J. Phys. Chem. 1995, 99, 17174-17180
17174
A Nanosecond Transient Absorption Study of Photoinduced Heat Generation in Microcrystals of PMetal-Free Phthalocyanine Dispersed in Polymer Films H. Van Mingroot, L. Viaene, M. Van der Auweraer, and F. C. De Schryver’ Katholieke Universiteit Leuven, Laboratory f o r Molecular Dynamics and Spectroscopy, Department of Chemistry, Celestijnenlaan 200 F, B- 3001 Heverlee, Belgium
M. Ichikawa, H. Fukumura, and H. Masuhara Department of Applied Physics, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Received: July 10, 1995; In Final Form: September 6, 1995@
Nanosecond transient absorption spectra were recorded following pulsed nanosecond laser excitation at 640 nm of dispersions of 2-metal-free phthalocyanine in a film made either of copolymer of styrene and n-butyl methacrylate (70:30) or of bisphenol A-polycarbonate. The transient absorption spectra of the dye in a polymer matrix differ significantly from those obtained after laser excitation of phthalocyanine molecules in solution. Referring to the temperature difference spectra of the ground state absorption spectra and the effect of immersion of the polymer film into a solvent, the transient detected between 650 and 800 nm was assigned to a photoinduced thermal phenomenon.
Introduction Dye-doped polymer films are frequently used as charge generation and transport layers in xerographic systems because of their chemical and thermal stability and their photoactivity in the visible light range.’ In order to obtain precise information about the mechanism of charge generation in such systems, it is indispensable to investigate the photophysical and photothermal processes and the possible formation of transient intermediates upon laser excitation of the doped polymer films. The photophysical and photochemical properties of molecular microcrystals are different from those of the component molecules determined in gas and solution phases, since electronic excitation energy can be trapped in dimers or aggregates or can be delocalized over a large number of chromophores. Furthermore, efficient excitation energy transfer and hopping between trapping sites can be expected. Hence, the relaxation processes of electronic excited states in an organic molecular crystal depend on the crystal structure.’ Exciting a molecular crystal with an intense and strongly absorbed laser pulse creates locally a high concentration of excitons leading to efficient exciton-exciton interactions. These interactions play a major role in the deactivation processes of excited molecular crystals of phthalocyanine dyes.2 A significant heat generation in a small volume can be expected for laser excitation at 640 nm because of the high optical absorption coefficient at the excitation wavelength and the quenching of monomer fluorescence due to exciton-exciton annihilation and trapping by nonfluorescing aggregates. It is the aim of the present work to investigate to which extent photothermal processes occur in the case of pulsed laser excitation of dispersions of X-metal-free phthalocyanine in polymer films. A photoinduced thermal phenomenon caused by rapid heating and cooling of the sample upon laser photolysis was reported for Ti02 powder^.^ Diffuse reflectance laser flash photolysis studies of microcrystalline samples of substituted pyrazolines confirmed the importance of thermal processes as the main contribution to the transient absorption spectrum when a high
* To whom correspondence should be addressed. Abstract published in Advance ACS Abstmcts, November 1, 1995.
