Excited-State Dynamics of a Benzotriazole Photostabilizer: 2-(2

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

Excited-State Dynamics of a Benzotriazole Photostabilizer: 2-(2#-Hydroxy-5#-methylphenyl)benzotriazole Shiela Pijeau, Donneille Foster, and Edward G. Hohenstein J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04504 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Excited-State Dynamics of a Benzotriazole Photostabilizer: 2-(2′-Hydroxy-5′-methylphenyl)benzotriazole Shiela Pijeau,† Donneille Foster,† and Edward G. Hohenstein∗,†,‡ †Department of Chemistry and Biochemistry, The City College of New York, New York, NY 10031 ‡Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, NY 10016 E-mail: [email protected]

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 42

Abstract A large number of common photostabilizers are based on the 2-(2′ -hydroxyphenyl)benzotriazole structure. One common example is 2-(2′ -hydroxy-5′ -methylphenyl)benzotriazole, or TINUVIN-P. The excited-state dynamics of this molecule have been extensively characterized by ultrafast spectroscopies. These experiments have established that upon photoexcitation, TINUVIN-P exhibits excited-state proton transfer followed by a remarkably fast internal conversion. We simulate the excited-state dynamics using ab initio multiple spawning (AIMS) and a complete active space configuration interaction (CASCI) wavefunction with a correction from density functional theory (DFT) to generate the potential energy surfaces. We predict ultrafast proton transfer on the order of 20 fs followed by simultaneous twisting and pyramidalization until a seam of conical intersection is reached. Near the intersection seam population transfer to the ground state is highly efficient. The process is best described as ballistic wavepacket motion from the Franck-Condon point along a barrierless coordinate leading to the seam of intersection. Internal conversion is primarily mediated by a minimum energy conical intersection (MECI) with a high degree of pyramidalization. We posit that the presence of a nitrogen atom in the bond linking the phenyl to the benzotriazole allows for the rapid pyramidalization and the short excited-state lifetime.

Introduction Photostabilizers and photoprotectants that efficiently absorb UV light and reversibly dissipate its energy are of great importance. There are both naturally occurring 1–6 and synthetic examples 7,8 of molecules that exhibit this behavior. These molecules are components of sunscreens and cosmetics where they protect against DNA damage from exposure to UV light. 9–11 Photostabilizers are also common additives to polymers, paints, and coatings that help extend the longevity and performance of these materials. In these applications, molecules that strongly absorb UV light and quickly quench the excitation are needed. A fea-

2


Page 3 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ture common amongst many photostabilizers is the presence of an excited-state intramolecular proton transfer (ESIPT) pathway. 12,13 ESIPT reactions stabilize a photoexcited molecule by shuttling a proton between protonation sites in order to stabilize the electronic structure of the excited state. Thus, the ESIPT reaction will rearrange the internal hydrogen bonding structure of the molecule; in some cases, the newly formed hydrogen bonds are much weaker than those on the ground state. This makes new internal conversion pathways energetically accessible (often via twisting). A detailed understanding of the excited-state dynamics of these types of photostabilizers will aid in the design of new photostabilizers or guide modifications to existing molecules to improve their photophysical properties. A common class of photostabilizer is based on the 2-(2′ -hydroxyphenyl)benzotriazole framework. A few representative examples can be found in Fig. 1. These benzotriazole photostabilizers find applications in the protection of various polymers against UV light; in particular, the photostabilizers relevant to the present work have been applied to polyethylene, 7,8,14,15 poly(vinyl chloride), 16–20 styrene-based rubber, 21 and polyurethane. 22 Additionally, these photostabilizers have been applied to textiles to protect the fabric 23 as well as dyes that it may contain. 24–28 These molecules typically absorb in the near-UV and rapidly undergo ESIPT from the hydroxyphenyl moiety to the nearest nitrogen atom of the benzotriazole. The excited state is quenched by what it thought to be a twisting motion about the central carbon-nitrogen bond. There are many molecules in the benzotriazole family of photostabilizer that share the important features of an ESIPT pathway and a single carbon-nitrogen bond connecting the two aromatic systems. The different benzotriazole photostabilizers are distinguished by the substituents on the two ring systems. In some cases, multiple 2-(2′ -hydroxyphenyl)benzotriazole systems can be contained in a single molecule (as in Fig. 1d). Although substituents may be important to the practical applications of these photostabilizers, broadly speaking, the substituents are not expected to significantly affect the photochemistry of these molecules (although the precise details of the internal conversion process may differ between molecules).

