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Picosecond photphysics of lutetium bis-phthalocyanines
CHEMICALPHYSICSLETTERS
Volume 199.number 6
20 November 1992
Picosecond photophysics of lutetium bis-phthalocyanines Amaud Germain l and Thomas W. Ebbesen FundamentalResearch
Laboratories, NECCorporation,
Received 15 May 1992;in fti
34 Miyukigaoka. Tsukuba 305. Japan
form 3 September 1992
The excited-state properties of lutetium his-phthalocyanine (LuPc*) in solution ia three different redox states (LuP& LuPcZ and LuPc2 ) have been investigated. Upon excitation at either 354, 532 or 1064 nm, LuPc; deactivates through a low-lying doublet excited state of charge transfer nature with a 60 ps lifetime. No evidence for charge hopping between the twophthalocyanine cycles of the dimer was observed. LuPc$ and LuPc; also have short-lived excited states (60 and 36 ps, respectively) suggestinga common deactivation process, possibly vibrational relaxation via low-lyingcharge transfer states.
1. Introduction Bis-phthalocyanine or bit-porphyrin complexes have generated a lot of interest not only because they are considered relevant to understanding the photosynthetic chlorophyll dimer but also because they possess a number of unique properties [ l-141. For instance, lanthanide his-phthalocyaninespossessboth electrochromic and semiconducting properties [ l31. One of the keys to understanding the properties of these dimer structures is learning more about their electronic structure: the amount of interaction between the two aromatic rings, the role of the central metal ion, etc. Lanthanide bis-phthalocyanines such as the lutetium bis-phthalocyanine radical (LuPc; ) have been studied by a variety of techniques in order to understand the electronic distribution. For instance, the crystal analysis of LuPcl shows that it does not have a symmetrical structure, one of the phthalocyanine rings being more distorted out of plane than the other [ 141. Furthermore, the ground-state absorption spectrum of LuPci has features of both PC- and PC*except in the near-infrared, where charge transfer bands resulting from an intra-ring interaction appear Correspondence to: T.W. Ebbesen, Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka,Tsukuba 305, Japan. ’ On a NEC Traineeship on leave from ESPCI, Paris, France.
[41. From such data it is argued that the hole is mostly localized on one ring. On the other hand, recently one of the phthalocyanine rings was replaced with a naphthalocyanine (NC) in order to check whether this interpretation was correct [ 61. The spectroscopic properties of LuPci, LuNcPc’ and LuNc; were compared and it was concluded that the hole is delocalized over the two rings [ 6 I. Excited-state properties have also been useful in elucidating the electronic properties of bis-phthalocyanines [ 5, lo] and bis-porphyrins [ 7-91. The excitation of thin films of LuPc; showed a long lived transient with unusual decay kinetics attributed to the intermolecular interaction in the solid, Here we report the excited-state properties in solution for three electronic ground states of lutetium bis-phthalocyanine: LuPc; , LuPcZ+and LuPc; . These are the stable electrochromic states. The properties of the mono Lu phthalocyanine were also studied for the purpose of comparison.
2. Experimental The absorbance spectra were measured with a Shimadzu MPC-3100 (UV-visible-NIR) spectrophotometer. The electrosynthesis and cyclic voltammetry experiments were carried out with a Hokuto Denko HA-5OlG potentiostat/galvanostat and a Hokuto Denko HB105 function generator both in-
0009-2614/92/S 05.00 0 1992 Elsevier Science Publishers B.V. AU rights reserved.