0022-365419512099- 17174$09.00/0
laser flux was used to excite the ample.^ Nanosecond laser photolysis data of poly(N-vinylcarbazole) film could be interpreted by assigning the observed transient absorption tail to a hot band of the ground state absorption. The temperature rise time was less than 50 ns, and the film cooled in a few microsecond^.^ A hot band due to vibrationally excited molecules was confirmed to be the main transient species produced via exciton-exciton annihilation in the picosecond time range for a P-copper phthalocyanine pellet.6 Picosecond regular reflection spectroscopy was used to investigate the rapid temperature rise of a P-copper phthalocyanine pellet in vacuum upon laser excitation. The obtained absorption spectrum (at 1 ns after excitation) showed negative peaks between 540 and 640 nm and between 690 and 730 nm and positive peaks between 640 and 690 nm and above 730 nm. The positive peaks at 670 and 760 nm were assigned to the vibrationally excited ground state. To confirm these interpretations, the temperature dependence of the absorption spectra of P-copper phthalocyanine solid in the ground state was measured and related to the transient absorption spectrum. When the penetration depth at the laser excitation wavelength is small and the energy of the laser pulse is absorbed in a small volume, similar temperature rises occur are expected in transmission flash photolysis studies. Local heating of suspensions and colloids, where high values of the product of the extinction coefficient and the concentration can be found, can also be induced. This explains why organic and inorganic crystals can sublime or undergo thermal damage when excited with a highpowered pulsed l a ~ e r . ~ , ~ . ~ , ’ Light absorbed by a polymer film will be converted ultimately in either luminescence or heat, and heat transfer can occur either by blackbody radiation or by conduction.8 The latter leads to an increase of the temperature. As the temperature of the film becomes higher than that of the surroundings, a heat transfer will occur to the surroundings. This heat transfer will limit the temperature increase under stationary illumination. For pulsed illumination, this process will limit the temperature increase only if the relaxation time of the temperature of the film is shorter than the laser pulse. 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 47, I995 17175
Microcrystals of 2-Metal-Free Phthalocyanine
Experimental Section Samples. The sample of X-metal-free phthalocyanine (Dainippon) and the polymer films were provided by Agfa-Gevaert. The phthalocyanine dye microcrystals (with a thickness of 1 p m or thinner) are first dispersed for 72 h in dichloromethane together with the binder. The total concentration is about 10 wt % of the solid phase in the solvent with a 1:0.15 pigment/ binder weight ratio. PS (Himer SBM 600, Sanyo Chemicals) or low molecular PC (Makrolon CD 2000, Bayer) is used as the binder in the films which have a thickness of about 2 pm, After adding the exact amount of the binder to adjust the pigment concentration, the films are coated with a Braive wire rod coater (doctor blade coater) on a poly(ethy1ene terephthalate) substratum (PET) and dried for 1 h at 80 "C. The combination of the film which contains 20 wt % 2-metal-free phthalocyanine and the PET substratum has a thickness of 10 pm. Another type of substratum used is glass. In the case of polymer films (PS) on glass, the total thickness of the sample is about 3 mm, while the thickness of the 20% polymer film is 5 pm. Metal-free phthalocyanine has been reported to exist in at least three polymorphic forms @-, a-, and X-metal-free phthalo~yanine).~-I The 2- modification of metal-free phthalocyanine has been reported by BymeI2 and has been characterized by its unique X-ray diffraction pattem as well as its infrared and visible absorption spectra.I3 The particles of 2-metal-free phthalocyanine are in the shape of needles, the majority of which have a length in the 1 pm region or smaller (typically 200 nm). The inner particle structure allows for gases and solvents to penetrate into the grain boundaries of the phthalocyanine microcrystals. In this polymorph, the phthalocyanine molecules are stacked in columns two by two in aparallel plane dimer. In this model, the planar ring systems of the two phthalocyanine molecules in a dimer are parallel to one another. The centerto-center distance between the phthalocyanine molecules in the dimer is about 4 A, while the center-to-center distance between two successivephthalocyaninemolecules in two different dimers is about 5.4 A.13 The level of light scattering in the case of d e phthalocyanine doped polymer films can be judged from the transmittance at longer wavelenghts (900 nm) where no light absorption by the phthalocyanine chromophores occurs. In this region, the transmission amounts to only 14% (at 900 nm). This may be due to scattering of the incident light beam by the phthalocyanine microcrystals whose dimensions are in the 1 pm region. If a light beam is focused on a transparant sample, a part of the light is scattered in all directions by the molecules of the substance (Rayleigh or "elastic" scattering) and by small particles such as microcrystals (Tyndall scattering). The effects of stray light may lead to a low background transmission and, in some cases, to the appearance of false absorption peaks.I4 Ground State Absorption Spectroscopy (Room Temperature and Higher Temperatures). The ground state absorption spectra were recorded with a Perkin-Elmer Lambda 5 W / v i s spectrophotometer and with a Cary 17 UV/vis/NIR spectrophotometer (for measurements of ground state absorbance at longer wavelengths than 900 nm). In order to record the ground state absorption spectra of the polymer films at elevated temperature, a Mettler heating compartment containing the polymer film was arranged in the optical path of the sample light beam of the spectrophotometer. A film of only the PET substratum or glass was placed in the optical path of the reference light beam. Absorption measurements at higher temperatures were performed after thermal stabilization of the polymer samples.