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 42

The focus of our present work will be the most well-studied 2-(2′ -hydroxyphenyl)-benzotriazole photostabilizer: 2-(2′ -hydroxy-5′ -methylphenyl)benzotriazole (commonly known as TINUVINP). There have been ultrafast transient absorption experiments performed on TINUVIN-P as well as some previous theoretical work mapping the excited-state potential energy surface and possible quenching pathways. 29–40 The timescales associated with the excited-state dynamics are remarkable fast: excited-state proton transfer occurs with a time constant of 60 to 80 fs and internal conversion follows in 120 to 150 fs (in cyclohexane). The proton transfer time is consistent with many other similar molecules; 41–43 the rate of internal conversion, however, is unusually fast. Rapid internal conversion is an important characteristic for an effective photostabilizer, since it reduces the probability of excited-state electron or energy transfer. The origin of this effect in TINUVIN-P has been attributed to a twisting motion about the central carbon-nitrogen bond, 37 however, this explanation does not account for the accelerated rate relative to structurally similar molecules. For example, 2(2′ -hydroxyphenyl)benzothiazole (HBT) has a 2.6 ps excited-state lifetime in the gas phase and is expected to undergo internal conversion via a similar twisting motion. 44 However, HBT has a much longer 100 ps lifetime in cyclohexane (generally, HBT has excited-state lifetimes on the order of tens to hundreds of picoseconds in solution). 45,46 Simulation of the nonadiabatic dynamics of internal conversion in TINUVIN-P is needed to offer an explanation for this enormous difference in timescales. Previous theoretical studies have been primarily limited to computations mapping the excited-state potential energy surface of TINUVIN-P along proton transfer and twisting coordinates. 35–38 Earlier studies relied on truncated models for TINUVIN-P that omitted the methyl group 37 or that additionally removed the benzene ring from the benzotriazole moiety. 35,36 The neglect of the methyl group likely has a negligible impact on the photophysics of TINUVIN-P, whereas, simplifications of the benzotriazole structure may be more significant. As a result of recent hardware and algorithmic advances, such simplifications are no longer necessary. In this work, we simulate the nonadiabatic excited-state dynamics of TINUVIN-P. Our

4


Page 5 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


goal is to obtain a detailed mechanistic understanding of the ESIPT and quenching processes that occur following photoexcitation. We will address the unusually short excited-state lifetime of TINUVIN-P. To the best of our knowledge, this is the first report of an excited-state dynamics simulation of a benzotriazole photostabilizer. We perform an ab initio multiple spawning (AIMS) 47,48 simulation to obtain the first direct theoretical estimation of the excited-state lifetime of TINUVIN-P and to determine the precise mechanistic details of the quenching process. The potential energy surface used in the simulation is determined on-the-fly by a newly developed embedding method that uses a compact complete active space configuration interaction (CASCI) wavefunction to describe the electronic structure of the ground and excited states and a correction from density functional theory (DFT) to account for some of the dynamic electron correlation that is not included in the CASCI wavefunction. 49 This approach has proven to be quite useful for treating ESIPT reactions and is highly efficient, which allows for the simulation of the nonadiabatic dynamics.

Computational Details We apply a CASCI wavefunction where the orbitals are obtained from a fractional occupation number Hartree-Fock (FON-HF) 50,51 computation using Gaussian broadening of the orbital energy levels and a temperature parameter of 0.15 atomic units. The combination of FON-HF and CASCI is known as floating occupation molecular orbital CASCI (FOMO-CASCI). 52,53 This method is computationally efficient and highly stable in the vicinity of intersections between electronic states. As a result, it is ideally suited for applications in excited-state dynamics. Further, we use wavefunction-in-DFT embedding 54,55 to incorporate some of the missing dynamic electron correlation. The energy of the core (inactive) electrons is evaluated using the ωPBE functional and a mean-field embedding potential is applied. 49 The active space in the CASCI wavefunction consists of two electrons in two orbitals where the highestlying π orbital and lowest-lying π ∗ orbital are included. We have found that the minimal

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

active space is sufficiently flexible to describe the character of the S1 excited state in the regions of configurational space that are of present interest. Much of the correlation that one would hope to capture by enlarging the active space is instead contained in the DFT correction. For example, it might seem that σ and σ ∗ orbitals would need to be included in the active space in order to describe the proton transfer reaction. However, it has been found by us and others that inclusion of these orbitals does not necessarily offer significant improvement in the description of excited-state proton transfer processes over a minimal active space. 49,56 All CASCI computations in this work use a 6-31G∗ basis set. Excited-state dynamics simulations of TINUVIN-P are performed using the ab initio multiple spawning (AIMS) method. The mathematical details of the method are described elsewhere. 47,48 Our simulation of TINUVIN-P is initialized with 200 independent (uncoupled) nuclear basis functions, where the positions and momenta were sampled from a Wigner distribution corresponding to the TINUVIN-P ground-state vibrational wavefunction. The vibrational wavefunction was constructed under a harmonic approximation at the B3LYP/631G∗∗ level of theory. These 200 initial basis functions were propagated in time for 1 ps (using a 25 au, ∼ 0.6 fs, time step) or until the population of the first excited state fell below 0.01. Additional nuclear basis functions, which are spawned when regions of strong nonadiabatic coupling between the ground and first excited state are reached, are fully coupled. 57 Energies, analytic gradients and analytic nonadiabatic coupling elements are evaluated on-the-fly using our GPU-accelerated implementation 58,59 of FOMO-CASCI within the TeraChem quantum chemistry program. 60–64

Results & Discussion The first step in the photochemistry of TINUVIN-P is the excited-state proton transfer reaction that follows photoexcitation to the S1 state. To extract the progress of the proton transfer reaction from the time evolution of the 3N dimensional AIMS wavepacket, we in-

6


Page 6 of 42

Page 7 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


troduce a proton transfer coordinate (see Fig. 2) defined as the difference between the OH bond distance and the NH bond distance.

