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(1968). (9) C.Tanford, J. Phys. Chem., 78, 2469 (1974). (10) Q. A. Trementozzi, J . Polym. Sci., 23,887 (1957). (11) C. I. Carr, Jr., and B. H. Zimm, J. Chem. Phys., 18, 1616 (1950). (12) M. Emerson and A. Holtzer, J . Phys. Chem., 71, 1898 (1967). (13) K. Kuriyama, Kolloid Z. Z. Polym., 180, 55 (1962). (14) K. J. Mysels and L. H.Princen, J . Phys. Chern., 63, 1696 (1959). (15) H. F. Huisman, Proc. K . Ned. Akad. Wet., Ser. B, 67, 388 (1964). (16) M. Corti and V. Degiorgio, Chem. Phys. Left., 53, 237 (1978);Ann. Phys. (Pnrls), 3, 303 (1978). (17)W. Prlns and J. J. Hermans, Proc, K . Ned. Akad. Wet., Ser. B , 59, 162 (1956). (18) L. H. Princen and K. J. Mysels, J. Colloid Scl., 12, 594 (1957). (19) G. E. A. Aniansson, J . Phys. Chem., 82, 2805 (1978). (20) S Ikeda, S.Ozeki, and M. Tsunoda, J . Colloid Interface Sci., 73, 27 (198OJ1. (21) W. Prinssind J. J. Hermans, Proc. K . Ned. Akad. Wet. Ser. B, 59, 298 (1956). (22) P. Mukerjoe, J . Phys. Chem., 76, 565 (1972). (23) T. Klhara, J . Phys. Soc. Jpn., 8, 289 (1951).
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(24) P. Debye, Ann. N.Y. Acad. Scl., 51, 575 (1949). (25) K. W. Herrmann, J . Phys. Chem., 68, 1540 (1964). (26)T. Nakagawa, H. Inoue, K. Torl, and K. Kuriyama, J , Chem. Soc. Jpn., Pure Chem. Sect., 79, 1194 (1958). (27) C. W. Dwiggins, Jr., and R. J. Bolen, J. Phys. Chem., 65, 1787 (1961). (28) R. R. Balmbra, J. S. Clunie, J. M. Corkill, and J. F. Goodman, Trans. Faraday Soc., 60,979 (1964). (29) P. H. Elworthy and C. B. MacFarlane, J. Chem. Soc., 907 (1963). (30) P. H. Elworthy and C. MacDonald, KolbklZ. Z. Polym., 195, 16 (1964). (31) D. Attwood, J. Phys. Chem., 72, 339 (1968). (32) K. W. Herrmann, J . Phys. Chem., 66, 595 (1962). (33) L. D. Moore, J . Polym. Scl,, 20, 137 (1956). (34)L. H. Peebles, Jr., J . Am. Chem. Soc,, 80, 5603 (1958). (35) N. Eliezer and A. Silberberg, Biopolymers, 5, 95 (1967). (36) D. W. Tanner and G. C. Berry, J . Polym. Scl., 12, 941 (1974). (37) T. Matsuo and H. Inagaki, Makromol. Chem., 53, 130 (1962). (38) K. Walenfels, H. Sund, and W. Burchard, Biochem. Z., 335, 315 (1962). (39) P. Kratochvil, Collect. Czech. Chem. Commun., 30, 1 1 19 ('1985). (40) J. N. Phillips and K. J. Mysels, J . Phys. Chem., 59, 325 ('1955). (41) L. H. Princen and K. J. Mysels, J . Phys. Chern., 63, 1781 ('1959).
Electronic Structure of a Porphyrin Solid Film and Energy Transfer at the Interface with a Metal Substrate K, Tanlmura, T. Kawal, and T. Sakata" Instltute for Molecular Science, MyodaJi, Okazakl444, Japan (Recelved February 5, 1979; Revised Manuscrlpt Recelved August 6, 1979) Publlcatlon costs asslsted by the Institute for Molecular Sclence
The electronic structure and the interaction with a metal substrate of amorphous solid films of free base tetraphenylporphine (H,TPP) and its Zn derivative (ZnTPP) have been studied. The visible absorption spectra resemble those of solutions, although the Soret band is greatly weakened and broadened in the film.The lifetime of the SI state in an amorphous HzTPPfilm is less than 2 ne and is much shorter than that of solutions. This quenching is attributed to enhancement of the nonradiative decay rate in the solid phase. The typical effect of the metal substrate on the film is a strong quenching both of sensitized chlorin emission in ZnTPP and of fluorescence in H2TPP. Forster type energy transfer to the metal substrate explains most of the quenching, but an additional long-range effect in the H2TPP film is attributed to exciton diffusion within the film. The metal-dye system is discussed in terms of a device for solar-energy conversion.
