Article pubs.acs.org/JPCA

Excited-State Vibrational Coherence in Perylene Bisimide Probed by Femtosecond Broadband Pump−Probe Spectroscopy Minjung Son, Kyu Hyung Park, Min-Chul Yoon,† Pyosang Kim, and Dongho Kim* Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea S Supporting Information *

ABSTRACT: Broadband laser pulses with ultrashort duration are capable of triggering impulsive excitation of the superposition of vibrational eigenstates, giving rise to quantum beating signals originating from coherent wave packet motions along the potential energy surface. In this work, coherent vibrational wave packet dynamics of an N,N′-bis(2,6-dimethylphenyl)perylene bisimide (DMP-PBI) were investigated by femtosecond broadband pump−probe spectroscopy which features fast and balanced data acquisition with a wide spectral coverage of >200 nm. Clear modulations were observed in the envelope of the stimulated emission decay profiles of DMP-PBI with the oscillation frequencies of 140 and 275 cm−1. Fast Fourier transform analysis of each oscillatory mode revealed characteristic phase jumps near the maxima of the steady-state fluorescence, indicating that the observed vibrational coherence originates from an excited-state wave packet motion. Quantum calculations of the normal modes at the low-frequency region suggest that lowfrequency C−C (CC) stretching motions accompanied by deformation of the dimethylphenyl substituents are responsible for the manifestation of such coherent wave packet dynamics.

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drawback compared to fluorescence detection techniques, which are exclusively sensitive to wave packets of the excitedstate origin,5,8,25 that it is challenging to unambiguously assign the origin of the beating signals, because ground-state vibrational coherence created by impulsive stimulated Raman scattering can also contribute to the overall oscillations observed in the transient absorption spectra.26 To challenge this issue, Pollard et al. suggested by developing the wave packet theory3 that the probe wavelength dependence of the oscillations can serve as key information to determining the minimum of the PES of the vibrational coordinates and, in turn, the nature of the observed oscillating features. Recently, we developed a femtosecond pump−probe spectrometer, employing a ∼30 fs pump pulse and a broadband probe (500−700 nm), which allows for a simultaneous acquisition of the transient absorption signals at a wide spectral range (>200 nm, at ∼0.5 nm intervals) with a charge-coupled device (CCD) detector. With the benefits of such balanced, simultaneous data acquisition at a wide spectral range, this spectrometer enables us to track down the coherent wave packet motions as well as evaluate the probe wavelength dependence of the oscillations with much enhanced accuracy and precision as compared to one-color or two-color coherence spectroscopies, in which tuning to the respective desired probe wavelength usually induces changes in the experimental condition and hinders quantitative analysis of the transient

INTRODUCTION Photoexcitation of molecules by employing coherent, ultrashort optical pulses often induces impulsive excitation of vibrations and generation of nonstationary wave packets through the coherent superposition of the vibrational manifolds.1−6 The development of time-resolved spectroscopic techniques using “vibrationally abrupt”,5 broadband laser sources enabled the direct observation of the evolution of such coherent wave packets by aid of a delayed probe pulse because oscillatory modulations appear in the transient signal amplitudes as the generated wave packets propagate along the potential energy surface (PES) with beating frequencies that correspond to the energy spacing between the vibrational levels.7−11 As such, femtosecond pump−probe spectroscopy with ultrashort laser pulses has been extensively utilized in the studies on the influence of coherent wave packet dynamics on the photophysics and chemical reactions of various molecular systems ranging from small organic and inorganic molecules7,12−14 to isolated photosynthetic complexes15−17 and other biological and biochemical systems composed of proteins and macromolecules.9,18−22 With regard to the investigation of coherent vibrational dynamics, femtosecond pump−probe spectroscopy has several advantages over resonance Raman scattering and femtosecond stimulated Raman scattering techniques, particularly when studying highly fluorescent molecules with small Stokes shifts, that it can effectively rule out the interference of fluorescence and strong Rayleigh scattering of the excitation beam.11,23,24 At the same time, however, this method suffers from a major © XXXX American Chemical Society