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Figure 1. (A, top) Ground state absorption spectrum of 20 wt % dispersion of X-metal-free phthalocyanine microcrystals in PS at different temperatures: 25- 60, 100, and 150 "C. (B, bottom) Influence of temperature on the ground state absorbance spectra (between 11 OOO and 15 OOO cm-I) of 20 wt % dispersion of X-metal-free phthalocyanine microcrystals in PS.
Excitation Source and Detection System Used in the TimeResolved Experiments. The excitation source is a DCM dye laser with a maximum output of about 25 mJlcm2 per pulse at 640 nm, pumped by the second harmonic (532 nm) of a Q-switched Nd3+: YAG laser (Type DCR3-G, Spectra-Physics). The light pulses provided by this setup have a pulse width of about 8 ns. A pulser (Pulseinheit MSP 05, Muller Elektronik-Optik) in combination with a 450 W flash lamp providing an intense 1- 10 ms light beam was used as an analytical light beam. This beam impinges into the polymer sample in a perpendicular geometry with respect to the excitation light source. The polymer sample is placed in the path of the excitation beam and analyzing beam in a 45" geometry. The analyzing light is transported by an optical fiber to a polychromator (SpectraF?~275 Acton Research Co. fl3.8) and recorded by an OMA III system (EG&G Instruments) equipped with a MCP gated intensifier that is sensitive to red light (Model 1420R-1024-G) and a diode array. The intemal clock of the OMA setup makes it possible to vary the time between the laser pulse and the collection of the spectrum. The delay increment can be varied between 1 ns and 100 ps. The gate width used in the experiments is 100 ns.
Results and Discussion Ground State Absorption Measurements At Room Temperature. The optical absorption spectrum at room temperature of a film containing a dispersion of small particles of X-metalfree phthalocyanine on PET is presented in Figure 1A (bottom spectrum). The spectrum shows a broad absorption band
17176 J. Phys. Chem., Vol. 99, No. 47, 1995 between 20 000 and 25 000 cm-' (Soret band) and two broad absorption bands between 11 000 and 20 000 cm-' with absorption maxima at 12 400 and 15 600 cm-I. Above 25 000 cm-I, the substratum (PS or PC) starts to absorb, disturbing the absorption measurements of the phthalocyanine microcrystals. The absorbance by the substratum is corrected for by the use of a film with an identical composition in the path of the reference beam. The low transmission at wavelengths where no absorption by the dye particles occurs is due to scattering by pigment particles which are not present in the polymer film placed in the reference beam. The light transmission of the films will be influenced not only by absorption of light by the pigment but also by light scattering by the pigment particles. The latter effect will be a complicated function of the absorption coefficient of the pigment, the refractive index of the pigment, the refractive index of the binder, and the particle size. In order to eliminate the effect of reflection and, to some extent, of scattering losses to the absorption spectra, some authors use a differential technique. l 5 Loutfy observed that a thin film (Ax = 0.23 pm) of X-metalfree phthalocyanine particles embedded in poly(viny1 acetate) absorbs at 13 000 and 16 200 cm-' when the fractional volume concentration of dye particles is 0.5 or higher. The minimum of the absorption spectrum is situated at 21 000 cm-I. The positions of the two absorption maxima and the absorption minimum are red shifted when the fractional volume concentration of the dye particles is lowered to 0.08. The two absorption maxima are shifted to 12600 and 16 100 cm-' at this dye concentration and the absorption minimum is shifted to 20 300 cm-'.I5 The values, reported here (Figure lA), are red-shifted in comparison to the results given in ref 15 for a sample with a pigment to polymer volume fraction of 0.08. The difference can be due to the fact that our measurements have been corrected in a different way for scattering and reflection of the incident beam. The differences between the absorption spectrum of the isolated metal-free phthalocyanine molecules in solution and X-metal-free phthalocyanine microcrystals in polymer film are mainly due to dimer formation of the phthalocyanine molecules. In a solution of metal-free phthalocyanine, the Q-band is split into two peaks (Q and QJ which are located at 14 300 and 15 000 cm-'. The additional peaks in the X-metal-free phthalocyanine films in comparison to solution spectra of metal-free phthalocyanine can be qualitatively accounted for by assuming that the X-metal-free phthalocyanine polymorph used here has a parallel plane dimeric structure.I3
Ground State Absorption Measurements at Elevated Temperatures. Figures 1A,B shows the temperature dependence (between 25 and 150 "C) of the ground state absorption spectrum of a polymer film (PS) on PET in which X-metal-free phthalocyanine i s dispersed. Heating the polymer film leads to a decrease in the ground state absorption in the regions of 19 200, 17 500, 16 100, and 12 400 cm-l and an increase in the region around 13 900 cm-I. It is not appropriate to discuss the depletion above 20 000 cm-I, since it belongs to another electronic transition. In Figure 2A, the ground state absorption difference spectra (1 1 000-25 000 cm-I) are presented. These difference spectra are obtained by subtraction of the ground state absorption spectrum of the polymer film at 25 "C from that at elevated temperature (60, 100, and 150 "C). Figure 2B shows the difference spectrum of a 20 wt % dispersion of X-metalfree phthalocyanine in PC on PET (25 and 150 "C). The difference spectra are characterized by positive and negative absorbance regions. The "depletions" are localized at about
Van Mingroot et al.
