Ultrafast Exciton Dynamics in a Branched Molecule Investigated by

10484

J. Phys. Chem. B 2004, 108, 10484-10492

Ultrafast Exciton Dynamics in a Branched Molecule Investigated by Time-Resolved Fluorescence, Transient Absorption, and Three-Pulse Photon Echo Peak Shift Measurements† O. Varnavski and T. Goodson III* Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202

L. Sukhomlinova and R. Twieg Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed: January 29, 2004; In Final Form: April 9, 2004

The excited-state dynamics of a branched octupolar molecule (tris-4,4′,4′′-(4-nitrophenilethynyl)triphenylamine (T-NPTPA)) with charge-transfer character is investigated by complementary measurements of time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift. The data are compared with those obtained for the linear molecule dimethylnitroaminotolane (DMNAT) representing a linear building block of the octupolar system. Time-resolved fluorescence as well as transient absorption experiments showed the presence of an intermediate C3 symmetry state for the trimer with a depolarization time faster than ∼50 fs. Three-pulse photon echo peak shift measurements for the trimer revealed a large initial fast component of the peak shift decay. The initial peak shift during the first 100 fs was found to be much higher for the trimer as compared to the linear building block. This is in accordance with the time-resolved anisotropy results suggesting a different character of the initial excitation of the trimer. The higher peak shift value for the trimer suggests a smaller total coupling of electronic transition frequency to the nuclear motions, which may be the result of the delocalized character of this initial excited state. A residual peak shift (∼2 fs) at long population times was also observed and was interpreted as an indication of the residual static inhomogeneity (inhomogeneous broadening) in the system.

Introduction Due to potential applications in light emitting materials as well as nonlinear optics, there has recently been a considerable degree of interest devoted to the use of organic branched (or multi-branched systems such as dendrimers) molecular structures in various areas of photonics.1-6 Indeed, this may be a promising direction as an important advantage of an organic branched structure is that a large number of chromophores can be brought together in a small volume in defined positions. Strong interactions between chromophores confined in close proximity may result in interesting properties important for numerous applications.1-6 Organic dendrimers and branched molecules have shown great promise for artificial energy funneling systems1-3 as well as light-emitting diodes.4-6 Another attractive application of strongly interacting chromophores in an organic branched architecture is for enhanced nonlinear optical effects.7-11 This also includes novel organic molecules, which contain strong charge-transfer character.12 For example, the large octupolar contribution to the hyperpolarizability, found for several donoracceptor molecules possessing C3 symmetry, initiated a high degree of interest to these systems for NLO applications. It was found in several of these systems that they do not possess a permanent dipole moment, which could eliminate the crystallization problem important for second-order nonlinear optics.7,8 Recently, it was also found that for certain branched donor†

Part of the special issue “Gerald Small Festschrift”. * To whom correspondence should be addressed. E-mail: tgoodson@ chem.wayne.edu.

acceptor systems the two photon absorption cross section may exhibit significant cooperative enhancement (σtimer/σmonomer up to 6.8).9,10 These investigations have demonstrated the contribution of multiple charge-transfer processes to the nonlinear optical process and have probed the importance of the relative positions of various donor and acceptor groups (and polarizable bridges between them also) in macromolecules. However, the details of the excited state formation in symmetrical donor-acceptor systems are not clearly understood. A considerable fundamental interest arises from the fact that such systems possess a charge-transfer character of the excited state, which may be accompanied by a stabilization of the excitation at one branch and breaking of the molecular symmetry.13-16 In this case, the relative time scales of the lifetime of the initially delocalized state (if it exists) and the decay time of the localized relaxed state are the key parameters for understanding the linear and nonlinear optical responses of the symmetrical donor-acceptor systems. Extensive studies of the nature of excited state of in C3 symmetry amino-substituted triphenylbenzene derivatives were performed by De Schryver and co-workers using transient absorption, time-resolved fluorescence, and microwave conductivity techniques13,14 They found that the bath-equilibrated polar excited state was localized on one branch.14 This excitation could in turn migrate between branches via a hopping mechanism.13 The excited states of the well-known organic light emitting diode system aluminum(III)tris(8-hydroxyquinoline) (Alq3) has also been thoroughly investigated6,15,16 Fluorescence, electroabsorption measurements, as well as computational studies suggested the lowest electronic

10.1021/jp0495996 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/03/2004

Exciton Dynamics in a Branched Molecule

J. Phys. Chem. B, Vol. 108, No. 29, 2004 10485

Figure 1. (top) Structure of DMNAT and T-NPTPA. (bottom) Spatial configuration and phase matching diagram for the photon echo experiment.

