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Probing the Ultrafast Solution Dynamics of a Cyanine Dye in an Organic Solvent Interfaced with Water Tapas Goswami,† S. K. Karthick Kumar,† Aveek Dutta,‡ and Debabrata Goswami*,†,‡ Department of Chemistry and Centre for Laser Technology, Indian Institute of Technology, Kanpur, UP - 208016, India ReceiVed: April 23, 2009; ReVised Manuscript ReceiVed: NoVember 17, 2009
Dependence of ultrafast dynamics on the excited state evolution and ground state recovery of a cyanine dye (IR125) in dichloromethane (DCM) solvent interfaced with neat water is presented. We use degenerate pump-probe transient absorption spectroscopy to show that the excited-state dynamics of the dye molecule is strongly dependent on the position of the measurements from bulk DCM solution to the solution near the water layer. The decay component of the transient corresponding to the excited state lifetime increases from bulk DCM solution to its interface with water. Such results show that the effect of the presence of water layer over the dye solution in DCM extends several micrometers, indicating the surfactant nature of the IR125 molecules, and provides us a measure of the penetration of water into the DCM layer. The initial ultrafast decay component (coherent spike) directly correlates to the pulse-width of our near-transform limited pulses used in these experiments. This approach of measuring the excited state decay of a dye across an immiscible liquid interface can provide important characteristics of microtransport across such interfaces. Introduction Interfaces across immiscible phases present an important paradigm that is relevant across various fields of science including chemistry and biology. While accurate characterization of interfaces where the inversion symmetry is broken, has been the subject of study through sum-frequency generation (SFG)1–3 spectroscopy, it would be also equally useful and important to explore and characterize the gradient of transport mechanism that exists across such interfaces. In the present work, we mimic the heterogeneous gradient environment in immiscible liquids when they are interfaced with each other and study the gradient of transport across such an interface. Our system is a cyanine dye solution in dichloromethane (DCM) interfaced with water. We have chosen DCM as a solvent of choice since it is immiscible with water. We study the femtosecond transient dynamics of the cyanine dye from bulk DCM solution to near the DCM-water interface. Specifically, we have studied how the transient absorption changes for the infrared fluorescent dye, indocyanine green (product name IR125, whose structure and absorption spectrum is shown in Figure 1) in DCM solvent, as we move from bulk solution to the DCM-water interface. In this particular model situation using degenerate pump and probe transition spectroscopy at 810 nm, the transient absorption signal is absent for IR125 when dissolved in water, ensuring a drastic contrast. An earlier study of transient absorption spectroscopy of a dye solution attached to human serum albumin has shown that the excited-state relaxation dynamics for confined water molecules have slower solvation times than bulk water because of the hindered motion of the molecules.4 Several groups have also studied solvation in liquids extensively.5–11 There are also some ultrafast studies at the liquid-liquid interface with transient * To whom correspondence
[email protected]. † Department of Chemistry. ‡ Centre for Laser Technology.
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Figure 1. (a) Molecular structure of the indocyanine green dye (IR125). (b) Ultraviolet-visible (UV-vis) spectra of IR 125 in DCM and in water at 0.5 × 10-5 M concentration.
