only 10% of accuracy. Its value 12.83 A2) is, however, quite close to the predicted one (14.75 As for the H4dimer, the values obtained for the Q bands are satisfactory. The predicted energies, taking into account the statistical mixture of isomers, are very close to the experimental values: we found predicted transitions peaking at 19 430,18 270, 17010, and 15355 cm-', to be compared with 19417, 18 182, 17 013, and 15 361 cm-'. The corresponding dipole lengths are predicted with only 20% accuracy. For porphyrin-like compounds, it seems that the stabilization energy results essentially from the interaction of the transition moment of the solute with the induced dipoles of the solvent. Neglecting the solvent effect for the Q bands significantly affects neither the predicted transition energies nor the corresponding dipole lengths. The small discrepancy between the calculated and
i2).
(23) Rimington, C.;Mason, S. F.;Kennard, 0.Spectrochim. Acro 1958, 12.65-77.
Tran-Thi et al.
TABLE Iv. c.leuLted Tlrndtiolrr for Trimera L IL d III b DCM Trimer I 15 290 15 340 15 380 16930 16980 ,17030 18 180 18 270 18 320 19 150 19460 19 550 23 100 23 193 24510 24510 24 873 24835
-0.0603 -0.0569 0.2709 0.0402 0.2885 0.3194 0 0 0 0 0 0 0.3661 0 0 0 -2.5221 0
15310 15 340 15 360 16 930 16980 17030 18210 18 270 18310 19 240 19 330 19530 23 090 23 170 24 260 24510 24 090 24 850
0.0062 0 0.2383 -0.0074 0 -0.3533 0 0.2402 0 0 -0.4644 0 0.3723 0 1.0339 0 1.8058 0
0 0 0 0 0 0 0.1216 0 -0.3151 0.1856 0 0.7628 0 -0.3409 0 0 0 2.6039
0.0036 0.0032 0.0734 0.0016 0.0832 0.1020 0.0148
O.oo00 0.0993 0.0344 O.oo00 0.5819 0.1340 0.1162
O.oo00 O.oo00 6.3610 6.7803
Trimer I1 -0.0127 0.1684 0.2219 0.0269 -0.2373 -0.3234 0.0163 0.2478 0.2705 0.0386 -0.4901 -0.6202 -0,3412 0.3638 0.0005 0.0002 2.545 2.581
0.0002 0.0284 0.1060 0.0008 0.0563 0.2294 0.0003 0.1 191 0.0732 0.0015 0.4559 0.3846 0.2550 0.1324 1.0689
O.oo00 9.7379 6.6616
Trimer 111 15 330 15 330 15 340 16970 16970 16 980 18 270 18 270 18270 19 375 19 320 19 460 23 145 23 145 24510 24510 24 860 24 850
0 -0.238 0.0004 0 0.329 o.Ooo1 -0.222 0 0 0.3849 0 0 0 0.3882 0 0 2.5568 0
-0.1351 0 0 -0.1618 0 0 0 -0.3543 0 0 0.6896 0 -0.3168 0 0 0 0 -2.567 5
0.0183 0.0566
O.oo00 0.0262 0.1082
O.oo00 0.0493 0.1255
O.oo00 0.1481 0.4755
O.oo00 0.1004 0.1507 O.oo00 O.oo00 6.5372 6.5921
experimental dipole lengths is mostly due to the lack of precision of (i) the B, and Bydipole lengths which were explicitly considwed as equal in the monomer; (ii) the Qy and Q, dipole lengths which are very weak. In the case of weakly interacting chromophores such as the basket handle compounds, it appears that if one talresinto account the role of the solvent, the exciton coupling theory allows one to predict the optical properties of the dimer and trimer from the monomer properties. As regards the singly linked dimer, its absorption band maxima remain unchanged with respect to the monomer, indicating the absence of any strong exciton coupling effect. A broadening of the Soret band is, however, observed which could be related to the contribution of different conformers due to the presence of the flexible chain.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1079
Photophysical Properties of Porphyrin TABLEV Fluoreacenee
a" HZTPP
propanol
H4
H6
P&30'P
Chang monomer dimer (7) dimer (6) dimer (5) Hamilton monomer (1) dimer (9) trimer (12)
I
I
I
,
kt9
solvent benzene benzene toluene
H2
(O.D.) 0.1
i
nm @,OS 650 (0.13 f 15%) 720 0.1 1 650 0.11 720 648 0.075 (0.09) 715 0.11
cyclohexane DMSO 652 718 DMSO 658 721 654 DCE 715 DMSO 659 723 655 DCE 714 DMSO 661 723 DCE 657 713 DMSO 652 718 benzene DCM
0.088 0.098
DCM DCM DCM DCM
619 628 630 630
0.094 0.035 0.021 0.007
DCM DCM DCM
619 0.09 628 0.004 635 0.0002
Fluorescence quantum yield. tained for air free solutions.
