Effect of Nonplanarity on Excited-State Proton Transfer and Internal

Article Cite This: J. Phys. Chem. A 2018, 122, 5555−5562

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Effect of Nonplanarity on Excited-State Proton Transfer and Internal Conversion in Salicylideneaniline Shiela Pijeau,† Donneille Foster,† and Edward G. Hohenstein*,†,‡ †

Department of Chemistry and Biochemistry, The City College of New York, New York, New York 10031, United States Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, New York 10016, United States

‡

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S Supporting Information *

ABSTRACT: Salicylideneaniline (SA) is a prototype for excited-state intramolecular proton transfer (ESIPT) reactions in nonplanar molecules. It is generally understood that the dominant photochemical pathway in this molecule is ESIPT followed by nonradiative decay due to twisting about its phenolic bond. However, the presence of a secondary internal conversion pathway resulting from frustrated proton transfer remains a matter of contention. We perform a detailed nonadiabatic dynamics simulation of SA and definitively identify the existence of both reaction pathways, thereby showing the presence of a secondary photochemical pathway and providing insight into the nature of ESIPT dynamics in molecules with nonplanar ground-state geometries.

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INTRODUCTION

Excited-state intramolecular proton transfer (ESIPT) is a fundamental photochemical reaction found in a variety of molecular systems across chemistry and biology. Chromophores capable of ESIPT are key design elements in synthetic photoswitches, optical storage devices, sunscreens, and photostabilizers.1−10 Examples of ESIPT also occur naturally in certain dyes and bioflavonoids.11−17 Many of the prototypical and most well-studied examples of ESIPT contain planar chromophores where this reaction occurs. In these cases, there is often a strong driving force promoting ESIPT, and the quantum yield of these reactions approaches unity. In a nonplanar molecule, however, there may be additional pathways competing with the ESIPT reaction. Since nonplanar molecules exhibiting ESIPT are less common, the photochemistry of these molecules is less well-characterized. A detailed understanding of the effect of nonplanarity is essential to the rational control of ESIPT in nonplanar molecules or in sterically constrained environments where nonplanar rotamers may exist. Of the nonplanar molecules that exhibit ESIPT, the aromatic Schiff base salicylideneaniline (SA) and its derivatives have perhaps been the most thoroughly studied both experimentally and theoretically.18−43 The ESIPT in SA is ultrafast and has been observed to complete within 50 fs of photoexcitation.29,31,34 Following ESIPT, SA undergoes twisting about its phenolic bond; the excited-state lifetime of SA is controlled by this twisting motion and is fairly short. Recent estimates of the excited-state lifetime from time-resolved photoelectron spectroscopy (TRPES) are on the order of 1 ps in the gas phase.34 However, the origin of the excited-state dynamics leading to these observations remains contentious. Spectroscopic investigations of SA have indicated the presence of two internal conversion pathways.27,34,39 There is agreement that the dominant deactivation pathway in SA results from twisting about the phenolic carbon−carbon bond that follows ESIPT (the a3 dihedral angle in Figure 1). It has © 2018 American Chemical Society

Figure 1. Coordinates of SA relevant to its photochemistry. We define three dihedral angles: twisting about the anilic carbon−nitrogen bond, a1 (a), twisting about the central carbon−nitrogen bond, a2 (b), and twisting about the phenol carbon−carbon bond, a3 (c). In the planar equilibrium geometry of the molecule, these dihedral angles have values of a1 = 0°, a2 = 180°, and a3 = 0°. We also define two bond lengths relevant to the proton transfer reaction: the rNH and rOH bond lengths in the enol structure (d); the proton transferred keto structure is shown in (e). The proton transfer coordinate is defined as the difference between these bond lengths: rPT = rOH − rNH.

been speculated by some that the rate and quantum yield of the ESIPT reaction are related to the twist of the anilino ring (the a1 dihedral angle in Figure 1).34 A secondary internal conversion pathway resulting from frustrated ESIPT and twisting about the central carbon−nitrogen bond (the a2 dihedral angle in Figure 1) has also been suggested.27 Early theoretical work on this system indicated that both decay pathways are viable, although estimates of the branching ratio were not obtained.32 This work leveraged one-dimensional quantum dynamics on potential energy surfaces generated with time-dependent density functional theory (TDDFT). More recently, full-dimensional surface-hopping dynamics simulaReceived: March 12, 2018 Revised: May 16, 2018 Published: May 31, 2018 5555

