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Two-dimensional Electronic Stark Spectroscopy Anton Loukianov, Andrew Niedringhaus, Brandon Berg, Jie Pan, S. Seckin Senlik, and Jennifer P. Ogilvie J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02695 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry Letters

Two-dimensional Electronic Stark Spectroscopy Anton Loukianov, Andrew Niedringhaus, Brandon Berg, Jie Pan, S. Seckin Senlik and Jennifer P. Ogilvie* 1

Department of Physics, University of Michigan, 450 Church St., Ann Arbor, MI 48109

Corresponding Author *[email protected]

TOC GRAPHICS

ABSTRACT: Characterizing ultrafast energy and charge transfer is important for understanding a wide range of systems, from natural photosynthetic complexes to organic photovoltaics. Distinguishing the kinetic processes of energy transfer and charge separation in such systems is challenging due to the lack of clear spectral signatures of charge transfer states, which are typically non-radiative. Stark spectroscopy has proven to be a valuable method for uncovering

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charge transfer states. Here we extend the dimensionality of Stark spectroscopy to perform twodimensional electronic Stark spectroscopy. We demonstrate the method on TIPS-pentacene in 3methylpentane at 77 K. The additional frequency dimension of two-dimensional Stark spectroscopy promises to enable the identification of charge transfer states, their coupling to other charge transfer and exciton states and their involvement in charge separation processes. KEYWORDS Nonlinear spectroscopy, Stark spectroscopy, electroabsorption spectroscopy, Multidimensional spectroscopy, two-dimensional electronic spectroscopy Characterizing ultrafast energy and charge transfer is important for our understanding of systems ranging from natural photosynthetic complexes to organic semiconductors. Two-dimensional electronic spectroscopy (2DES) has been shown to be a powerful method for studying electronic structure, dynamics and energy transfer in a wide range of systems1-3. Also a third order nonlinear spectroscopy, 2DES is closely related to transient absorption spectroscopy. In 2DES the excitation pulse in a transient absorption experiment is split into two pulses separated by a time t1. Following the second excitation pulse, a third pulse probes the sample after a “waiting time” t2. The complex signal field is resolved as a function of both t1 and the time following the third pulse (t3). The Fourier transform of the signal with respect to t1 and t3 yields the respective excitation and detection axes of the 2D spectrum for a given waiting time t2. At t2=0 the 2D spectrum provides rich information about electronic structure: the peak shapes along the diagonal provide a measure of homogeneous and inhomogeneous broadening, while cross-peaks indicate electronic coupling2. The waiting time evolution of the 2D spectrum reveals the relaxation dynamics and has been used to map energy transfer pathways1, 4. In many systems of interest, identifying charge transfer states, which are often nonradiative, is considerably more difficult,


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and theoretical studies have shown that 2D spectra are often relatively insensitive to charge separation pathways5. In Stark spectroscopy, also commonly referred to as electroabsorption spectroscopy, the linear absorption of the sample is measured with and without the application of a large DC (Stark) electric field and the differential linear absorption is reported. This approach, which is a one-dimensional measurement, has proven to be a valuable method for uncovering charge transfer (CT) states in photosynthetic reaction centers

6-8

and other systems

9-10

. The presence of

a large electric field Stark-shifts energy levels and also influences the movement of charge. A number of time-resolved measurements have used electric-field-modulation to investigate charge carrier generation and dynamics in a range of materials using photoluminescence11, photocurrent12, second-harmonic generation13 and pump-probe spectroscopies magnitude of the electric-field-dependent signal is often small

14

10, 14-17

. The

, necessitating a sensitive

spectroscopic method. Here we combine 2DES with Stark spectroscopy, extending the dimensionality of previous Stark spectroscopy measurements by performing differential 2DES with and without the application of a Stark electric field. The resulting two-dimensional electronic Stark spectroscopy (2DESS) resolves the Stark shifts of the system along both excitation and detection frequency axes as a function of waiting time. Like Stark spectroscopy, the method promises to reveal CT states. The additional frequency dimension of 2DESS should reveal coupling of these states to other states of the system. For systems where the Stark field modifies the charge separation kinetics, the waiting time dependence of the 2DESS spectrum promises to directly report the difference in kinetics, highlighting the states involved in the charge separation process. The same experimental setup can readily perform transient-grating electronic Stark spectroscopy (TGESS). Given that transient-grating spectroscopy offers


