4234
J. Phys. Chem. 1986, 90, 4234-4238
SPECTROSCOPY AND STRUCTURE Upper Excited-State Emission of a Covalently Linked Porphyrin Dlmer Youkoh Kaizu, Hideaki Maekawa, and Hiroshi Kobayashi* Department of Chemistry, Tokyo Institute of Technology, 0-okayama. Meguro- ku. Tokyo 152, Japan (Received: July 8, 1985; In Final Form: April 2, 1986) Zinc(I1) and aluminum(II1) complexes of the covalently linked porphyrin dimer 5,S-[1,3-propanediylbis(2-oxy-lphenylene)]bis[ 10,15,20-tris(4-rnethylphenyl)-21H,23H-porphine] (o,o’-C,((TTP)Zn), and o,o’-C3((TTP)AlCl),) emit not only from the SI (the lowest excited singlet) state but also from the T I (the lowest excited triplet) state, both of which conform to those of the corresponding monomers. The dimer exists in two different conformations and exhibits two Soret peaks. The blue component is ascribed to a “stacked” conformation of the two porphyrin moieties and the red component to a “twisted” conformation. The metal complexes of the dimer also emit from the S2 (the second excited singlet) state, the excitation spectrum of which is in good agreement with the red-component Soret band; this suggests that S2 fluorescence arises only from the twisted conformer. The diacid emits S2fluorescence, while the free base does only weakly. However, S2 emission of the free base dimer is observed with a dilute acidic solution where PH2-PH,, PH2-PH42+,and PH:+-PH42+ (PH,, free base; PH42+,diacid) coexist. The excitation spectrum exhibits peaks not only in the Soret band of the diacid but also in the Soret band of the free base. It is concluded that an intramolecular energy transfer from the higher S2 state of the free base moiety in PH2-PH42+to the lower S2 state of the diacid counterpart yields S2 emission of the diacid moiety. Introduction The Soret band of the p-oxo dimer [(TPP)NbI20, (TPP: 5,10,15,2O-tetraphenylporphine)is shifted to the blue in comparison with (TPP)NbO(CH,COO) and exhibits a long tailing to the red of the Soret band’ as has been observed in other p-oxo dimers2v3as well as in covalently linked “face-to-face” d i m e r ~ . ~ J The exciton coupling in the “face-to-face” stacking structure results in an allowed state at higher energy than the monomer state and a forbidden state at lower energy. Regardless of the sizable exciton coupling in the SIstate and the greater spin-orbit coupling within the niobium(V) ion, the dimer emits not only S1fluorescence but also phosphorescence. A rather intense S2fluorescence was observed for (TPP)NbO(CH,COO) while such fluorescence could not be detected for [(TPP)Nb]203.1 In the p-oxo dimer, exciton couplings are possible not only between the component S2states but also between the component SI and S2 states and give rise to a fast relaxation to the SImanifold. In the present work, we observed S2emission from a covalently linked porphyrin dimer. This particular dimer is more conformationally flexible than the dimers previously used for the studies on exciton coupling between porphyrin moieties.6 Experimental Section 5,5’-[1,3-Propanediylbis(2-oxy-l-phenylene)]bis[10,15,20tris(4-methylphenyl)-21H,23H-porphine](o,o’-C,((TTP)H,),) ((TTP)H2) were prepared and 5,10,15,2O-tetra-p-tolylporphine and purified by literature methods.’,* Chlorin contamination was oxidized by 2,3-dichloro-5,6-dicyano-p-benzoquinone.g Zinc (o,o’-C,((TTP)Zn),, (TTP)Zn)’’ and chloroaluminum complexes (1) Ohno, 0.;Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1985, 82, 1779. (2) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977,25,139. (3) Kaizu, Y.; Misu, N.; Tsuji, K.; Kaneko, Y.; Kobayashi, H. Bull. Chem. SOC.Jpn. 1985, 58, 103. (4) Collman, J. P.; Chong, A. 0.; Jameson, G. B.; Oakley, R. T.; Rose, E.; Schmitton, E. R.; Ibers, J. A. J . Am. Chem. SOC.1981, 103, 516. ( 5 ) Collman, J. P.; Anson, F. G.; Banes, C. E.; Bencosme, C. S.; Geiger, C.; Evitt, E. R.; Kreh, R. P.; Meier, K.; Pettman, R. B. J. Am. Chem. SOC. 1983, 105, 2694. (6) Selensky, R.; Holten, D.; Windsor, M. W.; Paine, J. B., 111; Dolphin, D.; Gouterman, M.; Thomas, J. C. Chem. Phys. 1981, 60, 33. (7) Little, R. G. J . Heterocycl. Chem. 1978, 15, 203. (8) Little, R. G.; Anton, J. A,; Loach, P. A.; Ibers, J. A. J. Heterocycl. Chem. 1975, 12, 343. (9) Barnett, G. H.; Hadson, M. F.; Smith, K. M. J . Chem. SOC.Perkin, Trans. 11975. 1401.