RPT = ROH − RNH

(1)

By integrating the square of the AIMS wavefunction over all other degrees of freedom, we obtain the nuclear density as a function of the proton transfer coordinate. Notice that the introduction of the proton transfer coordinate is solely for the purpose of analysis and does not affect the time evolution of the wavepacket. In Fig. 3, we show the progress of the nuclear density on the S1 state along this proton transfer coordinate. The proton transfer reaction begins immediately upon excitation and proceeds rapidly. We find that proton transfer is essentially complete within 50 fs of photoexcitation. In order to extract a rate constant for this process, we must introduce an operational definition of the reactant and product states (the enol and keto structures). In this analysis, we take all structures with negative values of the proton transfer coordinate to be in the enol state and structures with positive values of the proton transfer coordinate to be in the keto state. Fitting an exponential function to the decay of the population of the S1 enol state,

Nenol = e−t/τPT ,

(2)

yields a time constant, τPT , of 20 fs. This ultrafast proton transfer is consistent with the barrierless proton transfer coordinate predicted by both FOMO-CASCI (used to determine the potential energy surface for the dynamics simulations) as well as the result of CC2 computations by Sobolewski et al. 37 For proton transfer to proceed this rapidly, it is unlikely to be preceded by the significant bond rearrangement that has been found to control the rate of other ultrafast proton transfer reactions. 43,65–67 At this point, we simply note that the estimate of the proton transfer time from our simulations is much shorter than the experimental result of 60 to 80 fs; we will consider possible sources for this discrepancy after 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

discussing the internal conversion process. Following proton transfer, TINUVIN-P begins to twist about its central carbon-nitrogen bond and pyramidalize about the nitrogen in the 2-position of the benzotriazole moiety. The definition of these coordinates is shown in Fig. 2. We define the dihedral (or twisting) angle to be the angle formed between best fit planes through the non-hydrogen atoms of the phenyl and benzotriazole moieties. The pyramidalization angle is defined as the angle formed between a vector from the center of mass of the benzotriazole to the nitrogen in the 2-position and a vector from that nitrogen to the center of mass of the phenyl ring. In Fig. 4, we show the time-evolution of the AIMS nuclear density as a function of the dihedral and pyramidalization angles. Our simulations predict that progress along these coordinates immediately follows the completion of the proton transfer reaction. Further, the wavepacket evolves simultaneously along the dihedral and pyramidalization coordinates. This implies that there is no energetic barrier (real or apparent) between the S1 keto structure of TINUVIN-P and the seam of intersection with the ground electronic state. The S1 keto geometry is a minimum in Cs symmetry, but a saddle point overall. As a result, out of plane distortions should be expected to proceed unimpeded. Motion along the pyramidalization coordinate is quite pronounced. Pyramidalization and twisting of TINUVIN-P are equally important in our simulation of the excited-state dynamics. We suspect the nitrogen atom in the 2-position gains sp3 character following proton transfer thereby encouraging motion along the pyramidalization coordinate. The nuclear wavepacket reaches the seam of intersection within 250 to 300 fs after photoexcitation. Upon reaching the seam, internal conversion is highly efficient and only about 5% of the population remains in the S1 state after 400 fs. In Fig. 5, we show three minimum energy conical intersections (MECIs) relevant to the photodynamics of TINUVIN-P. The percent of the population transfer occurring near each MECI is listed. This is determined by comparing the geometries at which nuclear basis functions are spawned on the S0 state to each of the MECI geometries via their root-mean-square deviation (RMSD). 68 The fi-

8


Page 8 of 42

Page 9 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nal population (the norm of the nuclear wavefunction coefficient) associated with that basis function is used to quantify the population transfer that is mediated by that region of the intersection seam. This analysis shows that the population transfer process is dominated by MECI 1 in Fig. 5. Of the three MECI geometries, MECI 1 has the smallest dihedral angle and the least amount of strain on the intramolecular hydrogen bond. Relevant geometric parameters of the three MECIs are given in Table 1. MECI 1 also shows a significantly larger degree of pyramidalization than the other two MECIs (see Fig. 4). Interestingly, internal conversion occurs via the least energetically favorable MECI geometry. A combination of the ease of pyramidalization and the (partial) retention of the hydrogen bond guides the wavepacket towards this MECI. In Fig. 5, we show minimum energy paths from the S1 keto geometry to the three MECIs. The paths to MECI 2 and 3 are barrierless and decrease in energy monotonically. In the case of MECI 1, there is a minimum in the reaction path that is encountered prior to reaching the intersection. Note that this does not correspond to a minimum on the S1 potential energy surface. It is difficult to predict from the analysis shown in Fig. 5 which MECI geometry will be the most important to the internal conversion process. The reaction path to MECI 1 might be slightly steeper, initially, which causes the wavepacket to move in that direction after proton transfer is complete. Since this is largely speculative, the dynamics simulation proves extremely useful in unambiguously determining the involvement of each MECI. On the S1 potential energy surface of TINUVIN-P, there is no energetic barrier to proton transfer and, following proton transfer, there is no energetic barrier to simultaneous twisting and pyramidalization. That is to say, a barrierless minimum energy path exists connecting the Franck-Condon point to the vicinity of the intersection seam. The resulting excitedstate dynamics are best described as ballistic wavepacket motion from the Franck-Condon point through the S1 keto structure to the intersection seam. This behavior can be seen in Fig. 4. The wavepacket leaves the region of the planar keto structure 100 to 200 fs after excitation. The tail of the wavepacket reaches the intersection seam (in particular, MECI

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 42

1) approximately 250 fs after excitation. Internal conversion is extremely efficient near the intersection, and the excited state is rapidly quenched as the wavepacket approaches the intersection. As a result, the population of the S1 state is approximately unity for the first 250 fs and is followed by rapid exponential decay. These population dynamics are typical of ballistic wavepacket motion towards a conical intersection. 69 To model the time dependence of the S1 population, we fit the population obtained from our simulation with a delayed exponential function (the fit is shown in Fig. 6).

