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Synthesis and spectroscopic properties of phthalocyanine dimers in solution
20 October 1995
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 245 (1995) 23-29
Synthesis and spectroscopic properties of phthalocyanine dimers in solution A. Ferencz a, D. Neher a, M. Schulze a, G. Wegner a, L. Viaene b, F.C. De Schryver b a Max-Planck-lnstitutfiir Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany h Departement Scheikunde, Katholieke Universiteit Leuven, Celestijnenlaan 200G, 3001 Heverlee, Belgium
Received 6 July 1995; in final form 22 August 1995
Abstract A soluble dimer of a tetra-(methoxy)-tetra-(octyloxy)-substituted silicon phthalocyanine has been prepared. Optical absorption spectra of the compound in solution showed a pronounced solvatochromism, characterized by a rather sharp peak with maximum at 640 nm in aromatic solvents and a broad absorption band consisting of up to five individual peaks for non-aromatic solutions such as THF or chloroform. Nanosecond transient absorption spectra exhibited a long-lived photoinduced absorption band at 520 nm which can be assigned to triplet absorption. Large quantum yields for intersystem crossing ranging between 30% and 40% were observed. These findings are compared to the results on the monomeric and polymeric compounds and discussed in the framework of exciton coupling.
1. Introduction
Phthalocyanines (Pc) are a promising class of materials for electronic and photonic devices due to their superior chemical and physical stability. Coplanar face-to-face stacking of these w-conjugated macrocycles leads to extended one-dimensional systems with new electronic and optical properties. One particular polymer, the phthalocyaninato-polysiloxane (PcPS), has successfully been used in the fabrication of planar photodiodes and sensors [1,2]. Coplanar stacked Pc-dimers can be considered as model compounds lbr these one-dimensional extended systems. Extensive experimental and theoretical investigations have been performed on the electronic structure and optical properties of Pc-dimers [3-6]. As predicted by exciton coupling theory [7] the individual monomeric singlet levels split into pairs of new
states in the co-facial dimer. In the co-facial stacked geometry the optical transition from the H O M O to the lowest excited singlet state is allowed only to the upper dimer level which results in a blue-shift of the Q-band of around 3 0 - 4 0 nm for most Pc-dimers. As discussed by Orti et al. [6] the major factor influencing the splitting and thus the location of the dimeric Q-band are geometrical factors like the interring spacing and the staggering angle a. Theoretical calculations by Hush and Woolsey however predicted a rather small effect of a on the spectral shift [8] in the dimer. Since the direct transition from the lowest excited singlet level back to the ground state is optically forbidden, most Pc-dimers do not show any detectable fluorescence. Enhanced intersystem crossing to the triplet state can thus be expected. In this Letter we report on the synthesis and spectroscopic characterization of a RSi(tetra-
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00978-7
24
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
RzO R,O~
R20 R~O~ q
0Rt
~ OR2 R - ~ ) - sf'i"~ 0 - - ~ ) J'N" ~ N ,.J.'N"
Ra0
N
~0R1RzO R20
OR~
R=O
RI=CH3, R2=CsH17 Fig. 1. Molecular structure of the substituted Pcl,8-dimer.
(methoxy)-tetra-(octyloxy)-Pc)OSi(tetra-(methoxy)tetra-(octyloxy)-Pc)R dimer (Pc 1,8-dimer, see Fig. 1) as the minimal model compound for the PcPS polymer. In particular, the triplet absorption properties as studied by transient absorption will be outlined and compared to the properties of the monomeric and polymeric compounds.
2. Experimental
2.1. Synthesis Chemicals and solvents were obtained from Fluka or Aldrich. Purification is only necessary where described. Octa-(alkoxy)-phthalocyaninato-silicon dichloride was prepared according to Ref. [9] and thallium trifluoromethanesulfonate (T1SO3CF3) was prepared according to Ref. [10].
Octa-(alkoxy)phthalocyaninato-silicon dihydroxide (1). The hydrolysis of the alkoxy-substituted dichlorosilicon phthalocyanine can be easily performed by column chromatography with non dehydrated (i.e. water containing) solvents. A mixture of chloroform and methanol (98:2 v : v ) is appropriate for use with Merck silica gel for flash chromatography. Hydrolysis can be monitored by inspection of the additional OH-vibration (820 cm - ] ) in the infrared spectrum of the compound.