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Figure 2. (A, top) Ground state absorption difference spectra of 20 wt % dispersion of X-metal-free phthalocyanine microcrystals in PS
on PET (150, 100,60, and 25 "C). (B, bottom) Ground state absorption difference spectra of 20 wt 3'% dispersion of X-metal-free phthalocyanine microcrystals in PC (150 and 25 "C).
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Figure 3. Influence of temperature on the ground state absorption spectra (between 8300 and 25 000 cm-') of X-metal-free phthalocyanine microcrystals in PS (20 and 120 "C) deposited on glass.
12 400 and 17 900 cm-' in the case of X-metal-free phthalocyanine in the two polymers. The positive regions are localized in the region above 20 000 cm- and between 12 800 and 15 400 cm-I in the case of PC as polymer binder and between 13 000 nm and 15 400 cm-' in the case of PS as polymer binder. In contrast to the observations made for 2-metal-free phthalocyanine in PS, the positive values for X-metal-free phthalocyanine in PC are larger in the region above 20 000 cm-' than in the region between 12 800 and 15 400 cm-I. The temperature dependence of the ground state absorption spectrum (between 8300 and 25 000 cm-') of a sample, in which X-metal-free
J. Phys. Chem., Vol. 99, No. 47, 1995 17177
Microcrystals of 2-Metal-Free Phthalocyanine phthalocyanine is dispersed in PS and deposited on glass (Figure 3), shows that no absorption band below 12 400 cm-’ can be observed at any given temperature between 20 and 120 “C. This suggests that the observed experimental results are not due to 1-0 vibrational transitions which would occur after thermal population of the first, intramolecular,vibrational level of ground state molecules. In the absorption spectra, a bathochromic shift and an intensity loss of the absorption maximum are observed. The positive and negative regions in the ground state absorbance difference spectra are not due to the occupation of the high-frequency vibrations which are responsible for the vibrational progression in the absorption spectra. The relative occupation of the energy level at 750 cm-’ (which is the energy difference between the emission maximum and the 0- 1 vibration in the emission spectrum of metal-free phthalocyanine in tolueneI6) increases from 2 x to 9 x when the temperature is changed from 293 to 473 K, while the relative occupation of the energy level at 1500 cm-’ (which is the difference in energy between the emission maximum and the 0-2 vibration in the emission spectrum of metal-free phthalocyanine in toluenet6) increases from 4.3 x at 293 K to 8 x at 473 K. In both cases, the relative occupation of the higher energy level is still small at 473 K. Second, the vibrations are characterized by a small electron-phonon coupling, as suggested by the shape of the absorption spectrum. Hence, the 1-0 transitions will always be unimportant compared to the 0-0 and 1-1 transitions. A higher occupation of intermolecular, low energy, vibrational levels is more plausible at higher temperatures and leads to different interactions between phthalocyanine molecule and between the phthalocyanine molecules and the environment. As the absorption spectra are attributed to dimers, a different, intermolecular coupling can also change the oscillator strength. If the difference spectra were due to a redistribution of transition density between different vibrational bands, one would expect the total sum of the positive and negative regions underneath the difference spectra to be equal to zero. However, in the experimental difference spectra, this sum is systematically negative for each of the difference spectra. A multitude of effects can be the cause of the observed difference. First of all, thermal expansion of the phthalocyanine microcrystals, although partially compensated by the increase in path length due to thermal expansion of the film, could lead to a smaller optical density when the films are heated. The value of the absorption coefficient a is about 4 x lo4 cm-I. The penetration cm, and with a depth of the laser pulse is about 2.5 x pulse cross section of 0.2 cm2,the excited volume in the polymer sample is about 5 x cm3. With a typical value of 0.5 cal/(g K) for the specific heat of the dye microcrystals and in the unlikely assumption that there is no heat transport during the laser pulse duration, an estimate of the temperature rise after excitation of the dye doped polymer film is obtained. The temperature of the film would rise about 2400 K starting from 293 K upon absorbing a laser pulse with an energy of about 5 &/pulse, supposing that no heat transport occurs during the laser pulse. However, the real temperature rise will be substantially less due to heat dissipation in the film. Second, heating of the polymer films results in a change of the refractive index of the films, leading to a change in the level of light scattering. This is suggested by the temperature dependence of the transmittance around 11 000 cm-I where no appreciable absorption is observed. Finally, since the values of the differences in optical density are rather small, there is some uncertainty about the exact position of the base line. Transient Absorption Spectroscopy. Figure 4A,B sum-
