10534
J. Phys. Chem. 1992,96, 10534-10542
Femtosecond Spectroscopy of Vanadyi Phthalocyanines in Various Molecular Arrangementst Akira Termki,*J Department of Physics, Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan
Masahiro Howda,*Tatsuo Wada, Frontier Research Program, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351 -01, Japan
Hirokazu Tada, Atsushi Koma, Department of Chemistry, Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan
Akira Yamada, Hiroyuki Sasabe, Anthony F. Garito," Frontier Research Program, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351 -01, Japan
and Takayoshi Kobayashi* Department of Physics, Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan (Received: June 29, 1992; In Final Form: September 21, 1992)
The relaxation kinetics of excited electronic states of vanadyl phthalocyanines in various molecular arrangements was studied by femtosecond time-resolved spectroscopy. Four types of molecular systems were investigated, Le., phase I and phase I1 films prepared on quartz substrates, a thin film deposited on a potassium bromide (KBr) substrate by the organic molecular-beam-epitaxy (MBE)technique, and isolated molecules in a solution and in a host polymer. The excited-state dynamics was found to be strongly dependent on the molecular arrangements in these phases. Dominant relaxation processes were formation of tripdoublet and tripquartet states possessing lifetimes much longer than the lifetimes of hundreds of picoseconds in the isolated molecules, the exciton-xciton annihilation taking place in the picosecond regime in phases I and I1 with different rate constants, and the subpicosecond exciton decay in the film on KBr.
Introduction The research of optical nonlinearities of materials is of great interest both in the basic material science to clarify their origins and mechanisms and in applications to develop optical devices for the control of light. As materials suitable for various applications requiring large nonlinear optical susceptibility, ultrafast response, thermal and chemical stability, and flexible processing, organics are attracting attention because of the advantage of their superior flexibility in processing and manipulation. particularly, conjugated r-electron systems have been known to possess large optical nonliiearities and ultrafast optical responses.'v2 These attractive properties originate from the r electrons delocalized in one-dimemionally conjugated polymers or twodimensionally conjugated macrocyclic molecules. Among these systems, conjugated macrocyclic compounds possessing a ring r-electron conjugation, especially phthalocyanines, have recently emerged as promising materials and their nonlinear optical properties have been investigated extensively both in experiment and in t h e ~ r y . ~ - ' ~ In phthalocyanine molecules, it is known that various kinds of metals can coordinate to the center of their large aromatic rings and markedly change the properties of the m ~ l e c u l e .In ~ ~our previous studies, we have examined the nonlinear optical susceptibilities of third-harmonic generation (THG)in films of several metallophthalocyanines (VO, Sn, Co,and Ni) and metal-free (H,) ~hthalocyanine.~Among these phthalocyanines, vanadyl Authors to whom correspondence should bc addressed. 'This work was carried out at Frontier Research Program, RIKEN (The Institute of Physical and Chemical Research). r h n t address: Department of Chemistry, Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan. 'Resent address: Basic Research Department, Research and Development Center, Unitika Ltd., 23, Kozakura. Uji, Kyoto 61 1, Japan. 'I Permanent address: Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6396, U S A .
0022-3654/92/2096- 10534$03.00/0
phthalocyanine showed the largest x ( ~ )which , is in the same order of magnitude as those of conjugated polymers such as polydiacetylenes. For this reason intense interest is focused on this metallophthalocyanine. In their condensed phases, phthalocyanines can exist in several morphological forms with different stacking arrangements of the molecules.l3 Such a structural variation also causa modifications in their optoelectronic properties. Vanadyl phthalocyanine has been known to posseas three phases, phases I, 11, and 111, which were characterized by differential scanning calorimetry, X-ray diffraction, and optical absorption spectr~scopy.'~J~ The phase I1 crystal has been found to have a triclinic crystal structure with a slipped-stacked form. For phases I and 111, their crystal structures have not been determined yet because of a lack of suitable single crystals. For phase I, however, a cofacially-stacked form has been inferred from its blue-shifted absorption spectrum. Various properties of these phases have been studied extensively. Electronic states were studied by optical absorption and luminescence ~pectrosoopies'~ and electroabeorption ~pectroscopy,l'.~~ Studies of photoconductivity have reported higher efficiency of photogeneration of charge carriers in phase I1 than in phase I.I9 The structures of these polymorphs have also been studied by scanning electron microscopy (SEM)and infrared and Raman spectroscopies.20 Recently we have evaluated the third-order nonlinear optical susceptibilities x@)of these polymorphs by THG measurements and obtained larger ~ (in3phase I1 than in phase I at fundamental wavelengths of 1.9 and 1.5 pm.IoJ1 Thus,several linear and nonlinear optical properties of the molecular system have been found to be affected by the arrangement of the molecule. Since another important characteristic to be investigated for nonlinear optical materials is the time response of their nonlinearities, the relaxation kinetics of their excited electronic states, Le., dynamical properties of excitons created in these materials by short light pulses, is also an interesting topic to be studied. 0 1992 American Chemical Society