B is [ octa-( alkoxy )-phthalocyaninato ]-disilo xane (2). 0.5 mmol of compound 1 is refluxed with 0.55
mmol of T1SO3CF3 in 30 ml pyridine for 30 min. After that, the reaction mixture is given in a 100 ml mixture of water and methanol (1 : 1 v : v). The blue precipitate is filtered off, washed with methanol and dried under vacuum. The purification is performed by flash chromatography over silica gel. In the first step the unreacted monomer is separated from the oligomers using a mixture of chloroform and methanol (99:1 v : v ) as solvent. In the U V / V I S spectrum of the eluant the absorption at 680 nm (CHCI 3) vanishes upon complete extraction of the monomer. In the second step the dimer will be eluted by a mixture of chloroform and methanol (90:10 v : v ) or a mixture of chloroform, acetone and methanol (2 : 1 : 1 v : v : v). The solvent is evaporated and the product freeze dried from benzene (yield 85%). The product consists of almost pure dimer which is characterized by GPC, HPLC, mass-spectroscopy, cyclovoltammetry and U V / V I S spectroscopy. High molecular weight PcPS is hard to synthesize without oligomeric impurities following the procedures outlined in Ref. [10]. Instead of polymerizing the monomers directly it is convenient to polymerize the purified dimers [11] to get a pure, high molecular weight polymer:
Alkoxysubstituted phthalocyaninato-polysiloxane (3). In 50 ml of diglyme (freshly distilled over sodium) 0.2 mmol of 2 is reacted with 0.25 mmol of T1SO3CF3 for 72 h at 140°C. The reaction mixture is precipitated in methanol, filtered off using a PTFEmembrane filter (pore size of 5 ram) and finally dissolved in chloroform by washing the solution through the filter. For purification this precipitation procedure is repeated twice.
2.2. Optical spectroscopy and cyclovoltammetry Optical absorption spectra were recorded for different concentrations of the monomer, dimer and polymer in different solvents using a Perkin-Elmer Lambda 9 spectrophotometer. Spectroelectrochemical studies were carried out as described in Ref. [11] by placing a self-built electrochemical cell in the sample compartment of an HP8452A diode array spectrometer.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
25
2.3. Transient absorption experiments and laser-induced opto-acoustic spectroscopy The experimental apparatus has been described in detail in Ref. [12]. A Spectra Physics GCR 3 pulsed N d : Y A G laser was used as excitation source. The fundamental output was tripled to 355 nm using a SHG-2 Harmonic Generator. The pulse width was 8 ns and the maximum energy was 60 mJ per pulse. The analyzing light for the transient absorption experiments was provided by a pulsed 450 W xenon lamp (MiJller Elektronik-Optik). The transient spectra were analyzed simultaneously over the whole wavelength region using an OMA II system ( E G & G Instruments) with a gated amplifier. The experimental apparatus for the laser-induced opto-acoustic spectroscopy experiments was the same as described previously with a time resolution of the detector of 200 ns [13]. 2-hydroxybenzophenone was used as the reference compound.
3. Results and discussion
,~
Ber~er~
O
Dic~orobs'~zene
Chloroform
Dichfororn~hane TIP
TFFI M e[l~nol ,
r~OctanOl TPFIM ~hanollH20
300
46o
56o 660 76o Wavelength Inm
86o
9 0
Fig. 2. Absorptionspectra of the Pcl,8-dimer in different solvents• The concentrations of the solutions ranged between 1× 10-6 and 5 X 10 -6 m o l / l .
3.1. Ground state absorption properties The linear absorption spectra of the dimer in solution changed slightly with concentration indicating some degree of aggregation. Surprisingly, the spectra showed a pronounced solvatochroism as demonstrated in Fig. 2. The sharp peak at 640 nm, typical for Pc-dimers, is evident only in aromatic solvents such as benzene or toluene while considerable broadening into a broad band consisting of up to five equivalent transitions is observed for different non-aromatic solvents. From the following observations we can almost exclude the formation of aggregates causing the observed solvatochroism of the dimer: solvatochroism is well established even at rather low concentrations of 5 X 10 -7 mol/1. Furthermore absorption spectra measured at different temperatures in toluene first showed an increase in the absorption at 640 nm with increasing temperature followed by a decrease above T = 50°C. This decrease is untypical for aggregate formation. A close inspection of the absorption spectra for different solvents revealed peaks always at similar positions (e.g. 561,581,639, 677 and 714 nm for toluene and
561, 598, 637, 680, 711 nm for dichloromethane). The solvent thus does not influence the peak positions but leads rather to a redistribution of oscillator strength. It is not possible to give a conclusive physical explanation of the observed solvatochroism based on the current status of knowledge. One may, however, argue that solvent molecules in the direct vicinity of the dimer influence the excitonic and vibronic coupling between the w-conjugated rings in such a way that optically forbidden transitions became partly allowed under certain conditions. A theoretical framework which may be used to explain this effect has been published by Fulton and Gouterman [14,15].
3.2. Transient absorption (TA) experiments Transient absorption spectra recorded 100 ns after excitation with 354 nm are shown in Fig. 3 for solutions of the dimer in toluene, chloroform and THF. Solutions in THF or chloroform were photochemical not stable and, therefore, fresh solutions
26
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
? ~5
' 460 ' 500 ' 600 ' 700 ' wavelengfll/mn
'
Fig. 3. Transient absorption (TA) spectra of the Pcl,8-dimer in toluene (top), chloroform (middle) and THF (bottom) recorded 100 ns after excitation at 355 nm. The gate width was 100 ns and the laser energy 60 mJ/pulse. The concentrations of the solutions were adjusted so that the maximum of the ground state absorption (toluene: 639 nm, chloroform: 637 nm, THF: 560 nm) was 0.75.
were used for each recorded spectrum. In situ spectroelectrochemical studies on the P c l , 8 - d i m e r in solution showed that some degradation occurs in the cationic state accompanied by the formation o f the monomeric species. One possible step in the photochemical degradation process observed during the T A experiment m a y thus be photoproduction o f the radical cation o f the dimer. All spectra consisted o f a region with negative T A in the wavelength range of the original ground state absorption and a positive T A at wavelength of low ground state absorption between 350 and 550 nm.