evanescent grating techniques.12 However, to the best of our knowledge, there has not yet been any transient dynamical study from bulk solution to near a liquid-liquid interface. Femtosecond transient absorption spectroscopy has proven to be an efficient technique to study the excited-state dynamics of polyatomic molecules in solution phase.13,14 In a typical
10.1021/jp903753u 2009 American Chemical Society Published on Web 12/02/2009
Ultrafast Dynamics of a Cyanine Dye in DCM transient absorption experiment, an excitation of the molecule is initiated by a strong pump pulse, thus inducing photochemical or photophysical change, and its dynamics is followed by recording the absorbance of a weak probe pulse as a function of the time delay between the pump and probe pulses.15 In this experiment, the pump pulse is used to excite the small fraction of the molecules in the irradiated volume to the higher lying, photoreactive state. In order to obtain a sufficient signal-to-noise ratio as well as to avoid saturation by the probe, the number of molecules transferred to the photoreactive state has to be sufficiently large.15 An important point to note here is that our experiments are not geared to map out the structural details of the interface as is possible with the SFG studies since our large beam spot-size limits such resolution; however, they are geared to the study of the gradient of liquid diffusion across the interface as a result of the characteristics of the dye molecule. We think that our experimental efforts reveal certain issues of ultrafast dynamics of a dye solution in the presence of an immiscible polar solvent, which indicates that, although the interface layer is only of nanometer scale, its influence can exist over a much larger range due to transport across it. Our femtosecond excited-state dynamics of the IR125 dye molecule is strongly dependent on where it is measured from bulk DCM solution to the interface. A gradual change in the transient absorption signal is observed from DCM bulk medium to the DCM-water interface. The decay component that corresponds to the excited-state lifetime is observed to increase from the bulk dye solution to the dye solution in the presence of water. Materials and Method The laser system used in this experiment is a Ti:Sapphire multipass amplifier (Odin, Quantronix, Inc. USA), which operates at 810 nm with 50 fs full width at half-maximum (fwhm) pulses at 1 kHz with an energy of ∼1 mJ. It is seeded with a home-built Ti:Sapphire oscillator (K&M Laboratories, Inc. oscillator kit). The oscillator is pumped by a Nd:YVO4 (Verdi 5, Coherent, Inc.), resulting in femtosecond seed pulses with center wavelength of 810 nm and a spectral bandwidth of 50 nm fwhm at a 94 MHz repetition rate at an average power of 400 mW. The oscillator output is stretched and then fed into the Ti:Sapphire multipass amplifier, which is pumped by the second harmonic of a Nd:YAG laser operating at a repetition rate of 1 kHz (Corona, Coherent, Inc.). Our experiments involve the use of the amplified laser system for transient absorption studies (see Supporting Information, Figure S1). The laser pulse was split into two parts by an ultrafast beam splitter (50:50 BS). The transmitted beam after passing through a variable optical delay line controlled by a computer was used as a pump beam. The reflected beam from the front surface of the beam splitter was used as the probe beam, whose intensity was controlled by a variable neutral density (ND) filter (ThorLabs) such that the ratio of the pump to probe intensity was ∼100:1. The pump and probe beam are then overlapped and focused by a 20 cm focal length convex lens on a 1 mm quartz cell (Hellma, Inc.) filled with dye solution with a concentration of ∼0.5 × 10-5 M. The diffraction-limited spot sizes of the pump and probe beam at the foci are maintained at approximately 200 and 100 µm. The probe focus is kept smaller than the pump focus such that the probe samples a volume that was evenly excited by the pump. An overall spatial resolution of at best 100 µm, in terms of movement with respect to the interface into the bulk and vice versa, is possible in our
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Figure 2. Schematic diagram showing how to probe the dye molecule in DCM solution, starting from bulk to near the DCM-water interface. Neat water forms a layer above the dye solution in DCM and represents the +Z direction, the interface sets the zero, while the dye solution in DCM layer corresponds to -Z values.
experimental setup. After interaction with the sample, the transmitted probe beam was detected by an amplified silicon photodiode and displayed in a 600 MHz LeCroy digital oscilloscope (LeCroy 64X). The pump-induced transmittance change of the probe pulse as a function of the time delay between excitation and probe pulses was recorded. IR125 dye sample was purchased from Sigma-Aldrich and a solution of the dye in high-performance liquid chromatography (HPLC)grade DCM solvent (concentration ∼0.5 × 10-5 M) was freshly prepared. The interface between neat water and dye solution in DCM solvent is prepared by carefully adding distilled water using a microsyringe over the solution of IR125 dye in DCM in a 1 mm quartz cell (Figure 2). The quartz cell was placed on a XYZ stage (Newport). Next we probe the dye molecule in DCM solution at different Z positions when we approach from the bulk DCM solution to the DCM-water interface. As shown in Figure 2, Z is defined as zero where the probe transmittance is least as we approach from bulk DCM layer (negative Z direction) to the water layer (positive Z direction). Results Degenerate (810 nm pump and 810 nm probe) transient absorption spectra of IR125 dye in DCM solvent at different Z positions from bulk DCM to very close to the DCM-water interface was collected and is shown in Figure 3. At the zero delay, coherent artifact is dominant, and the time constant of the initial ultrafast decay, often referred to as the coherent spike, is insensitive to the position of probing from the bulk to the interface, as it only reflects pulse characteristics. The rising part of the transient absorption signals were fitted with a single exponential rise (Figure 3b), while the decay components corresponded to a triexponential decay (Figure 3c). The fitting of the rise and decay components was done by Matlab programming. We used the LevenbergsMarquardt algorithm, which is provided in the Matlab curve fitting toolbox. The first ultrafast decay component (τ1) can be assigned to the coherent artifact, the second longer decay component (τ2) can be assigned to the excited state relaxation processes, and the third decay component (τ3) can be assigned to the population relaxation to the ground state.16–21 As just discussed above, an unchanged τ1 indicates the consistency of our exciting laser pulse characteristics. The τ2 component, corresponding to the excited state lifetime, increases from 350 fs in the bulk DCM solution to 3.3 ps when we are at Z ) -100 µm (this is our spatial travel resolution limit of being
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Figure 4. Variation in the excited-state lifetime (τ2) of IR25 in DCM solution interfaced with water at different Z positions.