Femtosecond Spectroscopy of Phthalocyanines Femtosecond studies have been reported for several phthalocyanine~.~'-~ Greene and Millard showed that the initial exciton decay can be interpreted in terms of a (PI2) time-dependent annihilation rate of bimolecular interaction between two singlet excitons in a polycrystalline @-hydrogenphthalocyanine (H2Pc) by observing transient absorption of photoinduced excited states?' Ho and Peyghambarian studied a polycrystalline film of fluoroaluminum phthalocyanine (FAlPc) and explained the decay kinetics of bleaching due to exciton creation by a model taking into account an exciton-exciton annihilation process with a constant rate and an exciton-phonon coupling process.22 Casstevens et al. also observed excitation-intensity-dependentdecay of degenerate four-wave mixing (DFWM) signals from evaporated films of H2Pc and Langmuir-Blodgett (LB) films of silicon phthalocyanine ( S ~ P C ) DFWM .~~ measurements on SiPc derivatives in several molecular environments in LB and polymer-doped films were reported by Neher et al.,24 in which differences in the transient response were observed and an explanation in terms of the strength of the electronic coupling between phthalocyanine rings was proposed. More recently Williams et al. reported excited-state dynamics in FAlPc and ClAlPc thin films and discussed the existence of a subgap state below the first excited state.25 These reports, however, have not fully clarified the relaxation mechanisms of excitons yet, and also systematic studies of the dependence of the exciton dynamics on central metals and morphological forms have not been carried out yet, although they are basic studies to characterize optical properties of molecular systems. From the above viewpoints, the present study was focused on the effects of morphology on the relaxation mechanisms of excited electronic states in one of the metallophthalocyanines, vanadyl phthalocyanine, by means of femtosecond time-resolved p u m p probe spectroscopy. In addition to our previous brief report on phase I1 detailed discussion is given in this paper for four types of its morphological forms, Le., phase I, phase 11, a novel phase grown by the organic molecular-beam-epitaxy (MBE) technique, and isolated molecular systems in a solution and in a host polymer. Experimental Section
Sample preparation. For the present study, both unsubstituted vanadyl phthalocyanine (VOPC)'~J'and tert-butyl substituted were prepared. vanadyl phthalocyanine ((~-BU),VOPC)'~J~J~*~' The peripheral substituents were on the benzo rings at either the 2- or 3-positions. The average number of substituted tert-butyl groups, n, was 1.1, i.e., the product was a mixture of mono- and disubstituted derivatives with a ratio of 9:l determined by the elemental analysis. The tert-butyl substitution makes this compound soluble in common organic solvents such as chloroform, which is a great advantage in processing the compound. Samples we have studied can be classified into four types, Le., isolated molecular phase, phase I, phase 11, and a film deposited on an alkali halide substrate. Isolated molecular phases were prepared both as a solution and as a polymer film containing the molecule at low concentration. A solution sample of (t-Bu)l.lVOPcin chloroform was prepared in a 1-mm-pathlength cuvette. The concentration, ca. 8 X mol/L, corresponds to the mean intermolecular distance of 280 A, which is much longer than the diameter of the molecule, ca. 14 A, Although it is reportedthat dimerization of VOPc molecules takes place even at molecular concentrations as low as mol/L,16 (t-Bu),.,VOPc has high solubility in chloroform and the dimer fraction was estimated to be less than 1% at lo4 mol/L from the concentration dependence of the absorption spectrum. This fact ensures that the present solution sample is regarded as a monomeric phase. Similar molecular environments could be prepared also in the solid state by casting a chloroform solution of (~-BU)~.~VOPC mixed with a host polymer (polystyrene) at a relatively low concentration, 0.01 wt %, on quartz substrates. This concentration corresponds to 1.6 X lo4 mol/L. Aggregated systems were prepared in the form of thin films. Three kinds of such films were made on quartz substrates, which were either phase I or phase 11: a spin-coated film of
:i
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10535
, J>%3
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CPM Ring Dye Laser
Au;rrelator
Ar Ion Laser
V
Variable Delay
Variable Delay
Pump-probe Svstem O+dH
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Figure 1. Femtoseoond laser system for pumpprobe spectroscopy: CVL, copper vapor laser; G1, gain dye jet (rhodamine 6G/ethylene glycol, 2 X lo-' mol/L, 300 pm); SAl, saturable absorber jet (DODCI/ethylene glycol, 1.5 X lo-' mol/L, 50 pm); G2, gain dye flow cell (rhodamine 640/H20with 20 vol % of Ammonyx LO, 3 X 10-4 mol/L, 3 mm); SA2, saturable absorber jet (malachite green/ethylene glycol, 1 X lo-' mol/L, 500 pm); EG, ethylene glycol jet (1 mm); PSI, four-prism sequence made of silicate glass; PS2, four-prism sequence made of high-index glass (SFl 1); SH, mechanical shutter; X/2, half-wave plate; PL, GlanThompson polarizer; SP, spectrometer; MCPD, multichannel photodiode array; LS,loud speaker; PMT, photomultiplier tube, S, sample; A l , A2, apertures.
(t-Bu),.,VOPc doped in a host polymer, a similar film without the polymer, and a vapordeposited film of VOPc. (t-Bu),.,VOPc was cast on quartz substrates by spin-coating the compound dissolved in chloroform either with or without a host polymer. Polystyrene was used as the host polymer in the samples for the pumpprobe measurements because it provides the best stability of samples against intense laser-pulse irradiation among several popular polymers such as poly(methy1 methacrylate), polycarbonate, poly(viny1 acetate), and so forth. Thicknesses of the films with and without the host polymer were ca. 0.5 and 0.1 pm, respectively. These films,prepared by the above procedure, were in phase I and were changed into phase I1 by exposure to dichloroethane vapor in a desiccator for ca. 20 h at room temperature. Films of the unsubstituted VOPc were prepared on quartz substrates by vacuum vapor deposition because it is hardly soluble in any volatile solvent. Thickness of the films was measured to be ca. 0.1 pm. The sample obtained in this manner was phase I, which could be changed to phase I1 by heating it at 125 OC. X-ray diffraction measurements showed that these films are polycrystalline. Thin films of VOPc were grown also on a potassium bromide (KBr) crystal substrate by the organic MBE t e c h n i q ~ e . ~It~ . ~ ~ is known from the data of reflection high-energy electron diffraction (RHEED) that, in layers close to the surface of the substrate, epitaxially-stacked VOPc molecules form a square lattice reflecting the symmetry of the (001) surface of the substrate.28 The structural change of these samples was monitored by measuring the UV-visible absorption spectrum. Pump-lhbe Measur-ts. A schematic of the experimental setup for pump-probe measurements is shown in Figure 1. Femtosecond optical pulses obtained from a colliding-pulse mode-locked (CPM) ring dye laser,30containing a four-prism sequence for the compensation for group velocity dispersion in the c a v i t ~ , 3were ~ * ~amplified ~ by a six-pass amplifier pumped by a 5-W output of a copper vapor laser (CVL) (MLT/30/SP, Metalaser) operating at 10 kHz.33934The typical energy and the wavelength of the amplified pulses were 2 pJ and 620 nm, respectively. The pulses, broadened temporally by passing through the amplifier system, were compressed to 50-60 fs by a successive four-prism sequence. The output beam was split into two beams, one of which was used as a pump after passing through a variable
10536 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
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Terasaki et al.
