J. Phys. Chem. 1996, 100, 19441-19445
19441
Visible Luminescence of Dimethylaminobenzylidene-1,3-indandione Compounds Excited by Ultrashort Infrared Light Pulses V. Gulbinas, G. Kodis, and L. Valkunas* Institute of Physics, A. Gostauto St. 12, Vilnius 2600, Lithuania ReceiVed: February 9, 1996; In Final Form: July 11, 1996X
The origin of the nonlinear anti-Stokes luminescence effect in dimethylaminobenzylidene-1,3-indandione (DMABI) crystals and vacuum-evaporated films, i.e., the luminescence in the visible spectral region created by the infrared excitation, was investigated by means of the luminescence excitation stimulated by two excitation pulses separated in time and by means of ultrafast pump-probe spectroscopy. In contrast to the earlier proposed luminescence excitation model based on the low-energy exciton annihilation, the luminescence state was found to be excited via two-photon absorption through the virtual and real (characterized by 1 ps lifetime) intermediate states. Second harmonic generation and the pulse duration effect on the nonlinear luminescence are discussed. The exciton-exciton annihilation on a luminescent state manifold was observed at high excitation intensities.
Introduction Dimethylaminobenzylidene-1,3-indandione (DMABI) belongs to the class of polar derivatives of 1,3-indandione consisting of two chromophoric fragments connected via a CH bond. Owing to different electron donor and acceptor properties of fragments, an asymmetric charge distribution takes place. The dipolar character1,2 of the DMABI molecule and the excellent photoconductivity2 (iphoto/idark ) 104-105) of DMABI crystals have already been discussed in pioneering studies almost 25 year ago. Similar to many carbocyclic and heterocyclic organic compounds, DMABI crystals exhibit well-expressed polymorphism. Magomedova et al.3-5 have performed a detailed X-ray diffraction analysis of the crystalline structure and found that DMABI may emerge in three crystalline modifications. They also established that photoconductivity of DMABI crystals depends on their modification and found the R-modification as the most photosensitive.3 Gailis et al.6 have observed quite effective second and third harmonic generation stimulated by a YAG laser pulse in a number of polar 1,3-indandione derivatives, which shows their high hyperpolarizability. Later, unusually strong visible luminescence under excitation by infrared (IR) laser pulses has been observed in DMABI crystals and vacuum-evaporated films.7 This effect was named the nonlinear luminescence (NL) effect.7-9 The NL intensity recently was found to be correlated to the intensity of the IR absorption of the evaporated DMABI films, and it was explained as being caused by the low-energy intermolecular charge transfer (CT) excitons.8,9 The luminescence intensity under these conditions is quadratically dependent on the intensity of the IR excitation light, and it turns into a linear dependence at higher excitation intensities. According to the model proposed, the luminescent state is occupied during the annihilation of two CT excitons, created directly by the IR excitation. However, another possible explanation of the IR absorption of the films could be due to the light interference but not to the presence of the real states.10 This idea is supported by the absorption dependence on the thickness of the film as well as by the IR absorption data of the crystal structures. Thus, the origin of the IR absorption and that of the NL effect * E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)00394-2 CCC: $12.00
evidently remain undetermined. Our efforts to detect the light absorption by the CT states in DMABI films by means of picosecond pump-probe spectroscopy were not successful. A weak short-lived light-induced absorption in the red spectral region was detected only at extremely high excitation intensities, i.e., close to the damage threshold of the films. Time-resolved luminescence spectroscopy of DMABI films and crystals revealed that luminescence decay rate was dependent on the excitation intensity;11 however, any reasonable information about the intermediate state dynamics in these measurements was hardly accessible because the luminescence decay was determined by dynamics of both intermediate and luminescent states. Here we will show that these dynamics could be distinguished from each other if two excitation pulses separated in time are used. Transient absorption spectra of DMABI crystals with pico- and femtosecond time resolution will be also considered. A comparative analysis of the data obtained by both these methods will be concentrated on upon verification of the NL model8,9 based on the assumption of the exciton annihilation on the intermediate (IR) state and upon determination of the lifetime of the excitation in this intermediate state. Now we will briefly determine the two-pulse excitation technique. A similar method was also used by investigating the major charge carrier