DOI: 10.1021/acs.jpca.8b02426 J. Phys. Chem. A 2018, 122, 5555−5562

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The Journal of Physical Chemistry A

unsuccessful, SA twists about the central carbon−nitrogen bond (the a2 dihedral angle) until a conical intersection is reached and internal conversion occurs. The bifurcation of the initial wavepacket results from competition between the ESIPT reaction and twisting about the central carbon−nitrogen bond; the time evolution of the wavepacket along these two degrees of freedom is shown in Figure 2. When ESIPT occurs, the

tions have been performed using semiempirical configuration interaction potential surfaces.35 Surprisingly, these more detailed simulations indicated the presence of only a single photochemical pathway: ESIPT followed by internal conversion mediated by twisting about the phenolic carbon−carbon bond. As of yet, the discrepancy between these dynamics simulations and the experimental observations has not been resolved. Here, we address this question by performing full-dimensional ab initio multiple spawning (AIMS)44−46 quantum dynamics simulations of SA on potential energy surfaces determined on-the-fly using multireference complete active space configuration interaction (CASCI) wave functions with embedding corrections from density functional theory.47 This approach has been successfully applied to the nonadiabatic dynamics of several molecules that are closely related to SA.47−49 We observe two distinct deactivation pathways in our simulation and, for the first time, provide a theoretical estimate of the branching ratio between ESIPT and frustrated ESIPT. Further, we identify the deficiencies in the previous simulation35 that lead to the discrepancy between theory and experiment.

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COMPUTATIONAL METHODS

The potential energy surfaces used in this work were obtained on-the-fly with the floating occupation molecular orbital complete active space configuration interaction (FOMOCASCI) method (using a temperature parameter of 0.35 au and Gaussian broadening of the orbital energy levels).50,51 A minimal active space consisting of two electrons in two orbitals was used along with a 6-31G** basis set. To incorporate some of the dynamic electron correlation neglected by this approach, a DFT-embedding correction was applied.47 This correction was evaluated with the ωPBEh functional.52 Using this potential energy surface, an AIMS simulation of the excitedstate dynamics of SA was performed.44,45 The energies, analytic gradients,53 and analytic nonadiabatic coupling vectors54 of the FOMO-CASCI method were computed using the graphical processing unit (GPU) accelerated implementation in the TeraChem electronic structure package.55−59 The simulation consisted of 360 initial trajectory basis functions on the S1 electronic state; after considering spawning events, the total simulation included more than 2000 trajectory basis functions. The basis functions were propagated in time for 2 ps or until the population on S1 fell below 0.01; the time integration used 20.0 au time steps. Initial positions and momenta of the trajectory basis functions were sampled from a Wigner distribution of the ground-state harmonic vibrational wave function on the S0 electronic state computed at the B3LYP/631G** level of theory.

Figure 2. AIMS nuclear density on the S1 electronic state as a function of twisting about the central carbon−nitrogen bond, a2, and the proton transfer coordinate, rPT = rOH − rNH (see Figure 1). Snapshots of the density are shown every 50 fs for the first 350 fs following excitation. The AIMS density is averaged over all 360 trajectories. The horizontal black line represents a dihedral angle of 180°.

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RESULTS AND DISCUSSION We observe two distinct photochemical pathways in our AIMS simulation. At the Franck−Condon point, SA is in a nonplanar geometry characterized by a twist of approximately 37° about the anilic bond (the a1 dihedral angle in Figure 1). Over the first 100 fs following photoexcitation, SA planarizes about the anilic bond. The dominant photochemical pathway is characterized by an ESIPT reaction followed by twisting about the phenolic bond (the a3 dihedral angle); this motion leads to a conical intersection where quenching of the excited state occurs. A secondary pathway resulting from frustrated proton transfer is also present. When the ESIPT reaction is

twisting motion is impeded and SA becomes trapped in a planar S1 keto minimum. If SA is able to twist sufficiently, the ESIPT reaction is inhibited and the molecule continues to rapidly twist about the central carbon−nitrogen bond (the a2 dihedral angle). This twisting motion leads to a seam of conical intersection where deactivation of the excited state is highly efficient. The portion of the wavepacket trapped in the S1 keto minimum will decay via twisting about the phenolic bond (the a3 dihedral angle) on a somewhat longer time scale. These two pathways exhibit significantly different rates of internal conversion. When proton transfer occurs, the molecule 5556