excited state is localized on one of the quinolinolate ligands with charge being partially transferred from the phenoxide to the pyridyl side of the ligand.15,16 Time-resolved fluorescence anisotropy measurements were interpreted in terms of solventcontrolled mixing of several emitting states and rotational diffusion.15 An ultrafast inter-ligand relaxation in the initial stages after excitation has been reported for another metal complex Ru(bpy)3+2.17 Ultrafast polarization dynamics observed in this system suggest the existence of the intermediate shortlived electronic state delocalized over the ligands.17 This delocalized state possessed a high anisotropy value exceeding 0.4.17 The lifetime of the delocalized state was estimated to be approximately 100 fs, whereas population dynamics occurred on a picosecond time scale. Transient absorption anisotropy studies on polypyridyl complexes of Os(bpy)32+ also suggested the formation of a delocalized (or partially delocalized) initial state.18 The mechanism of the energy migration in a multi-chromophore system is determined by a delicate balance between disorder, homogeneous broadening dynamics, and electronic interaction.19 Thus, to understand the dynamics of energy transfer within a multi-chromophore system in detail, one needs the information concerning the electronic couplings, the siteto-site disorder of the excitation energies, the process of electron-phonon coupling, and the time scales of both environmental (solvent) relaxation and energy transfer. Many of these parameters can be directly obtained from femtosecond nonlinear and time-resolved fluorescence spectroscopy. Different types of femtosecond three-pulse scattering techniques including photon echo experiments have proven to be powerful methods for the purpose of investigating the fast photoinduced processes in the condensed phase.20-23 Recently, the three-pulse photon

echo peak shift (3PEPS) method in combination with transient gating (TG) and degenerate transient absorption (TA) measurements has been successfully used for investigations of solvation dynamics,24-26 intramolecular vibrations,24,27 dynamics of the reactive systems,28,29 energy transfer in both weakly coupled (Fo¨rster transfer),30,31 and strongly coupled (exciton relaxation) multi-chromophore systems.32-34 In this paper, we present the results of three-pulse-photon echo peak shift, time-resolved fluorescence, and transient anisotropy measurements in a branched donor-acceptor system possessing C3 symmetry in the ground state. A comparison with the linear donor-acceptor molecule representing a building block of the trimer is provided. In this contribution, we focus on elucidating the dynamics of energy redistribution and chargetransfer stabilization in the octupolar donor-acceptor molecule. Experimental Section A. Molecular Structures and Steady-State Spectra. The branched molecule we have investigated is the nitrogen-cored tris-4,4′,4”-(4-nitrophenilethynyl)triphenylamine (T-NPTPA). The parent molecule dimethylaminotalane (DMNAT) representing the linear building block of the trimer T-NPTPA was also investigated. The molecular structures of both molecules are shown in Figure 1. The synthesis and characterization of the trimer system has been described previously.11,35 Each nitroaminotolane segment of this trimer possesses donor-acceptor character, which is important for nonlinear optical applications.7,8 UV-visible absorption spectra were recorded with a HewlettPackard 8452A diode array spectrophotometer and the fluorescence spectra were measured with a Shimadzu RF-1501 spectrofluorophotometer.