The shape of the spectra in the region o f negative T A corresponds well to the optical absorption in the ground state. We, therefore, assign these features to the photoinduced ground state depletion. In the evolution o f the transient spectra as a function of time isosbestic points are observed indicating that there is only one transient species. This is further supported by the observation that the decay in the triplet absorption and the increase in the ground state absorption with time are single-exponential over a wide time range with similar rate constants for both increase and decay. The shape o f the spectra did not change as a function o f the laser energy used. Thus the formation of radical ions by bi-photonic processes can be excluded. Photoinduced radical pair formation has been frequently observed in T A experiments on different dimeric compounds. As an example photoexcitation in LB films o f octadecylrhodamine dimers first led to the occupation of the triplet state by intersystem crossing followed by a charge separation process into a radical pair [16]. Intramolecular charge transfer was further observed in porphyrin-phthalocyanine mixed dimers [17]. In solution, however, only a fast energy transfer to the phthalocyanine ring was detected with no evidence of a charge transfer process. W e could further exclude photoinduced radical pair formation as the main source o f the positive T A signal by comparing the T A spectrum to the ground state absorption o f the electrochemical oxidized dimer in
Table 1 Properties of the triplet state of Pcl,8-monomer, -dimer and the PcPS polymer in different solvents as obtained from transient absorption (TA) experiments Compound Solvent Areax (nm) a 7 (p,s) b ( _ 10%) Percentage c Pc 1,8-dimer Pc 1,8-dimer Pc 1,8-dimer
toluene chloroform THF
520 520 515
90 100 130
30 35 40
Pc 1,8-monomer Pc 1,8-monomer Pc 1,8-monomer
toluene chloroform THF
525 525-575 515
210 180 180
15 50 45
PcPS
chloroform
d
d
d
Maximum wavelength of the positive TA peak. b Lifetimes as determined from the decay constants of the triplet absorption. c Percentage of the molecules converted to the triplet state on excitation with pulses of 60 mJ. d No detectable transient absorption signal.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
solution. The latter shows the absorption peak at 580 nm, while no such peak can be observed in our TA spectra. We propose that the transient species observed in the TA experiment is the triplet state, populated via intersystem crossing upon excitation. It is important to note that the TA spectrum of the Pcl,8-dimer in THF or chloroform showed a depletion over the whole wavelength region of the original ground state absorption. If the linear absorption spectrum consisted of contributions from isolated dimer molecules and from aggregates and if these constituents possess different quantum yields of intersystem crossing to the triplet manifold one would expect that specific transition bands are more efficiently bleached in the TA experiments. Additional transient absorption experiments were performed on the Pcl,8-monomer in toluene, chloroform and THF solution and on the PcPS polymer in chloroform. The results are summarized in Table 1. As in the case of the dimer the monomer was photochemical unstable in chloroform and THF. No photochemical reaction was observed for the PcPS solution. As shown in Fig. 4 the TA spectra of the monomer corresponded well to the photoinduced absorption spectrum of the dimer. The resemblance of the monomer and dimer TA spectra is an indication that the excitonic coupling has only minor effects on the triplet properties. This is in agreement with phosphorescence studies on rhodamine B dimers which showed a splitting of the dimeric triplet state of less than 2 c m - 1 [ 18].
'
400
'
500 600 wavelength / nm
700
Fig. 4. Transient absorption (TA) spectra of the Pcl,8-monomer in toluene (top), chloroform (middle) and THF (bottom) 100 ns after excitation at 355 nm. The gate width was 100 ns and the laser energy 60 m J / p u l s e . The concentrations of the solutions were adjusted to give a maximum ground state absorption of 0.75.
xO
27
-o.1 I
m I
-0.21 0
2'0 ' 4'0 laser energy / mJ
'
6'0
Fig. 5. Influence of the laser energy on the transient absorption at 639 nm of the Pc 1,8-dimer in toluene. The ground state absorption at 639 nm was 0.75.
