Defects Cause Sub-gap Luminescence from a Crystalline Tetracene

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Defects Cause Sub-gap Luminescence from a Crystalline Tetracene Derivative R. Eric McAnally, Jon A Bender, Laura Estergreen, Ralf Haiges, Stephen E. Bradforth, Jahan M Dawlaty, Sean T Roberts, and Aaron S. Rury J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02718 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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The Journal of Physical Chemistry Letters

Defects Cause Sub-gap Luminescence from a Crystalline Tetracene Derivative R. Eric McAnally,† Jon A. Bender,‡ Laura Estergreen,† Ralf Haiges,† Stephen E. Bradforth,† Jahan M. Dawlaty,† Sean T. Roberts,‡ and Aaron S. Rury∗,¶ †Department of Chemistry, University of Southern California, Los Angeles, CA, USA 90089 ‡Department of Chemistry, University of Texas at Austin, Austin, TX, USA 78712 ¶Department of Chemistry, Wayne State University, Detroit, MI, USA 48202 E-mail: [email protected]

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Abstract We use steady-state and ultrafast nonlinear spectroscopies in combination with density functional theory calculations to explain light emission below the optical gap energy (Eo ) of crystalline samples of 5,12-diphenyl tetracene (DPT). In particular, the properties of vibrational coherences imprinted on a probe pulse transmitted through a DPT single crystal indicate discrete electronic transitions below Eo of this organic semiconductor. Analysis of coherence spectra lead us to propose structural defect states give rise to these discrete transitions and sub-gap light emission. We use the polarization dependence of vibrational coherence spectra to tentatively assign these defects in our DPT samples. Our results provide fundamental insights into the properties of mid-gap states in organic materials important for their application in next generation photonics and opto-electronics technologies.

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Fundamental knowledge of the optical and electronic properties of semiconductors formed from molecular constituents remains essential for understanding how these materials may be used in future technologies. Despite this importance, gaps still exist in our ability to explain pervasive features in their optical spectra. Among these is the origin of the long, low energy tail in the photoluminescence (PL) emission spectra measured in many so-called organic semiconductors (OSCs), including anthracene, 1 tetracene, 2 and rubrene. 3,4 Despite the ubiquity of these tails in measured spectra, the physical mechanism driving their appearance remains unclear. Theory shows the overlap integrals between vibrational states of the participating electronic potential energy surfaces dictate the emission spectra of a single conjugated, polyaromatic molecule. 5 The spectral dependence of these Franck-Condon factors depend on the difference in the equilibrium geometry of the molecule upon changing its electronic state. One can simply sum the contribution of each factor in modeling the emission process. The low energy tails of emission spectra of individual molecules in the gas phase or diluted in a solution arise from broadening mechanisms caused by the surrounding environment. When a crystalline solid forms from molecules, the manner in which it interacts with light changes. A band of excitonic states forms from the excitation of a single molecule in the periodic background of ground state molecules. 6 The intermolecular transfer of this exciton can substantially change the light absorption and emission spectra relative to an isolated gas-phase molecule. There is also a quasi-continuum of states that extends upward from the valence band maximum and downward from these exciton states due to fluctuations of the molecules around their equilibrium positions. 7–10 In addition to this so-called dynamical disorder, new types of electronic excitations appear in the solid-state and contribute to light absorption and emission, such as excitons localized by structural imperfections. Parsing through these contributions makes the assessment of how and why tails appear in the emission spectra of OSCs a challenge. 11 In this study we introduce an experimental approach to more directly determine the

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origin of the emission tail in a crystalline tetracene derivative by coupling steady-state electronic and state-of-the-art nonlinear vibrational spectroscopic techniques with density functional theory (DFT) calculations. Our vibrational spectroscopic approach utilizes coherent structural evolution on ultrafast time scales to sensitively detect and characterize weakly absorbing mid-gap transitions in our sample, 5,12-diphenyl tetracene (DPT). The molecular structure of DPT is shown in the bottom inset of Figure 1. We find evidence of discrete absorptive transitions that overlap with the sub-gap tail of the photoluminescence (PL) emission spectrum of crystalline DPT. Based on a combined analysis of constructed vibrational coherence spectra, the results of DFT calculations, and the different physical mechanisms that can explain the steady-state spectra, we propose mid-gap states stemming from edge and screw dislocations cause a substantial portion of the spectral tailing in the PL emission. By clarifying how the electronic structure of individual molecules differ from the semiconductors they form, these results provide fundamental insights into the optical properties of a group of materials central to next generation technologies in photonics and opto-electronics. We estimate the optical-gap energy, Eo , between the band formed from a lattice of ground state DPT molecules and the exciton band-edge from steady-state spectra using Tauc’s model of light absorption, α(ν) =