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Wavelength /nm Figure 2. Absorption spectra at room temperature: (a) ( ~ - B U ) ~ , ~ V O P C in chloroform (8 X lo-' mol/L), (b) phase I and (c) phase I1 of (t-Bu),,,VOPc doped in polystyrene at 10 wt W,and (d) VOPc deposited on KBr. The absorbance was multiplied by 0.3 in a.
optical delay line and the other was used as a broad-band probe after being focused into an ethylene glycol jet to generate continuum pulses.3s The intensity of the pump beam was controlled by a half-wave plate-polarizer combination. Two detection schemes were employed. The transmittancechange spectrum was measured by using a multichannel spectrometer (USP-500, Unisoku) and a mechanical shutter (VS14S2T1, Vincent Associates), which blocked the pump beam at ca. 5 Hz, as shown in Figure 1. For measurements of the transient transmittance change at a fixed probe wavelength, a phase-sensitive-detection technique was utilized. In this method, the output of a photodetector, which observed the intensity of the transmitted probe after a monochromator, was fed to a lock-in amplifier (SR530, Stanford Research Systems) and the signal synchronized with the chopping frequency (1-2 kHz) of the pump beam was recorded. The average power of the pump was monitored with a tiny portion reflected by a glass plate to discriminate signals. In both schemes, the chirp in the continuum probe was corrected by using cross-correlation traces obtained by sum-frequency generation between the pump and the probe. Most of the measurements were carried out at room temperature. For measurements at low temperatures, a continuous-flow type liquid He cryostat (CF-1204, Oxford) was used.
Results aod Discussion A l m r p t h Spectra. An absorption spectrum of a chloroform solution of (~-BU)~,~VOPC is shown in Figure 2a. A sharp and strong Qband transition appears at about 700 nm, and the next higher excited state, B band, appears around 350 nm. The shoulders appearing at 700-800 cm-I and 1500-1600 cm-'higher than the sharp peak of Q band are attributed to the vibrational structures. The polystyrene film doped with (t-Bu),.,VOPc at 0.01 wt % also showed an absorption spectrum similar to that of the solution. Absorption spectra of the films of phases I and I1 containing are shown in Figure 2b and c, re10 wt 5% of (~-BU)~,~VOPC spectively. The absorption peaks in the Q-band region are split, shifted, and broadened compared with those of the solution. This suggests that in rather dense spin-coated films, and also in vapordepoeitcd films described later, the phthalocyanine molecules aggregate and there exist strong intermolecular interactions. The marked difference between the absorption spectra of phases I and I1 is the appearance of a peak at 810 nm after solvent vapor treatment, which is due to differences in the molecular arrangements between the two phases. The absorption spectra of films without the host polymer and of VOPc films deposited on quartz substrates had essentially the same structures both in phases I and 11, except for the slight differences in the peak wavelength, the absorbance ratio of the peaks, and the magnitude of the shoulder at the longer wavelength side of the Q band. Similarity of the absorption spectra of VOPc and (~-BU)~,~VOPC suggests that the structure of the molecular aggregates is insensitive to the tert-butyl substitution. However, the one with four substituents, (~-Bu)~VOPC, existed only in phase I and could never be transformed to phase I1 because of the steric hindrance.
0.0
-
650
700
750
800
850
Sa,
Wavelength /nm
Figure 3. Temperature dependence of the absorption spectra: (a) phase I and (b) phase I1 of ( ~ - B U ) ~ , ~ V O doped P C in poly(methy1 methacrylate) at 40 wt ?%and (c) VOPc deposited on KBr. -, 10 K ---,295 K.
In phase I there appears two peaks in the Q-band region. The small blue shift of the transition can be explained by the cofacially associated dimer f o r m a t i ~ n . ' ~ This . ~ ~ -speculation ~~ of the molecular stacking structure of phase I is also supported by Raman and IR spectra.20 In phase I1 the crystal structure is found to have triclinic PT symmetry and have a staggered and slipped-stack struct~re.~IJ~ This structure gives rise to both the blue shift and the red shift of the Q-band transition because of the interaction between obliquely aligned exciton dipoles.36 However, the simple dipole-dipole interaction model is not sufficient to explain such a large red shift of the absorption peak by ca. 2000 cm-I from that of m o n ~ m e r s . l ~ * ~ ~ The temperature dependence of the absorption spectra is shown in Figures 3a and b for phases I and 11, respectively, of (tBU)~,~VOPC doped in poly(methy1 methacrylate) (PMMA) at 40 wt 9%. The spectrum of phase I remained unchanged, while in a phase I1 film, a significant red shift and sharpening were observed only at the lowest-energy absorption peak. These are common spectral features of all the phase I and I1 samples of polymer-doped, undoped, and vacuumdeposited films. The red shift of the absorption is probably caused by the denser packing of molecules at lower temperatures. These observations suggest that the lowest-energy exciton in phase I1 films has a nature different from other excitons and is strongly affected by intermolecular interaction. The absorption spectrum of VOPc deposited on KBr shown in Figure 2d is completely different from those of phases I and 11. It has a peak at ca.770 nm, which is narrower than the other films and red shifted by ca. 1300 cm-l from the monomer peak. This spectral feature has not been reported in the previous works. This might indicate a new stacking arrangement with a square lattice structure of VOPc, which is affected by the surface structure of the substrate because of the epitaxial growth of the thin film. The temperature dependence of its absorption spectrum is shown in Figure 3c. The spectrum at low temperature has red-shifted and sharpened peaks compared with that at room temperature, which was also seen for the lowest-energy absorption peak in the phase I1 films. And, in this case, the main peak split by ca. 60 cm-l. The shoulder that appeared around 810 nm can be regarded as a weak contribution of a phase 11-like excitonic transition. These are the features of the electronic transitions in VOPc on KBr, which make it distinct from phases I and 11. Excited-State Dynamics. 1. Isolated Molecular Phase. Differential transmission spectra (DTS) of a chloroform solution
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10537
Femtosecond Spectroscopy of Phthalocyanines 0.4
,
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0.1
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-0.2
0.0
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650
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Figure 4. Differential transmission spectra of (r-Bu)l,lVOPcin chloroform at time delays of (a) 0.1, (b) 1, and (c) 100 ps. Excitation, 620 nm.