NS1 =

   1

if t < t0

  e−(t−t0 )/τ

(3)

if t ≥ t0

The parameter t0 can be interpreted as the time needed for the tail of the wavepacket to reach the intersection seam and population transfer to S0 to begin. The parameter τ describes the rate of decay that is observed once the wavepacket has reached the intersection. In the case of ballistic motion, the rate of decay is related to the delocalization of the wavepacket and internal conversion efficiency. We obtain values of t0 = 260 fs and τ = 55 fs for these time constants. We find that the delayed exponential is the simplest function that provides an accurate representation of the simulation results. The rate constants obtained from experiment parametrize a sequential first-order kinetics model of the photochemistry of TINUVIN-P. 33 The time constant assigned to proton transfer is in the range 60-80 fs; the constant associated with internal conversion is in the range 120-150 fs. 40 The proton transfer rate obtained from our simulation does not agree particularly well with experiment; we estimate the proton transfer time to be approximately 20 fs. However, we believe that the spectroscopic feature associated with the 60-80 fs experimental time constant cannot be exclusively attributed to the proton transfer reaction. Since the planar S1 keto geometry is a saddle point, the nuclear wavepacket is not trapped in a local minimum and, therefore, there is no well-defined spectroscopic signature of the S1

10


Page 11 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


keto state. The kinetics model and experimental rate constants suggest that, at most, half of the population is in the S1 keto state at one time (see Fig. 6). In molecules with distinct timescales for proton transfer and internal conversion, it is more obvious what feature should be assigned to proton transfer. In the case of TINUVIN-P, it is difficult, if not impossible, to isolate a distinct spectroscopic signature associated exclusively with the proton transfer reaction. In Figure 6, we present the populations obtained from our simulation, the modeling of these populations with delayed exponential functions, and populations obtained from the experimentally determined kinetics model using a range of rate constants. The first important observation is that our simulation is in good agreement with experiment in the prediction of the time needed for the excited-state processes to complete. Both the kinetic modeling and the simulation results agree that the excited-state population falls below 0.1 sometime between 350 and 400 fs after excitation. This implies that the behavior we observe at shorter times occurs on a timescale consistent with the experimentally determined excitedstate lifetime. As previously discussed, we predict a much faster proton transfer reaction; the immediate result is that the S1 keto population grows very rapidly and remains roughly constant until the wavepacket reaches the intersection and internal conversion begins (around 260 fs). The theoretical populations cannot be adequately modeled using a sequential firstorder kinetics model; there is no choice for the rate constants that will approximate delayed exponential decay. Although the sequential kinetics model is able to describe changes in the transient absorption signal, 33 our simulation suggests that the this model does not adequately describe the details of the wavepacket dynamics that give rise to the signal. To test our hypothesis that the experimental transient absorption measurements are not necessarily inconsistent with our prediction of ultrafast proton transfer, we report a simulated transient absorption spectrum in Fig. 7. There are several caveats regarding this simulated spectrum. First, we include only contributions from stimulated emission; it is not possible to extract contributions due to ground- and excited-state absorption from our simulation in

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a meaningful way. Second, the emission energies are overestimated relative to experiment (due to the level of electronic structure theory applied in the dynamics simulation), so direct quantitative comparisions with experiment are not possible. These issues will be discussed in detail, later. In the theoretical spectrum, the energies are evaluated at the centroid of the trajectory basis functions (TBFs) on the S1 state and the intensity is taken to be proportional to the square of the transition dipole moment multiplied by the norm of the wavefunction coefficient associated with the TBF. The resulting signal is convolved with Gaussian functions in time and energy having full-width at half-maximums of 10 fs and 0.2 eV, respectively. This convolution is solely for the purpose of data smoothing and is not intended to mimic the instrument response function. Following photoexcitation, there is a pronounced Stokes shift associated with the ESIPT reaction. Along with the shift in emission energy, there is also an increase in intensity related to an increase in the magnitude of the transition dipole moment. As a result, a maximum in emission intensity is reached approximately 100 fs after excitation at 2 eV. At longer times, the emission energies begin to decrease and the intensity decays as the wavepacket approaches the MECI and internal conversion occurs. On the basis of the spectrum presented in Fig. 7, one might expect to be able to track the progress of the proton transfer reaction via an emission transient at 2 eV. This transient and others at higher energy are shown in Fig. 8; these transients correspond to the horizontal lines in Fig. 7. From a multi-exponential fit of the emission transient, a rise-time of 63 fs is deduced (at an emission energy of 2 eV). This is significantly longer than the 20 fs proton transfer time obtained from a direct geometric analysis of the wavepacket; this time constant is also in much better agreement with the most recent experimental estimates of the proton transfer time (60-80 fs). 40 It is also possible to use a higher energy transient at the vertical excitation energy of the S1 keto geometry (in Cs symmetry) of approximately 2.5 eV. A similar analysis of this transient leads to a time constant of 45 fs; this represents a middle ground between experiment and our geometric analysis of the wavepacket.