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resulting from frustrated proton transfer and decay through the enol MECI has not been measured experimentally; however, previous theoretical estimates using a 1-D model potential predict the time constant to be 38 fs.32 This seems to be unreasonably fast, and the present estimate of 250 fs obtained from these full-dimensional computations is expected to be more realistic. To extract the time scale of the proton transfer reaction, exponential functions with and without preexponential factors are fit to the time-dependent population of the S1 enol state (see Figure 4); time constants of 46 and 52

tends to planarize and the wavepacket is trapped in a metastable keto minimum on the S1 electronic state; 80% of the trajectories follow this pathway. The other 20% of the trajectories do not undergo proton transfer and instead decay via the twisted minimum energy conical intersection (MECI). As is evident in Figure 3, the excited-state lifetime of the

Figure 3. Population of the S1 electronic state for the first 2 ps following excitation is shown in green. The total population of S1 is divided into the part that arises from trajectories exhibiting proton transfer (shown in red) and those that do not undergo proton transfer (shown in blue); the population decay of these trajectories is modeled as delayed exponential decay. The decay of the total S1 population is modeled as a sum of those two functions.

Figure 4. Population of the enol state of SA for the first 150 fs following excitation. The enol state is defined as the set of coordinates with a negative value of the proton transfer coordinate: rPT = rOH − rNH. See Figure 1 for a definition of the proton transfer coordinate. The population is obtained by integrating the AIMS nuclear wave function over all negative values of rPT. To isolate the time constant associated with proton transfer, only AIMS trajectories that exhibit proton transfer are included in this analysis.

trajectories that undergo excited-state proton transfer is considerably longer than those that do not. To quantify the difference in time scales, we separate the S1 population into the group of trajectories that decay via the keto MECI and those that decay via the twisted enol MECI. This classification is determined by the geometry of the first basis function spawned by each initial trajectory basis function; if the function is spawned at an enol geometry, the parent function and all of its descendants are classified as following the enol decay pathway (and vice versa for basis functions that initially spawn at a keto geometry). This approach to the classification of trajectory basis functions is not entirely unambiguous, since an initial basis function could, in principle, spawn functions at both MECIs. However, that behavior is observed for only 1 of 360 initial trajectory basis functions and less than 0.04% of the overall population is subject to ambiguous classification. To quantify the rate of internal conversion, the change in population is modeled as delayed exponential decay. ⎧ if t < t0 ⎪1 NS1 = ⎨ ⎪ −(t − t 0)/ τ if t ≥ t0 ⎩e

fs are obtained, respectively. This is in remarkably good agreement with the experimental estimates of the excited-state proton transfer rate in the gas phase. The experimental rates of 40 ± 20 to 50 ± 20 fs are obtained from by TRPES.34 The variation of the time constants again results from dependence on the wavelength of the pump pulse. The predicted rates of ESIPT are also in good agreement with earlier experimental measurements that find the time scale of the proton transfer reaction to be roughly 50 fs;29,31 the present result is also in excellent agreement with a previous theoretical prediction of 50 fs.32 Competition between proton transfer and twisting motions about the central carbon−nitrogen bond leads to the bifurcation of the wavepacket. Following photoexcitation to the S1 state, motion along either the proton transfer or twisting coordinate is favorable. Once significant progress is made in one of these directions; motion in the other direction becomes unfavorable. This can be seen clearly in Figure 5 where initial progress along the proton transfer coordinate occurs at geometries with 180° twist angles, and at later times, around 100 fs after excitation, proton transfer occurs at geometries with twist angles between 140° and 160°. The more rapid proton transfer tends to outcompete the twisting motion and dominates the dynamics; about 80% of the population undergoes proton transfer and decays via the keto MECI (Figure 3). Analysis of the initial positions of each trajectory basis function suggests that the initial value of the proton transfer coordinate may be correlated with the success of the

(1)

This is the simplest functional form that we have found to accurately describe the population dynamics observed in our simulation. We take the excited-state lifetime to be the sum of the onset time, t0, and the rate of exponential decay, τ. Our simulation predicts the trajectories that decay through the keto MECI to have an excited-state lifetime of approximately 800 fs; those that decay via the enol MECI have an excited-state lifetime of approximately 250 fs. The longer lifetime (corresponding to the majority of the trajectories) is in reasonable agreement with recent gas-phase measurements of the lifetime: 0.97 ± 0.1 to 1.17 ± 0.1 ps, depending on the choice of pump wavelength.34 The shorter time constant 5557