A(hν−Eo )n . hν

In this model, α(ν) is the absorption

coefficient in units of cm−1 , ν is the photon frequency, and A is a constant determined by the D.C. conductivity of the material. 11,12 The value of n relates to the density of states near the edges of energy bands participating in light absorption. 11 One can extract Eo by plotting the Tauc spectrum, [α(ν)hν]1/n , and extrapolating the linear region of the resulting curve to its intercept with the photon energy/frequency axis. While typically applied to non-crystalline semiconductors, we believe the Tauc analysis is meaningful in the case of DPT since charge carrier transport characteristics of high quality crystalline OSCs, such as pentacene, resemble those of amorphous inorganic materials. 13 Figure 1 compares the quantity [α(ν)hν]1/2 calculated from the steady-state absorbance

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spectrum of a ensemble of DPT single crystals to the PL emission spectrum of a single crystal excited at 18800 cm−1 (532 nm). α(ν) is estimated by dividing the measured absorbance spectrum by the sample length (1 mm). For the PL spectrum both the incident and detected field polarizations lie along the horizontal (H) direction, as indicated by the top inset of Figure 1. H is parallel to the c-axis of the DPT crystal while the b-axis makes an angle of ∼10◦ with respect to the vertical direction,V. Since we do not have experimental measures of the density of states in DPT, we currently set n = 2 in the Tauc spectrum for simplicity and allow for refinement following more extensive studies. We find Eo = 14259 cm−1 using regression analysis of the linear rise of the Tauc spectrum. Surprisingly, this spectral position corresponds to a frequency far below the vibronic origin of the S0 →S1 transition of individual DPT molecules in the condensed phase, found near 20000 cm−1 . 14,15 Such a shift in Eo would necessitate a substantial change in the structure of each DPT molecular constituent unlike anything observed in solids formed other linear poly-aromatic molecules. 1–4 Furthermore, Roberts et al. have shown Eo shifts minimally upon changing the dielectric environment from a small molecule solvent to an amorphous film of other DPT molecules. 15 Therefore, we propose the difference between the absorption edge of individual and crystallized molecules stems from significant delocalization of excitons in the crystalline form of this material. 6 Comparison of the PL spectrum of Figure 1 to Eo indicates most of the light emission lies above Eo . However, we find substantial PL emission at energies below Eo , shown as the red shaded region of Figure 1. This shaded region may appear in the PL of crystalline DPT due to processes like dynamical disorder or new states like those stemming from structural defects. To further assess the possible origins of the sub-gap PL emission, we utilize vibrational coherence spectroscopy, a nonlinear ultrafast spectroscopic technique already used to characterize the physical properties of several material systems. 16–20 The conceptual design of our experiment is based on a simple pump-probe geometry. However, instead of a resonant pump pulse designed to induce a transient electronic population, we use a non-resonant

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/2 c [103 cm-1] Figure 1: Comparison of the Tauc spectrum, [α(ν)hν]1/2 , of an ensemble of 5,12-diphenyl tetracene (DPT) single crystals in a spectroscopic cell (solid blue) to the photoluminescence spectrum of a DPT single crystal excited at 18800 cm−1 in a polarized configuration (solid red). Dotted black line highlights the region of the Tauc spectrum used to estimate of the optical-gap energy, Eo , of crystalline DPT, as described in the text. The red shaded region indicates PL emission from states below Eo . Top inset: image of the crystal used in the reported ultrafast spectroscopic measurements with vertical (V) and horizontal (H) axes indicated in blue and red, respectively. The polarized PL spectrum of the main figure corresponds to the HH configuration. Bottom inset: the structure of a DPT molecule.