585
Volume 199,number 6
CHEMICAL PHYSICSLETTERS
terfaced with a NEC PC9801 computer. For the photophysical studies, a picosecond laserflash photolysis apparatus was used which has been described in detail elsewhere [ 15] .,Basically it consists of a Quantel YGSOl-1ODpYAG laser which gives 20 ps pulses at 10 Hz with a choice of wavelengths from the fundamental output at 1064 nm; 266,354 and 532 nm. One part of the 1064 nm pulse is used to generate a pulse of white light (in a range of about 400 to 900 run) by passingthe beam through a DzO-Hz0 solution. This serves as a probing beam for measuring the change in absorbance at a given delay after excitation of the sample. The energy of the 353 and 532 nm excitation pulses was typically between 0.3 and 3 mJ per pulse focused on a 20 mm2 area of the sample,After the sample, the probing light passed through a McPherson 2035 spectrometer and was collected by a Princeton Instruments DDA-512 array. The DDA-512 was connected to a Princeton Instrument Controller ST-120 which was controlled and operated by a NEC Power Mate 1 Plus computer. The final recordings of the transient spectra of the phthalocyanines were achieved by averaging typically 400 shots (i.e. 40 s). A flowing solution was used in conditions where photo-degradation was observed. Ultrapure argon was used to degas the solutions although the presence of air was found to have no effect on the results presented here. The lutetium phthalocyanines were synthesized from 2,3-dicyanonaphtalene (Tokyo Kasei Co. ) and lutetium acetate (Aldrich Chem. Co) in the presence of DBLJ (7,8 diazabicyclo[ 5,4,0]undeo7en, Merck). Various synthetic methods were tried but we found that the method reported by De Cian et al. [ 141 was the most useful guideline to follow. The phthalocyanines were separated by chromatography column with silica gel. The green LuPci was the first fraction eluted in a silica gel chromatography column using CH2C12t 2% CH30H, leaving LuPcOAc (Lu phthalocyanine with acetate ligand) blocked on top of the column. After changing the solvent to CHzClzt 20% CH,OH, LuPcOAc was recovered. LuPc: and LuPcr were prepared from LuPc; using standard electrochemical techniques. A solution of 1.7x 1OS5 M LuPcZwith 0.03 M TBAP (electrolyte) in CH2C12was introduced into a three compartment electrochemical cell. Then under argon, 900 and - 300 mV (versus Ag/AgCl ) were imposed for gen586
20 November 1992
erating respectively the oxidized and reduced spcties. The electrolysis was continued until only trace amounts of the initial product were left in the solution. The photophysical measurements were carried out on such freshly prepared solutions. In all the transient absorption experiments, the concentrations of the studied specieswere kept as low as possible (typically 5 X 10m6M ) , not only to minimize the presence of aggregatesbut also to assure accuracy in the recorded transient absorption spectra by having a low absorption ( u 0.1) in the excitation depth [ 161.Sincemultiphotonic events can be a major source of error when using the high power of picosecond lasers (gigawatt), the effect of the laser intensity was always checked systematically to ensure that the observed phenomena were clearly due to monophotonic excitation. The samples were checked for emission using a SPEX Fluorolog 2 but none could be observed except in the case of the LuPc; samples. It appeared at first that it was due to LuPc; because the excitation spectrum matched its absorption spectrum quite well. However after careful checking, it appeared that residual H,Pc impurity was responsible for the weak emission.
3. Results and discussion 3.1. Ground-stateabsorption spectra
The ground-state spectra of LuPc;, LuPc$ , and LuPcz between 250 and 1500 nm are shown in fig. 1. Since phthalocyanines are known to aggregateeasily in solution, Beer’s law was verified for all the four species down to = lop6 M. The absorbance spectra deviated slightly from linearity with concentration, indicating the presence of aggregates.The molar absorption coefficients at the main peaks in the visible were extrapolated by taking the initial linear slope of the absorption versus concentration curves. The resulting values, given in table 1, can be compared to the absorption coefficients reported in the literature. Much smaller values for LuPc; and LuPc: are reported by Corker et al. [ 111, however their spectrum of the oxidized species LuPc2+is impure and contains the LuPc, peak at 660 nm. The maximum molar absorption coefficients for LuPcz in DMF
Volume 199,number 6
CHEMICALPHYSICSLETTERS
Table 1 Absorption maxima (A,) and molar absorption co&icients (e) of the various Lu phthalocyanine species
1
II , LUPC; ’
LuPcOAc
; I’
Solvent
Lx c (nm) (!I mol-’ cm-‘)
CH&lz/TBAP 0.03 M
660
1.82x 10J
LuPcq
415 692 870
5.1x10’ 5.0x 10’ 2.8~10’
LUPCi
333 616 700
1.48~10’ 1.28x 10’ 6.7x10’
343 675
7.0x104 1.80x 10’
LuPci
LuPcGAc
iA Lu PC;
u
400
20 November 1992
L
%bO
1200
nm
•l
Fig. 1. Ground-state absorption spectra of LuPcGAc, LuPc; , LuPc; and LuPcl
( 1.27~ lo5 M” cm-‘) reported by Nicholson and Galiardi (see ref. [ 111) and in CHzCll ( 1.25X lo5 M-’ cm-’ ) given by Castaneda et al. [ 31 are in exact agreement with our value (table 1). In the case of LuPc; , the maximum extinction coefficient is in agreement with the average of the literature values (1.55x105 [3], 2x105 [4]).