Within the experimental resolution we could not detect any photoinduced absorption signal in the chloroform solution of the PcPS polymer. Since PcPS also does not show any detectable fluorescence the relaxation to the ground state must occur via non-radiative internal conversion. The linear absorption spectrum of PcPS shows a broad tail which extends far to the NIR region. Temperature-dependent conductivity data on PcPS-LB films further indicate the existence of low-lying trapping states [19]. If some of these states are located below the lowest excited triplet level the quantum yield for intersystem crossing must be strongly reduced in favour of direct internal conversion. As shown in Fig. 5 the depletion of the ground state saturates at higher laser energy. By comparing the original ground state absorption and the TA spectra at the wavelength of the maximum ground state absorption, the percentage of molecules in the triplet state after one laser pulse can be estimated. The figures range from 15% for the monomer in toluene over 30%-40% for the dimer to 50% for the monomer in chloroform (Table 1). If we assume that all molecules have been excited within one laser pulse, these figures can approximately be regarded as the quantum yields for intersystem crossing. Since the monomer and dimer are not photochemically stable in chloroform and THF the rather high quantum yields of triplet formation for the monomer in these solvents should be interpreted with care. The positive TA bands of the monomer in chloroform and THF are indeed rather broad and featureless when compared to the TA spectra of the monomer in toluene or to the dimeric spectra as shown in Fig. 3.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
28
3.3. Laser-induced opto-acoustic (LIOAS)
spectroscopy
The absence of any detectable phosphorescence of the Pc 1,8-dimer even in isopentane glass at 77 K at wavelengths shorter than 800 nm suggests that the triplet energy is lower than 12500 cm -~. LIOAS experiments on the dimer in toluene support this suggestion. The fraction a of the energy of the absorbed photons converted into heat within the time resolution of the apparatus was 0.91 __+0.03. The following relationshi p exists between a and the energy of the lowest excited triplet state [13]: O~Eex = [ Eex - E(S1) ] -'[- (~f[ E(Zg ) - E ( S 0 ) ] -'1- (/),sc[E(S1) - - E ( T , ) ] "{- 6 i c g ( S l ) ,
(1) with Eex = 28169 cm -1 (355 nm). Assuming a splitting between the dimeric singlet states of about 3000 c m - l [8] and the energy of the upper state equal to 15625 cm-1 (640 nm) we obtain an estimate for the energy of the lowest singlet state in the Pcl,8-dimer E(S ~) of about 12625 c m - l (790 nm). Assuming the fluorescence quantum yield ~f = 0, the intersystem crossing quantum yield 4hsc = 0.30 and the internal conversion yield ~ic = 0.70, a triplet energy E(T 1) of 8451 cm -1 (1183 nm) can be calculated. For PcPS in chloroform a = 0.98 _+ 0.03, which is in agreement with the observed absence with the observed photoinduced triplet formation. If we presume that E(T~) is the same also for the polymer (due to the small effect of the excitonic coupling on the triplet manifold) the lowest singlet state in PcPS must be located even below 8451 cm - I which is equivalent to a splitting of the polymeric singlet state of about 10000 cm -~. This is in rough agreement to an estimation for E(S l) of 8448 cm -1 based on exciton coupling theory [20].
4. Conclusion The results presented above support the model of the electronic structure of co-facial stacked phthalocyanine rings [20]. The pronounced solvatochroism of the dimer in different aromatic and non-aromatic solvents indicates a complicated coupling mecha-
nism between the Pc rings, which strongly effects the singlet spectrum but has only a minor effect on the triplet absorption properties. Both the monomer and the dimer show a comparable location of the triplet absorption in the transient absorption spectra. The absence of either fluorescence and detectable photoinduced absorption in the case of the polymer can be explained by internal conversion through low-lying singlet states.
Acknowledgement The authors would like to thank M. Katayose for his efforts in optimizing the synthesis, N.R. Armstrong, University of Arizona, for help in performing the electrochemical and spectroelectrochemical studies and C. Kryschi, University Diisseldorf, for many fruitful discussions. AF gratefully acknowledges support by the Kekul6 Scholarship of the Stiftung Stipendien-Fonds des Verbandes der Chemischen Industrie. The Belgian authors thank the Ministry of Wetenschapsbeleid for support (IUAP-II-16 and IUAP-III-040). FCDS thanks the Alexander yon Humboldt-Stiftung for support during his stay in Mainz.
References [1] Th. Saner, W. Caseri, G. Wegner, A. Vogel and B, Hofmann, J. Physik D 23 (1990) 79. [2] L. Kalvoda, R. Back, A. Ferencz, D. Neher and G. Wegner, Mol. Cryst. Liq. Cryst. (1995) in press [3] G. Teng, N.S. Dalai, Synth. Metals 32 (1989) 63. [4] D.W. de Wulf, J.K. Leland, B.L. Wheeler, A.J. Bard, D.A. Bartzel, D.R. Dininny and M.E. Kenney, Inorg. Chem. 26 (1987) 266. [5] E. Ciliberto, K.A. Doris, W.J. Peitro, G.M. Reisner, D.E. Ellis, I. Fragala, F.H. Herbstein, M.A. Rather and T.J. Marks, J. Am. Chem. Soc. 106 0984) 7748. [6] E. Orti, J.L. Bredas and C. Clarisse, J. Chem. Phys. 92 (1990) 1228. [7] M. Kasha, Radiation Res. 20 (1963) 55. [8] N.S. Hush and I.S. Woolsey, Mol. Phys. 21 (1971) 465. [9] E. Orthmann and G. Wegner, Angew. Chem. Intern. Ed. Engl. 25 (1986) 114. [10] W. Caseri, T. Saner and G. Wegner, Makromol. Chem. Rapid Commun. 9 (1988) 651. [11] A. Ferencz, N.R. Armstrong and G. Wegner, Macromolecules 27 (1994) 1517.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
[12] Ph. Van Haver, M. Van der Auweraer, L. Vieane, F.C. De Schryver, J.W. Verhoeven and H.J. van Ramesdonk, Chem. Phys. Letters 198 (1992) 361. [13] Ph. Van Haver, L. Vieane, M. Van der Auweraer and F.C. De Schryver, J. Photochem. Photobiol. A 63 (1992) 265. [14] R.L. Fulton and M. Gouterman, J. Chem. Phys. 35 (1961) 1059. [15] R.L. Fulton and M. Gouterman, J. Chem. Phys. 41 (1964) 228(/.