12


Page 12 of 42

Page 13 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Finally, we note that transients at even higher energy exhibit ultrafast behavior. We believe that the transient at 2 eV is the most comparable with experiment. At this emission energy, the rise in the signal is associated with proton transfer as well as a small amount of progress along the twisting and pyramidalization coordinates towards the MECI. As we suggested, the lack of a true S1 keto minimum leads to ambiguity in the assignment of the transient absorption spectrum. We do not find any features in the simulated spectrum that are similar to the population dynamics of the S1 keto state shown in Fig. 6. We offer this analysis of the theoretical stimulated emission transients as preliminary evidence that the ultrafast proton transfer time (20 fs) obtained from the geometric analysis of the wavepacket might not be incongruous with experimental measurements suggesting a slower process (60-80 fs). Finally, we note that resonance Raman spectra of TINUVIN-P show the most significant enhancement of a vibrational mode at 1422 cm−1 indicating wavepacket motion along this coordinate immediately following photoexcitation. 70,71 The half-period of this vibration, 12 fs, is not inconsistent with the ultrafast proton transfer observed in our simulation. A direct, quantitative comparision between theory and experiment is needed to answer this question more definatively. Unfortunately, it is extremely difficult to obtain quantitative predictions of transient absorption spectra from first principles. For this purpose, ultrafast time-resolved fluorescence experiments could be performed, similar to recent measurements of excited-state proton transfer in 10-hydroxybenzo[h]quinoline (HBQ) and 2-(2′ -hydroxyphenyl)benzothiazole (HBT); 43,72 time-resolved photoelectron spectroscopy of gas-phase TINUVIN-P would be another attractive alternative. 42,73 For the purpose of obtaining rigorous comparisions with theory, these spectroscopies are preferable to transient absorption because the experimental observable in these techniques can be more easily and accurately obtained from ab initio simulation. A final possibility would be to apply timeresolved infrared spectroscopy to directly probe the vibrational relaxation associated with the proton transfer event in analogy to experiments performed on HBT. 74,75 The accuracy of our simulation is primarily limited by the accuracy of the excited-state

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

potential energy surface. We applied a CASCI wavefunction to capture static electron correction and a DFT correction to account for dynamic electron correlation. This approach has been demonstrated to be accurate for excited-state proton transfer reactions. 49 To verify that the performance of this method is satisfactory in the present case, we compare with second-order approximate coupled-cluster singles and doubles (CC2) and state-averaged complete active space self-consistent field (SA-CASSCF) computations. We compare with CC2 because it offers a more robust treatment of dynamic electron correlation than either FOMO-CASCI or SA-CASSCF at modest, O(N 5 ), computational expense. It has been widely applied to excited-state proton transfer reactions 76 (including TINUVIN-P) 37 and has even been demonstrated to properly describe the topology of potential energy surfaces near a conical intersection. 77 We expect CC2 to provide an accurate treatment of the singlyexcited S1 state of TINUVIN-P. We also compare with SA-CASSCF using a minimal (2,2) active space similar to what is used in the dynamics simulation as well as a larger (14,12) active space that contains a more complete treatment of the π system. We use CC2 to test for sensitivity with respect to the inclusion of dynamic electron correlation and SA-CASSCF to test the active space dependence of our results. These comparisons are shown in Fig. 9. The vertical excitation energies obtained with our FOMO-CASCI wavefunctions are overestimated as compared to CC2 and are, unsurprisingly, similar to SA-CASSCF with a (2,2) active space; the larger (14,12) active space improves the vertical excitation energy, but not dramatically. The most important comparisons, in the context of the present work, are between the relative energies on the S1 state. These show good agreement between CC2, large active space SA-CASSCF and our DFT-corrected FOMO-CASCI method. The proton transfer reaction energy (the difference between the Franck-Condon point and the S1 keto geometry) agrees to about 0.1 eV when CC2 and our DFT-corrected FOMO-CASCI method are applied; the large active space SA-CASSCF computations predict a somewhat smaller reaction energy. Of importance to the internal conversion process, all four methods we test predict the same energetic ordering of the MECI geometries. The DFT-corrected

14


Page 14 of 42

Page 15 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FOMO-CASCI method and SA-CASSCF with the larger active space are in good agreement with the possible exception of MECI 1, where DFT-corrected FOMO-CASCI overstabilizes this geometry. Finally, we compare the MECI geometries obtained with the DFT-corrected FOMO-CASCI method and SA-CASSCF; the agreement between these methods is quite good. There are slight differences between the geometries optimized with SA-CASSCF and DFT-corrected FOMO-CASCI for MECI 1, which might lead it its overstabilization. Overall, the level of agreement between the CC2, SA-CASSCF, and FOMO-CASCI excited-state potential energy surfaces and MECI geometries is quite good and inspires confidence that the results of the AIMS simulation are physically reasonable. Our present work appears to be in good agreement with the conclusions drawn by Sobolewski et al. from an analysis of the excited-state potential energy surfaces. 37,39 The ballistic wavepacket motion observed in our simulations is in agreement with their predictions. Another important point of agreement is in the choice of orbitals for our active space. We can only describe an excited state with ππ ∗ character, since we do not include any nonbonding or σ orbitals in our active space. Both the computations performed by Sobolewski et al. and our CC2 computations suggest the involvement of a single, ππ ∗ state. Further, we also agree with their prediction that, once twisting occurs, the π and π ∗ orbitals localize on the phenol and benzotriazole structures resulting in an excited state that can be described as a charge-transfer excitation from the phenol to the benzotriazole (see Fig. 10). Our primary disagreement with the work of Sobolewski et al. is that they report a single conical intersection geometry that is most structurally similar to our MECI 2 geometry (see Fig. 5); our dynamics simulation suggests that this MECI is not responsible for the majority of the population transfer to the ground state. Instead, we find that the MECI 1 structure, which shows a smaller degree of twisting and a larger pyramidalization, is primarily responsible for the internal conversion of TINUVIN-P (see Table 1 for the relavent geometric parameters of the three MECIs). As a result, the importance of pyramidalization in the excited-state dynamics may be somewhat understated by Sobolewski et al. It should be noted, however, that