relaxation in GaAs crystals.12 Two ultrashort light pulses separated in time have been used for the excitation of the sample, and an integral luminescence stimulated by both pulses was analyzed as a function of the delay time between them and of the excitation energy. Due to the square dependence of the luminescence state population on the IR excitation intensity (the excitation conditions to the intermediate state), an increase in the luminescence intensity, when excitation pulses coincide in time or are separated by the time interval shorter than the lifetime of the intermediate states, can be expected. Luminescent state excitation density N(∆t), created by two IR light pulses separated in time by the delay ∆t, due to the process of the interaction of excitations in the intermediate state,9 can be evaluated as follows:
N(∆t) ∝ ∫-∞[n1(t) + n2(t - ∆t)]2 dt ) ∫-∞n12(t) dt + ∞
∞
∫-∞∞n22(t-∆t) dt + 2∫-∞∞n1(t) n2(t-∆t) dt
(1)
where n1(t) and n2(t) are the intermediate state population © 1996 American Chemical Society
19442 J. Phys. Chem., Vol. 100, No. 50, 1996
Gulbinas et al.
densities created by the first and the second pulses, respectively. Thus, they are proportional to the light intensity, i.e.
n(t) ∝ ∫-∞I(t′) f (t-t′) dt′ t
(2)
where I(t′) is the excitation pulse intensity and f(t) is the intermediate state decay function. The first and the second integrals in eq 1 determine the luminescent state populations created by both pulses independently, i.e., when delay between the excitation pulses is much longer than the lifetime of the intermediate state. The third integral defines the increase in the luminescent state population due to the interaction of the excitations on the intermediate state created by different pulses. If the lifetime of the intermediate state is shorter than the excitation pulse duration, the delay time dependence of the third integral turns into a correlation function of the pulses; otherwise, this dependence is characterized by the function f(t), and thus, the lifetime of the intermediate state can be evaluated.
Figure 1. Absorption spectrum of 1.9 µm thickness DMABI film (dashed line) and 2 mm thickness R-modification DMABI crystal (solid line). The structural formula of the DMABI molecule is presented as an insert.
Materials and Methods For the two-pulse excitation technique, 15 ps duration pulses of the basic light (1078 nm) or the second harmonic light (539 nm) generated by the low repetition rate passively mode-locked Nd:YAlO3 laser were used. The luminescence was recorded by the photomultiplier operating in the analog regime. The response time of the photomultiplier was slow in comparison with the luminescence decay time and the delay time between excitation pulses; therefore, the signal obtained was proportional to the luminescence energy and was additive to the luminescence input of both pulses. Time-resolved pump-probe absorption measurements were performed by two different spectrometers. Kinetics of the light-induced absorption relaxation was measured by means of the femtosecond spectrometer based on the Ti: sapphire laser. Excitation of the samples was carried out at 800 nm by the amplified, 200 fs duration pulses, and probing was realized by continuum light generated in water. Because of the limited optical quality of the samples, the probing was possible only in the narrow spectral region (670-750 nm) where the continuum light was of the highest intensity. Transient absorption spectra were also measured by means of the absorption spectrometer with 2 ps time resolution, based on the homemade passively mode-locked, feedback-controlled Nd:glass laser. The samples in this case were excited by the basic radiation of the laser at 1055 nm. High-intensity continuum pulses made it possible to probe the induced absorption change within 600-900 nm. A vacuum-evaporated DMABI film of 1.9 µm thickness was used in two-pulse excitation experiments, and a DMABI crystal of R-modification of approximately 1 mm thickness was used in pump-probe experiments. Providing the excitation in a transparent spectral region of the crystal, we created the homogeneous distribution of the excitations in the thick crystal, and thus, it became possible to detect the absorption changes of 2-3 orders of magnitude smaller than those in a thin film. The film and the crystal absorption spectra were measured by means of a Beckman spectrophotometer (UV 5270). Experimental Results Absorption spectra of the DMABI film (1.9 µm) and of the R-modification crystal under investigation (the β-modification spectrum is similar) are shown in Figure 1. The film possesses an intensive absorption in the blue-green spectral region (400550 nm), evidently consisting of several bands, the origin of which so far is not completely clear. Absorption is also present
Figure 2. DMABI film integral luminescence energy dependencies on delay time (on a picosecond scale) between two IR excitation light pulses at various excitation intensities.