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improved sensitivity compared to pump-probe spectroscopy and enables phase-resolved measurements18, we expect that TGESS offers advantages over electric-field-modulated pumpprobe spectroscopy. Here we demonstrate 2DESS and TGESS on TIPS-pentacene in 3methylpentane at 77 K. TIPS-pentacene was chosen for its large Stark response and recent interest in this material for photovoltaic applications19. The shift in energy ∆Ε of an electronic transition in response to an applied electric field F is given by

(1) where ∆µ and ∆α are the respective changes in dipole moment and polarizability between the initial and final states20. The large ∆µ associated with CT states enables their observation by Stark spectroscopy. The sensitivity of Stark spectroscopy to ∆α provides insight into the interactions between a molecule and its local environment. A Stark spectrum is frequently decomposed into a linear combination of first and second derivatives of the linear absorption spectrum with respect to frequency. For isolated transitions from nonoriented, immobilized samples, the first and second derivative contributions to the Stark spectrum are associated with ∆α and ∆µ respectively20. Additionally, a zero derivative contribution may be present due to a change in the transition dipole moment in the presence of the Stark field. In equation (1), F is the local field experienced by a molecule which differs from the macroscopic field due to intermolecular interactions and solvation effects

20-21

. We avoid the dependence on a particular

solvation model by describing the local field as F = F , where is a dimensionless scalar local-field correction factor and F = / is the external field, calculated as an applied voltage


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drop over a plate separation

20

. This approach straightforwardly generalizes to nonlinear

interactions22.

Figure 1. (a) Flat chemical structure of TIPS-pentacene molecule. (b) Top panel: Stark spectrum of TIPS-pentacene in 3-methylpentane at 77 K and 2.2/f MV/cm applied field is shown as a solid black line. Bottom panel: linear absorption spectrum of TIPS-pentacene in 3-methylpentane at 77 K. Dashed lines show a least-squares fit to the linear absorption spectrum (see Supplementary Information for details). A fit of scaled first and second derivatives of these lineshapes is shown as a red dashed line in the top panel; all parameters other than the amplitude of the derivatives were held fixed. The linear absorption and Stark spectrum of TIPS-pentacene in 3-methylpentane at 77 K are shown in Figure 1. The dominant peak at 1.93 eV in the linear absorption spectrum is assigned to the S0→S1 transition of TIPS-pentacene in 3-methylpentane, while the higher energy peaks are consistent with two vibrational progressions of frequency ~250 cm-1 and ~1165 cm-1, giving rise to transitions at 1.96 eV, 2.07 eV and 2.10 eV. These vibrational modes are in


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reasonable agreement with intramolecular modes identified in Raman studies of pentacene

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and TIPS-pentacene25, as well as with recent 2DES studies of thin films of TIPS-pentacene and other pentacene derivatives26. We fit the linear absorption spectrum using pseudo-Voigt and Lorentzian lineshapes as detailed in the Supplementary Information, yielding the fit shown in Figure 1. Two low-amplitude transitions were also identified at ~2.25 eV and ~2.28 eV in the linear absorption spectrum but lie outside of the spectral range of our 2DESS measurements. The Stark spectrum in the S0-S1 region resembles the first derivative of the linear absorption spectrum, indicating that ∆α dominates the Stark response. Although second derivative features have been included in the model, their contribution to the quality of the fit is negligible (see details of the fitting in the Supplementary Information). These observations are consistent with recent Stark spectroscopy measurements in solid pentacene films that report ∆α contributions associated with the vibrational progression of the Frenkel exciton states27. This is in contrast to earlier work in which a combination of ∆α and ∆µ contributions were invoked to account for the respective first- and second-derivative features in the Stark spectrum of solid pentacene films 9. For our solution studies we expect the dipole moment change to be dominated by ∆α due to the absence of intermolecular CT states that were proposed to give rise to the ∆µ contributions in the film studies 9. The experimental setup for 2DESS and TGESS is shown in Figure 2 and employs a hybrid diffractive-optics/pulse-shaping approach for the 2DES measurements28. The output of a regenerative amplifier (SpectraPhysics Spitfire Pro) operating at 500 Hz was used to pump two dual-stage non-collinear optical parametric amplifiers (NOPAs)29 to provide the pump (centered at 620 nm, 60 nm FWHM bandwidth) and probe (centered at 620 nm, 85 nm bandwidth) pulses.