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MECI 2 is the most energetically favorable of the three MECI geometries that we identified, so on the basis of static calculations, it is reasonable to report this geometry alone; this only serves to illustrate the important role that dynamics simulations play in the understanding of excited-state processes. The agreement between the present work and that of Sobolewski et al. follows in part from the use of CC2 to understand the photophysics of TINUVIN-P in their work and the use of CC2 to validate the DFT-corrected CASCI method we used to generate the potential energy surfaces in our dynamics simulation. Out of necessity, early computational work on benzotriazole photostabilizers neglected the benzene ring that is part of the benzotriazole moiety. 35,36 The general aspects of the excitedstate dynamics are captured within this model: proton transfer is followed by twisting and pyramidalization leading to a seam of intersection where the excited state is quenched. However, some of the more specific conclusions drawn from computations on truncated models of benzotriazole photostabilizers are not representative of the full TINUVIN-P molecule. For example, Paterson et al. report a barrier to proton transfer. This could be a result of the truncated model or the method they apply (CASSCF); regardless, the presence of a barrier to proton transfer does not agree with our results, the results of Sobolewski et al., or the experimental rate constants. Another issue with the truncated model is that it destabilizes the ππ ∗ state relative to the nπ ∗ state. In the truncated models, after proton transfer occurs, the character of S1 changes from ππ ∗ to nπ ∗ . Our simulations as well as computations with CC2 do not predict the nπ ∗ excitations to be important to the excited-state dynamics on the S1 potential surface. 37 Other conclusions drawn from the truncated models are consistent with what we observe in our simulation. In particular, Paterson et al. discuss the internal conversion as a twisted intramolecular charge transfer (TICT) process (see Fig. 10). 36 The proton transfer reaction increases the charge transfer character of the S1 state; the twisting motion creates an S1 state that can be described as a charge transfer excitation from the phenyl to the benzotriazole moiety. Another possible description of this process is as a type of photoinduced proton coupled electron transfer (PCET); such an explanation has been

16


Page 16 of 42

Page 17 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


offered for similar molecules. 78 Due to the size of the TINUVIN-P molecule (28 atoms), a semi-classical trajectory-based approach is required to treat the nonadiabatic dynamics. We apply the AIMS method; the relationship between this approach and other commonly applied methods (surface-hopping, Ehrenfest, etc.) is discussed elsewhere. Since proton tunneling is not expected to be important in this system (due to the barrierless proton transfer reaction coordinate), we do not include this effect in the AIMS simulation. 79 Our simulation begins with 200 independent nuclear basis functions and, during the course of the simulation, approximately 1200 additional nuclear basis functions are “spawned.” The computational expense associated with this simulation necessitates a low-scaling electronic structure method such as the DFTcorrected FOMO-CASCI that we have applied. We have previously demonstrated that this method can be evaluated in O(N 2 ) time without introducing additional approximations (local molecular orbitals, approximate electron repulsion integrals, etc.). Absent from our simulation is the ground-state recovery dynamics; there are several reasons for this omission. First, to capture the recovery dynamics within the framework of our AIMS simulation, the 1200 spawned basis functions would need to be propagated in time for several picoseconds. This would increase the computational expense of the simulation by a considerable amount (likely greater than a factor of 10). Next, our choice of molecular orbitals for the CASCI wavefunction would need to be changed. The FON-HF orbitals we apply are remarkably well-behaved during the excited-state dynamics and can be evaluated with similar effort as canonical Hartree-Fock orbitals. Unfortunately, these orbitals do not perform well for ground state dynamics and discontinuities in the potential energy surface are frequently encountered as a result. An alternative to FON-HF orbitals is needed if the recovery of the ground state is considered. Finally, there is a more fundamental concern about the electronic structure method we use to generate the potential energy surface. In the present work, we have hidden the errors resulting from overestimated vertical excitation energies, since they tend to affect relative energetics on the ground state more significantly than the relative energetics on the

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

S1 state. Since only the excited-state dynamics are considered, these errors do not obviously manifest in our simulation. If the recovery of the ground state is included, one must consider the relative energetics of both the ground and excited state and it is no longer acceptable to treat the excited state accurately at the expense of the ground state. The extent to which these errors will affect a simulation of the ground state recovery dynamics remains an open question that we plan to pursue in the future. An additional limitation resulting from the failure to treat the recovery dynamics is that we cannot extract a complete theoretical transient absorption spectrum from our simulation. By stopping the AIMS trajectories once the population of the excited state is less than 0.01, the propagation of each nuclear basis function stops at different points in time. As a result, contributions to the transient absorption signal resulting from ground state absorption cannot be determined in a meaningful way.

Conclusions Upon photoexcitation, TINUVIN-P undergoes an ultrafast proton transfer along a barrierless reaction coordinate. Our simulation predicts proton transfer with a time constant of 20 fs. There is no stable keto state on the excited-state potential surface, therefore, twisting and pyramidalization about the central carbon-nitrogen bond in TINUVIN-P immediately follows proton transfer. Our simulation predicts ballistic wavepacket motion from the Franck-Condon point towards a seam of intersection with the ground state. Once the wavepacket reaches the seam, population transfer is highly efficient and the excited state is rapidly quenched. We predict an excited-state lifetime for TINUVIN-P of roughly 300 fs. Our simulation agrees well with experiment at long times, but significant discrepancies exist at short times. The experimental proton transfer time is in the range 60 to 80 fs. We attribute this difference to the ballistic wavepacket motion and barrierless path connecting the Franck-Condon point to the intersection seam. Since there is no stationary point associated with the S1 keto geometry, the assignment of a transient absorption signature to the S1