in the near-IR region. The origin of this absorption, as was already mentioned in the Introduction, is still a subject of discussions. The crystal is transparent in the IR and red spectral regions (at wavelengths longer than 650 nm). Only weak absorption bands were observed in a transparent region, while at shorter wavelengths the optical density of the crystal was very highsit was too high to be measured. Assuming the absorption coefficient of the crystal in the visible spectral region to be similar to that of the vacuum evaporated film, the optical density of the crystal can be assumed to exceed 104. Thus, the absorption bands observed in the transparent region are about 104 times less intensive than those of the strong absorption bands. Similar weak absorption bands in the IR region were also observed in high concentration solutions of DMABI (not shown). According to the quantum chemical calculations of the DMABI molecular spectrum,9,13 these bands cannot originate from the molecular electronic excitations; therefore, we assume them as being due to transitions into higher excited molecular vibrational states. Experimental results obtained by the two-pulse excitation technique are presented in Figures 2 and 3. Figure 2 shows the luminescence energy measured at 655 nm, plotted as a function of the delay time between two IR pulses of equal intensities. Results are shown for different excitation intensities. The luminescence energy is normalized to the sum of lumines-
Visible Luminescence of DMABI Compounds
Figure 3. DMABI film integral luminescence energy dependence on the delay time (on a nanosecond scale) between two IR excitation pulses at 25 mJ/cm2 energy density of each pulse.
cence energies created by separate pulses. An increase in the luminescence energy can be achieved when the excitation pulses coincide with each other in time (within the accuracy of the pulse duration). At low excitation intensity the luminescence energy at zero delay is almost twice as high as the sum of the luminescence energies created by the separate pulses. When the delay time is much longer than the pulse duration, the normalized luminescence energy falls down to unity. The peak width equals to 23 ( 3 ps; i.e., it corresponds to the width of the autocorrelation function of 15 ps duration pulses used in these experiments. By increasing the excitation intensity, the normalized luminescence energy at longer delay times decreases approaching the value of the luminescence created by a single separate pulse. The intensity of the peak decreases, and it almost disappears at the highest excitation intensity. The luminescence is regained when the delay between pulses is increased on a nanosecond time scale, as is shown in Figure 3. At 12.5 ns delay it approximately reaches the normalization value. No luminescence energy dependence on the delay time between pulses on a short time scale was observed when 539 nm wavelength excitation pulses were used. Nevertheless, at high excitation intensities, the luminescence intensity created by two pulses is not additive, and the luminescence dependence on the delay time on a long time scale is similar to that observed at the IR excitation. Figure 4 shows the wavelength dependence of the induced transient absorption, measured by means of the Nd:glass spectrometer (1055 nm of the excitation wavelength and 2 ps of the pulse duration). The spectrum corresponds to zero delay time between excitation and probe pulses. A wide structureless induced absorption is observed in the transparent region. The induced absorption monotonously decreased when the probe wavelength is increased. The decrease in the induced absorption at short wavelengths approaching the absorption threshold is probably caused by the bleaching of the ground state absorption, which diminishes the effect of the induced absorption. The absorption decay kinetics measured with 2 ps time resolution is close to the correlation function of the excitation and probe pulses, irrespective of probe wavelength, indicating a very fast induced absorption decay. The decay kinetics at 700 nm measured with the 200 fs time resolution spectrometer at 800 nm excitation is shown in Figure 5. The kinetics is apparently nonexponential. The maximum observed at short delay times is limited by the pulse duration, and later exponential decay with 1 ps mean time is evidently seen (see insert). The induced absorption decay kinetics is not sensitive to the excitation
J. Phys. Chem., Vol. 100, No. 50, 1996 19443
Figure 4. Zero delay time transient induced absorption spectrum of R-modification DMABI crystal excited by 2 ps pulses at 1055 nm.