Introduction Special attention has recently been attracted, because of interest in solar-energy conversion mechanisms, to the interesting phenomena exhibited by organic dyes at their interface with other materials. A solid thin film of dye in contact with mlids (metal, semiconductor, and insulators) is one of the most common systems in which these interfacial phenomena have been ~ t u d i e d . l -Such ~ an organic film plays a central role in the conversion of light into other forms of energy through, in general, the following processes: capture of photons and the transfer of the excitation energy to the interfaces where carrier generation and/or chemical reactions mainly take place. Thus, clarification of excited-state behavior of dye film interfaces with other solids, liquids, and gases is crucial for elucidating the mechanisms of the various kinds of reactions concerned, and hence for developing a system with highconversion efficiency. Nevertheless, detailed properties of such excited states of dye films with interfaces are far from being well understood. In this paper, we study the electronic structure of the porphyrin film and its interaction with a metal substrate. The use of porphyrin (tetraphenylporphine and its metal derivatives in this study) as an organic dye gives us several advantages, since a great deal of data has already been 0022-3654/80/2084-0751$01 .OO/O
reported for porphyrin molecules in solutions as to their electronic structure, photochemical behavior, etcS8Properties of solid thin films of porphyrins are also interesting as simple analogues to the chlorophyll aggregates in photo~ynthesis.~J~
Experimental Section Tetraphenylporphine (H,TPP) of chlorin-free grade and the Zn derivative (ZnTPP) were obtained from Strem Chemicals, Inc. and were used after recrystallization three times from ethanol. Quartz plates polished to optical grade and platinum (99.9%) plates were used as substrates after cleaning their surfaces by an ultrasonic cleaner in tetrahydrofran. Films were prepared in vacuo (1 X lo4 torr) with a controlled deposition rate of 1 A/s. The thickness of the film was monitored and determined by a Sloan thickness meter (DTM 200). The optical absorption spectrum of the film deposited on quartz was measured by a Hitachi 556 spectrophotometer. The amount of porphyrin deposited was found to be the same for both substrates, which was confirmed by measuring the solution spectra of materials recovered from the film. Excitation of the porphyrins was made by means of a 100-W tungsten lamp with a grating monochromator (Nickon P-250). Exciting light intensity was calibrated @ 1980 American Chemical Society
752
The Journal of Physical Chemisty, Vol. 84, No. 7, 1980
1
T-----7-----7------r
T---
11
Tanimura, Kawai, and Sakata
1 LnTPC
1
i
1
- 2-
-_--
IC)
LOO
600 WAVELENGTH
500 WAVELENGTH
600 700 I nrn 1 Flgure 1. Optical absorption spectrum of the ZnTPP film (400 A) on quartz (solid curve) and the spectrum of ZnTPP recovered from the film In benzene (5 mL) (dotted curve). The Inset shows the absorption spectrum of ZnTPC-benzene (from ref 13).
with a rhodamine B quantum counter. Luminescence was detected with another grating monochromator of the same type and an HTV R-636 photomultiplier and recorded on a recorder (Riken Denshi D-72BP). The exciting light in most experiments traveled at an angle of 65' to the normal of the sample slide, and the luminescence was detected at right angles to the exciting beam. An attempt was made to measure the decay time of the luminescence by using a photon-counting system with a nanosecond pulser (ORTEC SP-3X). It was found, however, for both ZnTPP and H2TPPfilms, that luminescence decay times were less than 2 ns, which was too fast to permit determination of precise values.