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Figure 6. Differential transmission spectra of phase I1 (f-Bu),,,VOPc doped in polystyrene (10 wt 5%) at time delays of (a) 0.04, (b) 1, and (c) 10 ps. Excitation, 620 nm.
t: 0.3 0.2
730
Wavelength /nm
4
5
Delay /ps
Figure 5. Transmittance change of (t-Bu)l,lVOPc in chloroform as a function of time delay up to 50 ps (top) and 5 ps (bottom). Excitation, 620 nm; probe, 700 nm. 0,data; -, fitting curvts with a constant term and an exponential term with a time constant of 0.7 ps.
of (Z-BU)~~~VOPC are shown in Figure 4. The excitation wavelength, 620 nm, lies in the tail of the higher energy side of the Q-band absorption. Significant bleaching, Le., absorption saturation, was observed at the Q-band transition. At the higher energy side of the Q band, comsponding to the transparent region between the Q band and the B band, very weak induced excited-state absorption was observed. A transient response of the bleaching peak at 700 nm is shown in Figure 5. This trace could be fitted to a function consisting of an exponential term with a decay-time constant of 0.7 f 0.2 ps and a constant term with a fraction of ca. 0.8 of the peak of the signal. The molecule of vanadyl phthalocyanine is paramagnetic bea m of an unpaired electron in the dmorbital of the central metal ion (V@+) coupled with the diamagnetic phthalocyanine ligand (Pc2-). Owing to this electron coupling, the singlet states of the ligand become singdoublet, and the triplet states become trip doublet and tripquartet (Kramers degenera~y).~~ Therefore, in this molecule, the ground (21') and the first-excited Q-band (ZQ) states are singdoublet, and a tripdoublet (2T1)and a tripquartet (VI) state lie between 21' and Q16 For the observed 0.7-p decay, there are two possible processes. One is the relaxation process from ZQ to 2TI,and the other is the thermalization process3g between 2Tl and 4TI. For the former process, since the relaxation from ZQ to 2T1is a spin-allowed process, it can take place much faster than the ordinary singlet-to-triplet relaxation. The decay rate can be estimated as follows. The quantum yield of fluorescence has been measured to be of the order of lV in the vanadyl phthalocyanine molecule.16 The natural radiative lifetime of the ZQ state is calculated to be about 15 ns from the absorption spectrum (emx
-
-
2 X lo5L/molan, Av 500 an-')by using the Strickler-Berg relation.40 Thus, the relaxation time is estimated to be 15 fs. It seems that 0.7 ps is too long to be attributed to this process. However, since the above estimation can easily contain an error of 1 order of magnitude, this possibility cannot be excluded. For the latter case, such a process was observed in solutions of paramagnetic copper and silver protoporphyrins. Because copper and silver have an unpaired electron as in vanadyl, their electronic energy-level schemes are similar to that of a vanadyl phthalocyanine molecule. In these protoporphyrins, the thermalization between 2T1and 4TIhas been reported to take place with time constant of ca. 450 and 11 ps for copper and silver porphyrins, respectively.4I Because the rate of this process surely depends strongly on the central metal and the ligand, it is possible that it takes place with a 0.7-p time constant in the prarent system. But, on the other hand, it might be too fast for the spin flip accompanying the 2TIto 4T1transition. Thus, in the present stage, we cannot clearly assign the observed decay to either of these two processes. However, it is safe to say that the total relaxation from ZQ to 2T1and VI, as a whole, takes place with time constant of 0.7 ps. The relatively large amplitude of the constant term persisting for longer than hundreds of picoseconds is said to be due to the high yield of 2Tl and 4T1. Their lifetimes, which have not been determined yet because of the low phosphorescence yields, are much longer than the temporal range of observation (-100 p). But they are shorter than 100 ps, the period of excitation pulses, because no transmittance change was observed at negative delays in Figure 5 . It is consistent with the phosphorescence lifetime of 3 ps observed in copper phtl~alocyanine.~~ The experiment was also carried out on a dilutely doped polystyrene film of the same molecule. The results were similar to those obtained for the above solution sample. 2. Phase ZZ. Figure 6 shows the DTS of a phase I1 film of (Z-BU)~.~VOPC doped in polystyrene (10 wt %). The outstanding feature is that significant bleaching appears at the lowest-energy absorption peak (ca. 8 10 nm), while the higher energy absorption peaks bleach very weakly. This is because excitons created by the femtosecond pulses at 620 nm undergo very rapid internal conversion to the lowest excited state and/or because induced excited-stateabsorption in this energy region cancels the bleaching signals. In the spectral range 500-600 nm, corresponding to the window of the ground-state absorption, induced excited-state absorption was observed, which was much weaker than the bleaching signal. The transmittance change at 8 10 nm for excitation intensities of 20 GW/cm2 is shown as a function of time delay in Figure 7. The decay curve consists of three components: a fast (subpicosecond) component, a slower (tens of picoseconds) component, and a much slower (longer than hundreds of picoseconds) com-
10538 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 0.21
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Figure 7. Transmittance change of phase I1 (~-BU),,~VOPC doped in polystyrene (10 wt 5%) as a function of time delay up to 50 pa (top) and 5 p (bottom). Excitation, 620 nm (20 GW/cmZ); probe, 810 nm. 0, data; -, fitting curves for the model with a rl/z dependent bimolecular decay rate and a constant monomolecular decay rate, where (hy’)-I = 1.1 X lod s1/2 and &-I = 50 p. 1
1
1
1
1
1
1