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ultrafast pulse to drive coherent superpositions of vibrational states, known as vibrational coherences, in the ground state of the material via an impulsive stimulated Raman scattering (ISRS) mechanism. 21,22 Following ISRS excitation, we measure the dynamic response of the material to these coherences with a probe pulse detected by a charge-coupled device affixed to a spectrograph. We refer to the resulting energy-dependent amplitude and phase as the vibrational coherence spectrum (VCS) for each normal mode. 23–25 In contrast to the peaks and line shapes associated with steady-state linear spectra, the spectral dependence of vibrational coherences possess anti-peaks in their amplitudes that denote the presence of absorptive transitions whose energies are modulated by the coherent motion of the material’s atoms. In addition, the spectral shifts of the oscillations’ phases further indicate the presence of these transitions. By characterizing the anti-peaks and phase shifts in the VCS associated with different vibrations we can identify mid-gap absorptive transitions in DPT crystals. Direct access to this density of transitions is especially important to determine the origin of the sub-gap emission observed in Figure 1. Figure 2 shows the ultrafast change in the transmission of a probe pulse centered at 12578 cm−1 (795 nm) through a bulk DPT crystal following excitation by a 40 fs, 7150 cm−1 (1400 nm) Raman pump pulse. The signal has been integrated over probe energies between 11090 cm−1 and 13510 cm−1 , at the edge of the spectral tail seen in Figure 1. Immediately following the nonlinear response of DPT due to the temporal overlap of the pump and probe pulses the measured time-domain wave form shows coherent oscillations whose amplitudes rise above and drop below ∆T = 0. This experimental baseline indicates we form no transient electronically excited DPT population during the measurement. In Figure 2, both the pump and probe electric fields are polarized along the horizontal direction, H, as indicated in the top inset of Figure 1. We combine experimental and computational analyses to determine the origin of the oscillations in Figure 2. On the experimental side, we applied a singular value decomposition analysis to filter the data, windowed the resulting wave form with a Gaussian, and performed

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Probe Delay [ps] Figure 2: Ultrafast change in the transmission of a probe pulse through a single crystal of 5,12-diphenyltetracene (DPT) following a pump pulse centered at 7150 cm−1 integrated over probe energies from 11090 cm−1 and 13510 cm−1 for both pump and probe electric field polarizations aligned to the horizontal direction, as indicated in the top inset of Figure 1.

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a fast Fourier transformation (FFT), as previously described. 18 The results of the FFT for both HH and VV polarization configurations are compared in the top panel of Figure 3. This comparison shows the dominant contributions to both signals come from components whose frequencies correspond to values below 50 cm−1 . However, several smaller amplitude signals extend to frequencies corresponding to values of almost 400 cm−1 . The bottom panel of Figure 3 shows the spontaneous Raman scattering spectrum of the same DPT crystal excited at ωexc /2πc = 12739 cm−1 (785 nm). Comparison of the top and bottom panels of Figure 3 shows the features present in the impulsively excited wave form of Figure 2 correspond to Raman-active excitations of DPT, as anticipated above. The cut-off of features below ∼80 cm−1 in the spontaneous Raman spectra stems from a band-stop filter designed to block Rayleigh scattering from the sample. Computationally, we compare Raman-active features found at frequencies corresponding to values of 39, 322, and 392 cm−1 in Figure 3 to DFT calculations of the vibrational spectrum of DPT. The latter two of these features appear in both ISRS and spontaneous Raman measurements while the former is the dominant contribution to ISRS measurement in the HH polarization configuration. After calculating the vibrations of DPT as explained in the Methods section, we find three modes at 39, 338 and 442 cm−1 , which are close to the experimental features of interest. The motions of carbon (C) atoms in each of these calculated vibrations are shown in the top panels of Figure 4. 26 Inspection shows the lower frequency vibration is a librational mode that rotates the entire molecule around the short axis of the tetracene core, sometimes called a libron. 27 The 338 cm−1 mode is an in-plane compression mode of the tetracene core while the 442 cm−1 mode combines out-of-plane motion of the C atoms at the exterior of the tetracene core with torsional motion of the other C atoms. With these motions, we classify the 39 cm−1 libron as a intermolecular mode while its higher frequency counterparts are clearly intramolecular vibrations. In the bottom panels of Figure 4 we examine the VCS associated with the 39 cm−1 and 392 cm−1 coherences for the HH polarization configuration over a narrow window between

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ω/2πc [cm−1] Figure 3: Top panel: dominant frequency components of the ultrafast transient absorption signal measured in a 5,12-diphenyltetracene single crystal excited at 7150 cm−1 (1400 nm) and probed between 11090 cm−1 (900 nm) and 13510 cm−1 (740 nm) for pump and probe polarizations aligned along the vertical (blue) and horizontal directions, as indicated in the top inset of Figure 1. Bottom panel: spontaneous Raman spectra of a DPT single crystal excited at 12740 cm−1 (785 nm) for incident and scattered electric fields polarized in the V (blue) and H (red) directions, respectively.