r,b* ns
(15.7
ref 0.2) 33
12.4 13
14b
8.9 (10.7)
34
12.2
35
14a
0.13 0.13
11.6
0.1 0.077
11.3
0.066 0.055
11.3
0.044 0.12 11.9 9.2
Figure 6. Transient differential spectrum of excited H2monomer = 532 nm, c = 3.14 X l o d M. 20 ns); solvent, DMSO bXc
9 9 9 9
to.'
1Oa 1Oa 1Oa
Fluorescence lifetime. Values ob-
Radiative properties The fluorescence yield of the singly linked compound is mostly identical to that of the componding monomer (seeTable V). This result is consistent with the absorption data and indicates that the two chromophores behave independently of each other. For weakly interacting chromophores such as the basket handle dimer and trimer, if one knows the exciton energies, the oscillator strength associated to each transition and the distribution of molecules in each excited state at equilibrium (kT = 200 cm-l), one is also able to predict the fluorescence quantum yield of the dimer and trimer from the monomer radiative properties. The exciton coupling theory predicts only 8% and 13% of decrease of the fluorescence yield, respectively, for the dimer (1/ l mixture of dimers I and 11) and trimer (1/2/1 mixture of trimers I/II/III). The expcrimental data show in fact a much larger decrease, which is around 3540% for the dimer and 56-58% for the trimer (Table V). Very strong fluorescence quenching was also found for other cofacial dimers. Chang% found that decreasing the interplanar distance of the cofacial dimer from 6.4 to 5.4 and 4.2 A led respectively to a decrease of the fluorescence yield from 0.035 to 0.021 and 0.007. For 1,2-phenyIene-bridged face to face dimers, Osuka et al. found 75% less fluorescence for the (Zn), dimer and 91% less for the (H2)2dimer, with respect to their monomer.24 In the above-citad cases and in numerous others in the literature, important red shifts of the fluorescence maxima of dimers were reported, and charge transfer or pure exciton interaction was alternately invoked to account for the change in their radiative properties. Sanders et al.lo reportedin a recent work the p i b i l i t y of weakening the exciton coupling between two covalently and (24) Osuka, A.; Nakajima, S.;Napta, T.; Maruyama, K.;Toriumi. K.
Angew. Chrm., Int. Ed. Engl. 1991, 30, 582-584.
1
400
500
600
Figure 7. Transient differential spectrum of (---) excited H4dimer, c = 2.2 X 10" M; (-) excited & trimer, c = 2.3 X lod M; solvent, DMSO, hXc = 532 nm, r = 20 ns.
doubly linked octaalkyl zinc porphyrin chromophores by complexing the metal with strongly binding ligands of different size. In the unit, the two chromophores are maintained face to face by two floppy chains. With the presence of the ligand, the dimer &ret band shifted to the red, and the fluorescence was partially recovered. However, for Sanders's dimers, one cannot completely reject the possibility of existence of charge-transfer states. If any charge-transfer state exists, it should also affect the nonradiative processes of excited dimers and trimers and particularly the intersystem conversion which is the most important decay pathway of singlet excited states in porphyrins. As no systematic study of the singlet and triplet radiationless decay pathways in porphyrin oligomers has been reported,it is therefore of great interest to determine the triplet yield and kinetics decay of the basket handle compounds. Nonr8di8liveDecryPathwaya F~nnthe~osecondabsorption spectroscopy, transient differential spectra of excited solution of H2 monomer, H4dimer, & trimer, and Poc30'P in DMSO were
Tran-Thi et al.
1080 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 A(0.D.) + 0.1
,
'
0.01
i
I
650
V 6 0 0h
l
l
~
400
so 0
Figure 8. Transient differential spectrum of excited P&3o/p (t = 20 ns); solvent, DMSO bxc = 532 nm, c = 2.7 X lod M.