CHIC12/CH~OH3%
toionization is sufficiently small so that the excitedstate spectrum and its lifetime could be measured without significant error. In fig. 2 the transient spectrum between 450 and 830 nm recorded immediately after excitation at 354 nm is shown. The decay profile is shown in the insert yielding a lifetime of ~60 ps (1.8~10’~ s-l). No other transient spectra were observed. This short-lived excited state corresponds to the lowest excited doublet state of charge transfer nature of the LuPcj radical, as will be discussed later. Its lifetime was also checked in acetone and was found to have the same value within experimental error. So the short lifetime of this radical is clearly reflecting intrinsic properties of the molecule. The transient spectrum and lifetime were the
AA
3.2. Excited-state properties 3.2.1. Picosecond transient absorption measurements - LuPC;. Initially, we found that LuPci easily
photoionized to LuPc: at laser intensities as low as 0.5 mJ/pulse (354 nm). The process is not monophotonic since the yield of LuPc: increases much faster than expected from a linear dependence on the laser intensity. At 0.3 mJ/pulse, the contribution to the transient signal from the multiphotonic pho-
Fig. 2. Transient differential absorption (M) spectrum I& corded immediately after excitation of LuPcj with the 20 ps 354 nm laser pulse. Insert: Decay profile of the transient (vertical axis A.4inau).
587
same when 532 and 1064 run excitation wavelengths were used. Finally the effect of temperature on the lifetime was checked in a limited range between 10 and 30°C but no variation could be detected within experimental error. - LuPcz+. The picosecond measurements on LuPc: revealed no degradation of LuPc: so that laser intensities as high as 1.5 mJ/pulse could be used without any problems. The transient absorption spectrum immediately after the excitation is shown in fig, 3. The decay kinetics shown in the insert give a lifetime of z 60 ps. As with LuPg, these results are independent of the excitation wavelength (354 and 532 nm). No other excited state was observed. In view of its immediate appearance after excitation and its short lifetime, we assign this state to the S, of LUPC: . - LuPc; . LuPcz is particularly sensitive to photoionization (yielding LuPci ) when using 354 nm
excitation light. Using 532 nm excitation pulses ( %5 mJ/pulse) clean results could be obtained, however, the transient absorption signals are relatively weak due to the low extinction coefficient at this excitation wavelength. Fig. 4 shows the transient spectrum measured for LuPc? immediately after excitation. It decays rapidly with a rate 2.8~ 10” s-’ ( = 36 ps) as shown by the time dependence in the insert (fig. 4). No other transient was observed. For the same reasons as given for LuPc: , this transient is assigned to s, of LUPCF. - LuPcOAc. Upon excitation of LuPcOAc (in
660
760
nm
Fig. 3. Transient differential absorption (AA) spectrum recorded immediately after excitation of LuPc: with the 20 ps 354 mn laser pulse. Insert: Decay profile of the transient (vertical axis AAin au).
588
20November 1992
CHEMICAL PHYSICS LETTERS
Volume199,number6
-On3 0 500
600
700
"In
Fig. 4. Transient differential absorption (AA) spectrum recorded immediately after excitation of LuPc,- with 20 ps 532nm laser pulse. Insert: Decay profile of the transient (vertical axis AA inau).
CH2C12+3%CHaOH) at either 354 or 532 nm, a single transient absorption indicating loss of the ground state was always observed. Furthermore this transient spectrum showed no decay. Measurement of the ground-state spectrum after the laser experiments revealed that LuPcOAc had undergone photodegradation. It appears that the photodegradation is monophotonic. LuPcOAc was also destroyed when left exposed to ambient light. So in the case of LuPcOAc no photophysical properties could be measured. 3.2.2. Discussion
There has been a great deal of interest in the photophysics of his-phthalocyanine and bis-porphyrin complexes because, among other things, they are considered analogues to the chlorophyll dimer of the photosynthetic reaction center [ 7-91. Knowledge of the charge distribution and charge dynamics in such dimer structures should help us understand the electron transfer mechanisms in the natural photosynthetic system. In the case of LuPc; , it could also help us understand the mechanics of charge transport in the solid state where it behaves as an intrinsic semiconductor [ 1,21. One of the key questions with regards to the radical sandwich structures, such as LuPc; having an unpaired electron, is whether the hole is localized in one ring or delocalizedover the two rings. It has been argued on the basis of hole localization that the absorption spectrum of LuPc; is composed of the sum of the spectra of the neutral and radical cation moieties plus the near-infrared bands due to charge trans-
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CHEMICALPHYSICS LETTERS
fer between the two rings [4]. This is supported by the structure of the complex where one ring is more distorted out of plane [ 141. At the same time, it is expected that the charge must be hopping back and forth as shown schematically by the following reaction [ 45 ] :
.[
,kct
1.