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Figure 5. AIMS nuclear density on the S1 electronic state divided into contributions from from trajectory basis functions that decay via the keto MECI (in red) and those that decay via the enol MECI (in blue). The densities are plotted as a function of the carbon−nitrogen dihedral angle, a2, and the proton transfer coordinate. The horizontal black line represents a dihedral angle of 180°, and the vertical black line divides the enol and keto structures.

proton transfer reaction. Perhaps unsurprisingly, the portion of the wavepacket located initially at more negative values of the proton transfer coordinate has a lower ESIPT quantum yield than the portion of the wave packet with more initial progress along the reaction coordinate. Trajectories that follow the keto pathway have initial values of the proton transfer coordinate of −1.34 ± 0.04 Å, while those that follow the enol pathway have an initial value of −1.42 ± 0.08 Å. The initial values of the twist angles are less predictive of the outcome of the reaction (177.5 ± 0.6° for the keto pathway and 176.9 ± 1.0° for the enol pathway). A qualitative picture of this process is that SA begins to planarize about its anilic bond and twist about its central carbon−nitrogen bond immediately following excitation (see Figures 5 and 6). If the proton transfer reaction can complete before the carbon−nitrogen twist becomes too significant, this twisting motion will cease and SA becomes trapped in the planar S1 keto minimum. Since the proton transfer reaction is unfavorable in the twisted molecule, if the reaction does not occur within approximately the first 100 fs, it is unlikely to occur at all (Figure 5). When SA is trapped in the keto minimum, motion along the anilic and phenolic twisting coordinates eventually leads to a conical intersection (Figure 6). Although the keto MECI is characterized by a twist about the phenolic angle alone, the seam of intersection can also be reached by simultaneous twisting about both the anilic and phenolic dihedral angles. The enol pathway can be described entirely as twisting about the central carbon−nitrogen bond; there is very limited motion along the proton transfer coordinate or phenolic twist angles and motion along the

anilic dihedral angle ceases once it has become planar (see Figures 5 and 6). The surface-hopping dynamics simulation of Thiel and coworkers identified only a single photochemical pathway for SA.35 Their simulation predicts ESIPT to be complete and to occur while SA is planar. This is in obvious disagreement with our AIMS simulation as well as previous work by others.32,34 The source of the discrepancy seems to be related to the choice of electronic structure method used to determine the potential energy surfaces. Thiel and co-workers apply semiempirical orthogonalization model 2 with multireference configuration interaction (OM2/MRCI).60−62 The authors report a perfectly planar ground-state minimum for SA predicted by this semiempirical method; all ab initio methods that we have tested predict a nonplanar ground-state minimum with anil twist angles (angle a1 in Figure 1) of roughly 30°−40° (geometric parameters of the optimized ground-state minimum are provided in the Supporting Information). This error severely damages the ability of OM2/MRCI to describe the photophysics of SA since, as demonstrated by Stolow and coworkers, the barrier (or lack thereof) to proton transfer is tied to the anil twist angle indicating that proton transfer is most efficient when SA is in planar geometries.34 By starting their dynamics simulation at a planar geometry of SA, Thiel and coworkers35 inadvertently biased their simulation toward successful ESIPT reactions and lost the ability to observe the secondary pathway. It should be noted that this appears to be an error in the OM2/MRCI method itself, rather than an error resulting from a misapplication of the method. This result does, however, suggest a strategy for controlling the quantum yield of 5558


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Figure 6. AIMS nuclear density on the S1 electronic state divided into contributions from trajectory basis functions that decay via the keto MECI (in red) and those that decay via the enol MECI (in blue). The densities are plotted as a function of the anilic dihedral angle, a1, and the phenolic dihedral angle, a3. The black lines represent dihedral angles of 0°.

Figure 7. Energies of salicylideneaniline computed with several different methods at geometries relevant to its photochemistry. The excitation energies at certain geometries are shown next to the arrow and reported in eV. Relative energies on the S1 state are given relative to the Franck− Condon point (the S0 enol twist geometry) and are reported in eV. Energies are computed with FOMO-CAS(2/2)-CI/6-31G** including a DFT correction from ωPBEh (shown in normal print), SA-CAS(2/2)-SCF/6-31G** (shown in bold print), SA-CAS(12/11)-SCF/6-31G** (shown in italicized print), CC2/cc-pVDZ (shown in blue print), and MS-CAS(2/2)-PT2/6-31G** (shown in red print). Geometries are optimized at the respective level of theory, except in the case of CC2 and MS-CASPT2, where the DFT-corrected CASCI geometries are used.