from 3 ps in the bulk solution to 4.4 ps near the interface. However, this change is overwhelmed by the change in τ2, which makes τ2 the main observable parameter in our study of the gradient of liquid diffusion across interface. The rise-times became longer (290 fs) at positions that are ∼100 µm to the interface as compared to that in the bulk solution (79 fs). If we transit from bulk solution to a position very near to the interface, i.e., at Z ) -100 µm, we still record a measurable pump-probe signal. But with another 100 µm move, which put us in the interface position (Z ) 0), where the center of symmetry is broken, we cannot measure any pump-probe signal, and the transmission energy is much less, indicating that we have reached the interface. A further transit of 100 µm from the interface puts us at the water layer, where the probe transmission is more, and there is no transient signal at this wavelength. Such a change in the transmission energy of the probe at the interface can be attributed to our conjecture that the dye is preferentially lined up in the interface in the transport process across it, which we discuss further in the following section. In Figure 4, we show how the change in excited lifetime (τ2) accelerates as we approach the interface. We have also measured the transient absorption spectra when the dye solution is in equal concentration in both layers, and the time constants are observed to be roughly the same from bulk to near the interface. Discussion
Figure 3. (a) Degenerate transient absorption spectra of IR125 dye in DCM solvent at different positions of measurement from bulk solution to interface with neat water. Solid lines are the exponential rise and triexponential decay fits for calculating the associated rise and decay time scales. (b) Rise components are shown separately for better comparison. (c) Decay components at different Z positions shown for comparison.
close the DCM-water interface as discussed in the previous section). The τ3 component also shows an increase as it changes
Our main aim in the present paper is to study the excitedstate evolution and ground-state recovery of the dye molecule in the presence of a more polar water layer using degenerate pump probe transient absorption technique. The presence of a water layer over the dye solution in DCM may lengthen the excited-state lifetime of the dye for various reasons. For example, excited-state relaxation can occur through many pathways. One of the pathways involves forming a shell around the molecule by the solvent (solvation). The rate of relaxation through solvation becomes slower from the bulk medium to the liquid-liquid interface. In the bulk medium, the dye molecule is equally surrounded by the solvent molecules, forming an infinite number of solvation shells around the dye molecule. Since the IR125 molecule has a hydrophilic part, it preferentially attaches to the water molecule. In the presence of water, therefore, the dye molecule will feel some attraction from the water layer, and this attractive force decreases from
Ultrafast Dynamics of a Cyanine Dye in DCM
Figure 5. (a) Schematic illustration of the solvation shell around an IR125 dye molecule in the bulk solution, which shows that the dye molecule is equally surrounded by the solvent molecules. (b) Schematic of an IR125 molecule near the interface where the solvation shell is no longer intact.
the interface to the DCM bulk. Hence, near the water interface, the dye molecule will not be able to keep the solvation shell intact, and, if we excite the dye molecule at this position, it will remain in the excited state for a longer duration. However, near the interface, there is a delay in the formation of the solvation shell as the attractive force from water destroys the solvation shell (Figure 5).22,23 We use DCM as a solvent for the IR125 dye in preference to water since this dye readily undergoes polymerization and aggregation in water even at low concentrations.24–33 The photostability of the dye in water is also lower than that in organic solvents such as DCM, methanol, and dimethyl sulfoxide (DMSO).24 However, in these organic solvents, the dye
J. Phys. Chem. B, Vol. 113, No. 51, 2009 16335 remains appreciably monomeric, even at fairly high concentrations.24 Fortunately, there is a significant shift in the dye absorption maxima as soon as there is any dimerization or aggregation, and also a red shift is expected with increasing solvent refractive index from water to DCM solvent, which serves as an important marker for our experiments.24,29,34–37 The absorbance of the dye solution in water is almost an order of magnitude smaller than that in DCM at 810 nm for our experimental concentration of