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12500 and 13250 cm−1 . First and foremost, this examination shows several discrete antipeaks in the coherence amplitudes that coincide with shifts of each coherence’s phase. While the nonlinear optical process of spectrally broadening the probe pulse can introduce sharp structure in its spectrum, we have verified that the VCS do not derive from this structure, as seen in Figure S1 of the Supporting Information. In addition, the dips in the VCS of DPT only appear in spectral regions for which we measure detectable PL signal, as shown in Figure S2. Taken with our vibrational assignments above, these features indicate that both inter- and intramolecular vibrations of DPT modulate the energies of absorptive transitions whose spectral positions overlap the tail of the PL spectrum shown in Figure 1. Thus, the VCS provide evidence of discrete light absorption transitions that couple the ground state of crystalline DPT with mid-gap states. Based on the significant red-shift of these discrete absorptive transitions relative to molecular DPT, we believe they appear due to the presence of molecular excitons localized by structural imperfections. The characteristics of the VCS and polarized PL emission spectra also support this conclusion. The bottom panels of Figure 4 indicate each type of vibration couples differently to the transitions present in the interrogated spectral window. To demonstrate this point concretely, let us consider the anti-peaks and phase shifts found at 12620 cm−1 and 12590 cm−1 for the 39 cm−1 and 392 cm−1 inter- and intramolecular coherences, respectively. We propose these features correspond to the same electronic transition that couples to both vibrations given their close proximity in frequency and similar VCS characteristics. Upon close comparison one finds the anti-peak of the 39 cm−1 coherence amplitude around 12620 cm−1 is broader than that of the 392 cm−1 coherence around 12590 cm−1 . In addition, the phase shift of the 39 cm−1 coherence is more shallow around 12620 cm−1 than that of the 392 cm−1 coherence around 12590 cm−1 . In the theoretical framework pioneered by Champion and co-workers, these facts indicate the lower frequency librational mode couples more strongly to this transition via a Huang-Rhys mechanism than its intermolecular counterpart. 23–25,28 This difference in coupling also explains evidence for only two transitions coupled to

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(a) ω/2πc = 39 cm−1

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ωprobe/2πc [103 cm−1] Figure 4: Top panel: vector representation of the motions of carbon atoms comprising Raman-active vibrations found at 39 (left), 338 (middle), and 442 (right) cm−1 through density functional theory electronic structure calculations. Bottom panels: comparison of the spectral dependence of amplitude (blue) and phase (red) of the coherence of the 392 cm−1 vibration of a 5,12-diphenyltetracene single crystal excited in the HH polarization configuration to that of the 39 cm−1 vibration for probe frequencies corresponding to values between 12500 cm−1 and 13250 cm−1 .

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the 39 cm−1 coherence at 12975 cm−1 and 13060 cm−1 while evidence for four transitions appears for the 392 cm−1 coherence in the region between 12880 cm−1 and 13025 cm−1 . The stronger coupling to the 39 cm−1 libration blurs each pair of transitions into a single anti-peak in the coherence amplitude while the weaker coupling to the 392 cm−1 vibration allows one to resolve the distinct presence of each pair in its VCS. Furthermore, we see that phase shifts around each pair of dips in the 392 cm−1 coherence oppose one another. This effect likely arises from a difference in the sign of the nuclear displacement of the excited state participating in each transition, i.e. a positive displacement for one with negative displacement for the other. Therefore, as the VCS of each pair of transitions merge together due to increased coupling to the 39 cm−1 libration, we would expect the overall phase shift to nearly cancel. The middle panel of Figure 4 shows this expected near cancellation of the phase shift in the VCS of the 39 cm−1 coherence. The polarization dependence of the VCS also provides meaningful insight into the origin of the states that give rise to mid-gap absorptive transitions in crystalline DPT. The polarization selection rules for excitonic absorption are dictated by the orientation of each molecular site and provide a probe of the coupling of structure to each excitonic transition measured in the VCS. 6 If amplitude anti-peaks and phase shifts appear at the same probe frequency for pulses aligned along both the H and V directions, then this indicates a single transition whose dipole moment must project onto both H and V. In contrast, if VCS features appear at different probe frequencies for different probe polarization directions, then this indicates separate transitions whose dipole moments must project onto only one lab frame direction. Thus, by comparing the ability of a given vibration to modulate the absorption of differently polarized probe pulses, we can assess how the molecular transition dipole moments align relative to the structure of the bulk crystal lattice. The top and middle panels of Figure 5 compare the VCS of the 322 cm−1 coherence for HH and VV excitation, respectively. We find this mode couples to four transitions in the band between 11250 cm−1 and 12250 cm−1 for HH excitation, and three transitions appear