Figure 5. Induced absorption at 700 nm decay kinetics of R-modification DMABI crystal excited by 200 fs duration pulses at 800 nm: squares, maximal excitation intensity; triangles, 5 times attenuated; circles, 15 times attenuated. Insert shows the decay kinetics at maximal excitation intensity and calculated kinetics by assuming two-component decay (solid line) containing a pulse duration limited component and an exponential decay with 1.01 ps lifetime. Dotted and dashed lines show both components separately.
intensitysit does not change its form when the excitation pulse is attenuated. Unfortunately, limited optical quality of the crystal did not allow us to measure the induced absorption spectrum with femtosecond time resolution and to determine the spectra of the short and 1 ps lifetime components. The spectrum measured by the picosecond spectrometer due to insufficient time resolution apparently contains contributions of both components. Discussion The fact that the luminescence peak at zero delay time between two excitation pulses (Figure 2) corresponds to the autocorrelation function of excitation pulses means that the lifetime of the intermediate states ought to be much shorter than the pulse duration. It practically rules out the mechanism of the luminescence state occupation proposed by Valkunas et al.,9 which was based on the nonlinear annihilation of the intermediate state excitations. The annihilation should be unrealistically fast in order to create significant population of the luminescent state during the shorter than 15 ps lifetime of the intermediate state excitations. Therefore, it seems reasonable to assume that the luminescent state is occupied by means of two-photon
19444 J. Phys. Chem., Vol. 100, No. 50, 1996
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Figure 6. Induced absorption at 650 nm in the R-modification DMABI crystal dependence on the excitation energy density.
absorption via the virtual state or by two-step absorption via some short-lived real state. Pump-probe investigations yield more information about the intermediate states. The transient induced absorption could originate from both the luminescent state or the intermediate state taking part in two-photon or two-step absorption. Induced absorption components with different lifetimes could originate also from different states. The population densities of the luminescent and intermediate states should have different dependencies on the excitation intensity. The intermediate state is occupied directly by the IR light; therefore, its population should be proportional to the excitation intensity, while the twophoton or two-step processes suggest a quadratic intensity dependence of the luminescence state population. Figure 6 shows the induced absorption at 650 nm dependence on the excitation pulse energy, measured by the picosecond spectrometer. Linear dependence evidently indicates that the induced absorption originates from the intermediate state. Induced absorption decay kinetics measured with femtosecond time resolution at different excitation intensities differs in intensity, but not in shape. This confirms that both induced absorption components belong to the intermediate states. The short relaxation time component can be evidently related to the virtual intermediate state population. The two-photon absorption through the nonresonant virtual intermediate states takes place only when two light quanta interact with the same molecule simultaneously; therefore, the time profile of the absorption induced by the IR pump pulse corresponds to the time profile of the pump pulse irrespectively of the pump pulse duration. The induced absorption component with 1 ps lifetime could be related to the resonant intermediate states. Weak stationary absorption bands observed in DMABI crystal imply that such states could be vibrational states excited resonantly by the IR light, while the luminescent state, thus, is excited during the second step. The relaxation time of the induced absorption being equal to 1 ps is typical for vibrational energy redistribution in large molecules. Due to different relaxation times of intermediate states, both two-photon absorption channels give different contributions for light pulses of different duration. The luminescence state occupation via two-photon absorption through the virtual states is always proportional to the square of the light intensity: Nv ∼ ∫I2(t) dt, while occupation through the real states Nr could be expressed as