the rate being determined by the activation barrier to the structural changes. In a recent spectroscopic study using an asymmetric lutetium bis-phthalocyanine Lu (NC)(PC), it was argued that the hole is delocalized over the two rings because its absorption spectrum is not exactly the sum of the spectra of the two moieties [ 61. The LuPc; excited state that is observed (fig. 2) must correspond to the first excited state of charge transfer nature ( Dlct ) since it is observed even when exciting the phthalocyanine near its infrared charge transfer bands at 1064 nm. This spectrum is in good agreement with that reported in the literature for LuPc; in the solid state [ 5 ] although its decay is different. In the solid state, the transient spectrum is reported to disappear with a non-exponential decay having a first half-life of 275 ns [ 51, considerably shorter than that observed in solution here. The remaining question is what is the main pathway among the non-radiative decay processes. Two possibilities seem most likely. One is the rapid vibrational deactivation via close-lying low-energy charge transfer states. The existence of such low-energy states can be seen from the absorption peaks in the near infrared part of the ground-state LuPci spectrum (fig. 1) and have been assignedto ring-toring charge transfer (RRCT) [ 41. Such a deactivation process has been suggestedby Holten and coworkers for the rapid deactivation of non-radical Ce(IV) porphyrin dimers [ 7,8]. This is also probably the main deactivation path of D,ct state of LuPci observed by the transient absorption measurements. The other process, not available in the symmetric Ce( IV) porphyrin dimers, is charge hopping between the two cycles in the LuPc; sandwich. As mentioned earlier this is expected to be fast and
20 November 1992
is probably partly responsible for the non-radiative deactivation of the upper excited state D, ( S, ), leading among other things to the formation of the charge transfer state D,ct. In any case, the intra-molecular charge transfer rate after excitation must be faster or equal to 5 X IO’Os-‘, the upper limit of our time resolution. The lifetimes of the excited states of LuPcZ+(S, ) and LuPcr (St) are similar to LuPc; (D,ct), hinting at a possible common decay process despite the electronic differences. It is unlikely that ligand to metal charge transfer plays a role since Lu”’ has a closed-shell electronic structure as pointed out by Markovitsi et al. [ 41. So low-energy states as seen in the near infrared bands (fig. 1) might be the determining factor, allowing for rapid vibrational deactivation to the ground state for all three redox states.
References [ 1] P. Turek, P. Petit, J.-J. Andre, J. Simon, R. Even, B. Boudjema, G. Guillaud and M. Maitrot, J. Am. Chem. Sot. 109 (1987) 5119. [ 21 M. Bouvet and J. Simon, Chem. Phys. Letters 172 ( 1990) 299. [ 31F. Castaneda, C. Piechocki, V. Plichon, J. Simon and J. Vaxiviere,Electrochint.Acta31 (1986) 131. [4] D. Markovitsi,T.-H. Tran-Thi,R. Even and J. Simon,Chem. Phys Letters 137 (1987) 107. [ 51T.-H.Tran-TM,D. Markovitsi,R. Even and J. Simon,Chem. Phys. Letters 139 (1987) 207. [ 61 N. Ishikawa, 0. Ohno and Y. R&u, Chem. Phys. Letters 180 (1991) 51. [ 71 X. Yan and D. Holten, I. Phys. Chem, 92 ( 1988) 409. [ 810. Bilsel, J. Rodriguez and D. Holten, J. Phys. Chem. 94 ( 1990) 3508. [9] 0. Bilsel, J. Rodriguez, D. Holten, G.S. Girolami, S.N. Milam and K.S. Suslick, J. Am. Chem. Sot. 112 (1990) 4075. [ 10] G. Ferraud&in: Phthalcqaaines properties and application, eds. C.C. Leznoff and A.B.P. Lever (VCH Publishers, New York, 1989) p. 291. [ II] G.A. Corker, B. Grant and N.J. Clecak,I. Electrochem. Sot. 126 (1979) 1339. [12]A.T.ChangandJ.C.Marchon,Inorg.Chint.Acta53 (1981) L241. [ 131M. L’Her,Y. Cozien and J. Cotutot-Coupez, J. Electroansl. Chem. 157 (1983) 183. [ 141A. De Cian, M. Moussavi, J. Fischer and R. Weiss,Inotg. Chem. 24 (1985) 3162. [lS]T.W. Ebbesen,Rev. Sci. Instrum. 59 (1988) 1307. [ 16]M. Bazin and T.W. Ebbesen, Photochem. Photobiol. 37 (1983) 675.