4t
I
Ihlll
1::
0.03
/
450
I
400
450
400
440
Figure 10. T-T absorption spectra for different yield values. (a) H2 monomer: A,h = 0.80; 0,h = 0.85; 0, & = 0.90. (b) H,dimer: 0, = 0.36; 0 , & = 0.40. (c) H6trimer: El, h = 0.36; 0, h = 0.40. Extinction coefficient versus wavelength.
TABLE VI: Triplet Properties ~~
TT,C
compd H2TPP
:
H2 H4
solvent benzene toluene propanol DMSO DMSO
@.I
ms
0.82 f 0.05"
, , ,A
nm
e,
M-'
cm-l
430
8.30 X lo5
426 424
1.66 X lo5 3.76 X lo5
1.35O 0.84 f 0.05b 0.85 f 0.05d 0.40 f 0.05d 0.49 f 0.05' 0.40 0.04d 0.40 f 0.04f 0.41 f 0.048 0.70 O.OSd
1.60 1.50
* I 0.5 0.6 0.7 0.8
-yield
Figure 9. T-T absorption spectra of PoC3o'P for different triplet values. Solvent, DMSO.
obtained (see Figures 6-8). The triplet quantum yield was determined for each compound, using both the isosbestic points method and the trial method.25 Briefly, a triplet yield value was first determined by the isosbestic points method and the T-T absorption spectrum was reconstructed from this value. For underestimated values, the T-T spectrum usually shows negative optical densities and abnormal trough over the ground-state absorption domain. The triplet yield is therefore progressively increased until no abnormal trough exists any more in the T-T spectrum. For the H2 monomer, no abnormal trough was found by the isobestic points method which directly gave aT= 0.85 f 0.04, a value consistent with the ones previously given for H2TPP.26 With the same method, we obtain for the singly linked dimer aT = 0.70 (Figure 9). Surprisingly, very low triplet yield values, ipT = 0.40, were found for both H4dimer and H6 trimer (Figure 10). In the absence of exciton coupling between singlet and triplet excited states, one can reasonably assume that the triplet absorption band of a dimer is the sum of the absorption bands of a triplet monomer and a ground-state monomer. The differential optical density, in this case, is related to the extinction coefficients ~
DMSO
PoC3o'P
DMSO
*
1.40
422.5
4.1 X lo5
1.75
419
3.2 X lo5
@Reference25. Reference 32. First-order kinetics in air-free solutions. dSuccessive trial method. 'From eq 14 at X = 430 nm. 'From eq 15 at X = 430 nm. gFrom eq 16 at X = 430 nm.
x nm
~
H6
~~~
(25) Carmichael, L.; Hugh, G. L. J. Phys. Chem. Ref.Dum 1986, 25,
11-20. ..
(26) Pekkarinen, L.; Linschitz, H. J. Am. Chem. Soc. 1960, 82, 2407-241 1.
(tMT'!aMM) of the monomer, and to tDMand triplet yield R of the dimer by
ADO(X) = R{&X)
+ &(A) - &A))lc
(14)
where 1 is the optical path length and c the concentration of the solution. The same assumption is applied to determine the triplet quantum yield of the trimer. In this case, depending on the localization of the excitation, either on one of the extemal chromophore or in the central one, we obtain
We found approximately the same triplet yield for the dimer and trimer by the different methods (Table VI). From these data, it can be concluded for all the studied compounds that the excitation energy of the triplet remains localized on one chromophore. The absorption spectrum of the triplet excited state of the dimer (or trimer) corresponds in fact to the absorption of one chromophore in the triplet state and the other (or two others) in the ground state, thus explaining the blue shift observed for the dimer and trimer with respect to the monomer. On the other hand, from Table VI, it appears that all the T-T &ret absorption bands of the H2 monomer, H4dimer, I& trimer, and PoC3oT in DMSO are red-shifted compared to the corresponding ground-state absorptions. The red shift is comparable for the H2 monomer and Poc30'P (respectively, 1.9 and 1.5 MI),
Photophysical Properties of Porphyrin but increases from the H2 monomer to the H4 dimer (4.2 nm) and Hb trimer (7.1nm). One can also notice that the triplet B bands are not strongly broadened like the singlet ones (see Figures 2 and 10). The oscillator strengths of the studied dimers and trimer are roughly two and threetimes that of the monomer (Table VI). The triplet yield of the singly linked dimer is only slightly decreased with respect to the monomer. On the contrary, one can see that the intersystem crossing process is less efficient in the face to face dimer and trimer comparatively to the monomer, and that internal conversion or/and a third nonradiative pathway prevail. From the experimental values of the singlet lifetime, fluorescence, and triplet quantum yields, one can calculate for the basket handle compounds values of kf, kk, and ki, corresponding respectively to the singlet disappearance rate via fluorescence, intersystem crossing, and internal conversion. In the case of composite molecules, we define k’as the apparent rate constant corresponding to the sum of internal conversion and some other radiationless decay rate whose nature will be discussed. for the monomer kf = 1.1 x 107 s-1