10486 J. Phys. Chem. B, Vol. 108, No. 29, 2004 B. Time-Resolved Fluorescence. Time-resolved fluorescence measurements were carried out using a fluorescence upconversion system that has been described in detail elsewhere.3,11,36,37 Briefly, the sample solution was excited with frequency-doubled light from a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics). This produces pulses of approximately 100 fs duration in a wavelength range of 385-430 nm. The polarization of the excitation beam for the anisotropy measurements was controlled with a Berek compensator. The horizontally polarized fluorescence emitted from the sample was up-converted in a nonlinear crystal of β-barium borate using a pump beam at about 800 nm that was first passed through a variable delay line. This system acts as an optical gate and enables the fluorescence to be resolved temporally with a time resolution of about 200 fs (pump-excitation 800/400 nm cross correlation function had a fwhm of 180 fs). Spectral resolution was achieved by dispersing the up-converted light in a monochromator and detecting it by using a photomultiplier tube (Hamamatsu R1527P). Chloroform solutions with concentrations of ∼1.8 × 10-4 (T-NPTPA) and ∼5.4 × 10-4 M (DMNAT) in a rotating 1 mm-quartz cell were used in these experiments. C. Three-Pulse Photon Echo and Transient Absorption Measurements. The light source for the photon echo and transient absorption experiments was a cavity-dumped Kerr lens mode-locked Ti:sapphire laser pumped by a frequency doubled YVO laser (Millennia, Spectra Physics), which was assembled in our laboratory. A four-prism resonator design, similar to that developed in N. Scherer’s research group, was used in the laser construction.38 For the Ti:sapphire system, TS laser model from Kapteyn-Murnane Laboratories, L.L.C. was used. We added two additional prisms and two mirrors and rearranged the cavity in accordance with the design described in the literature.38 As compared to the distances of the optical elements separation reported by Scherer and co-workers,38 we increased slightly the distance between P1 and P2 as well as between P3 and P4 (Figure 1 in ref 38) keeping the overall cavity length corresponding to 77 MHz pulse repetition rate. This modification was connected with the use of a different Ti:sapphire rod. A fused silica Bragg cell was located at the end of the longer arm of the resonator between two focusing mirrors. The acoustooptic Bragg cell (3 mm thick) was driven by the cavity dumper driver (N13389 and N64389-Syn respectively, both from NEOS Technologies), which, in turn, was locked to the signal from the fast photodiode (ThorLabs, DET210) that samples the laser pulse train after the output coupler. The cavity-dumper efficiency was about 30%. A cavity-dumped laser pulse had a duration of 19 fs assuming a Gaussian intensity envelope function and negligible chirp as measured by a second-order interferometric autocotrrelator (CDP). The pulse spectrum was centered at ∼830 nm. The cavity dumped beam was focused into a 0.5 mm BBO crystal to convert the fundamental beam into the second harmonic at ∼415 nm. Unless mentioned otherwise, the pulse repetition rate was fixed to 77 kHz. In the 3PEPS experiment, both echo signals at phase-matching conditions k1 - k2 + k3 and -k1 + k2 + k3 are simultaneously recorded, whereas the time period τ between the pulses 1 and 2 is scanned (Figure 1). Half of the distance between the intensity peaks of these two signals is called the “peak shift” which is the essential quantity deduced from the experiment. Thus, the echo peak shift is obtained from the echo profiles and then recorded as a function of population period T between the second and the third pulse. The third pulse creates the superposition state, which may lead in case of a proper phase evolution to the photon echo formation. Photon echo peak shift

Varnavski et al. is sensitive to the extent of the correlation of the phase evolution between the first and the second coherence periods, which is, in turn, sensitive the transition frequency dynamics during the population period. During the population period T, the system is in a diagonal population state, and thus, the peak shift decay reflects processes that cause the transition frequency changes within the spectral window of the laser pulse. In our setup, three beams of equal intensities (∼0.5 nJ per pulse in one beam at the sample) were generated with the aid of thin beam splitters (1 mm-thick quartz substrate, CDP). One pulse (k1) traveled a fixed delay, whereas the other two pulses (k2 and k3) traveled variable delays formed with retro-reflectors mounted on DC-motor driven delay stages (Newport ILS100CCHA) controlled via a Newport ESP7000 motion controller. The three beams were aligned after the delay stages to form the equilateral triangle beam geometry (8 mm sides) and were focused into the 440 mm-quartz sample cell using thin singlet lens (f ) 18 cm). Chloroform solutions with concentrations of 3.5 × 10-4 (T-NPTPA) and ∼1.05 × 10-3 M (DMNAT) were used in these experiments. Care was taken to equalize the total amount of dispersive material in each beam path by insertion of compensation quartz plates (CVI) at the appropriate places in the setup. The two-third-order nonlinear signals into the k1 - k2 + k3 and -k1 + k2 + k3 phase matching directions were spatially filtered and directed to two photomultipliers (Hamamatsu Photo Sensor Modules H6780). The electrical signals from photomultipliers were measured by two lock-in amplifiers (Stanford Research, SR830), which were referenced to the chopper (SR540) inserted in the k1 beam. Many measurements were performed in order to confirm reproducibility of the signal. Special attention was paid to the residual peak shift value. The uncertainties of the peak shift value can be estimated at approximately (0.5 fs over the entire population period up to 100 ps. In a transient absorption (TA) measurement, only two beams were used, and a neutral density filter attenuated the intensity of the probe beam by a factor of 10 relative to that for the pump beam.11 The photodiode (ThorLabs DET110) connected to the lock-in-amplifier was used as a detector of the probe beam in TA experiments. A computer (LabView) program was developed to carry out the translation stage control and data acquisition in all experiments. Results and Discussion Figure 2 shows the absorption spectra of T-NPTPA and linear building block DMNAT in chloroform along with the spectrum of the excitation laser pulse used in the photon echo experiments. The main absorption peak for the trimer T-NPTPA is at 422 nm, and a small additional peak (shoulder) at 374 nm is also observed. The absorption spectrum of the DMNAT representing the linear building block of the trimer is quite similar to the spectrum of the trimer.35 The low energy peak has chargetransfer character for both systems, and this peak for the trimer T-NPTPA is slightly red shifted relative to the low energy peak of the monomer DMNAT (presumably due to inter-segment interaction).35 The emission spectra of the trimer and monomer are very broad possessing large, solvent polarity dependent Stokes shifts ranging up to a few thousand wavenumbers.11 The Stokes shift increases with the increasing solvent polarity in a similar manner for both trimer and monomer system.11 This suggests that the polar character of the relaxed excited state possesses approximately the same amount of charge transferred from donor to the acceptor group.11,13,14