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Volume 199.number 6
20 November 1992
Picosecond photophysics of lutetium bis-phthalocyanines Amaud Germain l and Thomas W. Ebbesen FundamentalResearch
Laboratories, NECCorporation,
Received 15 May 1992;in fti
34 Miyukigaoka. Tsukuba 305. Japan
form 3 September 1992
The excited-state properties of lutetium his-phthalocyanine (LuPc*) in solution ia three different redox states (LuP& LuPcZ and LuPc2 ) have been investigated. Upon excitation at either 354, 532 or 1064 nm, LuPc; deactivates through a low-lying doublet excited state of charge transfer nature with a 60 ps lifetime. No evidence for charge hopping between the twophthalocyanine cycles of the dimer was observed. LuPc$ and LuPc; also have short-lived excited states (60 and 36 ps, respectively) suggestinga common deactivation process, possibly vibrational relaxation via low-lyingcharge transfer states.
1. Introduction Bis-phthalocyanine or bit-porphyrin complexes have generated a lot of interest not only because they are considered relevant to understanding the photosynthetic chlorophyll dimer but also because they possess a number of unique properties [ l-141. For instance, lanthanide his-phthalocyaninespossessboth electrochromic and semiconducting properties [ l31. One of the keys to understanding the properties of these dimer structures is learning more about their electronic structure: the amount of interaction between the two aromatic rings, the role of the central metal ion, etc. Lanthanide bis-phthalocyanines such as the lutetium bis-phthalocyanine radical (LuPc; ) have been studied by a variety of techniques in order to understand the electronic distribution. For instance, the crystal analysis of LuPcl shows that it does not have a symmetrical structure, one of the phthalocyanine rings being more distorted out of plane than the other [ 141. Furthermore, the ground-state absorption spectrum of LuPci has features of both PC- and PC*except in the near-infrared, where charge transfer bands resulting from an intra-ring interaction appear Correspondence to: T.W. Ebbesen, Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka,Tsukuba 305, Japan. ’ On a NEC Traineeship on leave from ESPCI, Paris, France.
[41. From such data it is argued that the hole is mostly localized on one ring. On the other hand, recently one of the phthalocyanine rings was replaced with a naphthalocyanine (NC) in order to check whether this interpretation was correct [ 61. The spectroscopic properties of LuPci, LuNcPc’ and LuNc; were compared and it was concluded that the hole is delocalized over the two rings [ 6 I. Excited-state properties have also been useful in elucidating the electronic properties of bis-phthalocyanines [ 5, lo] and bis-porphyrins [ 7-91. The excitation of thin films of LuPc; showed a long lived transient with unusual decay kinetics attributed to the intermolecular interaction in the solid, Here we report the excited-state properties in solution for three electronic ground states of lutetium bis-phthalocyanine: LuPc; , LuPcZ+and LuPc; . These are the stable electrochromic states. The properties of the mono Lu phthalocyanine were also studied for the purpose of comparison.
2. Experimental The absorbance spectra were measured with a Shimadzu MPC-3100 (UV-visible-NIR) spectrophotometer. The electrosynthesis and cyclic voltammetry experiments were carried out with a Hokuto Denko HA-5OlG potentiostat/galvanostat and a Hokuto Denko HB105 function generator both in-
0009-2614/92/S 05.00 0 1992 Elsevier Science Publishers B.V. AU rights reserved.