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Figure 5: Top panels: comparison of the spectrally dependence of amplitude (blue) and phase (red) of the coherence of the 322 cm−1 vibration of a 5,12-diphenyltetracene single crystal excited the HH polarization configuration (top panel) to those excited in the VV polarization configuration (bottom panel) for probe energies between 11700 cm−1 and 12100 cm−1 . Bottom panel: comparison of the microscopic crystal structure of 5,12-diphenyl tetracene (DPT) to the lab-frame horizontal (H) and vertical (V) directions used to define polarization states in the spectroscopic measurements reported above. Note that the long and short axes of the tetracene core of DPT do not align with the H and V directions. 14 ACS Paragon Plus Environment

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for VV excitation. Interestingly, the positions of the transitions found in each polarization configuration do not match completely. We find evidence for transitions at 12050 cm−1 and 11900 cm−1 (marked by asterisks in Figure 5) for both HH and VV configurations while an additional transition appears at 11940 cm−1 for VV excitation. In addition, two transitions appear at 11700 cm−1 and 11791 cm−1 for HH excitation without evidence for either transition in VV. We propose two broad conclusions from the polarization dependence of the 322 cm−1 VCS. First, amplitude anti-peaks and phase shifts at the same frequency for both polarization configurations suggest the presence of structural defects that preserve the orientation of DPT molecules in the crystal lattice. The transition dipole moment of a DPT molecule likely lies along the short axis of its tetracene core given this alignment in tetracene itself and a similar S0 →S1 energy in molecules of each species. 29 The bottom panel of Figure 5 shows that the short axis of DPT’s tetracene core makes a ∼35◦ angle with the V lab frame direction in its room temperature crystal structure. Given this fact, we expect to observe structurally modulated absorption of probe electric fields polarized along both the H and V directions for sub-gap excitonic states in a portion of the lattice that preserves this crystal structure. The features of the polarized VCS observed at 12050 cm−1 and 11900 cm−1 in the top and middle panels of Figure 5 meet this criterion and likely indicate mid-gap excitonic states of whose transition dipole moments align with the undisturbed crystal lattice. Second, transitions whose appearance in the VCS depends on the polarization configuration indicates significant reorientation of their associated dipole moments relative to the structure of Figure 5. As stated above, the direction of the transition dipole moment must at least partially align with the electric field of the probe pulse for structurally modulated absorption to occur. We would anticipate observing evidence of a transition whose dipole moment aligns along H or V only when the probe polarization matches that lab frame direction. Since the polarization selection rules for absorption in crystalline DPT likely stem from the orientation of individual molecules at each crystal lattice, the appearance of the 11700