(O,~’-C~((TTP)-A~CI)~.~H~O, (TTP)A1Cl.H20)3 were made according to literature methods. o,o’-C,((TTP)Zn), and (TT.P)Zn were purified by column chromatography on dry-packed silica gel with chloroform as eluent and recrystallized from chloroand (TTP)A1CLH20 form/ hexane. O,O’-C~((TTP)A~C~)~-~H~O were purified by chromatography using a wet-packed Sephadex LH-20 column with methanol as eluent and recrystallized from acetone/hexane. All the compounds used in the present work were identified by elemental analysis and characterized by NMR, IR, and visible absorption and emission spectra. Absorption spectra and second-derivative absorption spectra were taken on a Hitachi 330 spectrophotometer. Secondderivative spectra were obtained by measurements of the difference spectra for two different wavelengths with an interval AA = 2 nm using a slit width of 2 nm. Fluorescence emission and excitation spectra as well as polarized excitation spectra were measured on a Hitachi 850 spectrofluorometer equipped with a Hamamatsu Photonics photomultiplier R928. The emission and excitation responses were calibrated by use of a concentrated ethylene glycol solution of rhodamine B (8 g/dm3).” Emission spectra were also corrected by means of standard solutions.12 The degree of fluorescence polarization was corrected for the instrumentally induced polarization as described in the 1iterat~re.I~ Polarized excitation spectra were obtained with A1 complexes in glycerol at 8 OC, Zn complexes doped in cellulose acetate films, and the porphyrin free bases in 2-methyltetrahydrofuran (MTHF) at I1 K. The fluorescence lifetimes were measured by the single-photon-counting method on a PRA nanosecond fluorometer system. The excitation wavelength was set at 400 nm (bandwidth, 16 nm) with a Jobin-Yvon monochromator H-10. Emission was detected through a glass filter (Toshiba 0-57)by a Hamamatsu Photonics photomultiplier R928 and counted on a Norland Model 5300 multichannel analyzer. The lifetime was determined by fitting the decay curve exponentially with an iterative least-squares reconvolution method. (10) Adler, A. 0.;Longo, F.R.; Kampas, F.; Kim, J. J . Inorg. Nucl. Chem.
1970, 32, 2443.
(1 1) Melhuish, W. H. J . Res. Null. Bur. Stand. Secr. A 1972, 76A, 547. (1 2) Lippert, E.; Nagele, W.; Seibold-Blankenstein, I.; Staeger, U.;Voss, W. A . Anal. Chem. 1959, 120, 1. (13) Azumi, T.; McGlynn, S.P. J . Chem. Phys. 1962, 37, 2413.
0022-3654/86/2090-4234$01.50/00 1986 American Chemical Society
Excited-State Emission of a Porphyrin Dimer
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4235
I a
IO
20
30
IO
20
4
30
Wavenumber / I O’cm-’ Figure 1. Absorption (-)
and fluorescence (---) spectra of the porphyrin dimers o,o’-C,((TTP)H,), in dichloromethane (Ia), o,o’-C~((TTP)Zn), in benzene (Ib), and O,~’-C,((TTP)AICI)~ in methanol (IC) and their corresponding monomers (TTP)H, in dichloromethane (IIa), (TTP)Zn in benzene (IIb), and (TTP)AICl in methanol (IIc).