Terasalci et al. is due to bottleneck states, e.g., triplet statates, photocamers, and/or trap states, possessing lifetimes much longer than the time range of observation. In our experimental condition, the monomolecular decay rate, K(t), is considered to be a constant, k,because there are no energy acceptors in the samples. On the other hand, r(t)can be either a constant, y, or a certain timedependent rate, yr ll2, dependm on the interaction mechanism between excitons which causes quenching. The constant decay rate reflects the annihilation of excitons during their hopping migration in the crystallines, while the inverse of the square-root time dependence describes the annihilation via the long-range dipole-dipole interaction between excitons4w or that in the manner of the motion-limiteddithion.@ Similar observations were reported by other group. Ho and Peyghambarian studied a polycrystalline film of FAIPC,~~ which had an absorption spectrum similar to our phase I1 films. They explained the decay of the bleaching by assuming a constant decay rate for r(t)and K(t). On the other hand, Greene and Millard21 studied a polycrystalline film of &H2Pc and found out that the initial decay of the induced excited-state absorption signal is described by a timedependent (f exciton-exciton annihilation rate, although they did not include a monomolecular decay term. In the former model, where r(t) = y and K ( t ) = k, the time-dependent exciton density, eq 2, is reduced as f o l l o ~ s : ~ ~ * ~ exp[-kt] n (3) no 1 + (nor/k)(l - exp[-krI)
_ --
On the other hand, in the latter model, where r(t)= y W 2 and K ( t ) = k, it is expressed as follows: exp[-kt] -n= (4) / ~ )[(kt)I/*] no 1 ( 2 ~ y ’ / k l erf
+
4.2
0.0
0.2
0.4
0.6
0.5
1.0
/ps Figure 8. Bleaching decay of phase I1 ( ~ - B U ) ~ . ~ V Odoped P C in polystyrene (10 wt %) at two excitation intensities (0, 30 GW/cm2; X, 10 GW/cm2). Excitation, 620 nm; probe 810 nm. -, calculated curves, where = 0.7 X la“ d2and &-I = 50 p. -,calculated curves, where (hy’)-l = 2.1 X la“ d2and k’= 50 p.
--
ponent. Because the fast decay rate appears to be excitationintensity dependent as shown in Figure 8, it is considered to represent a bimolecular process, Le., exciton-exciton annihilation.21-23*43suThe slower decay is independent of excitation intensity and attributed to a monomolecular decay without interaction between excitons. The long-lived component is presumably due to bottleneck states such as triplet states. The exciton population density, n, obeys the following rate equation: dn = un,(t)ZL(t) - r(r)n2- K(t)n dt
(1)
where r(t)is the excitondecay rate via bimolecular annihilation p~wxssesand K(t) is the decay rate without interaction between excitons. The first term of the right-hand side of the above equation is the pumping rate, where ZL(t) is the excitation-laser photon flux, y&r) is the ground-state population density, and u is the absorption cross section. For a delta-function excitation pulse, the above equation gives the following timedependent exciton density at t > 0:
where is the initial exciton density at time zero. When the decay rate via the bimolecular processes is much greater than that of monomdecular ones, the differential transmission AT/T is given by the sum of a term proportional to n and a constant term,which
where erf [XI= Jfdu exp[-u2]. By the analysis reported in the it was found that, although by the former model previous we are able to fit the data for either short times or long times but not over the full time range from 0 to 100 p, the latter model can explain the data over the full time range of observation. Fitting of the data in Figure 7 yielded the solid curve with parameters of (%y’)-l = 1.1 X lod s1i2,k-I = 50 f 20 p, and a constant of ca. 10% of the maximum value of the data. When the volume fraction of the phthalocyanine aggregates in the polymer sample is considered, the value of is estimated to be 4 X l W 5cm3d 1 2 . These parameters are consistent with the excitation-intensity dependence of the bleaching decay shown in Figure 8. As discussed previously,26 the other phase I1 films prepared differently, Le., a spin-coated film of (t-Bu),,,VOPc without host polymer and a vapor-depoaitedf h of VOPC, resulted in essentially the same kinetics, which were also well explained by the model including a timedependent exciton-exciton annihilationrate with almost the same values of parameters y’ and k. Therefore, we could conclude that the effect of steric hindrance by the twt-butyl substituent on the exciton decay rates is weak and that the effect of the host polymer is also weak at the present dopant Concentration (10 w t 96) because of formation of molecular aggregates with a local structure similar to that of the undoped films. Low-temperature experiments have been carried out on a spin-coatedfilm of (r-Bu),.,VOPc doped in polystyrene. The decay kinetics of the lowest-energy excitons remained unchanged even at 10 K,although its absorption spectrum undergoes a marked red shift and sharpening. The fact that the initial decay kinetics of excitons is described by the timedependent (t-l12) bimolecular-decay-rate model suggests that the excitons interact with each other via the longrange dipoledipole interacti~n~’*~”* or that they diffuse in a motion-limited manner.21,48The long-range dipole-dipole interaction model seems to be consistent with the fact that this decay rate was insensitive to temperature and also with out -tal condition, Le., no was about 2 X lom cm-’ under the typical excitation intensity used (20 GW/cm2). Since the molecular density is estimated to be approximately 1.65 X lo2’cm-3 from lattice parameters of the phase I1 VOPc crystal,I5this excitation
+
Femtosecond Spectroscopy of Phthalocyanines
The Journal of Physical Chemistry, Vol. 96, No.25, 1992 10539 0.15,
(a)
%
,
,
0
IO
,
,
,
I
20
30
40
50 -
0.0~s
0.0
4.I
0.2
0.10
0.05 0.2
,
I
0.00
0.04
0.02 550
600
650
700
750
Wavelength Inm
Figure 9. Differential transmission spectra of phase I (t-Bu)l,lVOPc doped in polystyrene (10 wt %) at time delays of (a) 0.0, (b) 0.2,and (c) 100 ps. Excitation, 620 nm.