To verify the accuracy of the potential energy surface used in our dynamics simulation, we compare relative energies of the S1 state at several geometries that are important to the photochemistry of SA. The results of this comparison are

ESIPT reactions in derivatives of SA. Structural modifications that reduce the anil twist angle at the Franck−Condon point should promote the ESIPT reaction and suppress the secondary pathway. 5559


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against more robust approaches to gauge their suitability for treating the problem at hand. Our results indicate that the secondary pathway due to frustrated proton transfer must be blocked in order to control the photochemistry of the SA molecule. The photochromic isomerization product of SA is produced by the main decay channel that follows successful proton transfer. The yield of the proton transfer reaction could be enhanced by the addition of substituents that lead the molecule to more planar ground-state geometries or that lower the barrier to proton transfer at nonplanar geometries. It may also be possible enhance the yield of the proton transfer reaction by introducing SA into sterically constraining environments; perhaps even a viscous solvent could be sufficient to alter the quantum yield of the ESIPT reaction. Finally, we note that due to the presence of multiple photochemical pathways, SA would make an intriguing candidate for coherent control experiments or other techniques aimed at manipulating the intrinsic photochemistry of a molecule.

shown in Figure 7. The CASCI method including a DFTembedding correction is compared to state-averaged complete active space self-consistent field (SA-CASSCF) with a minimal active space (two electrons in two orbitals) as well as a larger active space (12 electrons in 11 orbitals). We compare to the approximate second-order coupled-cluster singles and doubles (CC2) method; CC2 has been shown to provide a robust treatment of ESIPT reactions63 and a reasonable treatment of electronic states near conical intersections.64 Finally, we also compare to multistate complete active space second-order perturbation theory (MS-CASPT2)65,66 to provide a more rigorous treatment of dynamic electron correlation near regions of intersection. The accuracy of the DFT-corrected CASCI approach compares favorably to these more traditional approaches in its description of relative energies on the S1 potential energy surface. All four methods agree that the ground-state minimum has a twisted geometry, while the keto minimum on the S1 state has a planar minimum; this is essential for a qualitatively correct description of the photodynamics of SA. Both our DFT-corrected CASCI approach and CC2 are in good agreement with respect to the ESIPT reaction energy on the S1 state (i.e., the energy difference between the Franck− Condon point and the S1 keto minimum). Also promising is the excellent agreement between SA-CASSCF and our DFTcorrected CASCI in the estimation of the relative energies of the minimum energy conical intersections. This agreement suggests that the minimal active space is indeed sufficient to characterize the region of the S1 potential energy surface accessed in our dynamics simulation. The energetics of the MECI geometries are in reasonable agreement with MSCASPT2 for the enol intersection, but the DFT-corrected CASCI (and all CASSCF methods) tend to overstabilize the keto intersection. This overstabilization of the keto intersection may account for the slightly underestimated excited-state lifetime we predict. As expected for CASCI-based methods with small active spaces, the vertical excitation energies are significantly overestimated. Although this is certainly not ideal, as has been demonstrated by us and others,48,49,67 it is the relative energetics of the excited state that are of primary importance to the excited-state dynamics. We expect that the overestimated vertical excitation energies may accelerate the excited-state dynamics to some extent.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b02426. Energies and optimized geometries of salicylideneaniline (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.G.H.). ORCID

Edward G. Hohenstein: 0000-0002-2119-2959 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support for this project was provided by the Martin & Michele Cohen Fund for Science and PSC−CUNY Award #60719-00 48, jointly funded by The Professional Staff Congress and The City University of New York. Computational resources were provided through a Research Cluster Grant from Silicon Mechanics: Award SM-2015-289297.

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CONCLUSIONS We have identified two distinct photochemical pathways present in the excited-state dynamics of SA. The major pathway is characterized by an ESIPT reaction and a planar S1 keto photoproduct with an excited-state lifetime of about 1 ps (in the gas phase). The minor pathway results from frustrated proton transfer and has a much shorter excited-state lifetime (250 fs). This resolves the apparent discrepancy between theory and experiment with regard to the existence of the secondary reaction pathway. Deficiencies in the electronic structure method applied in the previous dynamics simulations systematically biased the dynamics toward the ESIPT reaction and generation of the S1 keto product. Although dynamical simulations provide tremendous insight into photochemical processes, their success is inexorably linked to the quality of the potential energy surface (or surfaces) used in the simulation. Since the methods used to determine these potential surfaces often contain quite severe approximations (particularly for larger molecules when the surfaces are generated on-the-fly), it is essential to carefully benchmark these approximate methods

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