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Figure 4. Transient absorption spectra of 20 wt % dispersions of X-metal-free phthalocyanine microcrystals in polymer film at room temperature after 640 nm excitation in air (A, X-metal-free phthalocyanine in PC; B, X-metal-free phthalocyanine in PS) at different times after excitation (100 and 10 000 ns).
marizes the transient absorption spectra obtained at different delay times (100 ns and 10 ,us) after 640 nm excitation of dispersions of X-metal-free phthalocyanine in PC (A) or PS (B). The observed values of the difference in optical density before and after laser excitation are rather small. One broad positive absorption band with an apparent maximum at 13 500 cm-’ is observed at 100 ns after excitation (640 nm) and decays on a time scale of several tens of microseconds. In the same time range, the negative peak at 17900 cm-l becomes less important. The shape of the transient spectrum is not changed dramatically over a period of several tens of microseconds, and the intensity of the positive and negative bands decreases gradually and simultaneously at times larger than 100 ns after excitation. When the experiments were repeated for a PS film directly deposited on a glass substratum, identical transients were obtained, and they decayed on a similar time scale. In general, the transient absorption spectra are expected to be the superposition of the ground state depletion spectra and the absorption spectra of excited states and/or radical ions. In all cases, the negative regions in the transient absorption spectra must coincide with the “depletion” regions in the ground state absorption difference spectra. Considering the similarity of the decay of the positive peak in the region of 13 500 cm-I and of the decrease of the depletion at 17 900 cm-I, it is concluded that only one transient species and the depleted ground state are observed in the transient spectra. If one considers the ground state absorption spectrum, the absorbing transient should be characterized by a ground state
17178 J. Phys. Chem., Vol. 99, No. 47, 1995 depletion above 20 000 cm-I and in the region between 11 800 and 19 000 cm-I. Hence, the positive spectrum in the region of 13 500 cm-' represents an absorption band due to a transient produced after pulsed laser excitation. The minimum in the spectrum at 17 900 cm-l reflects the ground state depletion in the 11 800-19 200 cm-I region, while the absence of a netto transient at 13 000 cm-' is related to a competition between ground state depletion and transient absorption. The X-metalfree phthalocyanine microcrystals have an intense ground state absorption in the 12 500 cm-' region due to the formation of parallel dimers. The transient absorption spectrum cannot be assigned to singlet-singlet absorption since the decay of this species should be completed within 100 ns of the time resolution of our experimental setup.* A solution of metal-free phthalocyanine in toluene has a singlet lifetime of only a few nanoseconds at room temperature.I6 The singlet decay will even be enhanced in the phthalocyanine microcrystals due to trapping by nonfluorescent dimers, exciton-exciton annihilation, and heating of the sample by the energy of the impinging laser pulse." The transient absorption spectrum cannot be assigned to triplet-triplet absorption since the transient absorption spectrum shows no absorption band in the 20 000 cm-' region, which is typical for a solution of metal-free phthalocyanine in a mixture of dimethylacetamide and water (70:30).'8 In principle, higher triplet states which can be reached by triplet-triplet absorption at 20 000 cm-' may be split by exciton-exciton interactions. However, no absorption bands are found at wavelengths within 200 or 300 cm-' of the triplet-triplet absorption maximum in solutions of metal-free phthalocyanine. Another possibility to explain the data in the 13 000-20 000 cm-' region is to refer to the processes of efficient charge generation in the microcrystals of metal-free phthalocyanine. Photocurrents and charge generation processes in phthalocyanine microcrystals and evaporated thin films have been studied extensively.' A more efficient charge generation in the microcrystals may be due to thermal effects caused by heating of the sample by the energy