29
[16] E. Vuorimaa, M. Ikonen and H. Lemmetyinen, Thin Solid Films 214 (1992) 243. [17] T.H. Tran-Thi, J.F. Lipskier, M. Simoes and S. Palacin, Thin Solid Films 210/211 (1992) 150. [18] R.W. Chambers, T. Kajiwara and DR. Kearns, J. Phys. Chem. 78 (1974) 380. [19] M. Gurka, Diploma Thesis, University of Heidelberg (1994). [20] Th. Sauer, W. Caseri and G. Wegner, Mol. Cryst. Liquid Cryst. 183 (1990) 387.
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 245 (1995) 23-29
Synthesis and spectroscopic properties of phthalocyanine dimers in solution A. Ferencz a, D. Neher a, M. Schulze a, G. Wegner a, L. Viaene b, F.C. De Schryver b a Max-Planck-lnstitutfiir Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany h Departement Scheikunde, Katholieke Universiteit Leuven, Celestijnenlaan 200G, 3001 Heverlee, Belgium
Received 6 July 1995; in final form 22 August 1995
Abstract A soluble dimer of a tetra-(methoxy)-tetra-(octyloxy)-substituted silicon phthalocyanine has been prepared. Optical absorption spectra of the compound in solution showed a pronounced solvatochromism, characterized by a rather sharp peak with maximum at 640 nm in aromatic solvents and a broad absorption band consisting of up to five individual peaks for non-aromatic solutions such as THF or chloroform. Nanosecond transient absorption spectra exhibited a long-lived photoinduced absorption band at 520 nm which can be assigned to triplet absorption. Large quantum yields for intersystem crossing ranging between 30% and 40% were observed. These findings are compared to the results on the monomeric and polymeric compounds and discussed in the framework of exciton coupling.
1. Introduction
Phthalocyanines (Pc) are a promising class of materials for electronic and photonic devices due to their superior chemical and physical stability. Coplanar face-to-face stacking of these w-conjugated macrocycles leads to extended one-dimensional systems with new electronic and optical properties. One particular polymer, the phthalocyaninato-polysiloxane (PcPS), has successfully been used in the fabrication of planar photodiodes and sensors [1,2]. Coplanar stacked Pc-dimers can be considered as model compounds lbr these one-dimensional extended systems. Extensive experimental and theoretical investigations have been performed on the electronic structure and optical properties of Pc-dimers [3-6]. As predicted by exciton coupling theory [7] the individual monomeric singlet levels split into pairs of new
states in the co-facial dimer. In the co-facial stacked geometry the optical transition from the H O M O to the lowest excited singlet state is allowed only to the upper dimer level which results in a blue-shift of the Q-band of around 3 0 - 4 0 nm for most Pc-dimers. As discussed by Orti et al. [6] the major factor influencing the splitting and thus the location of the dimeric Q-band are geometrical factors like the interring spacing and the staggering angle a. Theoretical calculations by Hush and Woolsey however predicted a rather small effect of a on the spectral shift [8] in the dimer. Since the direct transition from the lowest excited singlet level back to the ground state is optically forbidden, most Pc-dimers do not show any detectable fluorescence. Enhanced intersystem crossing to the triplet state can thus be expected. In this Letter we report on the synthesis and spectroscopic characterization of a RSi(tetra-
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00978-7
24
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
RzO R,O~
R20 R~O~ q
0Rt
~ OR2 R - ~ ) - sf'i"~ 0 - - ~ ) J'N" ~ N ,.J.'N"
Ra0
N
~0R1RzO R20
OR~
R=O
RI=CH3, R2=CsH17 Fig. 1. Molecular structure of the substituted Pcl,8-dimer.
(methoxy)-tetra-(octyloxy)-Pc)OSi(tetra-(methoxy)tetra-(octyloxy)-Pc)R dimer (Pc 1,8-dimer, see Fig. 1) as the minimal model compound for the PcPS polymer. In particular, the triplet absorption properties as studied by transient absorption will be outlined and compared to the properties of the monomeric and polymeric compounds.
2. Experimental
2.1. Synthesis Chemicals and solvents were obtained from Fluka or Aldrich. Purification is only necessary where described. Octa-(alkoxy)-phthalocyaninato-silicon dichloride was prepared according to Ref. [9] and thallium trifluoromethanesulfonate (T1SO3CF3) was prepared according to Ref. [10].