density corresponds to the condition in which one exciton is created on about every eight molecules. From the value of y’, the transition-dipole moment between excited states was estimated to be about 3 D, while that between the ground and the first-excited state was estimated to be about 3.2 D from the absorption spectrum. The values of y’ determined in the present study are larger by more than 1 order of magnitude than that for a polycrystalline film of B-H2Pc, 1.0 X 10-l6cm3 s-1/2?1 One possible reason for this discrepancy is that the value for b-H2Pc was underestimated because of the extremely high excitation density in the experiment. The other possibility is that the exciton in the phase I1 form of vanadyl phthalocyanines might have a larger transition-dipole moment. The electroabsorption suggest spectroecopyI79l8and the photoconductivity measurements~g the intermolecular chargetransfer nature of excitons in phase I1 vanadyl phthalocyanines. It is possible that such a feature is related to their faster decay rates. 3. Phuse I. In F i e 9, DTS of a phase I film of (t-Bu),,,VOPc doped in polystyrene (10 wt 9%) are shown. Contrary to those of phase I1 samples, bleaching of the absorption appeared over the full spectral range of the Q band, which has absorption peaks at ca. 650 nm (the higher exciton level) and 700 nm (the lower exciton level), and also induced excited-state absorption appeared at the shorter wavelength side, 5 0 0 6 0 0 nm, which was stronger than that observed in phase I1 samples. The recovery of the ground state was slower than that of phase 11. These differences between the two phases are clearly due to their different stacking arrangements and hence differences in the nature of the excitons. Measurement was carried out also on phase I films prepared differently, Le., a spin-coated film of (~-BU)~,,VOPC without the host polymer and a vapor-deposited film of VOPc, in order to examine the effect of the host polymer as well as the steric hindrance of tert-butyl substituents. The observed transient spectra appeared to be asentially the same for these three phase I films, as was also the case in phase I1 films. The following discussion is focused on the spin-coated film of (t-Bu),.,VOPc doped in polystyrene. The temporal evolution of the transmittance change at 700, 650,620, and 550 nm is shown in Figure 10. The density of the initially created excitons, n ,, was estimated to be 3 X lozocm-3 by considering the volume fraction of the phthalocyanine aggregates in the sample, when the incident laser intensity was 20 GW/cm2. The data at 620 nm were taken by using not continuum but attenuated amplified pulses for the probe without a monochromator in front of the detector. This allowed averaging over a spectralwidth of the probe pulses around 620 nm and elimination of the effect of the probe spectral shift due to the induced phase and the artifact due to the high spectral resolution compared with the inverse of the temporal resolution determined This artifact is manifested, for example, by the pulse durati~n.~z~’
0.00
0.00 4.01
4.02 4.03 4.04 -10
1
0
1
2
3
4
5
Delay /ps
Delay Ips
Figure 10. Transmittance change of phase I ( ~ - B U ) ~ , ~ V O doped P C in polystyrene (10 wt %) as a function of time delay up to 50 ps (a-d) and 5 ps (a/+ 0, data; -, fitting curves. Excitation, 620 nm (20 GW/ cm2): (a, a’) probe, 700 nm. The fitting curve is for the model with a t-1/2dependent bimolecular and a constant monomolecular decay rates, = 2.5 X 10” d2and k-l = 10 ps; (b, b’) probe, 650 nm. where (r&)-’ The fitting curve consists of a constant term and two exponential terms with a rise time constant of 1.5 ps and a decay time constant of 10 ps; (c, c‘) probe, 620 nm, which was split off from the excitation beam and attenuated. The fitting curve consists of a constant term and three exponential terms with rise time constants of 0.4 and 4.5 ps and a decay time constant of 70 ps; (d, d’) probe, 550 nm. The fitting curve consists of a constant term and an exponential term with a time constant of 15 PS.
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Delay ips Figure 11. Bleaching decay of phase I ( ~ - B U ) ~ , ~ V O doped P C in polystyrene (10 wt %) at two excitation intensities (0,30 GW/cm2, X, 5 GW/cm2). Excitation, 620 nm; probe, 700 nm. -, calculated curves, where = 1.5 X lod s1l2and k-’ = 10 ps. ---, calculated curves, where (n,,y’)-I = 9.0 X 10“ s1/2 and k-l = 10 ps.