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J. Phys. Chem. B, Vol. 108, No. 29, 2004 10487

Figure 2. Absorption spectrum of T-NPTPA along with the spectrum of the excitation pulse.

One method of probing directly the interactions of the branch chromophores is to utilize time-resolved anisotropy decay measurements.3,18,35,39-42 The time-resolved fluorescence anisotropy of the N-NPTPA at 600 nm is shown in Figure 3a, whereas Figure 3b shows the anisotropy dynamics in the degenerated pump-probe experiment at ∼415 nm. Timeresolved isotropic fluorescence is also shown in Figure 3a. The initial fluorescence anisotropy in symmetrical multi-chromophore systems theoretically can be quite high, up to 0.7 for planar system.43 In the pump-probe experiments, this value was suggested to be ∼0.4 in the case of negligible additional excitedstate absorption for a single chromophore.44,45 In our measurements, the initial anisotropy did not exceed 0.4 for both fluorescence and pump-probe experiments. In case of fluorescence measurements, the low initial value can be connected to the fact that the fluorescence upconversion method provides information about already relaxed (Stokes-shifted, mostly dephased) states, for which the anisotropy is expected to be lower than a dipole coherent superposition value of 0.7. The finite time resolution of the measurement further suppresses the initial anisotropy in both fluorescence and transient absorption measurements. The observed residual anisotropy before rotational diffusion ∼0.1 corresponds to the complete relaxation between the chromophores in a planar molecule with symmetry higher than C3. It is clearly seen from Figure 3 that the time scale of the anisotropy decay in both fluorescence and transient absorption experiments was very fast about 50 fs (in case of the monomer no fluorescence anisotropy decay from the level ∼0.4 has been observed on this time scale11). This result unambiguously reflects the strong interaction between the branches in T-NPTPA, which leads the fast optical depolarization immediately after excitation.3,35 In a previous report,35 we estimated the interaction strength between the segments in the T-NPTPA and characterized the regime of the excitation energy transport in this system using a phenomenological model.19 It was shown that use of an incoherent hopping model could not explain such fast anisotropy decay and that significant coherent character of the interaction should be taken into account.35 That suggests that the initial excited state is delocalized over the branches of the T-NPTPA molecule. Comparison of the emission spectra of DMNAT and T-NPTPA in solvents of different polarity showed remarkable similarity of the trimer and monomer emission spectra in each particular solvent.11 This indicates that the relaxed excited state of the trimer T-NPTPA is a polar state, which is expected to be localized on one segment of the trimer similar to some other

Figure 3. (a) Fluorescence anisotropy dynamics of the T-NPTPA (solid squares). Time-resolved fluorescence intensity (solid circles, IF) and the instrument response function (dots, IRF) are also shown. Excitation and detection wavelengths are 410 and 600 nm, respectively. (b) Transient absorption anisotropy dynamics of the T-NPTPA. Instrument response function is also shown (dots). Inset shows the transient absorption components polarized perpendicular and parallel with respect to the pump.

trimers with intramolecular charge transfer.13-18 Consequently, the ground-state molecular C3 (or higher) symmetry can be broken in the relaxed polar excited state.14,17,18 The ability to probe the connection between the charger stabilization process and the fast coherent energy migration process would be highly desirable in order to understand the mechanism of the enhanced NLO properties. To this aim, 3PEPS measurements were carried out for both the trimer T-NPTPA and linear DMNAT molecules. Different types of photon echo measurements with varying sensitivities to all aspects of the dynamics have recently been described in the literature.20-23,46-53 Applications of photon echo spectroscopy ranged from semiconductor physics to protein dynamics.23,50,51 The three-pulse photon-echo peak shift (3PEPS) was first measured by Ippen and co-workers in a dye-doped polymer glass.20 This particular technique has proven to be a valuable coherent spectroscopy tool since it has a good sensitivity, high dynamic range, and provides, under certain conditions, direct insight into the energy gap correlation function M(t).21,52 The representative integrated three-pulse stimulated echo intensity signals for various population periods T for the T-NPTPA are shown in Figure 4a. The signals were collected in the phase-

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Varnavski et al.