of the impinging laser pulse. If these species are to contribute to the transient absorption spectra of the phthalocyanine doped polymer films, the cation and anion of 2-metal-free phthalocyanine should be observed in the spectra. There is no contribution of these two species observed here.'6.'8 An alternative way to explain the absorbing transient is the occurrence of photothermal effects. Phthalocyanines in microcrystals are characterized by a weak luminescence, suggesting that the absorbed light is mainly converted into heat through the processes of internal conversion and intersystem crossing. Due to the combination of the high laser intensity used and the high local concentration of the phthalocyanine molecules in the microcrystals, those processes will be enhanced by the occurrence of singlet-singlet and triplet-triplet annihilation. Since the phthalocyanine molecules are inhomogeneously distributed in the polymer film and since the density of the incident photons in the excitation beam (-3 x 102'/cm3)is comparable to the density of the molecules in the microcrystals in the film (-lo2' molecules/cm3 or 0.2 g/cm3), one can expect that multiphotonic processes will extensively contribute to the photophysical deactivation pathways of the microcrystals in the film. A similar interpretation has been used to explain the experimental results for a film of p-metal-free phthalocyanine.* Due to the high extinction coefficient of the phthalocyanine molecules at the excitation wavelength, all light absorption occurs in a layer of a few hundred nanometers thickness, and the heat will be generated in an extremely small volume. This leads to changes in the interactions between the phthalocyanine
Van Mingroot et al. molecules and the surrounding polymer film and to the structure of the phthalocyanine dimer structure. Now, we point out a coincidence between the thermal difference spectra and the transient absorption spectra which supports the hypothesis of the photothermal effect. This indicates that the transient absorption spectrum can be assigned to photothermal effects. Because of the pulsed laser excitation, the temperature of the sample is locally raised to high values in a short period of time. Similar observations were made for /?-copper phthalocyanine solids where efficient mutual interactions between excited states lead to a hot band.6 Furthermore, this important temperature increase will change the relative values of the rate constants of internal conversion and intersystem crossing as well as shorten the singlet and triplet decay time. This could be the reason why no triplet-triplet absorption is observed. Another effect, which should be taken into account in the interpretation of the observed transient absorption spectra, is the probable creation of a temperature gradient in the microcrystals in the polymer sample. The excitation light penetrates the dye microcrystals and is attenuated by light absorption and scattering. While, on the one hand, heat generation will be the most important at the surface of the crystals where the incident beam is not yet attenuated by absorption and scattering, the heat dissipation by transport to the polymer binder will be the most efficient close to the crystal surface. In the thermal relaxation of the system, two steps can be discriminated. In a first step, thermal diffusion will equalize the temperature within the microcrystals and the surrounding polymer film. In a second step, the polymer film will cool by transfemng heat to the substrate. The heat which is generated in a small volume of the film, is transferred to a volume of about of 2 x cm3, leading to a temperature increase of about 40 K. The first step occurs between 10 ns and 10 ,us after excitation; the second is expected to be much slower. Such heating and cooling processes of the other polymer film were measured directly by time-resolved, spectroscopic, and interferometric technique^.^,'^ During the process of heating and cooling of the polymer film, the analyzing light beam penetrates the polymer film to a different depth in the film as a function of wavelength. This means that, at different wavelengths, the analyzing beam is probing regions in the microcrystals with a different average temperature. In order to get an idea of the time scale of the heat dissipation and to make a distinction between the contribution of chemical transients and that of the hot band to the transient absorption spectra, transient absorption measurements were performed on the same polymer samples in the time range of 100 ns to a few tens of microseconds after excitation, following the immersion of the sample in water. In Figure 5A, the transient absorption spectrum of a film of 20 wt % X-metal-free phthalocyanine in PC immersed in water is