as the antisymmetric shape of DTS near zero delay observed around the excitation wavelength (620 nm) in Figure 9a, which can be explained qualitatively by the simulation based on the theory of transient hole burning when the pump pulse is detuned from the peak of an inhomogeneously broadened absorption line.s2.54 The decay kinetics at 700 nm was dependent on the excitation w 11, and consistedof three components intensity, as shown in F as in the case of the bleaching observed in phase I1 samples. We attribute the fast component to an exciton-exciton annihilation process, the slower component to a monomolecular nonradiative decay process, and the slow component to formation of bottleneck states such as triplet states. The data were fitted in two ways as described in the preceding section by assuming the temporal
10540 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
evolution of exciton density represented by eqs 3 and 4. Both models could fit the data well over the full observed time range in this case. However, because similar data taken at low temperatures between 10 and 50 K showed only a slight change in the response curves of the bleaching from that at room temperature, the mechanism of exciton+xciton annihilation is attributed to the long-range dipole-dipole interaction, as in phase I1 films, rather than collision of excitons after hopping migration, in which activation energy is usually needed in the case of localized excitons. The values of T’ and k-’were estimated as ca. 1 X 10-15cm3 and 10 f 5 ps, respectively, and the fraction of the constant term as ca. 0.3 of the peak value of the data. These parameters give the fitting curves shown in Figures loa, a’ and 11. From the value of T’, the transition-dipole moment between excited states was estimated to be about 0.8 D. This value, as well as T’, is smaller than that of phase I1 by a factor of about 4,whereas that between the ground and the first-excited state, which was estimated to be about 3.5 D, is almost the same as that of phase 11. The response of the bleaching at 650 nm, Figure lob, b’, behaved differently from that at 700 nm. The bleaching took place at the same time as the rise of the excitation pulse and even after this fast rise, it continued to grow more slowly. This component with a rise time constant of a few picoseconds probably corresponds to the creation of the excitons due to the annihilation of excitons in the lower energy level (700 nm). That is, a portion of the excitons in the lower exciton level annihilated via the bimolecular process was once excited into a higher singlet exciton state and repopulated at this higher exciton level in the Q band (650nm) after the successive rapid internal conversion. Under higher excitation intensity, the rising component had a faster rate and a larger fraction. This observation also supports the assignment. After this initial decay, the bleaching signal decays with time constant of ca. 10 ps and about 0.7 of the peak signal remains for much longer than 100 ps. The signal fraction of the long-lived component is much larger than that observed at 700 nm. This suggests that the decay from the higher to the lower exciton level in the Q band is much slower than usual internal conversion processes. And also, excitons in the higher energy level do not appear to undergo exciton-txCiton annihilation by interacting with each other. Similar slow rise of bleaching following excitation was also observed in the data at 620 nm, Figure lOc, c‘. However, because this response was insensitive to excitation intensity, it is attributed to the broadening of the burned spectral hole centered at 650 nm to the blue side of the absorption band, Le., spectral cross-relaxation5557in the excited state. This process appears more clearly in DTS, in which the wavelength giving A T / T = 0 shifts from 620 nm at zero delay (Figure 9a) to 615 nm at the delay of 0.2 ps (Figure 9b) and to 600 nm at the delay of 100 ps (Figure 9c). The data in Figure lOc, c‘ was fitted to a triexponential function plus a constant term, for instance. It yielded rise time constants of 0.4 f 0.1 and 4.5 f 1 ps, a decay time constant of 70 f 20 p, and a fraction of the constant term of about 50% of the peak ATIT. The decay kinetics of the induced excited-state absorption at 550 nm, shown in Figure lod, d’, had a decay rate of 15 f 3 ps and a constant term of 0.7 of the peak value of the data. Thus the observed induced absorption at 550 nm is considered to be taking place from the higher exciton level in the Q band, with a lifetime of ca.10 ps, and bottleneck states, with lifetimes of much longer than 100 ps. In the present study it was found that two types of excitons created in the phase I form, corresponding to the two absorption peaks in the Q band, behave completely different. The lower energy excitons (700nm) relax by the exciton-exciton annihilation due to the dipoltdipole interaction more rapidly than the higher energy excitons (650nm), which interact only weakly with each other and have a long lifetime ca. 10 ps. 4. Films Deposited on KBr. In Figure 12, DTS at three different time delays between pump and probe pulses are shown. The outstanding The excitation density, n,was ca.2 X 1oM feature of these spectra are the appearance of a strong bleaching
0.4
-0.2
Terasaki et al.
,
I
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,
u
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700
800
754
850
Wavelength Inm
Figure 12. Differential transmission spectra of VOPc deposited on KBr at time delays of (a) 0.1, (b) 1, and (c) 10 ps. Excitation, 620 nm.
0.00
t
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0.01
e
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3
4
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Delay Ips Delay Ips Figure 13. Transmittance change of VOPc deposited on KBr as a function of time delay up to 50 ps (a-c) and 5 ps (a’-’). 0 , data; -, fitting curves. Excitation, 620 nm (20 GW/cm2): (a, a’) probe, 780 nm. The fitting curve consists of a constant term and two exponential terms with time constants of 0.4 and 3.5 ps; (b, b’) probe, 730 nm. The fitting curve consists of R constant term and two exponential terms with time constants of 0.4 and 3.5 ps; (c, c‘) probe, 620 NII, which was split off from the excitation beam and attenuated. The fitting curve consists of a constant term and two exponential terms with time constants of 0.3 and 3.0 ps.
L
I
-0.2
0.0
‘ 0.2
I 0.4
I
f 0.6
i 0.8
I 1.0
Delay Ips Figure 14. Bleaching decay of VOPc deposited on KBr at two excitation intensities (0,30GW/cm2; X, 5 GW/cm2). Excitation, 620 nm; probe,
780 nm. -, calculated curves with a constant term and two exponential terms with time constants of 0.4 and 3.5 p.
around 780 nm, the peak of its absorption spectrum, and a signal of induced excited-state absorption at a little higher energy region around 730 nm immediately after the excitation within the time resolution.
P
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4.2 0.4
,
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The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10541
Femtosecond Spectroscopy of Phthalocyanines
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Figure 16. Temperaturedependent time evolution of the transmittance change of VOPc deposited on KBr. Excitation, 620 nm; probe, 780 nm. The data are normalized to unity at the maximum value. The bottom is shown in a magnified scale.
Wavelength /nm
Figure 15. Differential transmission spectra of VOPc deposited on KBr at 10 K at time delays of (a) -0.06, (b) 0.1, (c) 1, (d) 10, and (e) 80 ps. Excitation, 620 nm.