Figure 4. (a) Integrated 3-pulse photon echo signals for T-NPTPA for the following population periods: T ) 0 fs, T ) 367 fs, and T ) 80 ps. (b) Integrated 3-pulse photon echo signals for DMNAT for the population periods T ) 0 fs and T ) 80 fs.

matched directions -k1 + k2 + k3 and k1 - k2 + k3. Peak shift values were determined by fitting Gaussians to the two 3PE signals and taking half the distance in time between the maxima of these fits. At short population periods comparable with the duration of the instrument response function, the photon echo pulse shape appreciably deviated from a Gaussian function (Figure 4). In this area, we used the procedure of a partial Gaussian fit of the leading part of the first pulse and of the trailing part of the second pulse.27 At short population periods, we also scan pulse delays in such a way that the population time was fixed between pulses 1 and 3 at τ < 0 and then between 2 and 3 at τ > 0 (“3PEPS-scan” in terms of ref 49). That was accomplished by starting at τ ) 0 with a synchronous movement of both motorized delay stages 2 and 3 to keep the time interval between pulses 2 and 3 constant. Even though time constants obtained from 3PEPS measurements are not very sensitive to the excitation pulse duration at the sample cell,21 this parameter is important for quantitative measurements. A simple method of second harmonic autocorrelation trace for the measurement of pulse duration is not straightforward in the spectral region around 400 nm due to the lack of a good nonlinear crystal for this region. To estimate the pulse width at the sample cell, we performed, similarly to ref 51, the fitting of the rising edge of transient absorption signals of β-carotene, two other molecules tris-4,4′,4′′-(4-nitrophenylethynyl)triphenylamine (see Figure 1) and amino-tristyrylbenzene, which were assumed not to have any intrinsic rise-time feature in their dynamics. All three substances showed the rising edge corresponding to the pulse duration ∼50 fs for Gaussian pulse intensity profile. We also compared the two pulse echo pulse width for β-carotene with that numerically calculated for the finite pulse duration and very short dephasing time (2 fs)54. This procedure gave us the pulse duration 46 fs in a good agreement with the pump-probe estimation. Using this value for the pulse duration and the pulse spectrum measured at the sample position we estimated the time bandwidth product for the excitation pulse as ∆t∆n ) 0.52. Even though this is somewhat higher than for Gaussian chirp-free (transform limited) pulse (0.44), it remains

Figure 5. Three pulse photon echo peak shift data for T-NPTPA (solid circles) and for DMNAT (open diamonds) on a log scale. For short population periods 100 ps were too small to supply reliable peak shift data at longer delays. For relatively long population times, 3PEPS follows transition frequency correlation function M(t) closely.21,24,52 However, care should be taken about particular fast time constants in the direct fitting of τp(T): they do not directly measure M(t).24 In this case, the numerical simulation can provide more accurate details of time scales and interactions strength with the bath.21,24 To calculate various linear and nonlinear signals such as absorption spectrum or the photon echo signal, we start from a model for the transition frequency correlation function M(t)

M(t) )

〈δω(0)δω(t)〉

(2)

〈δω2〉

where ∆ω(t) is the fluctuating part of the electronic transition frequency for each chromophore relative to its central frequency 〈ω〉. The quantity M(t) can be used for the description of the memory of the electronic transition frequency and system dynamics when the characteristic frequencies of transition frequency fluctuations are small compared to kT.64 This hightemperature approach can be applied to describe the coupling of the electronic transition to many intermolecular (solvation) modes at room temperature. A more general approach describing the case of low temperature and/or intramolecular highfrequency coupled modes implies using the spectral density C(ω) and the Brownian oscillator model to describe the system’s dynamics and spectroscopic parameters.64-66 The main spectroscopic signals can be most conveniently calculated using the complex line shape function g(t)

g(t) ) P(t) + iQ(t) ) 〈∆ω2〉

∫0t dt1 ∫0t

1

dt2 M(t2) + iλ

∫0t dt1 M(t1)

(3)

or it can be also expressed in terms of spectral density C(ω)64

g(t) )

∫0∞ dω

1 - cos(ωt) ω2

C(ω) + (pω kT )

coth

i

∫0∞ dω

sin (ωt) C(ω) (4) ω2

In eq 3, P(t) and Q(t) are the real and imaginary parts of g(t), 〈∆w2〉 is the coupling strength (fluctuation amplitude), and λ is the reorganization energy. The absorption spectrum of the system can be calculated by taking the real part of the Fourier transform of exp[-g(t)]64