18


Page 18 of 42

Page 19 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


keto state likely includes progress towards the intersection seam (in addition to the proton transfer reaction). Our simulation of the excited-state dynamics of TINUVIN-P suggests a new analysis of the experimental result to determine if a 20 fs proton transfer time and ballistic wavepacket motion could be consistent with the measured transient spectrum. We have identified the source of the unusually fast internal conversion in TINUVIN-P to be the barrierless path to the intersection seam consisting of simultaneous twisting and pyramidalization. It is likely the presence of the nitrogen atom connecting the two ring systems that encourages pyramidalization. Of the three MECIs identified, the one with the largest degree of pyramidalization is the most kinetically accessible. This is in spite of the fact that the other two MECIs, which are more twisted and less pyramidalized, are energetically favored. TINUVIN-P is a prototype for the 2-(2′ -hydroxyphenyl)benzotriazole family of photostabilizers. It is expected that all of these molecules undergo excited-state proton transfer, however, it remains uncertain what factors affect their internal conversion processes. It is possible that other substituents (see Fig. 1) change the nature of the excited-state dynamics relative to TINUVIN-P, even though the electronic structure of these molecules should remain quite similar. For example, the large nuclear motions associated with simultaneous twisting and pyramidalization might be disfavored by the addition of heavy t-butyl groups. Work to study the excited-state dynamics of some molecules related to TINUVIN-P is currently underway in our laboratory. The combination of the DFT-corrected CASCI potential energy surfaces and AIMS dynamics is a powerful tool for understanding the nonadiabatic dynamics of photoexcited molecules. Due to the low scaling of the method, O(N 2 ), it can be easily applied to the study of these larger molecules. Another remaining question is how solvent affects the internal conversion process. It is possible that the presence of solvent will introduce disorder that slows the excited-state proton transfer reaction. Solvent friction may also affect the mechanism of internal conversion by hindering the twisting and pyramidalization motions. In the present case, inclusion of solvent through a hybrid quantum mechanics/molecular mechanics (QM/MM) approach should provide a reasonable descrip-

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 42

tion of those effects. Finally, we are working to extend the simulations in order to study the ground state recovery dynamics and directly predict the transient absorption spectrum.

Acknowledgement 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 number SM-2015-289297.

Supporting Information Available Energies and optimized geometries of TINUVIN-P MECIs are provided.

This material is

available free of charge via the Internet at http://pubs.acs.org/.

References (1) Sengupta, P. K.; Kasha, M. Excited State Proton-Transfer Spectroscopy of 3Hydroxyflavone and Quercetin. Chem. Phys. Lett. 1979, 68, 382–385. (2) McMorrow, D.; Kasha, M. Intramolecular Excited-State Proton-Transfer in 3Hydroxyflavone - Hydrogen-Bonding Solvent Perturbations. J. Phys. Chem. 1984, 88, 2235–2243. (3) Strandjord, A. J.; Barbara, P. The Proton-Transfer Kinetics of 3-Hydroxyflavone: Solvent Effects. J. Phys. Chem. 1985, 89, 2355–2361. (4) Meredith, P.; Riesz, J. Radiative Relaxation Quantum Yields for Synthetic Eumelanin. Photochem. Photobiol. 2004, 79, 211–216.

20


Page 21 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Simkovitch, R.; Huppert, D. Excited-State Intramolecular Proton Transfer of the Natural Product Quercetin. J. Phys. Chem. B 2015, 119, 10244–10251. (6) Baker, L. A.; Horbury, M. D.; Greenough, S. E.; Allais, F.; Walsh, P. S.; Habershon, S.; Stavros, V. G. Ultrafast Photoprotecting Sunscreens in Natural Plants. J. Phys. Chem. Lett. 2016, 7, 56–61. (7) Heller, H. J.; Blattmann, H. R. Some Aspects of the Light Protection of Polymers. Pure Appl. Chem. 1972, 30, 145–166. (8) Heller, H. J.; Blattmann, H. R. Some Aspects of Stabilization of Polymers Against Light. Pure Appl. Chem. 1973, 36, 141–162. (9) Tan, E. M. M.; Hilbers, M.; Buma, W. J. Excited-State Dynamics of Isolated and Microsolvated Cinnamate Based UVB Sunscreens. J. Phys. Chem. Lett. 2014, 5, 2464– 2468. (10) Baker, L. A.; Greenough, S. E.; Stavros, V. G. A Perspective on the Ultrafast Photochemistry of Solution-Phase Sunscreen Molecules. J. Phys. Chem. Lett. 2016, 7, 4655–4665. (11) Luo, J.; Liu, Y.; Yang, S.; Flourat, A. L.; Allais, F.; Han, K. Ultrafast Barrierless Photoisomerization and Strong Ultraviolet Absorption of Photoproducts in Plant Sunscreens. J. Phys. Chem. Lett. 2017, 8, 1025–1030. (12) Formosinho, S. J.; Arnaut, L. G. Excited-State Proton Transfer Reactions II. Intramolecular Reactions. J. Photochem. Photobiol. A 1993, 75, 21–48. (13) Douhal, A.; Lahmani, F.; Zewail, A. H. Proton-Transfer Reaction Dynamics. Chem. Phys. 1996, 207, 477–498. (14) Jipa, S.; Zaharescu, T.; Setnescu, R.; Setnescu, T.; Brites, M. J. S.; Silva, A. M. G.;