Octa-(alkoxy)phthalocyaninato-silicon dihydroxide (1). The hydrolysis of the alkoxy-substituted dichlorosilicon phthalocyanine can be easily performed by column chromatography with non dehydrated (i.e. water containing) solvents. A mixture of chloroform and methanol (98:2 v : v ) is appropriate for use with Merck silica gel for flash chromatography. Hydrolysis can be monitored by inspection of the additional OH-vibration (820 cm - ] ) in the infrared spectrum of the compound.
B is [ octa-( alkoxy )-phthalocyaninato ]-disilo xane (2). 0.5 mmol of compound 1 is refluxed with 0.55
mmol of T1SO3CF3 in 30 ml pyridine for 30 min. After that, the reaction mixture is given in a 100 ml mixture of water and methanol (1 : 1 v : v). The blue precipitate is filtered off, washed with methanol and dried under vacuum. The purification is performed by flash chromatography over silica gel. In the first step the unreacted monomer is separated from the oligomers using a mixture of chloroform and methanol (99:1 v : v ) as solvent. In the U V / V I S spectrum of the eluant the absorption at 680 nm (CHCI 3) vanishes upon complete extraction of the monomer. In the second step the dimer will be eluted by a mixture of chloroform and methanol (90:10 v : v ) or a mixture of chloroform, acetone and methanol (2 : 1 : 1 v : v : v). The solvent is evaporated and the product freeze dried from benzene (yield 85%). The product consists of almost pure dimer which is characterized by GPC, HPLC, mass-spectroscopy, cyclovoltammetry and U V / V I S spectroscopy. High molecular weight PcPS is hard to synthesize without oligomeric impurities following the procedures outlined in Ref. [10]. Instead of polymerizing the monomers directly it is convenient to polymerize the purified dimers [11] to get a pure, high molecular weight polymer:
Alkoxysubstituted phthalocyaninato-polysiloxane (3). In 50 ml of diglyme (freshly distilled over sodium) 0.2 mmol of 2 is reacted with 0.25 mmol of T1SO3CF3 for 72 h at 140°C. The reaction mixture is precipitated in methanol, filtered off using a PTFEmembrane filter (pore size of 5 ram) and finally dissolved in chloroform by washing the solution through the filter. For purification this precipitation procedure is repeated twice.
2.2. Optical spectroscopy and cyclovoltammetry Optical absorption spectra were recorded for different concentrations of the monomer, dimer and polymer in different solvents using a Perkin-Elmer Lambda 9 spectrophotometer. Spectroelectrochemical studies were carried out as described in Ref. [11] by placing a self-built electrochemical cell in the sample compartment of an HP8452A diode array spectrometer.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
25
2.3. Transient absorption experiments and laser-induced opto-acoustic spectroscopy The experimental apparatus has been described in detail in Ref. [12]. A Spectra Physics GCR 3 pulsed N d : Y A G laser was used as excitation source. The fundamental output was tripled to 355 nm using a SHG-2 Harmonic Generator. The pulse width was 8 ns and the maximum energy was 60 mJ per pulse. The analyzing light for the transient absorption experiments was provided by a pulsed 450 W xenon lamp (MiJller Elektronik-Optik). The transient spectra were analyzed simultaneously over the whole wavelength region using an OMA II system ( E G & G Instruments) with a gated amplifier. The experimental apparatus for the laser-induced opto-acoustic spectroscopy experiments was the same as described previously with a time resolution of the detector of 200 ns [13]. 2-hydroxybenzophenone was used as the reference compound.
3. Results and discussion
,~
Ber~er~
O
Dic~orobs'~zene
Chloroform
Dichfororn~hane TIP
TFFI M e[l~nol ,
r~OctanOl TPFIM ~hanollH20
300
46o
56o 660 76o Wavelength Inm
86o
9 0
Fig. 2. Absorptionspectra of the Pcl,8-dimer in different solvents• The concentrations of the solutions ranged between 1× 10-6 and 5 X 10 -6 m o l / l .
3.1. Ground state absorption properties The linear absorption spectra of the dimer in solution changed slightly with concentration indicating some degree of aggregation. Surprisingly, the spectra showed a pronounced solvatochroism as demonstrated in Fig. 2. The sharp peak at 640 nm, typical for Pc-dimers, is evident only in aromatic solvents such as benzene or toluene while considerable broadening into a broad band consisting of up to five equivalent transitions is observed for different non-aromatic solvents. From the following observations we can almost exclude the formation of aggregates causing the observed solvatochroism of the dimer: solvatochroism is well established even at rather low concentrations of 5 X 10 -7 mol/1. Furthermore absorption spectra measured at different temperatures in toluene first showed an increase in the absorption at 640 nm with increasing temperature followed by a decrease above T = 50°C. This decrease is untypical for aggregate formation. A close inspection of the absorption spectra for different solvents revealed peaks always at similar positions (e.g. 561,581,639, 677 and 714 nm for toluene and
561, 598, 637, 680, 711 nm for dichloromethane). The solvent thus does not influence the peak positions but leads rather to a redistribution of oscillator strength. It is not possible to give a conclusive physical explanation of the observed solvatochroism based on the current status of knowledge. One may, however, argue that solvent molecules in the direct vicinity of the dimer influence the excitonic and vibronic coupling between the w-conjugated rings in such a way that optically forbidden transitions became partly allowed under certain conditions. A theoretical framework which may be used to explain this effect has been published by Fulton and Gouterman [14,15].