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Both pump and probe pulses were compressed with grism compressors30; for the pump pulse this serves to enable broader bandwidth operation of the Dazzler pulse-shaper, which produces a collinear 16 fs pulse pair separated by the desired t1 and with relative phase ϕn. A separate grism compressor provided the full pulse compression for the probe pulse (17 fs). The pump was chopped and advanced relative to the probe by delay t2 using a retroreflector on a linear translation stage. The pump and probe were focused onto a diffractive optic to generate the background-free passively-phase-stable 2DES geometry and a delayed local oscillator for heterodyne detection. The beams, with parallel relative polarizations and pulse energies of 34 nJ and 30 nJ (pump and probe respectively), were spatially overlapped and focused to a 180 µm spot on a Stark sample cell in a liquid nitrogen immersion cryostat (Janis)31. To prepare the sample, 6,13-Bis(triisopropylsilylethynyl pentacene) (TIPS pentacene) was dissolved in 3-methylpentane such that the final optical density was 0.3 at 1.93 eV at room temperature for a 50 µm pathlength cell. All reagents were purchased from Sigma Aldrich. The concentrated sample was loaded into a custom sample cell composed of etched conductive (ITO) cover slides separated by a (nominal) 50 µm polyimide spacer and sealed on three sides with Norland 68 optical adhesive. After loading, the cell was sealed on the remaining side by a layer of glycerol. An arbitrary waveform generator (AWG) (Agilent 33500B) synchronized to the laser trigger was used to generate a variable-amplitude low-pass-filtered square-wave with period of 24 ms and 50% duty cycle. The AWG output was used to drive a high voltage power supply (TReK 10/10B-HS) that induced an applied electric field of up to 2.2/f MV/cm in the sample. The heterodyned signal was detected with a spectrometer (Horiba Jobin-Yvon iHR320 and PIXIS 1340x100B CCD) was read-out at every laser shot (500 Hz).


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The 2DESS data was generated as follows. For each t1 delay, the external field was modulated (Figure 2c) such that a group of six phase cycles (ϕ1-ϕ6) was collected for each halfcycle of the modulation. For 10% of the CCD integration time the probe was blocked with a shutter to collect background. The total CCD integration time per t1 was 5 s. At each delay t2, 60 t1 points were collected from 0 fs to 390 fs (step size of 6.61 fs). A full 2DESS spectrum took about 5 minutes to acquire. After spectral interferometry, phase-cycling, and Fouriertransformation of the t1 axis, the isolated rephasing and non-rephasing spectra were combined and the data “phased” to obtain 2D absorptive electronic spectra28. The difference between the field-on and field-off spectra obtained in this way yield the 2DESS spectrum. TGESS can be performed with this setup by setting t1=0 and scanning t2 to study electric-field-dependent kinetics. The TGESS signal is a subset of the 2DESS data and can be automatically extracted via phase-cycling from 2DESS measurements28. Since TGESS does not require scanning of the t1 delay, it can also be performed without the need for the pulse-shaper shown in Figure 2.


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Figure 2. Experimental setup for two-dimensional electronic Stark spectroscopy. (a) The hybrid pulse-shaping/diffractive optics-based 2DES setup28, where PS = pulse-shaper, DO = diffractive optic, HV = high voltage, S=sample. The lenses are shown for simplicity only; in the setup a spherical mirror (f=500 mm) is used to focus the beams onto the diffractive optic (DO) and a patterned spherical mirror (f=200 mm) is used to relay the focus at the DO to the sample position. The reflective coating is etched away to selectively attenuate the local oscillator. (b) The Stark cell design. (c) The timing diagram for the sequence of phase-cycled optical pulses and the applied electric field in a 2DESS experiment. Figure 3 shows the 2DES and 2DESS spectrum of TIPS-pentacene at 77 K at a waiting time of t2=1 ns. The 2DES spectrum shows positive features along the diagonal at energies consistent with the linear absorption spectrum. In 2DES spectra positive features arise from ground state bleach (GSB) and stimulated emission (SE) signals, while negative features indicate excited state absorption (ESA). At early waiting times, positive signals at the cross-peak position