The timedelay dependence of the bleaching signal at 780 nm is shown in Figure 13a, a'. In marked contrast to the decay kinetia of phase I and I1 films, the initial decay of this bleaching signal appeared to be insensitive to excitation intensity, as shown in Figure 14. Fitting of the response curve to a biexponential function including a constant term yielded time constants of 0.4 f 0.1 and 3.5 f 0.5 ps, whose fraction were ca. 0.62 and 0.32, respectively. The fraction of the constant term was ca. 0.06of the peak of the signal. Fitting of the data at 730 nm in Figure 13b, b' resulted in almost the same parameters except that the ratio of the constant term was ca. 0.1 of the peak value of the data. The induced excited-state absorption lasting for longer than hundreds of picoseconds suggests the transition not only from the first-excited state but also from bottleneck states. The data in Figure 13c, d were taken by the pumpprobe method using the same amplified pulses (620 nm) for both excitation and probe without a monochromator in front of the photodetector. In this c88t,induced excited-state absorption appeared at early times and it tumed to bleaching after 2 ps. The obtained decay time constants were the same as those of the above within experimental mor. At this wavelength, the induced absorption is predominantly due to the excitons populated in the first-excited state and the contribution of bottleneck states is much weaker than at 730 nm. From these data, the following model is proposed. The excitons initially created by 620-nm pump pulses rapidly relax to the lowest-energy excited state around 780 nm. These excitons relax to a lower lying level and/or the ground state with a time constant of ca. 0.4 p,and formation of a bottleneck population is followed with a time constant of ca. 3.5 p. The bottleneck state lives for longer than the time range of our observation, 100 ps. The absence of the exciton-exciton annihilation process is probably becauseof the rapid (0.4 p) nonradiative decay of excitons, which takes place faster than the interaction between excitons at the present density level, due to the strong exciton-lattice coupling in the relatively ordered crystal film compared with phase I or I1 films, which are in amorphous structures. Figure 15 shows DTS at 10 K. Significant bleaching appeared around 780 nm immediately after excitation. Also,induced excited-state absorption was observed at several wavelengths. The structure in these DTS is similar to that of the absorption spectrum shown by a solid line in Figure 3c. This is because the bleaching signal, which has the same structure as the absorption spectrum, overlaps with rather structureless induced absorption signals in this spectral region. The transmittance change was observable
-
even after 80 ps, Le., for which the decay was slower than that observed at room temperature. The temperature dependence of the time evolution is much clearer in Figure 16, which shows the bleaching at 780 nm as a function of time delay for 10, 100, and 295 K. The initial relaxation time constants, 0.4 and 3.5 ps, remained almost unchanged even at 10 K. However, significant differences are seen after ca. 10 ps. At room temperature the bleaching recovered monotonically, whereas at 10 K, it starts to increase again at 10 ps, and the data at 100 K traced between the two. This observation, which was not the case in phases I and 11, suggests the existence of a temperature-dependent prows of the formation of excitons at the lowest energy of the Q band. However, details of these kinetics still remain to be clarified. In this sample, the feature of DTS was similar to those of the phase I1 films in that a strong bleaching signal appears only at the lowest-energy edge of the Q band immediately after excitation at the high-energy edge of the band. However, the relaxation mechanisms of excitons are significantly different. This suggests that the molecular stacking arrangement affects not only interaction between excitons in different molecules but also the nonradiative monomolecular decay mechanism of excitons.
Summary Femtosecond time-resolved measurements revealed that the excited-state dynamics in vanadyl phthalocyanine is strongly dependent on the four types of molecular arrangements as follows. In isolated molecules, the relaxation from the first excited singdoublet (zQ) to the tripdoublet (2T,) and tripquartet (4T,) was found to take place with a time constant of 0.7 ps. The recovery of the ground-state population was very slow (>>lo0ps) because of the high yield of the population of tripdoublet and tripquartet states. In phase I1 films, the excitons at the lowest-energy Q-band absorption peak were created immediately after excitation and relaxed to the ground state much faster than isolated systems because of an exciton-exciton annihilation process due to the long-range dipoledipole interaction. The rate of this bimolecular decay had a time dependence, t-1/2, and its coefficient y' was estimated to be 4 x io+ cmz 0. In phase I films, two types of excitons were created after excitation. The initial relaxation of the lower energy excitons was interpreted to be an exciton-exciton annihilation process as in phase 11. The coefficient y' of the time-dependent bimolecular decay rate was estimated to be 1 X l(r15m3&I2, which is smaller than that of phase I1 by a factor of about 4. This is mainly due to the differences in the transition dipole moment between excited states in phases I and 11. On the other hand, the higher energy excitons created at 650 nm did not show fast decay via the bimolecular annihilation and relaxed more slowly with time constant of ca. 10 ps. In VOPc deposited on KBr, in marked contrast to the above cases of phases I and 11, excitons created at the bottom of the Q band showed very rapid intrinsic monomolecular decay with time
10542 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 constant of 400 fs. This is probably due to the stronger exciton-lattice coupling in the more ordered crystal phase. Since the crystal structures and the nature of electronic transitions of these materials have not been fully understood yet, the relationship between arrangements of molecules and the dynamia of excitons remained to be clarified. However, it is important to notice that the same molecules can show markedly different dynamics of the excited electronic states. Although only one compound, vanadyl phthalocyanines, was studied in the present paper, such phenomena are likely to be observed also in many other molecular systems.
Acknowledgment. The authors arc grateful to Drs. K. Ishikawa, S.Kahihara, J. P. Sokoloff, T. Hattori, and E. Tokunaga for the stimulating discussion, to Messn. A. Kaneko and T. Yamamoto for their help in some of the measurements for sample characterization, and to h. M.Yoahizawa and M.Taiji for their helpful advice on the experimental techniques. This work was supported in part by a Grant-in-Aid for Encouragement of Young Scientists (02952075) from the Ministry of Mucation, Science, and Culture of Japan and by a Fellowship of the Japan Society for the Promotion of Science for Japanese Junior Scientists to A. Terasaki.
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