585
Volume 199,number 6
CHEMICAL PHYSICSLETTERS
terfaced with a NEC PC9801 computer. For the photophysical studies, a picosecond laserflash photolysis apparatus was used which has been described in detail elsewhere [ 15] .,Basically it consists of a Quantel YGSOl-1ODpYAG laser which gives 20 ps pulses at 10 Hz with a choice of wavelengths from the fundamental output at 1064 nm; 266,354 and 532 nm. One part of the 1064 nm pulse is used to generate a pulse of white light (in a range of about 400 to 900 run) by passingthe beam through a DzO-Hz0 solution. This serves as a probing beam for measuring the change in absorbance at a given delay after excitation of the sample. The energy of the 353 and 532 nm excitation pulses was typically between 0.3 and 3 mJ per pulse focused on a 20 mm2 area of the sample,After the sample, the probing light passed through a McPherson 2035 spectrometer and was collected by a Princeton Instruments DDA-512 array. The DDA-512 was connected to a Princeton Instrument Controller ST-120 which was controlled and operated by a NEC Power Mate 1 Plus computer. The final recordings of the transient spectra of the phthalocyanines were achieved by averaging typically 400 shots (i.e. 40 s). A flowing solution was used in conditions where photo-degradation was observed. Ultrapure argon was used to degas the solutions although the presence of air was found to have no effect on the results presented here. The lutetium phthalocyanines were synthesized from 2,3-dicyanonaphtalene (Tokyo Kasei Co. ) and lutetium acetate (Aldrich Chem. Co) in the presence of DBLJ (7,8 diazabicyclo[ 5,4,0]undeo7en, Merck). Various synthetic methods were tried but we found that the method reported by De Cian et al. [ 141 was the most useful guideline to follow. The phthalocyanines were separated by chromatography column with silica gel. The green LuPci was the first fraction eluted in a silica gel chromatography column using CH2C12t 2% CH30H, leaving LuPcOAc (Lu phthalocyanine with acetate ligand) blocked on top of the column. After changing the solvent to CHzClzt 20% CH,OH, LuPcOAc was recovered. LuPc: and LuPcr were prepared from LuPc; using standard electrochemical techniques. A solution of 1.7x 1OS5 M LuPcZwith 0.03 M TBAP (electrolyte) in CH2C12was introduced into a three compartment electrochemical cell. Then under argon, 900 and - 300 mV (versus Ag/AgCl ) were imposed for gen586
20 November 1992
erating respectively the oxidized and reduced spcties. The electrolysis was continued until only trace amounts of the initial product were left in the solution. The photophysical measurements were carried out on such freshly prepared solutions. In all the transient absorption experiments, the concentrations of the studied specieswere kept as low as possible (typically 5 X 10m6M ) , not only to minimize the presence of aggregatesbut also to assure accuracy in the recorded transient absorption spectra by having a low absorption ( u 0.1) in the excitation depth [ 161.Sincemultiphotonic events can be a major source of error when using the high power of picosecond lasers (gigawatt), the effect of the laser intensity was always checked systematically to ensure that the observed phenomena were clearly due to monophotonic excitation. The samples were checked for emission using a SPEX Fluorolog 2 but none could be observed except in the case of the LuPc; samples. It appeared at first that it was due to LuPc; because the excitation spectrum matched its absorption spectrum quite well. However after careful checking, it appeared that residual H,Pc impurity was responsible for the weak emission.
3. Results and discussion 3.1. Ground-stateabsorption spectra
The ground-state spectra of LuPc;, LuPc$ , and LuPcz between 250 and 1500 nm are shown in fig. 1. Since phthalocyanines are known to aggregateeasily in solution, Beer’s law was verified for all the four species down to = lop6 M. The absorbance spectra deviated slightly from linearity with concentration, indicating the presence of aggregates.The molar absorption coefficients at the main peaks in the visible were extrapolated by taking the initial linear slope of the absorption versus concentration curves. The resulting values, given in table 1, can be compared to the absorption coefficients reported in the literature. Much smaller values for LuPc; and LuPc: are reported by Corker et al. [ 111, however their spectrum of the oxidized species LuPc2+is impure and contains the LuPc, peak at 660 nm. The maximum molar absorption coefficients for LuPcz in DMF
Volume 199,number 6
CHEMICALPHYSICSLETTERS
Table 1 Absorption maxima (A,) and molar absorption co&icients (e) of the various Lu phthalocyanine species
1
II , LUPC; ’
LuPcOAc
; I’
Solvent
Lx c (nm) (!I mol-’ cm-‘)
CH&lz/TBAP 0.03 M
660
1.82x 10J
LuPcq
415 692 870
5.1x10’ 5.0x 10’ 2.8~10’
LUPCi
333 616 700
1.48~10’ 1.28x 10’ 6.7x10’
343 675
7.0x104 1.80x 10’
LuPci
LuPcGAc
iA Lu PC;
u
400
20 November 1992
L
%bO
1200
nm
•l
Fig. 1. Ground-state absorption spectra of LuPcGAc, LuPc; , LuPc; and LuPcl
( 1.27~ lo5 M” cm-‘) reported by Nicholson and Galiardi (see ref. [ 111) and in CHzCll ( 1.25X lo5 M-’ cm-’ ) given by Castaneda et al. [ 31 are in exact agreement with our value (table 1). In the case of LuPc; , the maximum extinction coefficient is in agreement with the average of the literature values (1.55x105 [3], 2x105 [4]).