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between two transitions indicate a common ground state between them. The dominant peak at 1.93 eV in the 2DES spectrum arising from the S0→S1 transition exhibits persistent diagonal elongation at t2=1 ns reflecting inhomogeneous broadening. The lower amplitude, higher energy diagonal peaks and cross-peaks are consistent with the transitions assigned to two vibrational progressions in our fitting of the linear absorption spectrum. Above the diagonal the 2DES spectrum shows a strong ESA signal at a probe energy of ~2.15 eV. The 2DESS data show positive and negative features at a number of different excitation/detection energies. As a differential (field on – field off) measurement, the signed features in a 2DESS spectrum have several origins. They may arise from frequency shifts of the molecular transitions, as well as changes in transition dipole moment with applied field. Reorientation effects may also play a role but are negligible here where samples are frozen at 77 K. Positive 2DESS features may reflect increased positive 2DES signal amplitude (from GSB or SE) or a decrease in negative signal (ESA) in the presence of the field, while negative 2DESS features will show the opposite of these trends. The 2DESS signal near the S0→S1 transition resembles a first-derivative shape along the diagonal, consistent with the Stark spectrum. The relatively weaker higher energy diagonal peaks show negligible signal, but the cross-peaks above and below the diagonal are clearly visible, due to amplification from the intense S0→S1 transition. The positive and negative features of the cross-peaks are consistent with a similar first-derivative type of shift of the higher energy peaks of the vibrational progression, consistent with expectations that these transitions shift in the same way as the fundamental electronic transition. As in the Stark spectrum, these first derivative shapes along the diagonal are consistent with the TIPS-pentacene transitions being dominated by changes in ∆α. In the Supplementary Information we provide a comparison of the Stark spectrum with the diagonal


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slice from the 2DESS data at 50 fs and 1 ns. The 2DESS spectrum also shows a large positive signal with small negative features above the diagonal at a probe energy of ~2.15 eV. The 2DES spectrum shows a strong negative feature due to ESA at this location. We note that this ESA transition is shifted in a significantly different way from the other transitions in the 2DESS spectrum, suggesting that it is highly sensitive to the Stark field and may have some CT character. Like Stark spectroscopy, the 2DESS method requires immobilization of the molecule of interest, which can be achieved in a glassy matrix or by thin film preparation20. Our choice of 3methylpentane as the solvent for this study was motivated by the need to obtain a high quality optical glass at 77 K, as well as sufficient optical density over the 50 µm pathlength cell. We note that in this solvent, the vibrational progression is considerably better pronounced than in studies of TIPS-pentacene films26. In thin films or solids, intermolecular interactions lead to excitonic coupling and CT states as well as rich photophysics such as singlet fission. We expect that the combination of Stark spectroscopy and 2DESS will enable the identification of CT states and their coupling to other transitions. While CT states can be identified by their Stark response, charge separation kinetics may also be altered by the applied field, and will be evident in the waiting time dependence of the 2DESS signal. In analogy to previous studies that have used electric-field modulation to study charge carrier generation and kinetics17 using pump-probe or other time-resolved spectroscopies, TGESS offers a high sensitivity approach. In the case of TIPS-pentacene in 3-methylpentane, no charge separation is observed, as is evident from the TGESS data shown in Figure 4. Following the initial changes in dipole moment and polarizability associated with photoexcitation, there is little time evolution of the signal over the


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~1 ns of the experiment with the exception of modulation due to vibrational wavepacket motion within the first few picoseconds.

Figure 3. Two dimensional spectroscopy of TIPS-pentacene in 3-methylpentane at 77 K. (a) Normalized 2D electronic spectrum (2DES) at a waiting time of t2=1 ns at a contour spacing of 0.01 units, omitting 3% of full-scale around zero. (b) 2D electronic Stark spectrum (2DESS) at 2.2/f MV/cm applied field and a waiting time of t2=1 ns. The spectrum was multiplied by the same normalization factor as in (a). Contours have spacing of 0.001 units, omitting 3% of full-


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scale (0.1 units) around zero. To obtain this data, 60 t1 points were collected with an integration time of 5 s per t1 for a total acquisition time of about 5 minutes.