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cm−1 and 11791 cm−1 transitions only for HH excitation and the 11940 cm−1 transition only for VV excitation suggest the states giving rise this these absorption processes derive from strongly reoriented molecular sites. The vibrational coupling we infer from the VCS indicates the presence of discrete states that absorb light inside the optical-gap of crystalline DPT. The energies of transitions between these discrete states overlap with the spectral tails seen in Figure 1. Furthermore, the polarization dependence of the VCS suggest some of these discrete transitions correspond to molecular sites that do not preserve the structure of the lattice determined by x-ray diffraction. Based on these conclusions, we propose that the transitions we observe in VCS arise from structural defects in DPT crystals. Given that the energy of these transitions overlaps with the low energy portion of the emission spectrum in Figure 1, it is likely that emission from this spectral regions contains significant contributions from structural defects. To further solidify this proposal we consider and rule out other possible explanations of transient changes in probe pulse absorption under ultrafast coherent structural evolution of the sample. First, as shown by many authors dating back to Urbach’s observations, 30–32 fluctuations about the average positions of atoms in a crystal lattice cause the appearance of temperaturedependent tails in the material’s absorption spectrum. Based on this dynamical physical picture, we would expect structural fluctuations to substantially broaden the anti-peaks and phase shifts associated with a vibration that modulates Eo of the material. However, we measure evidence of discrete absorptive transitions substantially below Eo in the VCS. Furthermore, dynamical disorder cannot significantly reorient the molecular constituents of crystalline DPT, yet our polarization-dependent VCS data reveal transitions that indicate some molecular dipoles are oriented in manners that strongly differ from DPT’s crystal lattice. Polarization-dependent PL spectra also support the conclusion that dynamical disorder cannot explain the VCS shown in Figures 4 and 5. Since dynamical disorder occurs on the

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ωemission/2πc [10 cm ] Figure 6: Anisotropy of the HH and HV polarized photoluminescence of a 5,12diphenyltetracene single crystal excited at 18800 cm−1 (532 nm). time scales of molecular vibrations and cannot significantly displace atoms from their positions in the crystal lattice, one would expect this effect would distribute some light emission into a lower band of energies while preserving the polarization anisotropy dictated by the orientation of the transition dipole moments in the undistorted crystal lattice. Figure 6 shows the 18800 cm−1 (532 nm)-excited PL emission anisotropy [r = (IHH − IHV ) / (IHH + 2IHV )] of DPT at 298 K. Inspection of this spectrum shows while r>0.25 for frequencies above 14000 cm−1 , the anisotropy weakens by a factor of 4 in the tail of the PL spectrum. One explanation of this difference is that delocalized electronic states in the dynamically disordered lattice cause the emission at frequencies possessing a sizable anisotropy and weakly anisotropic PL emission stems from defect states. The states stemming from structural defects could play this role in the excited state dynamics, as seen by the polarization dependent properties of the 322 cm−1 VCS. Trapping in defects with transition dipole moments oriented differently from the DPT’s crystal lattice would not preserve the initial polarization state of

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the excitation field and cause depolarization of the light emission like that shown in Figure 6. Second, oxidization of solid-phase polyaromatic hydrocarbons has been known to cause sub-gap PL emission. Figure S3 shows that our samples possess less than a 1% density of oxidized DPT molecules. Furthermore, we do not expect oxidized DPT molecules to significantly affect the observed VCS. Previous studies on rubrene (5,6,11,12-tetraphenyl tetracene) have shown deterministic oxidization introduces defects states ∼1200 cm−1 (150 meV) above the ground state. 33,34 The oxidized impurities then act as states that can accept an excited electron in combination with the emission of light below the optical gap of rubrene. Given the structural similarity of rubrene and DPT, one would expect oxidation of DPT to produce defects in a similar energy range as observed in rubrene. In that case, the accepter states stemming from these defects would not be appreciably populated at room temperature and transitions from these states to higher lying ones should not significantly contribute to structurally modulated absorption central to our results. In contrast, the presence of structural defect states just below the exciton band would support sub-gap light emission and structurally modulated light absorption at the energies measured in the spectra of Figures 1, 4, and 5. The analysis used above to propose defects cause the tail in the PL spectrum of Figure 1 also motivates the assignment of these defects. Our analysis of the PL emission spectra indicates the excited electronic density of DPT delocalizes significantly relative to amorphous thin films studied previously. 15 In addition, we anticipate the defects whose structural imperfections further delocalize electronic density would result in discrete electronic states below the exciton band minimum, Eo . Furthermore, specific types of structural defects would give rise to the appearance of distinct polarization dependence in transitions from the ground state to these defect states. For instance, the appearance of the same transition in different polarization configurations suggests structural defects that preserve the orientations of molecule in DPT’s crystal lattice. These transitions could stem from edge dislocations