3.2. Transient absorption (TA) experiments Transient absorption spectra recorded 100 ns after excitation with 354 nm are shown in Fig. 3 for solutions of the dimer in toluene, chloroform and THF. Solutions in THF or chloroform were photochemical not stable and, therefore, fresh solutions
26
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
? ~5
' 460 ' 500 ' 600 ' 700 ' wavelengfll/mn
'
Fig. 3. Transient absorption (TA) spectra of the Pcl,8-dimer in toluene (top), chloroform (middle) and THF (bottom) recorded 100 ns after excitation at 355 nm. The gate width was 100 ns and the laser energy 60 mJ/pulse. The concentrations of the solutions were adjusted so that the maximum of the ground state absorption (toluene: 639 nm, chloroform: 637 nm, THF: 560 nm) was 0.75.
were used for each recorded spectrum. In situ spectroelectrochemical studies on the P c l , 8 - d i m e r in solution showed that some degradation occurs in the cationic state accompanied by the formation o f the monomeric species. One possible step in the photochemical degradation process observed during the T A experiment m a y thus be photoproduction o f the radical cation o f the dimer. All spectra consisted o f a region with negative T A in the wavelength range of the original ground state absorption and a positive T A at wavelength of low ground state absorption between 350 and 550 nm.
The shape of the spectra in the region o f negative T A corresponds well to the optical absorption in the ground state. We, therefore, assign these features to the photoinduced ground state depletion. In the evolution o f the transient spectra as a function of time isosbestic points are observed indicating that there is only one transient species. This is further supported by the observation that the decay in the triplet absorption and the increase in the ground state absorption with time are single-exponential over a wide time range with similar rate constants for both increase and decay. The shape o f the spectra did not change as a function o f the laser energy used. Thus the formation of radical ions by bi-photonic processes can be excluded. Photoinduced radical pair formation has been frequently observed in T A experiments on different dimeric compounds. As an example photoexcitation in LB films o f octadecylrhodamine dimers first led to the occupation of the triplet state by intersystem crossing followed by a charge separation process into a radical pair [16]. Intramolecular charge transfer was further observed in porphyrin-phthalocyanine mixed dimers [17]. In solution, however, only a fast energy transfer to the phthalocyanine ring was detected with no evidence of a charge transfer process. W e could further exclude photoinduced radical pair formation as the main source o f the positive T A signal by comparing the T A spectrum to the ground state absorption o f the electrochemical oxidized dimer in
Table 1 Properties of the triplet state of Pcl,8-monomer, -dimer and the PcPS polymer in different solvents as obtained from transient absorption (TA) experiments Compound Solvent Areax (nm) a 7 (p,s) b ( _ 10%) Percentage c Pc 1,8-dimer Pc 1,8-dimer Pc 1,8-dimer
toluene chloroform THF
520 520 515
90 100 130
30 35 40
Pc 1,8-monomer Pc 1,8-monomer Pc 1,8-monomer
toluene chloroform THF
525 525-575 515
210 180 180
15 50 45
PcPS
chloroform
d
d
d
Maximum wavelength of the positive TA peak. b Lifetimes as determined from the decay constants of the triplet absorption. c Percentage of the molecules converted to the triplet state on excitation with pulses of 60 mJ. d No detectable transient absorption signal.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
solution. The latter shows the absorption peak at 580 nm, while no such peak can be observed in our TA spectra. We propose that the transient species observed in the TA experiment is the triplet state, populated via intersystem crossing upon excitation. It is important to note that the TA spectrum of the Pcl,8-dimer in THF or chloroform showed a depletion over the whole wavelength region of the original ground state absorption. If the linear absorption spectrum consisted of contributions from isolated dimer molecules and from aggregates and if these constituents possess different quantum yields of intersystem crossing to the triplet manifold one would expect that specific transition bands are more efficiently bleached in the TA experiments. Additional transient absorption experiments were performed on the Pcl,8-monomer in toluene, chloroform and THF solution and on the PcPS polymer in chloroform. The results are summarized in Table 1. As in the case of the dimer the monomer was photochemical unstable in chloroform and THF. No photochemical reaction was observed for the PcPS solution. As shown in Fig. 4 the TA spectra of the monomer corresponded well to the photoinduced absorption spectrum of the dimer. The resemblance of the monomer and dimer TA spectra is an indication that the excitonic coupling has only minor effects on the triplet properties. This is in agreement with phosphorescence studies on rhodamine B dimers which showed a splitting of the dimeric triplet state of less than 2 c m - 1 [ 18].