Figure 4.

Normalized transient grating Stark spectroscopy of TIPS-pentacene in 3-

methylpentane at 77 K, recorded with a 2.2/f MV/cm applied field. In this case, 100 t2 points were collected with a camera integration time of 10 s per t2 with a hyperbolic spacing. The data acquisition took 17 minutes. In conclusion we have demonstrated 2DESS and TGESS on TIPS-pentacene at 77 K. We expect that 2DESS will be useful for characterizing the electronic structure of molecular systems by augmenting the information about the change in dipole moment and polarizability of transitions available from Stark spectroscopy. The additional frequency dimension of 2DESS may help distinguish dipole moment and polarizability changes in samples with overlapping transitions, enabling the identification of CT states and their coupling to other transitions. The additional waiting time dimension of 2DESS may enable identification of charge separation processes and the states that drive them. The electric-field dependent kinetics can also be characterized with TGESS, which offers a high sensitivity alternative to electric-field-modulated pump-probe spectroscopy, a method that has been useful for understanding charge


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photogeneration, mobility and dynamics in photovoltaic materials17. In combination with 2DES, we expect the 2DESS and TGESS methods to further our understanding the electronic structure, energy transfer and charge separation processes in natural and artificial light harvesting systems. ACKNOWLEDGMENT A.L., J.P., A.N. and J.P.O. acknowledge support from the Office of Basic Energy Sciences, the US Department of Energy under grant number DE-FG02-07ER15904 and DE-SC0016384. S.S.S. acknowledges support from the National Science Foundation (PHY-1305450). We also thank the group of Steven Boxer for advice regarding sample preparation and the application of large Stark fields. REFERENCES 1. Brixner, T.; Stenger, J.; Vaswani, H. M.; Cho, M.; Blankenship, R. E.; Fleming, G. R. Two-dimensional spectroscopy of electronic couplings in photosynthesis. Nature 2005, 434, 625-628. 2. Jonas, D. M. Two-dimensional Femtosecond Spectroscopy. Annual Review Of Physical Chemistry 2003, 54, 425-463. 3. Lewis, K. L. M.; Ogilvie, J. P. Probing Photosynthetic Energy and Charge Transfer with Two-dimensional Electronic Spectroscopy. Journal of Physical Chemistry Letters 2012, 3, 503510. 4. Ogilvie, J. P.; Kubarych, K. J. Multidimensional Electronic and Vibrational Spectroscopy: An Ultrafast Probe of Molecular Relaxation and Reaction Dynamics. Advances in Atomic, Molecular, and Optical Physics 2009, 57, 249-321. 5. Gelzinis, A.; Valkunas, L.; Fuller, F. D.; Ogilvie, J. P.; Mukamel, S.; Abramavicius, D. Tight-binding model of the photosystem II reaction center: application to two-dimensional electronic spectroscopy. New Journal of Physics 2013, 15, 075013. 6. Steffen, M. A.; Lao, K. Q.; Boxer, S. G. Dielectric Asymmetry In The Photosynthetic Reaction-Center. Science 1994, 264, 810-816. 7. Frese, R. N.; Germano, M.; de Weerd, F. L.; van Stokkum, I. H. M.; Shkuropatov, A. Y.; Shuvalov, V. A.; van Gorkom, H. J.; van Grondelle, R.; Dekker, J. P. Electric field effects on the chlorophylls, pheophytins, and beta-carotenes in the reaction center of photosystem II. Biochemistry 2003, 42, 9205-9213. 8. Romero, E.; Diner, B. A.; Nixon, P. J.; Coleman, W. J.; Dekker, J. P.; van Grondelle, R. Mixed Exciton-Charge-Transfer States in Photosystem II: Stark Spectroscopy on Site-Directed Mutants. Biophysical Journal 2012, 103, 185-194. 9. Sebastian, L.; Weiser, G.; Bassler, H. Charge-transfer transitions in solid tetracene and pentacene studied by electro-absorption. Chemical Physics 1981, 61, 125-135.


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