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whereby an additional plane of molecules is inserted into the lattice. This extra plane of constituents moves the molecules closer, further delocalizes electronic density, causes a reduction in the energies of exciton states stemming from the edge dislocations, and red-shifts the PL emission. Previously, Ahn et al. proposed linear stacking defects similar to edge dislocation in anthracene created excitonic states that delocalize over as many as 10 molecular sites and cause light emission red-shifted from the 0-0 vibronic band of anthracene by 1000’s of cm−1 . 1 In contrast, the case of a transition that appears only in one polarization configuration of the VCS could correspond to states introduced by screw dislocations in the crystal, as have been recently observed in the homo-epitaxy of rubrene thin films. 35 Molecules near the center of a screw dislocation can become strongly compressed and reoriented relative to the DPT’s crystal lattice. In the case of the energetic material cyclotrimethylene trinitramine, Pal and Picu have shown that a molecular site substantially rotated relative to the surrounding lattice can be favored energetically over both vacancies and interstitial point defects. 36 Thus, the polarization dependence of the VCS may provide direct evidence of the effect of rotationally distorted constituents in an OSC on exciton delocalization. Such distortions likely play a key role in the charge and energy transport efficiency in addition to the excited state dynamics of this class of materials. Further work using state-of-the-art computational approaches may be able to provide theoretical estimates of the energies of these defects to compare with the experimental results above. However, that work is beyond the scope of this study. In conclusion, we have developed an optical spectroscopic approach to assess the origin of a ubiquitous feature of optical spectra in organic semiconductors. Based on a simple pump-probe geometry, we induced coherent structural evolution to directly and sensitively probe very weakly absorbing transitions in the optical-gap of 5,12-diphenyl teatracene. We found discrete electronic states lie at ∼12000 cm−1 above the ground state of crystalline DPT and overlap with spectral tails of its steady-state photoluminescence spectra. A combined analysis of the vibrational coherence spectra and results of electronic structure calculations motivates a physical picture in which these discrete states stem from structural defects that

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could correspond to edge and screw dislocations. Based on the simplicity of both the experimental and computational approaches, there is good reason to believe vibrational coherence spectroscopy can establish the origin of weak transitions found other in materials with different properties. Such transitions could originate from impurities that drain charging capabilities of battery materials, ’dark’ states that trap energy and charge carriers in biological and bio-inspired photo-catalytic materials, and localized charge transfer excitons in cuprate materials that support high temperature superconducting phases, to name a few examples. In these and other cases, studying the coupling of electronic and nuclear structure using vibrational coherence spectroscopy will reveal additional fundamental and essential physical insights into the importance of structural dynamics in the properties of many fascinating classes of materials.

Methods 5,12-diphenyl tetracene (DPT) was synthesized as described previously. 14,15 DPT crystals were grown through vacuum train sublimation in a three zone tube furnace. The system is first evacuated to 10−7 Torr using a molecular drag pump. Then, the three oven zones are set to 250, 210, and 170 ◦ C, with the solid DPT sample housed in glass tubing, placed in the hottest zone. Over the course of several hours the DPT sublimes at reduced pressure and the molecular vapor transported down the tube where it condenses in zone 2. Slow transport and growth allows for direct crystal formation condensed from the molecular vapor. The crystals are retrieved by breaking the glass and scraping the tube walls. Steady-state transmission spectra were taken by placing an ensemble of single crystals in a 1 mm path length cell and placing the cell in an integrating sphere (Shimadzu UV-2000). Polarized photoluminescence measurements were made on macroscopic single crystals using a spectroscopic microscope system (Horiba XploRA Plus). Single crystals were placed in a nitrogen-purged cryostat at room temperature (Linkam Scientific Instruments) to prevent oxidative damage.