σa ∝

∫-∞ ∞ dt exp[-i(ω - 〈ω〉)t] exp[-g(t)]

(5)

whereas the third-order photon echo signal in the impulsive limit can be expressed by24,57,64

10490 J. Phys. Chem. B, Vol. 108, No. 29, 2004

SPE(τ,T) ∝

∫0∞ dt exp[-δin2(t - τ)2] exp{-2[P(τ) - P(T) +

P(t) + P(τ + T) + P(T + t) - P(τ + T + t)]} cos2[Q(T) + Q(t) - Q(T + t)] (6) where din represents the width of the static inhomogeneous distribution. The peak shift value (position of the maximum, tp) was calculated from photon echo signal SPE(t,T) as a function of population period T. As the calculation of the echo profile in case of a finite pulse width took much longer computer time, most of the fitting and modeling procedures were performed in the impulsive limit. The effect of the finite pulse width was estimated by convoluting the pulse electric field envelope (which was taken to be of Gaussian shape with s ) 33 fs) with the response functions with proper time ordering21,24,67 at several population periods (T points), comparing the 3PEPS values with those obtained in the impulsive limit. The ratio 3PEPSconv/ 3PEPSinpuls was weakly dependent on the population period T (at least for some particular important range of interactions strengths and correlation function decay times), and we corrected the impulsive limit results by this factor (1.702 for large T) similar to the procedure used by Fleming et al.21,58 We modeled the overdamped solvent components using a sum of Gaussian (inertial) and exponential (diffusion) terms in the correlation function

M(t) )

{[

( )

〈∆ωg2〉 exp -

( )]}/

t2 t + 〈∆ωe2〉 exp 2 τ τg e

{(〈∆ωg2〉 + 〈∆ωe2〉)} (7)

where 〈∆ω2〉 is the coupling strength for each term and τg and τe are time constants for Gaussian (inertial) and exponential (diffusive) components. To estimate contribution of underdamped components we used the underdamped Brownian oscillator model with Cb(ω) ) 2λbωb2ωγb/π{(ω2 - ωb2)2 + ω2γb2}, where ωb is the oscillator frequency and γb2 is the damping constant.64 We calculated then the contribution to g(t) using eq 4. The peak shift value was calculated from photon echo signal SPE(τ,T) (eq 7) as a function of population period T. The result obtained from the model was then corrected for the finite laser pulse width using the factor 1.702 as described above. The three-pulse photon echo peak shift for the T-NPTPA can be reproduced with the following parameters of the model (the result is shown in Figure 6): λg ) 106 cm-1, τg ) 75 fs, λe ) 112 cm-1, τe ) 250 fs, ωb ) 100 cm-1, λb ) 497 cm-1, γb ) 25 cm-1. The use of the underdamped Brownian oscillator was not only to reproduce the damped oscillations, which are seen in 3PEPS-dynamics, but it was also necessary to simulate the absorption spectrum. Using just solvent overdamped modes, it was not possible to reproduce the 3PEPS decay and obtain a reasonably broad absorption spectrum at the same time. In effect, this Brownian oscillator model qualitatively represents a sum of several intramolecular vibrational modes, which are not exactly known for this trimer molecule. The exact nature of the molecular motions involved in these modes is not known. However, fluorescence and excitation spectra of a tolane system in supersonic jet68 as well as for low-temperature solid matrixes69 have indicated the possibility of low-frequency large amplitude torsional motions around the sC≡Cs bond. Fundamentals as well as overtone signatures were detected in the region 17150 cm-1 including the strong line ∼96 cm-1 suggestively representing two quanta of the torsional vibration in the electronic excited state (T02).68,69 In the donor-acceptor sub-