ki, = 7.3 x 107 s-1
ki,
1.7 X
lo6 s - ~
for the dimer
kf = 6.8 X IO6 s-’ ki, = 3.5 x 107 s-1
k’= 4.5 X
lo7 s-I
for the trimer
kf = 4.9 x 106 s-I
ki, = 3.5 X lo7 s-l k’= 4.8 X lo7 s-l It can be noticed that values of k’ are of the same ordei of magnitude as those of ki, for the dimer and trimer. The question which arises is whether or not chargetransfer processes could occur in composite molecules with identical chromophores. Ring to ring charge transfer was in some cases invoked to account for the fluorescence quenching in dimers. However, radical pairs were only formed in the case of hybrid compounds. One can roughly estimate the energy of the lowest ring to ring charge-transfer (RRCT)states from the ring oxidation and reduction potentials of the H4 dimer,27using Weller’s equation2*
where (AG& is the energy of the chargetransfer state in solvent x (DCE), and El,,- the monoelectronic oxidation and reduction potential of the dimer in solvent m (DMSO), t the static dielectric constant, r the radius of the two ionic species, and r12 the distance between them. With r taken equal to the Onsager solute cavity radius (r = a = 7.6 A), we found (AGC& = 2.04 eV, which places the chargetransfer state 0.16eV above the lowest singlet excited state of the dimer (1.88eV or 659.6 nm). The difference of energy between the two states is sufficiently weak to provide another decay pathway for the singlet excited states of the dimer and trimer. (27) (a) The oxidation and reduction of H2 monomer, HI dimer, and H6 trimer lo-’ M in dichloroethanein presence of n-But4NPF6,was performed with a glassy carbon as the working elcctrcdc and an Ag electrode as reference. The Ed and E, values of the dimer and trimer are nearly the same than that of the monomer: End = -1.53 eV, E,,, = 0.75 eV. Instrumentation and experimental produrea were described earlier in: Gurutin, C.; Lexa, D.; Mommteau, M.; SavQnt, J. M.; Xu, F. Imrg. Chem. 1986,25,4294-4307. (b) Yan, X.; Holten, D. J . Phys. Chem. 1988, 92, 409-414. (28) Weller, A. 2.Phys. Chem. 1982, 133, 93-98.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1081
IU
I
I
Nuclearcrmdlnates
Flgm 11. Potential surface diagram for S, SI,and CT states of the H4 dimer.
It is also possible that a mixing of the CT states into the S1could enhance nonradiative transitions between S1 and So.This last scheme has been recently proposed by Wasielevski et al.29for a chlorophyll-porphyrin heterodimer for which the chargetransfer states estimated from the redox potentials are virtual states lying above the singlet ones. Usually, the nonradiative decay from So S1in porphyrin-like Ab initio SCF-MO-CI molecules are inefficient (10-15’%). calculation of both So and SI for porphine and chlorophyll-like compounds show that the r-electron populations at each atom in the r system do not differ significantly between S, and S1. The similar electron distributions for So and S1 suggest that the potential surface of S1 is imbricated in the So one, which results in a very small Franck-Condon factor with a consequent low rate of internal conversion. On the other hand, for a chargetransfer state, the electronic distribution would be rather different from the So and S1 and should result in a displacement of the nuclei in the CT state relative to those of the excited and ground states. Thus, it is possible that the potential energy surface for the CT state could intersect the surfaces of S1 and So.Wasielevski et al. argue that the S1and Sostates could be coupled via the CT state, which results in an enhancement of the So S1 nonradiative decay. Their reasoning is, however, based on a potential surface diagram which has no clear physical meaning. We believe that the avoided crossing between the potential surfaces for CT and So states, as presented on their diagram, is incorrect and should be replaced by that of Figure 11. In the present case, the CT state is slightly above the S1state and the corresponding potential surface shows a local minimum. Due to the coupling between the SIand CT state, the fluorescence from SIwould be partially reduced, and the So SI internal conversion pathway enhanced via nonradiative decay of CT states to So.The existence of such low virtual CT state