presented. Immersing the polymer film in a solvent will probably change the rate of heat dissipation to the environment after excitation at 640 nm. Thus, a change in the decay rate of the hot band is expected, and a possible identification of the hot band and the chemical transients that may contribute to the transient absorption spectra is aimed for. The transient absorption spectrum presented in Figure 5A shows a broad positive band in the 18 200-22 200 cm-' region with a maximum at about 19 200 cm-' and two negative bands at 25 000 and 15 400 cm-'. On the basis of the observed decay rate of the transient absorption at 19 200 cm-' (about a few microseconds) and the resemblance of the transient absorption spectrum to the transient absorption spectrum of metal-free phthalocyanine in solution,'6 the observed positive absorption
Microcrystals of X-Metal-Free Phthalocyanine
J. Phys. Chem., Vol. 99, No. 47, 1995 17179
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in the film through which water molecules can penetrate the film. In other words, a capillary water flow along the surface of the phthalocyanine microcrystals in the polymer film may occur. These water molecules will be a heat bath and will influence furthermore the thermal contact between the phthalocyanine microcrystals and the polymer. As the penetration efficiency of a particular solvent in the polymer films is a function of the contact angle between the solvent molecules and the phthalocyanine microcrystals, different results can be expected for different solvents. The heat transfer from the phthalocyanine microcrystals, dispersed in PC, to the solvent molecules is rapidly accomplished. The increased triplet-triplet absorption suggests that immersion of the sample in water enhances not only the second step of the cooling process but also the thermal equilibration inside the (polycrystalline) microcrystals and between the phthalocyanine microcrystals and the surrounding polymer film. Due to the more efficient generation of triplets with a lifetime that exceeds the time constant of the thermal equilibration, triplet-triplet annihilation will no longer lead to a large temperature increase. This will furthermore decrease the importance of the photothermal effects for samples immersed in water. When the film is in contact with air, the hot band can still be observed at a delay time of 10 ps with an intensity which is about 50% relative to the intensity of the hot band at a delay time of 100 ns in the case of X-metal-free phthalocyanine in PC. In the case of a water-immersed film of X-metal-free phthalocyanine in PC, no distinct hot band is observed, and the triplet-triplet absorption at 19 200 cm-’ has completely decayed at a delay time of only 5 ps. This fact proves that the second step in the cooling proces is undoubtedly fastened by immersion in water. In the case of phthalocyanine dispersions in PS, on the other hand , there is a difference between the transient absorption spectra recorded immediately after immersion in the solvent (water) and when the film has been in contact with water for some time. Figure 5B presents the transient absorption spectrum of X-metal-free phthalocyanine dispersed in PS immediately after immersion in water and after about 10 min. The transient absorption spectrum recorded immediately after immersion in water resembles the transient absorption spectrum of X-metalfree phthalocyanine dispersed in PS in contact with air. A negative absorption band, due to ground state depletion of the phthalocyanine dye in the film, at 17900 cm-’ and below 13 000 cm-’ can be observed, as well as a positive absorption band which is assigned to a thermal effect (hot band). After about 10 min, the transient absorption spectrum of the polymer film immersed in water has completely changed. A broad absorption band with a maximum at 20 000 cm-’ and a negative region at 16 000 cm-’ are observed in Figure 5B. The broad absorption band can probably be assigned to triplet-triplet absorption, while the ground state depletion of the phthalocyanine molecules in the film is observed at 16 000 cm-I. The change of the transient absorption spectrum as a function of the time after immersion in the solvent is probably due to the penetration of solvent molecules into the PS polymer film. This penetration is probably slower in comparison to a similar process in the case of PC. Thus, one may conclude that the “quenching” of the hot band which is observed after laser excitation at 640 nm of the phthalocyanine-doped PS film is due to a rapid heat dissipation to water molecules that have slowly penetrated into the film. In the case of phthalocyanine doped PC film, this penetration is faster, so that “quenching” of the hot band occurs immediately after immersion in the solvent.