CHIC12/CH~OH3%
toionization is sufficiently small so that the excitedstate spectrum and its lifetime could be measured without significant error. In fig. 2 the transient spectrum between 450 and 830 nm recorded immediately after excitation at 354 nm is shown. The decay profile is shown in the insert yielding a lifetime of ~60 ps (1.8~10’~ s-l). No other transient spectra were observed. This short-lived excited state corresponds to the lowest excited doublet state of charge transfer nature of the LuPcj radical, as will be discussed later. Its lifetime was also checked in acetone and was found to have the same value within experimental error. So the short lifetime of this radical is clearly reflecting intrinsic properties of the molecule. The transient spectrum and lifetime were the
AA
3.2. Excited-state properties 3.2.1. Picosecond transient absorption measurements - LuPC;. Initially, we found that LuPci easily
photoionized to LuPc: at laser intensities as low as 0.5 mJ/pulse (354 nm). The process is not monophotonic since the yield of LuPc: increases much faster than expected from a linear dependence on the laser intensity. At 0.3 mJ/pulse, the contribution to the transient signal from the multiphotonic pho-
Fig. 2. Transient differential absorption (M) spectrum I& corded immediately after excitation of LuPcj with the 20 ps 354 nm laser pulse. Insert: Decay profile of the transient (vertical axis A.4inau).
587
same when 532 and 1064 run excitation wavelengths were used. Finally the effect of temperature on the lifetime was checked in a limited range between 10 and 30°C but no variation could be detected within experimental error. - LuPcz+. The picosecond measurements on LuPc: revealed no degradation of LuPc: so that laser intensities as high as 1.5 mJ/pulse could be used without any problems. The transient absorption spectrum immediately after the excitation is shown in fig, 3. The decay kinetics shown in the insert give a lifetime of z 60 ps. As with LuPg, these results are independent of the excitation wavelength (354 and 532 nm). No other excited state was observed. In view of its immediate appearance after excitation and its short lifetime, we assign this state to the S, of LUPC: . - LuPc; . LuPcz is particularly sensitive to photoionization (yielding LuPci ) when using 354 nm
excitation light. Using 532 nm excitation pulses ( %5 mJ/pulse) clean results could be obtained, however, the transient absorption signals are relatively weak due to the low extinction coefficient at this excitation wavelength. Fig. 4 shows the transient spectrum measured for LuPc? immediately after excitation. It decays rapidly with a rate 2.8~ 10” s-’ ( = 36 ps) as shown by the time dependence in the insert (fig. 4). No other transient was observed. For the same reasons as given for LuPc: , this transient is assigned to s, of LUPCF. - LuPcOAc. Upon excitation of LuPcOAc (in
660
760
nm
Fig. 3. Transient differential absorption (AA) spectrum recorded immediately after excitation of LuPc: with the 20 ps 354 mn laser pulse. Insert: Decay profile of the transient (vertical axis AAin au).
588
20November 1992
CHEMICAL PHYSICS LETTERS
Volume199,number6
-On3 0 500
600
700
"In
Fig. 4. Transient differential absorption (AA) spectrum recorded immediately after excitation of LuPc,- with 20 ps 532nm laser pulse. Insert: Decay profile of the transient (vertical axis AA inau).
CH2C12+3%CHaOH) at either 354 or 532 nm, a single transient absorption indicating loss of the ground state was always observed. Furthermore this transient spectrum showed no decay. Measurement of the ground-state spectrum after the laser experiments revealed that LuPcOAc had undergone photodegradation. It appears that the photodegradation is monophotonic. LuPcOAc was also destroyed when left exposed to ambient light. So in the case of LuPcOAc no photophysical properties could be measured. 3.2.2. Discussion
There has been a great deal of interest in the photophysics of his-phthalocyanine and bis-porphyrin complexes because, among other things, they are considered analogues to the chlorophyll dimer of the photosynthetic reaction center [ 7-91. Knowledge of the charge distribution and charge dynamics in such dimer structures should help us understand the electron transfer mechanisms in the natural photosynthetic system. In the case of LuPc; , it could also help us understand the mechanics of charge transport in the solid state where it behaves as an intrinsic semiconductor [ 1,21. One of the key questions with regards to the radical sandwich structures, such as LuPc; having an unpaired electron, is whether the hole is localized in one ring or delocalizedover the two rings. It has been argued on the basis of hole localization that the absorption spectrum of LuPc; is composed of the sum of the spectra of the neutral and radical cation moieties plus the near-infrared bands due to charge trans-
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CHEMICALPHYSICS LETTERS
fer between the two rings [4]. This is supported by the structure of the complex where one ring is more distorted out of plane [ 141. At the same time, it is expected that the charge must be hopping back and forth as shown schematically by the following reaction [ 45 ] :
.[
,kct
1.