allows a better understanding of the invariance of both singlet and triplet lifetimes. In the present scheme, a radiative transition So CT could be possible. It would be red-shifted. However, in our experimental conditions, we were unable to detect any red-shifted fluorescence of the dimer or trimer. Charge-transfer states have also been predicted for a tetraphenylporphyrin dimer linked via a phenyl group, with the help of the CNDO/S-CI method and using transition dipoles and charges of the monomer.36 For such head-to-tail dimer, the lowest CT state was found to be located approximately 1.2 eV above the lowest singlet excited state. This result is not surprising, since it was usually found that charge
-
-
-
-
(29) Wasielevski, M. R.; Johnson, D. G.; Niemczyk, M.P.; Gaines, G. L.; ONeil, M. P.; Svec, W. A. J . Am. Chem. Soc. 1990,112,6482-6488. (30) Dorough, G. D.; Miller, J. R.; Huennekens, F. M. J. Am. Chem. Soc. 1951, 173,4315-4320. (31) Msot-Ner, M.; Adler, A. D. J. Am. Chem. Soc. 1975,97,5107-5111. (32) Gradyushko, A. T.; Tsvirko, M. P. Opt. Spectmc. 1972,31,291-295. (33) Haniman, A.; Hosie, R. J. J. Photochem. 1981, IS, 163-167. (34) Gradyushko, A. T.; Sevchenko, A. N.; Solovov, K. N.; Tsvirko, M. P. Photochem. Photobiol. 1970, 11, 387-400. (35) Kikushi, K.; Kurabayashi, Y.; Kokobun, H.; Kaizu, Y.; Kobayashi, H. J. Photochem. Photobiol. A:Chcm. 1988,45,261-263. (36) Eriksson, S.; Killebring, B.; Larsson, S.; Martensson, J.; Wennerstrbm, 0. Chem. Phys. 1990,146, 165-177.
1082
J. Phys. Chem. 1992, 96, 1082-1087
transfer never occurs in singly linked dimers or heterodimers. For these compounds, exciton coupling or energy transfer is the dominant process. Another possible way of the singlet excited-state decay via internal conversion is related to the geometry of the dimer and trimer. As suggested earlier?* the presence of the basket handle chains, which are semiflexible, might lead to additional vibrational modes which would also enhance the radiationless decay.
Conclusion The systematic study of singly linked and doubly linked face to face porphyrin dimers and trimer allows us to conclude, that, if one takes into account the solvent effect, the molecular exciton theory can be successfully applied to the basket handle compounds, whose exciton interactions are weak. For composite molecules bearing identical chromophores, it clearly appears that there are no chargetransfer states lower than the singlet excited states. The deactivation pathways of the singlet states of the dimer and trimer
remain the same as for the monomer, but the decay rates are changed. The radiative (fluorescence) and intersystem crossing rates decrease on behalf of the internal conversion rate. The latter, which is enhanced by a factor of 3 in the doubly linked dimer and trimer, is explained in terms of either the existence of additional vibrational modes due to the presence of the flexible basket handle chains or the existence of virtual transfer states lying near above the singlet ones. The stabilizationeffect of the solvent was found to play a crucial role, not only in balancing the exciton coupling energy, but also in lowering the chargetransfer states. We believe that theoretical calculations using either ab initio CI or CNDO/S-CI methods to predict energies of excited and CT states and oscillator strengths would gain in reliability if they could directly include both charge-transfer characters and solvent effect. Such studies are, however, very demanding. They are already underway, but exclusively on small molecules, and should be extended to bigger ones.
Ionization Potentlals and Reactivity of Coinage Metal Clusters M. A. Cheeseman and J. R. Eyler* Department of Chemistry, University of Florida, Gainesville. Florida 3261 1-2046 (Received: June 21, 1991; In Final Form: September 24, 1991)
The ionization potentials of several homoatomic and heteroatomic coinage metal clusters have been determined utilizing charge-transfer bracketing and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The clusters studied were Ag,, Au,, Cum,and AgkCu,, where n = 2,3,5, m = 2,3, and k and 1 = 1, 2. Atomic ionization potentials were also verified for each of the above metals as a test of the bracketing method. This work represents one of the first measurements of adiabatic ionization potentials for any of the above clusters. A number of additional reactions were observed between several of the charge-transfer agents and metal cluster ions, including some which resulted in metal-metal bond cleavage.