Received: April 14, 2015

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DOI: 10.1021/acs.jpca.5b03571 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

modulated probe pulses were detected by a High Speed Spectrometer (Ultrafast Systems). The polarization angle between pump and probe beam was set at the magic angle (54.7°) in order to prevent polarization-dependent signals. In general experimental conditions, time resolutions of less than 40 fs were achieved (Figure S1 of the Supporting Information). To minimize chirp, thin absorption cells with a path length of 1 mm were used for all measurements. Time delays were averaged over 4000 laser pulses (2000 transient absorption spectra), and 12 independent matrices were collected and averaged followed by exclusion of the solvent coherence signals. The solvent signals were measured with neat dichloromethane in the identical absorption cell as used in the measurement of the sample. The raw transient absorption spectra were timecorrected by fourth-order polynomial fitting of the measured chirp data obtained with pure dichloromethane. Quantum Mechanical Calculations. Quantum mechanical calculations were performed by the Gaussian09 program suite 28 installed on a supercomputer (KISTI). Before calculation of the vibrational modes of DMP-PBI, geometry optimizations were carried out by the density functional theory (DFT) method with the Becke’s three-parameter hybrid exchange functional and the Lee−Yang−Parr correlation functional (B3LYP),29,30 employing the basis set 6-31G(d,p) for all atoms.31 For symmetry analysis, all degrees of freedom were optimized in the D2h symmetry. Calculation of groundstate vibrational frequencies of DMP-PBI was carried out by using the optimized output structures at B3LYP/6-31G(d,p). The geometry of DMP-PBI at its lowest singlet excited (S1) state was optimized with the time-dependent density functional theory (TD-DFT) method with the B3LYP functional and the 6-31G(d,p) basis set, starting from the ground-state optimized geometry. The polarizable continuum model using the integral equation formalism variant was applied where appropriate to represent the solvent (dichloromethane).

absorption data. By taking advantage of this technique, in this Article, we present the spectral signatures of a low-frequency vibrational coherence manifested in the broadband pump− probe spectra of an N,N′-bis(2,6-dimethylphenyl)perylene bisimide DMP-PBI (Figure 1a) and discuss their probe-

Figure 1. (a) Molecular structure of DMP-PBI. (b) Normalized steady-state absorption (black solid line) and fluorescence emission (black dashed line) spectra plotted with the laser pulse spectra (green, pump; magenta, probe) used for the femtosecond broadband pump− probe spectroscopy measurements.

wavelength-dependent phase inversion behavior as well as the vibrational origin of the coherent features. DMP-PBI is an imide-substituted derivative of perylene-3,4:9,10-bis(dicarboximide) (PBI) which has drawn growing attraction due to its excellent fluorescence behavior, chemical and photochemical robustness, and versatility as the building blocks for diverse molecular arrays and supramolecular architectures.27 Because this compound only consists of a single chromophore with noninteracting substituents, we can simply rule out any possible interference of electronic coherence as the origin of the observed beating signals and monitor exclusively the vibrational coherence originating from a PBI monomer. By combining the experimental results with computational analysis of the normal vibrational modes we show that the observed oscillatory signals reflect coherent nuclear motions on the excited-state PES of DMP-PBI.

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RESULTS Figure 1 shows the molecular structure and the steady-state absorption and fluorescence spectra of DMP-PBI along with the laser pulse spectra employed in the femtosecond broadband pump−probe measurements. The absorption and fluorescence spectra of DMP-PBI reveal the characteristic vibronic progression of the S0-S1 transition of a PBI monomer32 and are concentration-independent in dichloromethane (DCM), indicating the absence of intermolecular aggregation in our experimental condition (Figure S2 of the Supporting Information).33 The pump beam, centered at 522 nm, was tuned to have a good resonance with the S0-S1 electronic transition of DMP-PBI so that the vibrational manifolds in the S1 state could be coherently excited. The probe laser pulse employed in the broadband pump−probe measurements covers a broad range of 500−700 nm (Figure 1b), which spans the entire fluorescence spectrum and the 0−0 vibronic band of the absorption of DMP-PBI due to the small Stokes shift coming from its rigid molecular structure (Table 1).

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EXPERIMENTAL METHODS Femtosecond Broadband Pump−Probe Spectroscopy. The femtosecond pump−probe spectrometer consists of two independently tunable homemade optical parametric amplifiers (OPAs) pumped by a regeneratively amplified Ti:sapphire laser system (λmax = 800 nm, fwhm = 100 fs, 750 μJ/pulse, Hurricane-X, Spectra-Physics), operating at 1 kHz repetition rate and an optical detection system. The OPA is based on a noncollinearly phase-matching geometry, which was easily color-tuned by controlling the optical delay between white light continuum seed pulses (450−1400 nm) and the visible pump pulse (400 nm) produced by using a sapphire window and a β-barium borate (BBO) crystal, respectively. The generated visible OPA pulses had a pulse width of ∼30 fs and an average power of 5 mW at 1 kHz repetition rate in the range of 500−700 nm after fused-silica prism compressor. Two OPA pulses were used as the pump and probe pulses, respectively, for the pump−probe measurement. The time delay between the pump and probe beams was carefully controlled by making the pump beam travel along a variable optical delay (ILS250, Newport). By chopping the pump pulses at 500 Hz, the