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The structure of these crystals were determined by x-ray diffraction. The single crystal X-ray diffraction data were collected on a Bruker SMART APEX DUO 3-circle platform diffractometer with the x-axis fixed at 54.74◦ , and using Mo K α radiation (λ = 0.710 73 ˚ A) monochromatized by a TRIUMPH curved-crystal monochromator. The diffractometer was equipped with an APEX II CCD detector and an Oxford Cryosystems Cryostream 700 apparatus for low-temperature data collection. A red prism-like specimen of approximate dimensions 0.045 mm x 0.153 mm x 0.214 mm, was used for the X-ray crystallographic analysis. The crystal was mounted in Cryo-Loops using Paratone oil. A complete hemisphere of data was scanned on omega (0.5◦ ) at a detector distance of 50 mm and a resolution of 512 x 512 pixels. A total of 2060 frames were collected. The frames were integrated using the SAINT algorithm to give the hkl files corrected for Lp/decay. The integration of the data using a triclinic unit cell yielded a total of 16238 reflections to a maximum θ angle of 27.48◦ (0.77 resolution), of which 4581 were independent (average redundancy 3.545, completeness = 99.9%, Rint = 4.78%, Rsig = 5.55%) and 2262 (49.38%) were greater than 2σ(F2 ). The final cell constants of a = 8.7434(5) ˚ A, b = 11.0673(6) ˚ A, c = 11.7555(7) ˚ A, α = 99.5810(10)◦ , β = 104.7500(10)◦ , γ = 109.2960(10)◦ , volume = 998.36(10) ˚ A3 , are based upon the refinement of the XYZ-centroids of 3496 reflections above 20 σ(I) with 4.694◦ < 2θ < 57.82◦ . Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.885. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.9840 and 0.9970. The structures were solved by the direct method and refined on F2 using the Bruker SHELXTL Software Package, using the space group P-1 with Z = 2 for the formula unit C30 H20 . All non-hydrogen atoms were refined anisotropically. The final anisotropic full-matrix least-squares refinement on F2 with 271 variables converged at R1 = 6.40%, for the observed data and wR2 = 16.88% for all data. The goodness-of-fit was 1.012. The largest peak in the final difference electron density synthesis was 0.185 e− /˚ A3 and the largest hole was -0.201 e− /˚ A3 with an RMS deviation of 0.036 e− /˚ A3 . On the basis of the final model, the calculated density was 1.266 g/cm3 and

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F(000), 400 e− . Nonlinear ultrafast vibrational spectroscopy was performed with a 1 kHz Ti:S regenerative amplifier (Coherent Legend Elite) seeded by an 88 MHz self-mode locked titaniumdoped sapphire (Ti:S) oscillator (Coherent Vitara). The output of the regenerative amplifier at 12578 cm−1 (795 nm) was split into two arms to form beams for a simple pump-probe experimental geometry. One beam was used to drive an optical parametric amplifier (Coherent Opera Solo) to form pulses centered at 7150 cm−1 (1400 nm) possessing a duration close to 40 fs. The central energy of these pulses was a factor of 2 below that of the optical-gap of DPT. The other arm was heavily attenuated and softly focused into a 5 mm thick disc of sapphire to induce a small broadening of the pulse spectrum due to self-phase modulation. This broadening allowed probing of the spectral region between 11090 cm−1 (900 nm) and 13510 cm−1 (740 nm). Density functional theory calculations of the electronic structure of DPT were carried out using CRYSTAL14. 37 We used the Perdew-Burke-Ernzerhof generalized gradient functional for both exchange and correlation 38 as implemented for solids by Adamo and Barone 39 and polarizable electronic basis sets for hydrogen and carbon atoms. 40 The positions of the atoms of DPT were set to those found in the room temperature x-ray diffraction pattern. The irreducible Brillouin zone of DPT was set on a mesh according to Pack-Monkhorst sampling using a shrinking factor of 3 for the inverse of all three crystallographic directions. The Coupled Perturbed/Kohm-Sham algorithm was used to find the frequencies of both the IR and Raman-active vibrations of DPT. 41,42

Acknowledgement ASR and JMD acknowledge support through a University of Southern California start up grant, the AFOSR YIP Award (FA9550-13-1-0128) and the Rose Hills Foundation Research Fellowship. JAB and STR would like to acknowledge support from the Robert A. Welch

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Foundation (Grant F-1885) and a CAREER award from the National Science Foundation for materials support (CHE-1654404). The authors thank Dr. Jon Dieringer, Dr. Eric Driscoll, and Dr. Shayne Sorenson for contributions to the development of the software used in the reported ultrafast spectroscopic measurements and Prof. Mark E. Thompson for useful discussions. The authors declare no competing financial interests.

Supporting Information Available Comparison of the spectrum of the probe pulse to the amplitude of the 392 cm−1 vibrational coherence spectrum of a 5,12 diphenyl tetracene single crystal, comparison of the polarized photoluminescence of a 5,12 diphenyl tetracene single crystal excited at 18800 cm−1 to the amplitude of the 392 cm−1 , and the absorption spectrum of a solution of re-dissolved 5,12 diphenyl tetracene single crystals in dichloromethane can be found on-line:

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Defects Cause Sub-gap Luminescence from a Crystalline Tetracene

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