Introduction Clusters are currently the focus of several areas of research. It has been proposed that these species represent an intermediate phase between atoms and bulk solids. Studies of cluster reactivities have revealed similaritieswith and provided insight into such areas as surface catalysis, gas plasma and flame chemistry, and processes in interstellar space. The practical significance of clusters extends to the electronics industry, which is making use of progressively smaller metallic and semiconductor structures. Now that X-ray and electron beam lithography and high-resolution microscopy have made it possible to produce and study nanoscale devices, knowledge of the physical properties of clusters is urgently needed. As a result, research into the structure and other characteristics of these species has exploded over the past Mass spectrometric and matrix isolation studies have predominated as the main avenues of work, and they have complemented each other in their attempts to elucidate the structural and electronic properties, as well as the reactivity, of these species. However, there remains a paucity of data about many basic physical properties of even the simplest of clusters. This shortfall hinders the understanding of the species themselves, as well as the complete interpretation of the reactions which they undergo. It is therefore
of primary importance to acquire data on the fundamental physical properties of cluster species. A number of workers have noted differences in the reactivity of clusters as opposed to that of the bulk.611 Several physical properties have shown distinct variations, both as the size of the cluster changes and in comparison with corresponding values for the bulk and atomic These observations support the view that clusters do indeed compose a distinct phase of matter with unique properties. For this reason we have attempted to study those properties of clusters which can be suitably investigated by FTICR mass spectrometry.*e21 This endeavor has led to the (6) Davis, S.C.; Klabunde, K. J. Chem. Reu. 1982,82, 153. (7) Ray, U.; Jarrold, M. F. J . Chem. Phys. 1990,93, 5709. (8) Jarmld, M. F.; Ray, U.; Crtegan, K. M. J. Chem.Phys. 1990, 93,224. (9) Trevor, D. J.; Cox, D. M.; Kaldor, A. J. Am. Chem. Soc. 1990,212, 3742 and references therein. (10) Hamrick, Y. M.; Morse, M. D. J. Phys. Chem. 1989,93,6494 and
references therein. (11) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byme, N. E.; Flanagan, S.;Haley, M. M.; OBrien, S. C.; Pan, C.; Xiao, 2.;Billups,
W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J. Phys. Chem. 1990, 94, 8634. (12) Parks, E. K.; Klots, T. D.;Riley, S. J. J. Chem. Phys. 1990,92,3813. (13) Yang, S.; Knickelbein, M. B. J. Chem. Phys. 1990, 93, 1533. (14) Knickelbein, M. B.; Yang, S.;Riley, S.J. J . Chem. Phys. 1990,93, 94.
(1) Weltner, W., Jr.; Van Zee,R. J. In Proceedings of the Conference on Computational Chemistry: The Challenge of d and f electrons;Salahub. D. R., Zemer, M. C., Eds.;ACS Sympium Series 394; American Chemical Society: Washington, DC, 1989 (and references therein). (2) Gingerich, K. A.; Brewer, L.; Winn. J. S.; Miedema, A. R.; Ozin, J. Faraday Symp. Chem. Soc. 1980, 24 and references therein. (3) Morse, M. D. Chem. Rev. 1986, 86, 1049 and references therein. (4) Russell, D. H.. Ed.Gas Phase Inorganic Chemistry;Plenum Publishing: New York, 1989 (and references therein). (5) Weltner, W., Jr.; Van Zee,R. J. Chem. Reu. 1989, 89, 1713.
(15) Zh"mrman, J. A.; Eyler, J. R.; Bach, S. B. H.; McElvany, S.W. J . Chem. Phys. 1991, 94, 3556. (16) Zimmerman, J. A.; Bach, S. B. H.; Watson, C. H.; Eyler, J. R. J. Phys. Chem. 1991, 95,98. (17) Bach, S. B. H.; Eyler, J. R. J . Chem. Phys. 1990, 92, 358. (18) Pettiette, C. L.; Yang, S.H.; Craycroft, M. J.; Conceicao, J.; Laaksonen, R. T.; Cheshnovsky, 0.;Smalley, R. E. J. Chcm. Phys. 1988,8,5377. (19) Buchanan, M. V.; Comisarow, M. B. In Fourier Transform Mass Spectrometry: Evolution. Inmation, and Applications; Buchanan, M. V., Ed.;ACS Symposium Series 359; American Chemical Society: Washington, DC, 1987, p 1.
0022-3654/92/2096-1082%03.00/00 1992 American Chemical Society