Nr ∝ ∫-∞I(t)∫-∞I(t′) exp((t′-t)/τ) dt dt′ ∞
t
(3)
where τ is the lifetime of the real state. According to these relations, relative contributions of both channels depend on the pulse duration, when the duration is comparable with τ. When the pulse duration is much shorter (or longer) than this value, the relative contributions do not depend on τ any more. Contributions of both channels could be estimated from the induced absorption decay kinetics. For this purpose we decomposed the decay kinetics into the instantaneous and 1 ps relaxing components, as is shown in Figure 5 (insert). The luminescence state excitation through different channels is roughly proportional to the cross-correlation of these components with the excitation pulse shape. In the case of 200 fs duration pulses used for excitation, the two-photon absorption through the virtual state is dominating and comprises about 75%. Relative intensities of the induced absorption components obtained from the decomposition enable us to calculate the induced absorption decay kinetics for light pulses of any duration. Such calculations show that the two-photon absorption through the virtual state comprises about 80% for pulses much shorter than τ (1 ps), while both contributions are approximately equal when pulse duration is much longer than τ. Thus, the presence of the resonant states enhances the two-photon absorption in DMABI, but not more than 2 times, and hardly can explain the unusually strong anti-Stokes luminescence. The second harmonic generation, which was shown to be quite effective in 1,3-indandione derivatives,6 can also be responsible for nonlinear luminescence. This is because the second harmonic light, due to high absorption coefficient of DMABI derivatives at this wavelength, is reabsorbed immediately after being generated. Moreover, the second harmonic generation effect can hardly be distinguished from the two-photon absorption through the virtual states, as both processes being inertialess, show the same dependencies on the light intensity and the result in the probe pulse absorption during the pump pulse action. Thus, the second harmonic generation together with the twophoton absorption through the real states could explain the observed nonlinear luminescence. Two-photon absorption or the second harmonic generation explains the square dependence of the luminescence intensity on the IR excitation intensity but fails to explain the saturation which appears at high excitation intensities. In the two-pulse excitation investigations, the saturation leads to the normalized luminescence decrease observed at high excitation intensity when pulses both are separated on a picosecond time scale or overlap in time (Figure 2). A similar situation is also present under direct luminescence state excitation by green light pulses. This shows that the saturation is not related to the excitation mechanism but is caused by the saturation of the exciton density on the luminescent state manifold. The exciton-exciton annihilation is the most conventional concentration nonlinearity stimulating a decrease in the luminescence quantum yield at high excitation intensities. This nonlinear annihilation channel has been distinguished by investigating the luminescence kinetics11 at similar excitation intensities. Thus, the excitonexciton annihilation results in a decrease of the normalized luminescence below the normalization value in the two pulse experiment. The luminescence energy is regained when the delay time between pulses is longer than the luminescence state lifetime, and the excitations created by the first and the second pulses do not interact and annihilate. In the case of sample excitation by short pulses, the excitonexciton annihilation can be taken into account as follows:
dN/dt ) -kN - γ(t) N2
(4)
where k is the intrinsic exciton relaxation rate and γ(t) is the
Visible Luminescence of DMABI Compounds
J. Phys. Chem., Vol. 100, No. 50, 1996 19445