Varnavski et al. stituted tolane system, the contributions and frequencies of these vibrations can be different. However, the calculations and experiments on the donor-acceptor substituted tolane performed by Barzoukas et al.62 showed that barrier heights for the torsional movements are very similar to those for unsubstituted tolane. We can suggest roughly the same potential and, consequently, similar torsional frequencies in the region 15-150 cm-1 for dimethylnitroaminotolane. It is worth noting that a distinct line near ∼256 cm-1 was observed in the Raman spectrum of a tolane system70 as well as in it’s fluorescence excitation spectrum.68,69 It was assigned to a lag-type vibration. The frequency of this mode for unsubstituted tolane is beyond the spectrum of the Brownian oscillator model appearing in the best fit equation for 3PEPS decay of the T-NPTPA. However, the frequency of this mode can be different for donor-acceptor substituted tolanes, and it may contribute to the oscillatory structure observed in 3PEPS signal. We can conclude that the Brownian oscillator coupled with the optical transition of the T-NPTPA most probably reflects the contribution of lowfrequency torsional motion around the sC≡Cs bond.68,69 Some contribution from a low-frequency 1ag mode of a modified tolane system by donor-acceptor groups is also possible. It is also worth noting that the second time constant in the solvent response is very close to that found for chloroform by Horng et al. (285 fs) using time-resolved Stokes shift measurements for the probe molecule Coumarin 153.71 The contribution of a static inhomogeneous broadening in this model was calculated to be ∆in ) 529 cm-1. The possible origin of the long-lived inhomogeneity has been discussed above in connection with the nonzero residual peak shift value. The total reorganization energy of ∼715 cm-1 obtained with this simulation is still much smaller than half of the observed steady-state fluorescence Stokes shift approaching ∼8000 cm-1.35 This is in accordance with the existence of an intermediate state prior to the charge has been transferred. The 3PEPS experiment probes the reorganization energy of this intermediate state (within the effective parabolic potential energy surface), which is much smaller than that for final highly polar state observed in the steady-state experiment. To get more information about the role of the intersegment interactions in the trimer system dynamics, we performed 3PEPS measurements in the linear building block DMNAT. Strikingly, the peak shift value for the DMNAT at short population periods (3000 cm-1) leads to a very small peak shifts when used in numerical simulation procedure. The fact that we observed relatively good initial rephasing capability and moderate initial peak shift value for the trimer and did not observe such capability for the monomer strongly suggests the intermediate relatively weakly interacting state for the trimer. It should be also noted that the above qualitative consideration in the frame of the two-level system can be complicated by the presence of excited-state absorption associated with both the initial excited state and the excited state of the charge separated product.74 However, a different character of the initial excited state of the linear building block and the trimer is well demonstrated by very different initial echo peak shift values as well as by optical anisotropy decay measurements described above. Additionally, quantitatively similar results for the anisotropy dynamics of the fluorescence and the transient absorption (see Figure 3) for the trimer indicate a small contribution of the excited-state absorption to the initial dynamics.44 Conclusions Time-resolved fluorescence, pump-probe, and three-pulse photon echo measurements were used to investigate femtosecond dynamics of a branched octupolar architecture tris-4,4′,4′′-(4nitrophenilethynyl)triphenylamine (T-NPTPA) possessing chargetransfer character. An enhancement in the nonlinear transmission effect was reported for this trimer (T-NPTPA) in comparison to the linear building block.11 Obviously, this enhancement is related to a strong inter-branch interaction. However, the details of this process in octupolar donor-acceptor systems, important for nonlinear optical applications, remain unclear. The investigation of the utrafast dynamics of this molecule using timeresolved fluorescence and third-order nonlinear optical techniques such as transient absorption, transient grating, and threepulse photon echo gives a valuable information about chargetransfer processes, excitation delocalization, possible breaking of symmetry, and the structure of the spectral line broadening. Time-resolved fluorescence study of the T-NPTPA in comparison with the monomer building block DMNAT showed the presence of an intermediate C3 symmetry state in the trimer with very high depolarization rate, which evolved into a polar state localized on one branch within a few picoseconds. The first photon echo peak shift study of a branched octupolar molecule in comparison with it’s linear building block is