Experimental Results In Figure 1,the optical absorption spectrum of a ZnTPP film evapolated on quartz is shown, together with the spectrum in a benzene solution. The striking broadening and weakening of the Soret band is clear, which is a common feature in solid films of p ~ r p h y r i n s . ~ -Al ~red shift of the absorption peak (550 cm-l) is also observed for this band. The visible absorption bands of ZnTPP, on the other hand, are hardly affected by condensation of molecules, although a slight broadening takes place. The ratio of the oscillator strength of a band in the film to that in benzene solution is 0.95 for visible bands and 1.1for the Soret bands. An additional absorption band around 625 nm is observed in both spectra. In the inset of Figure 1 is shown the absorption spectrum of zinc tetraphenylchlorin (ZnTPC).13 Since the peak position of 625 nm is the same as that of the Q (0 band ) of ZnTPC, and chlorins are known as common impurities in porphines, one can attribute this band to a chlorin impurity. The concentration of this impurity is estimated to be 0.01 mole fraction from the present spectral data and from molar extinction coefficients previously reported for ZnTPP and ZnTPC.19J4 We have concluded that the structure of our ZnTPP film is amorphous, based on following experimental results: (1) No X-ray diffraction patterns were observed. (2) A ZnTPP film formed on a (100) surface of a KBr crystal by evap7 oration, which is crystalline, shows an essentially different absorption spectrum from that of the film on quartz.15 Figure 2 shows the emission spectra of two kinds of ZnTPP films, which are different from each other in the chlorin concentrations. The spectrum of ZnTPP in benzene isalso shown in the figure. Four emission bands can be resolved whose peaks are situated at 600,650,630, and 690 nm, respectively. The former two bands can be assigned to the fluorescence of ZnTPP, since the peak
700
I nrn I
Flgure 2. Emission spectra of a ZnTPP film containing less than 0.005 mole fractlon of ZnTPC, curve b, and of that containing 0.01 mole fraction of the chlorin, curve c, respectively, The spectra of ZnTPP in benzene (lo-' mol), curve a, Is also shown for comparison. Excitatlon was made at 555 nm. The spectra are not corrected for monochromator dlsperslon and photomultlpller response. WAVELENGTH
-500
600 1
nm
'
I 700
800
W
I Figure 3. Optical absorption spectra and the emission spectra of the HPTPPfilm on quartz (solid curve) and of the molecule in benzene (lo-' mol), respectively. Excitation was made at 525 nm. Emission spectra are not corrected for response function of the detection system. PHOTON ENERGY
(eV
positions are identical with those of emissions of ZnTPP in benzene. The 630- and 690-nm bands, which are then impurity emissions, become dominant in the film with 0.01 mole fraction of ZnTPC. Arrows in Figure 2 indicate peak positions of fluorescence bands of ZnTPC in benzene,13 which are quite close to those of the observed impurity emissions. It was found that the excitation spectrum of the 630-nm band follows well the absorption spectrum of ZnTPP. I t is, therefore, concluded that the sensitized luminescence of ZnTPC takes place predominantly in the ZnTPP film containing 0.01 mole fraction of ZnTPC. Figure 3 shows the absorption and emission spectra of a H,TPP film on quartz. The spectra of the molecule in benzene is also shown in the €igure for comparison. The structure of the HzTPP film can be concluded to be amorphous from experimental facts similar to those for the ZnTPP film. The red shift of the absorption peaks in the film with respect to those in solution is evident: 280 cm-I for Q, bands and 520 cm-l for Qy bands, respect(ive1y. These shifts in corresponding transitions are considered to be due to the change in the interaction energy of one molecule with all of the surrounding ones.16 The emission spectrum of the H2TPP film consists of three emission bands at 665, 732, and 770 nm. The former two are assigned to the intrinsic fluorescence of H,TPP, since these display approximately mirror-symmetrical correlation to the Q, absorption spectrum. The red shift of emission energies are also the same as those of the Q, bands. The supplemental emission band at 770 nm is tentatively at-
Electronic Struciture of a Porphyrin Solid Film
The Journal of Physical Chemistry, Vol. 84, No. 7, 1980 753
I
I
I
LO0
1 500
600
oL-J---
-r
I
WAVELENGTH l n m )
Flgure 4. The excitation spectra of the 664-nm fluorescence band in H,TPP film on quartz and on Pt. The optical absorption spectrum of the same film on quartz is also shown (broken curve).