The quantum yields of fluorescence were determined with reference to the yield of (TPP)Zn in benzene (&(Sl)= 0.033)14 using optically dilute solution^.^^ Phosphorescence spectra and decay lifetimes were measured in a Dewar assembly with the samples in rigid glass media at 77 K. Phosphorescence decays were measured through a Nikon P-250 monochromator by an R928 photomultiplier upon excitation by repetitive flash pulses of 5-ps duration from a stroboscope (Sugawara Type MS-210). The excitation wavelength was selected by a band-pass filter (A = 418 nm, AA = 8.5 nm). For A1 complexes, however, the excitation was achieved through a 5-cm-path saturated aqueous solution of CuS04.5H20. The decay signal was analogldigital converted and accumulated on a Kawasaki Electronica transient memory Model M-50E. The lifetimes were determined from the decay curves by use of the least-squares method. Solutions of the porphyrins were sealed after being purged with nitrogen gas just before the emission measurements. ‘HN M R spectra were taken on a JEOL FX-100 spectrometer using Me4% as an internal standard. Dichloromethane, chloroform, acetonitrile, methanol, ethanol, ethylene glycol, acetone, hexane, and M T H F used for solvents were purified by distillation. Benzene (Luminasol spectroscopic grade), CF3COOH (Tokyo Kasei), and glycerol (Merck fluorescence microscopy grade) were used without further purification. Deuterated solvents including CF,COOD were obtained from Merck.
Results and Discussion Figure 1 shows absorption and fluorescence spectra of the dimers o,o’-C3((TTP)H2),, O,O’-C,((TTP)Z~)~,and 0,0’-C3((TTP)AlCl), and their corresponding monomers (TTP)H2, (TTP)Zn, and (TTP)AlCl. The dimer Q (S,)band is twice as intense as the monomer Q band, while the dimer B (S,) band is rather broad and not quite twice as intense as the monomer B band. Figure 2 shows the corresponding second-derivative absorption spectra in the B region. The minima of the second-derivative spectra correspond to the absorption maxima, and thus two absorption maxima are present in the dimer B band. The porphyrin dimer used in the present work is conformationally flexible. However the two Soret peaks observed indicate (14) Quirnby, D. J.; Longo, F . R. J. Am. Chem. SOC.1975, 97, 51 11. (15) Demas,J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
3
Wavenumber/ 103cm-’ Figure 2. Second-derivative(-) and absorption spectra (---) in the Soret band of the dimers o,o’-C3((TTP)H,), in dichloromethane (Ia) o,~’-c~((TTP)zn)~ in benzene (Ib), and o,o’-C,((TTP)AICI), in methanol (IC) and their corresponding monomers (TTP)H, in dichloromethane (IIa), (TTP)Zn in benzene (IIb), and (TTP)AlCI in methanol (IIC).
‘I
twisted“ I,
“stacked”
I,
‘I
b”
Figure 3. Two most probable conformations of the porphyrin dimers based on computer calculations. For clarity, only the porphyrin meso phenyl groups in the linking bridge are shown.
that this particular flexible dimer is mainly in two different conformations. The 8.4 ppm pyrrole proton signal also shows a remarkable splitting in the dimer. An exciton coupling gives rise to a blue shift of the B band when two porphyrin moieties are ~ t a c k e d , while ~ . ~ the interaction is much reduced in a twisted conformation. The red component of the dimer Soret band is ascribed to the “twisted” conformation (“a”) and the blue component to the “stacked” conformation (“b”). The relative intensities of the two absorption peaks vary with temperature. An enhancement of the blue component is observed for o,o’-C,((TTP)AlCl)* in glycerol when the temperature is increased. Internal rotation around the carbon-carbon bonds in the chain allows two conformations of the porphyrin dimer that are rather free from steric hindrance. With the assumption of the local geometries found in metallotetrapheny1porphinesl6 and dimethoxybenzene,” free rotation around the carbon-carbon single bonds (16) Scheidt, W. R.In The Porphyrins I& Dolphin, D., Ed.; Academic: New York, 1978; p 463. (17) Goodwin, T. H.; Przybylska, M.; Robertson, J. M. Acro Crystallogr. 1950, 3, 279.
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The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
Kaizu et al.
TABLE I: Fluorescence Properties of Porphyrin Dimers and Their Corresponding Monomers comvd solvent br(S,)” o,o’-C,((TTP)Hz)z (TTP)H, o,o’-C,((TTP)Hz)2 (TWH2 o,~’-c,((TTP)zn)~ (TTP)Zn o,o’-C3((TTP)Zn), (TTP)Zn o,o’-C,((TTP)Zn), (TTP)Zn 0,0’-C3( (TTP)AICI), (TTP) AlCl 0,0’-C3( (TTP)AICI)z (TTP) AlCl o,o’-C~( (TTP)AICI), (TTP)AlCI o,o’-C~((TTP)H~+)~ (TTP) H4*+
0.15 0.21
0.10 0.15 0.14 0.17 0.12 0.17
OExcitation at 550 nm. *Excitation at 400 nm. CNoS2 emission detected (&
< 0.03
compd
medium
T;/ms
X,,,“/nm
ethanol ethanol
55 59 29 25
772 772 786 788
MTHF MTHF
X
&(S2)*/
lo4
C C C C
0.15 3.7 0.20 5.7 0.20 5.9 0.78 7.8 1.9 7.6 1.6 9.7 0.03 0.17
IO4; see text).