J. Phys. Chem. B, Vol. 108, No. 29, 2004 10491 reported. Three-pulse photon echo peak shift results for the trimer system T-NPTPA showed fast initial peak shift decay in about 40 fs followed by an oscillatory structure (∼100 cm-1) and the residual peak shift ∼2 fs. The residual peak shift is an indication of the residual inhomogeneity in the system suggesting different possible conformations of the polar localized state. The initial three-pulse photon echo peak shift (which is an indication of the coupling to the bath) was found to be much smaller for the linear building block as compared to that for the trimer system. The increased magnitude of the peak shift for the trimer system can be suggestively attributed to the delocalized character of the initial state of the trimer. Acknowledgment. We thank Prof. G. Scholes for helpful encouraging discussions. We also greatly appreciate Prof. N. Scherer for his valuable advise on the construction of cavitydumped femtosecond Ti:sapphire laser. T.G.III thanks the NSF and DARPA for partial support of this project. References and Notes (1) Kopelman, R.; Shortreed, M.; Shi, Z. Y.; Tan, W. H.; Xu, Z. F.; Moore, J. S.; Bar-Haim, A.; Klafter, J. Phys. ReV. Lett. 1997, 78, 1239. (2) Bar-Haim, A.; Klafter, J.; Kopelman, R. J. Am. Chem. Soc. 1997, 119, 6197. (3) Varnavski, O.; Samuel, I. D. W.; Pålsson, L.-O.; Beavington, R.; Burn, P. L.; Goodson, T., III. J. Chem. Phys. 2002, 116, 8893. (4) Markham, J. P. J.; Lo, S.-C.; Magennis, S. W.; Burn, P. L.; Samuel, I. D. W. Appl. Phys. Lett. 2002, 80, 2645. (5) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Burn, P. L.; Bassler, H. Synth. Met. 2001, 116, 357. (6) Tang, C. W.; Van Slyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (7) Brasselet, S.; Zyss J. J. Opt. Soc. B, 1998, 15, 257. (8) Verbiest, T.; Clays, K.; Samyn, C.; Wolff, J.; Reinhoudt, D.; Persoon, A. J. Am. Chem. Soc. 1994, 116, 9320. (9) Chung, S.-J.; Kim, K.-S.; Lin, T.-C.; He, G.-S.; Swiatkiewicz, J.; Prasad, P. N. J. Phys. Chem. B 1999, 103, 10741. (10) Drobizhev, M.; Karotki, A.; Rebane, A. Opt. Lett. 2001, 26, 1081. (11) Lahankar, S. A.; West, R.; Varnavski, O.; Xie, X.; Sukhomlinova, L.; Twieg, R.; Goodson, T., III. J. Chem. Phys. 2004, 120, 337. (12) Varnavski, O.; Leanov, A.; Liu, L.; Takacs, J.; Goodson, T., III. J. Phys. Chem. B 2000, 104, 179. (13) Latterini, L.; De Belder, G.; Schweitzer, G.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. Lett. 1998, 295, 11. (14) Verbouwe, W.; Van der Auweraer, M.; De Schryver, F. C.; Piet, J. J.; Warman, J. M. J. Am. Chem. Soc. 1998, 120, 1319. (15) van Velhoven, E.; Zhang, H.; Glasbeek, M. J. Phys. Chem. A 2001, 105, 1687. (16) Curioni, A.; Boero, M.; Andreoni, W. Chem. Phys. Lett. 1998, 294, 263. (17) Yeh, A. T.; Shank, C. V.; McCusker, J. K. Science 2000, 289, 935. (18) Shaw, G. B.; Brown, C. L., Papanikolas, J. M. J. Phys. Chem. A 2002, 106, 1483. (19) Leegwater, J. A. J. Phys. Chem. 1996, 100, 14403. (20) Weiner, A. M.;. DeSilvestri, S.; Ippen, E. P. J. Opt. Soc. Am. B 1985, 2, 654. (21) Joo, T.; Jia, Y.;. Yu, J.-Y.; Lang, M. J.; Fleming, G. R. J. Chem. Phys. 1996, 104, 6089. (22) Bigot, J. Y.; Portella, M. T.;. Schoenlein, R. W.; Bardeen, C. J.; Migus, A.; Shank, C. V. Phys. ReV. Lett. 1991, 66, 1138. (23) Schoenlein, R. W.; Mittleman, D. M.; Shiang, J. J.; Alivisatos, A. P.; Shank, C. V. Phys. ReV. Lett. 1993, 70, 1014. (24) de Boeij, W. P.; Pshenichnikov, M. S.; Wiersma, D. A. J. Chem. Phys. 1996, 100, 11806. (25) Passino, S. A.; Nagasawa, Y.; Joo, T.; Fleming, G. R. J. Phys. Chem. A 1997, 101, 725. (26) Larsen, D. S.; Ohta, K.; Fleming, G. R. J. Chem. Phys. 1999, 111, 8970. (27) Larsen, D. S.; Ohta, K.; Xu, Q.-H.; Cyrier, M.; Fleming, G. R. J. Chem. Phys. 2001, 114, 8008. (28) Groot, M.-L.; Yu, J.-Y.; Agarwal, R.; Norris, J. R.; Fleming, G. R. J. Phys. Chem. B 1998, 102, 5923. (29) Yang, M.; Ohta, K.; Fleming, G. R. J. Chem. Phys. 1999, 110, 10243. (30) Jimenez, R.; van Mourik, F.; Yu, J. Y.; Fleming, G. R. J. Phys. Chem. B 1997, 101, 7350. (31) Yang, M.; Fleming, G. R. J. Chem. Phys 1999, 111, 27.

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