tributed to an unknown impurity. The characteristic feature of the luminescence in the H2TPP film is that the intrinsic fluorescence is dominant, whereas sensitized chlorin emissions mainly take place in the ZnTPP film. In order to know the electronic structure of the porphyrin film on a platinum plate, the excitation spectrum of the luminescence was measured. It was confirmed that the emission r3pectrum of the film on Pt is essentially the same as that of the film on quartz, except for the intensity. The excitation spectrum for the HzTPP film on Pt is shown in Figure 4 and is compared with that on quartz. The relative emission intensities between films on Pt and on quartz are shown on an arbitrary scale. It is seen that peak positions of the film on Pt are the same as those on quartz. The emission intensity excited a t the Soret band relative to that at the QJ1-0) band, which is hereafter written as Is/tv,is about 1.9 for the film on quartz. This value is quite close to the ratio of the absorbed light at the respective wavelength, which is given by the ratio of (1e-@Ad),where is the absorption coefficient at A, and is calculated to be 1.85 by using absorption data. In the case (of the H2TPP film on Pt, Is/Iv is 1.45, which is smaller than the value of 1.9 for the film on quartz. This difference can be accounted for by the fact that the light reflected by the metal surface also contributes to the excitation of the film on Pt. Simple consideration shows that the absorbed light at h is proportional to (1- e-fAd)(l R2e-'Ad),when the reflectivity of Pt is R2 at A. Thus, 18/Iv in this case should be the ratio of this factor at the two wavelengths, if no additional wavelength-dependent effects occur for the excitation of fluorescence. By utilizing eA of the film on quartz and R2 at the wavelengths concerned (0.65 at 525 nm and 0.58 at 430 nm), the ratio in absorbed light is found to be 1.4. This value is also close to the observed emission-intensity ratio. One can, therefore, conclude that the electronic structure of the film on Pt is essentially the same as that on quartz, and that no additional wavelength dependent effects occur in fluorescence excitation. Figure 5a shows the fluorescence intensity of the HzTPP film as a function of film thickness. The intensity of the film on quartz saturates a t large film thickness, although the optical density at 525 nm is almost in proportion to the film thickness as shown in Figure 5b. It was confirmed that the emission intensity of the film, whose thickness is 500 A or more, is proportional to the exciting light intensity which was chainged by two decades. The saturation is not, therefore, due to dense excitation effects such as singletsinglet annihi1,ation. It was also found that the excitation of the film from the substrate side, or through the quartz plate, results in the similar film-thickness dependence of emission intensity to that shown in Figure 5a. This fact
+
/-p; 1
0
1
"1
3w
600
900
IAj Flgure 5. (a) Film thickness dependence of the fluorescence intensity of the H,TPP film on quartz (0)and on Pt(0).The error bar of a datum point Indicates estimated maximum error. Excitatlon was made at 525 nm. The intensity of the film on Pt is divided by the additional excitation factor of the metal substrate (1 R2e-'hd). (b) Film thickness dependence of the reflected light intensity at 525 nm (0)and of optical density (0) of the H,TPP film on quartz. The datum point of the reflected light at film thickness of zero corresponds to the llght reflected by a quartz plate. THICKNESS
+
shows that the emission does not occur only in some specific regions such as the surface layer of the film. Reflectivity and transmissivity is strongly sensitive to a film thickness and an incident angle in the case of thin film. In fact, it was found that reflected light intensity from the sample surface to which exciting light is incident at an angle of 45O increases with increasing film thickness as shown in Figure 5b. Although the nonlinear growth of the fluorescence intensity seen in Figure 5a is not clear at present, characteristic optical property of the thin film may be responsible for such a phenomenon. It is clear in Figure 5a that the fluorescence of the H2TPP film on Pt is quenched significantly. In order to analyze this quenching effect more quantitatively, we define the relative yield of luminescence, CP, of the film on Pt as Q, = I M / I Q ( l R2e-Qd)
+
where IM and IQare the emission intensities of the film on Pt and on quartz, respectively. The factor of (I R2e-