the rate being determined by the activation barrier to the structural changes. In a recent spectroscopic study using an asymmetric lutetium bis-phthalocyanine Lu (NC)(PC), it was argued that the hole is delocalized over the two rings because its absorption spectrum is not exactly the sum of the spectra of the two moieties [ 61. The LuPc; excited state that is observed (fig. 2) must correspond to the first excited state of charge transfer nature ( Dlct ) since it is observed even when exciting the phthalocyanine near its infrared charge transfer bands at 1064 nm. This spectrum is in good agreement with that reported in the literature for LuPc; in the solid state [ 5 ] although its decay is different. In the solid state, the transient spectrum is reported to disappear with a non-exponential decay having a first half-life of 275 ns [ 51, considerably shorter than that observed in solution here. The remaining question is what is the main pathway among the non-radiative decay processes. Two possibilities seem most likely. One is the rapid vibrational deactivation via close-lying low-energy charge transfer states. The existence of such low-energy states can be seen from the absorption peaks in the near infrared part of the ground-state LuPci spectrum (fig. 1) and have been assignedto ring-toring charge transfer (RRCT) [ 41. Such a deactivation process has been suggestedby Holten and coworkers for the rapid deactivation of non-radical Ce(IV) porphyrin dimers [ 7,8]. This is also probably the main deactivation path of D,ct state of LuPci observed by the transient absorption measurements. The other process, not available in the symmetric Ce( IV) porphyrin dimers, is charge hopping between the two cycles in the LuPc; sandwich. As mentioned earlier this is expected to be fast and
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is probably partly responsible for the non-radiative deactivation of the upper excited state D, ( S, ), leading among other things to the formation of the charge transfer state D,ct. In any case, the intra-molecular charge transfer rate after excitation must be faster or equal to 5 X IO’Os-‘, the upper limit of our time resolution. The lifetimes of the excited states of LuPcZ+(S, ) and LuPcr (St) are similar to LuPc; (D,ct), hinting at a possible common decay process despite the electronic differences. It is unlikely that ligand to metal charge transfer plays a role since Lu”’ has a closed-shell electronic structure as pointed out by Markovitsi et al. [ 41. So low-energy states as seen in the near infrared bands (fig. 1) might be the determining factor, allowing for rapid vibrational deactivation to the ground state for all three redox states.
References [ 1] P. Turek, P. Petit, J.-J. Andre, J. Simon, R. Even, B. Boudjema, G. Guillaud and M. Maitrot, J. Am. Chem. Sot. 109 (1987) 5119. [ 21 M. Bouvet and J. Simon, Chem. Phys. Letters 172 ( 1990) 299. [ 31F. Castaneda, C. Piechocki, V. Plichon, J. Simon and J. Vaxiviere,Electrochint.Acta31 (1986) 131. [4] D. Markovitsi,T.-H. Tran-Thi,R. Even and J. Simon,Chem. Phys Letters 137 (1987) 107. [ 51T.-H.Tran-TM,D. Markovitsi,R. Even and J. Simon,Chem. Phys. Letters 139 (1987) 207. [ 61 N. Ishikawa, 0. Ohno and Y. R&u, Chem. Phys. Letters 180 (1991) 51. [ 71 X. Yan and D. Holten, I. Phys. Chem, 92 ( 1988) 409. [ 810. Bilsel, J. Rodriguez and D. Holten, J. Phys. Chem. 94 ( 1990) 3508. [9] 0. Bilsel, J. Rodriguez, D. Holten, G.S. Girolami, S.N. Milam and K.S. Suslick, J. Am. Chem. Sot. 112 (1990) 4075. [ 10] G. Ferraud&in: Phthalcqaaines properties and application, eds. C.C. Leznoff and A.B.P. Lever (VCH Publishers, New York, 1989) p. 291. [ II] G.A. Corker, B. Grant and N.J. Clecak,I. Electrochem. Sot. 126 (1979) 1339. [12]A.T.ChangandJ.C.Marchon,Inorg.Chint.Acta53 (1981) L241. [ 131M. L’Her,Y. Cozien and J. Cotutot-Coupez, J. Electroansl. Chem. 157 (1983) 183. [ 141A. De Cian, M. Moussavi, J. Fischer and R. Weiss,Inotg. Chem. 24 (1985) 3162. [lS]T.W. Ebbesen,Rev. Sci. Instrum. 59 (1988) 1307. [ 16]M. Bazin and T.W. Ebbesen, Photochem. Photobiol. 37 (1983) 675.
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