exciton-exciton annihilation rate. If the latter value possesses a weak time dependence, i.e. γ ) constant, the solution of eq 4 for the exciton density as a function on initial exciton density created during the excitation pulse and time is as follows:
N(N0,t) )
N0 exp(-kt) 1 + N0γ/k(1 - exp(-kt))
(5)
The integral luminescence energy created by two very short pulses separated by delay time ∆t is proportional to the sum of two integrals:
E(∆t) ∝ ∫0 N(N10,t) dt + ∫∆t N(N20 + N1∆t,t) dt ∆t
∞
(6)
where N10 and N20 are exciton densities created by the first and the second pulses, respectively, and N1∆t is the exciton density which remained from the first pulse at the moment of the the second pulse. This dependence was calculated numerically in order to fit experimental data presented in Figure 3. The best fit was obtained by assuming that k ) 1.4 × 108 s-1 (measured by Jursenas et al.11) with the single fitting parameter N0γ ) 1.6 × 108 s-1. Difficulties by estimating the density of excitations in the case of the two-photon excitation and in the strong excitonexciton annihilation conditions did not allow to determine γ value separately. Conclusions Two experimental techniques for investigations of the nonlinear luminescence in the visible spectral region stimulated by the IR excitation in dimethylaminobenzylidene-1,3-indandione solids have been applied: (i) the two-pulse excitation technique and (ii) the ultrafast pump-probe spectroscopy. The twophoton absorption was determined as the main channel responsible for the stimulation of the luminescence, which is in contradiction with the originally assumed model based on the interaction of the CT states.9 It was found that two kinds of intermediate states, i.e., virtual and real intermediate states, take place in the two-photon absorption process. A 1 ps lifetime of the real states was determined from the femtosecond kinetic
measurements. The role of the virtual and real states in the two-photon absorption was discussed, and it has been shown that two-photon absorption through the virtual states becomes dominating when the IR light excitation pulses are shorter than 1 ps, while for longer pulses the absorption via the real and virtual states give similar contributions. Second harmonic generation was considered as an additional process which enhances the efficiency of the nonlinear anti-Stokes luminescence. The exciton-exciton annihilation on the luminescence state manifold was shown to be responsible for the luminescence saturation at high excitation intensities. Acknowledgment. The research described in this publication was made possible in part by Grant LE6000 from the International Science Foundation. The authors are grateful to E. Silinsh for providing us the DMABI samples and to V. Sundstro¨m for giving us the opportunity to use the femtosecond spectrometer for DMABI investigation. References and Notes (1) Weiler Feilhenhold, H.; Lowenstein, R. M. J.; Agranat, J.; Bergmann, E. D. Isr. J. Chem. 1969, 7, 99. (2) Dimond, N. A.; Mukherjee, T. K. Discuss. Faraday Soc. 1971, 51, 102. (3) Magomedova, N. S.; Kolninov, O. V.; Ruchadze, Y. G.; Zvonkova, Z. V. Zh. Phizich. Khim. 1975, 49, 1322 (in Russian). (4) Magomedova, N. S.; Zvonkova, Z. V. Kristallography 1978, 23, 281 (in Russian). (5) Magomedova, N. S.; Zvonkova, Z. V.; Neigauz, M. G.; Novakovskaja, L. A. Kristallography 1980, 25, 400 (in Russian). (6) Gailis, A. K.; Kolesnikov, V. A.; Silinsh, E. A. IzV. Akad. Nauk LatV. SSR, Ser. Fiz. Tekh. Nauk. 1978, 1, 20. (7) Gailis, A. K.; Durandin, A. D.; Silinsh, E. A. In Abstracts of 5th School of Organic Semiconductors, Chernowitzy, May 1988; Vol. 5, p 6. (8) Valkunas, L.; Gruodis, A.; Juodzbalis, D.; Urbas, A. Mol. Cryst. Liq. Cryst. 1993, 230, 163. (9) Valkunas, L.; Juodzbalis, D.; Urbas, A.; Gruodis, A.; Durandin, A.; Silinish, E. A.; Klimkans, A.; Larssons, S. AdV. Mater. Opt. Electron. 1993, 2, 221. (10) Jursenas, S.; Valkunas, L. Lith. J. Phys. 1995, 35, 575. (11) Jursenas, S.; Gruodis, A.; Kodis, G.; Chachisvilis, M.; Valkunas, L. Lith. J. Phys. 1994, 34, 361. (12) Rosen, D.; Donkas, A. G.; Budansky, Y.; Katz, A.; Alfano, R. R. Appl. Phys. Lett. 1981, 39, 935. (13) Balevicius, M.; Stumbrys, E.; Sorokolit, B.; Gruodis, A. Lith. J. Phys. 1995, 35, 20.
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