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Wavenumber (cm’l)
Figure 5. (A) Transient absorption spectra of 20 wt % dispersion of X-metal-free phthalocyanine microcrystals in PC film at room temperature 100 ns after excitation at 640 nm immersed in water. (B,C) Transient absorption spectra of 20 wt % dispersion of X-metal-free phthalocyanine microcrystals in PS film at room temperature 100 ns after 640 nm excitation (B, immediately after immersion in water; C, 10 min after immersion in water).
band in the 18 200-22 200 cm-’ region can possibly be assigned to triplet-triplet absorption of metal-free phthalocyanine molecules in the polymer film. The negative absorption band may be due to the ground state depletion of the phthalocyanine molecules in the film after laser excitation at 640 nm. The independance of the triplet-triplet absorption spectra upon concentration of the phthalocyanine molecules in solution suggests a similarity of the triplet-triplet absorption spectra of the monomer and dimer. On the other hand, the hot band with an apparent maximum at 13 500 cm-’, which is observed after excitation at 640 nm of the same polymer film in contact with air, has disappeared almost completely. If the positive absorption band at 19 200 cm-’ is assigned to triplet-triplet absorption of the phthalocyanine molecules in the film, one can draw the following conclusions. Immersing the polymer film in water “quenches” the hot band, which is observed after laser excitation of a dyedoped film in air, because the heat dissipation after laser excitation from the film to the solvent is more rapid than the transfer of heat of the excited film to air. The observation of the triplet-triplet absorption of metal-free phthalocyanine in the polymer film when immersed in water suggests the more efficient triplet generation and a slower triplet decay related to the faster cooling of the microcrystals. The rate-determining step in the process of heat dissipation can be the transfer of heat between the phthalocyanine microcrystals and the film (PS or PC) or the transfer between the film (PS or PC) and the environment. There may exist channels
17180 J. Phys. Chem., Vol. 99, No. 47, 1995
Conclusion Heterogeneity and interactions between the chromophores make an understanding of the photophysics of molecules in heterogeneous or “microscopically organized” media more complicated than in the gas phase or in homogeneous solution.*’ In the study of the photophysical and photochemical excitation energy dissipation pathways of molecular solids, thermal deactivation processes following photo-excitation should be taken into account, since this pathway of energy dissipation is possible for any system although their rates and yields are dependent on the investigated system. This study suggests the importance of considering the possibility of thermal effects in the interpretation of experimental results in the field of transient absorption spectroscopy of polymer films in which dyes with a high optical absorption coefficient at the excitation wavelength have been dispersed and in Langmuir-Blodgett films. Analogous effects have been observed in the field of laser transient diffuse reflectance spectro~copy~-~ and picosecond regular reflection spectroscopy.6 Materials with a high optical absorption coefficient are likely to experience a large temperature rise following pulsed excitation, and this should be borne in mind when considering experimental results.
Acknowledgment. H.V.M. thanks the Instituut voor Wetenschappelijk Onderzoek in Nijverheid en Landbouw (IWONL) for financial support. L.V. thanks the K. U. Leuven for financial support. M.V.d.A. is a “Onderzoeksdirecteur” of the Fonds voor Kollectief Fundamenteel Onderzoek (FKFO). Financial support by DWTC through IUAP-11-16 and IUAP-040 is gratefully acknowledged. H. M. thanks K. U. Leuven for the guest professorship (1994, April-May). References and Notes (1) Popovic, Z. D.: Hor, A,; Loutfy, R. 0. Chem. Phys. 1988, 127, 45 1. Popovic. 2. D.; Menzel, E. R. J. Chem. Phys. 1979, 71. 5090. Popovic, Z. D. J. Chem. Phys. 1982, 76, 2714. (2) Greene, B. I.; Millard, R. R. Phys. Reu. Lett. 1985, 55, 1331. Casstevens, M. K.; Samoc, M.; Pfleger J.; Prasad. P. N. J. Chem. PhFs.
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