0.2,
TABLE 11: Phosphorescence Lifetimes of Porphyrin Dimers o,o’-C3((TTP)AICI), (TTP)AICI o,o’-C3((TTP)Zn), (TTP)Zn
rp/ns 11.9 11.5 9.2 8.7 2.1 2.1 1.8 1.5 2.2 1.7 5.3 5.4 7.8 7.5 7.8 7.8 2.3 2.0
0.088 0.12 0.098 0.10 0.038 0.036 0.029 0.03 1 0.033 0.032
benzene benzene dichloromethane dichloromethane benzene benzene dichloromethane dichloromethane methanol methanol dichloromethane dichloromethane methanol methanol acetonitrile acetonitrile CF,COOH/ benzene C F3COOH/benzene
,
,
.
,
I
. .
,
.
I
T
Tt/ns 0.12 0.13 0.034 0.034
8.8 8.6 2.0 1.8
0.0
II
“Measured with rigid glass media at 77 K. bFluorescence yields and decay lifetimes at room temperature also measured.
and van der Waals radii for all the atoms in the porphyrin dimer, computer calculations can predict two possible conformations without much hindrance such as drawn in Figure 3 (“a” and “b”), which have dihedral angles of the porphyrin moieties of 80° and 56”, respectively. SI emission is a mirror image of the Q band regardless of whether the porphyrin is a monomer or a dimer, and the decay is a single exponential even in the dimer. Both the band shape and the decay lifetime of the SI emission of the zinc dimer are in good agreement with those of the monomer, while small differences in the quantum yields of the dimer and the monomer are observed both in the free base and in the aluminum complex. Table I summarizes the lifetimes and the quantum yields of Si fluorescence obtained in the present work. The metal porphyrin dimers exhibit TI emission bands that are coincident with those of the respective monomers. No sizable differences were detected in the decay lifetimes as shown in Table 11. Figure 1 also shows the S2emission bands of o,o’-C,((lTP)Zn), and o,o’-C3( (TTP)AICI), and their corresponding monomers. Excitation spectra of the monomer S2emission coincide with the respective absorption spectra up to the ultraviolet region. S2 emission of the free base porphyrins cannot be detected by the sensitivity used in the present work, for which a quantum yield is the detectable limit. However, a diffuse and of 0.03 X red-shift S2emission of the free bases can be observed by laser excitation.’* The excitation spectra of the dimer S2 emissions correspond well to the red components of &ret bands of the dimers and also to the Soret bands of the respective monomers. This suggests that S2emission comes only from the dimer in the twisted conformation. A p o x 0 dimer [(TPP)Nb],O,, in which two porphyrin moieties are in a “face-to-face” stacking, gives rise to a remarkable Davydov splitting in the Soret band and exhibits no S2emission but shows both Si and TI emissions.’ Table I also presents the quantum yields of S2emission. Diffuse Raman lines ascribed to solvent molecules appear in the S2 region.Ig The quantum yields of S, emission given in Table I were (18) Tobita, S.; Kaizu, Y.; Kobayashi, H.; Tanaka, I. J . Chem. Phys. 1984, 81, 2962.
15
20
25
Wovenumber / 103cm-’ Figure 4. Fluorescence polarization (I) and excitation spectra (11) of O,O’-C,((TTP)AICI)~(-) and (TTP)AICI (---) in glycerol at 8.0 “C (Ae,,, = 660 nm).
corrected for the solvent Raman scattering. It is noted in each case that the quantum yield of the dimer is much less than that of the corresponding monomer. The dimer in a conformation like “a” (twisted) emits since S2 SI internal conversion is retarded, while strong exciton coupling accelerates electronic relaxation in the dimer of conformation “b” (stacked). Figure 4 presents the fluorescence polarization spectra of 0,o’-C,((TTP)AICl), and (TTP)AICI in glycerol at 8 O C (viscosity, ca. 4000 cP; concentration of porphyrin