'
400
'
500 600 wavelength / nm
700
Fig. 4. Transient absorption (TA) spectra of the Pcl,8-monomer in toluene (top), chloroform (middle) and THF (bottom) 100 ns after excitation at 355 nm. The gate width was 100 ns and the laser energy 60 m J / p u l s e . The concentrations of the solutions were adjusted to give a maximum ground state absorption of 0.75.
xO
27
-o.1 I
m I
-0.21 0
2'0 ' 4'0 laser energy / mJ
'
6'0
Fig. 5. Influence of the laser energy on the transient absorption at 639 nm of the Pc 1,8-dimer in toluene. The ground state absorption at 639 nm was 0.75.
Within the experimental resolution we could not detect any photoinduced absorption signal in the chloroform solution of the PcPS polymer. Since PcPS also does not show any detectable fluorescence the relaxation to the ground state must occur via non-radiative internal conversion. The linear absorption spectrum of PcPS shows a broad tail which extends far to the NIR region. Temperature-dependent conductivity data on PcPS-LB films further indicate the existence of low-lying trapping states [19]. If some of these states are located below the lowest excited triplet level the quantum yield for intersystem crossing must be strongly reduced in favour of direct internal conversion. As shown in Fig. 5 the depletion of the ground state saturates at higher laser energy. By comparing the original ground state absorption and the TA spectra at the wavelength of the maximum ground state absorption, the percentage of molecules in the triplet state after one laser pulse can be estimated. The figures range from 15% for the monomer in toluene over 30%-40% for the dimer to 50% for the monomer in chloroform (Table 1). If we assume that all molecules have been excited within one laser pulse, these figures can approximately be regarded as the quantum yields for intersystem crossing. Since the monomer and dimer are not photochemically stable in chloroform and THF the rather high quantum yields of triplet formation for the monomer in these solvents should be interpreted with care. The positive TA bands of the monomer in chloroform and THF are indeed rather broad and featureless when compared to the TA spectra of the monomer in toluene or to the dimeric spectra as shown in Fig. 3.
A. Ferencz et al. / Chemical Physics Letters 245 (1995) 23-29
28
3.3. Laser-induced opto-acoustic (LIOAS)
spectroscopy
The absence of any detectable phosphorescence of the Pc 1,8-dimer even in isopentane glass at 77 K at wavelengths shorter than 800 nm suggests that the triplet energy is lower than 12500 cm -~. LIOAS experiments on the dimer in toluene support this suggestion. The fraction a of the energy of the absorbed photons converted into heat within the time resolution of the apparatus was 0.91 __+0.03. The following relationshi p exists between a and the energy of the lowest excited triplet state [13]: O~Eex = [ Eex - E(S1) ] -'[- (~f[ E(Zg ) - E ( S 0 ) ] -'1- (/),sc[E(S1) - - E ( T , ) ] "{- 6 i c g ( S l ) ,
(1) with Eex = 28169 cm -1 (355 nm). Assuming a splitting between the dimeric singlet states of about 3000 c m - l [8] and the energy of the upper state equal to 15625 cm-1 (640 nm) we obtain an estimate for the energy of the lowest singlet state in the Pcl,8-dimer E(S ~) of about 12625 c m - l (790 nm). Assuming the fluorescence quantum yield ~f = 0, the intersystem crossing quantum yield 4hsc = 0.30 and the internal conversion yield ~ic = 0.70, a triplet energy E(T 1) of 8451 cm -1 (1183 nm) can be calculated. For PcPS in chloroform a = 0.98 _+ 0.03, which is in agreement with the observed absence with the observed photoinduced triplet formation. If we presume that E(T~) is the same also for the polymer (due to the small effect of the excitonic coupling on the triplet manifold) the lowest singlet state in PcPS must be located even below 8451 cm - I which is equivalent to a splitting of the polymeric singlet state of about 10000 cm -~. This is in rough agreement to an estimation for E(S l) of 8448 cm -1 based on exciton coupling theory [20].
4. Conclusion The results presented above support the model of the electronic structure of co-facial stacked phthalocyanine rings [20]. The pronounced solvatochroism of the dimer in different aromatic and non-aromatic solvents indicates a complicated coupling mecha-
nism between the Pc rings, which strongly effects the singlet spectrum but has only a minor effect on the triplet absorption properties. Both the monomer and the dimer show a comparable location of the triplet absorption in the transient absorption spectra. The absence of either fluorescence and detectable photoinduced absorption in the case of the polymer can be explained by internal conversion through low-lying singlet states.
Acknowledgement The authors would like to thank M. Katayose for his efforts in optimizing the synthesis, N.R. Armstrong, University of Arizona, for help in performing the electrochemical and spectroelectrochemical studies and C. Kryschi, University Diisseldorf, for many fruitful discussions. AF gratefully acknowledges support by the Kekul6 Scholarship of the Stiftung Stipendien-Fonds des Verbandes der Chemischen Industrie. The Belgian authors thank the Ministry of Wetenschapsbeleid for support (IUAP-II-16 and IUAP-III-040). FCDS thanks the Alexander yon Humboldt-Stiftung for support during his stay in Mainz.
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