Table 1. Photophysical Parameters of DMP-PBI (c = 1 × 10−5 M) Measured in DCM at Room Temperature

DMP-PBI B

absorption peaks (nm)

emission peaks (nm)

Stokes shift (cm−1)

458, 489, 526

539, 579, 629

389.4

DOI: 10.1021/acs.jpca.5b03571 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 2. (a) Two-dimensional contour map of the time-dependent absorbance changes (bottom) together with the one-dimensional differential absorption spectra from 300 to 2000 fs (top). (b) Residual vibrational coherence profiles at the probe wavelengths of 525−600 nm (5 nm interval) after subtraction of the exponentially fitted population decays.

broadband pump−probe setup. In principle, in order to generate coherent wave packets associated with a molecular vibration of interest, the optical pulses used should have shorter pulse duration than the period of that mode, which means that our setup can hardly induce coherent nuclear motions at higher frequencies than presented here. While all other modes are of relatively weak intensities comparable to the noise level, two modes with the strongest intensities are found at approximately 140 and 275 cm−1, which is in excellent agreement with our findings by damped cosinusoidal fitting. Although slight deviations are observed in the individual FFT peak positions obtained at each probe wavelength, the maximum deviation for both modes lies within the frequency resolution of our FFT analysis (∼5 cm−1, see Table 2), which points to the high reliability of both frequency modes irrespective of the probe region. Using the real and imaginary data as well as other parameters acquired from the Fourier transformation, we obtained the information on the relative amplitude and phase of the two modes, which are shown in Figure 3 (panels b and c). Because each vibrational wave packet travels independently along different nuclear coordinates within the multidimensional PES in the absence of any coupling,9 we plotted the relative amplitudes and phases of the 140- and 275 cm−1 oscillatory modes separately as a function of probe wavelength and investigated their probe wavelength dependent behavior. Interestingly, both the relative amplitude and phase of these two dominant modes are found to be greatly sensitive to the probe wavelength. The FFT amplitude spectrum of the former mode reveals a sharp W-shaped probe wavelength dependence with two local minima at 560 and 582.5 nm, which well match the spectral positions where the two phase jumps are observed in our broadband pump−probe spectra. On the other hand, the amplitude spectrum of the 275 cm−1 oscillation appears much broader with relatively diminished intensities at the entire spectral window. In the phase spectra, despite the presence of phase drifts especially for the 275 cm−1 mode, several distinct phase jumps are observed, including those at nearly the same wavelengths as where the amplitude inversion occurs for the

The time-corrected broadband pump−probe spectra of DMP-PBI (Figure 2a) consist of intense negative groundstate bleach (GSB) and stimulated emission (SE) components at 510−630 nm and positive excited-state absorption (ESA) bands at 630−700 nm. Notably, clear low-frequency oscillations are manifested on top of the electronic population decay at 525−600 nm where the contribution from the SE is predominant. The residuals were obtained by subtracting the exponentially decaying electronic dynamics from the raw pump−probe spectra and revealed pure vibrational coherence features at 525−600 nm (Figure 2b). Although the residuals appear as a complex interference of several waveforms with different beating frequencies and phases, it is worth noting that two apparent phase inversions are observed at 555−560 nm and approximately 580 nm, respectively, the former of which roughly corresponds to the borderline between the GSB and the pure SE signals. The individual residuals were fitted to the sum of several damped cosine waves (Figure S3 of the Supporting Information), which reproducibly gave rise to two prominent oscillatory modes with the fitted period of 122 and 239 fs, respectively (i.e., approximately 273 and 140 cm−1 in frequency). Unlike previously reported molecular systems and complexes that revealed rapidly dephasing coherence features accompanied by fast population decays,25,34,35 both modes remain largely undamped throughout the entire time window (∼2000 fs) of our measurement, exhibiting the fitted damping time constants of >1 ps regardless of the probe wavelength (Table S1 of the Supporting Information). We conjecture that this long-lived coherent feature is associated with the long S1 lifetime (4.6 ns) of the monomeric DMP-PBI (Figure S4 of the Supporting Information) without any fast deactivation channels such as intramolecular energy or charge transfer processes.32 The fast Fourier transform (FFT) amplitude map of DMPPBI at 525−600 nm (Figure 3a) reveal multiple peaks with varying intensities, all of which reside in the low-frequency region of