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Marcelo-Curto, M. J.; Gigante, B. Chemiluminescence Study of Thermal and Photostability of Polyethylene. Polym. Int. 1999, 48, 414–420. (15) Basfar, A. A.; Idriss Ali, K. M. Natural Weathering Test for Films of Various Formulations of Low Density Polyethylene (LDPE) and Linear Low Density Polyethylene (LLDPE). Polym. Degrad. Stab. 2006, 91, 437–443. (16) Skowronski, T.; Rabek, J. F.; R˚ anby, B. Photodegradation of Some Poly(Vinyl Chloride) (PVC) Blends:

PVC/Ethylene-Vinyl Acetate (EVA) Copolymers and

PVC/Butadiene-Acrylonitrile (NBR) Copolymers. Polym. Eng. Sci. 1984, 24, 278– 286. (17) Vinhas, G. M.; Souto Maior, R. M.; de Almeida, Y. M. B. Radiolytic Degradation and Stabilization of Poly(Vinyl Chloride). Polym. Degrad. Stab. 2004, 83, 429–433. (18) Pimentel Real, L. E.; Ferraria, A. M.; Botelho do Rego, A. M. Comparison of Different Photo-Oxidation Conditions of Poly(Vinyl Chloride) for Outdoor Applications. Polym. Test. 2008, 27, 743–751. (19) Chaochanchaikul, K.; Sombatsompop, N. Stabilizations of Molecular Structures and Mechanical Properties of PVC and Wood/PVC Composites by Tinuvin and TiO2 Stabilizers. Polym. Eng. Sci. 2011, 51, 1354–1365. (20) Chaochanchaikul, K.; Rosarpitak, V.; Sombatsompop, N. Photodegradation Profiles of PVC Compound and Wood/PVC Composites Under UV Weathering. Express Polym. Lett. 2013, 7, 146–160. (21) Singh, R. P.; Desai, S. M.; Solanky, S. S.; Thanki, P. N. Photodegradation and Stabilization of Styrene-Butadiene-Styrene Rubber. J. Appl. Polym. Sci. 2000, 75, 1103–1114. (22) Singh, R. P.; Tomer, N. S.; Bhadraiah, S. V. Photo-Oxidation Studies on Polyurethane

22


Page 22 of 42

Page 23 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Coating: Effect of Additives on Yellowing of Polyurethane. Polym. Degrad. Stab. 2001, 73, 443–446. (23) Evans, B. A.; Water, P. J. Photoprotection of Wool by Application of Ultraviolet Absorber- Polymer Mixtures to its Surface. Text. Res. J. 1981, 51, 432–434. (24) Kehayoglou, A. H.; Tsatsaroni, E. G. Dyeing of Polyester Fabrics with Disperse Dyes in the Presence of a UV-Absorber. Dyes Pigm. 1993, 23, 53–63. (25) Tsatsaroni, E. G.; Kehayoglou, A. H. Dyeing of Polyester with CI Disperse Yellow 42 in the Presence of Various UV-Absorbers. Part II. Dyes Pigm. 1995, 28, 123–130. (26) Kehayoglou, A. H.; Tsatsaroni, E. G.; Eleftheriadis, I.; Loufakis, K. C.; Kyriazis, L. E. Effectiveness of Various UV-Absorbers in Dyeing of Polyester with Disperse Dyes. Part III. Dyes Pigm. 1997, 34, 207–218. (27) Tsatsaroni, E. G.; Kehayoglou, A. H.; Eleftheriadis, I.; Kyriazis, L. E. Effectiveness of Various UV-Absorbers on the Dyeing of Polyester with Disperse Dyes. Part IV. Dyes Pigm. 1998, 38, 65–75. (28) Tsatsaroni, E. G.; Eleftheriadis, I. UV-Absorbers in the Dyeing of Polyester with Disperse Dyes. Dyes Pigm. 2004, 61, 141–147. (29) Flom, S. R.; Barbara, P. F. The Photodynamics of 2-(2-Hydroxy-5-methylphenyl)benzotriazole in Low-Temperature Organic Glasses. Chem. Phys. Lett. 1983, 94, 488. (30) Wiechmann, M.; Port, H.; Laermer, F.; Frey, W.; Elaesser, T. Excited-State Proton Transfer in a Benzotriazole Photostabilizer Investigated by Femtosecond Spectroscopy. Chem. Phys. Lett. 1990, 165, 28–34. (31) Wiechmann, M.; Port, H.; Frey, W.; Lärmer, F.; Elässer, T. Time-Resolved Spectroscopy on Ultrafast Proton Transfer in 2-(2’-Hydroxy-5’-methylphenyl)benzotriazole in Liquid and Polymer Environments. J. Phys. Chem. 1991, 95, 1918. 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(32) Frey, W.; Elaesser, T. Femtosecond Intramolecular Proton Transfer of Vibrationally Hot Molecules in the Electronic Ground State. Chem. Phys. Lett. 1992, 189, 565–570. (33) Chudoba, C.; Lutgen, S.; Jentzsch, T.; Riedle, E.; Woerner, M.; Elsaesser, T. Femtosecond Studies of Vibrationally Hot Molecules Produced by Intramolecular Proton Transfer in the Excited State. Chem. Phys. Lett. 1995, 240, 35–41. (34) Chudoba, C.; Riedle, E.; Pfeiffer, M.; Elsaesser, T. Vibrational Coherence in Ultrafast Excited State Proton Transfer. Chem. Phys. Lett. 1996, 263, 622–628. (35) Estévez, C. M.; Bach, R. D.; Hass, K. C.; Schneider, W. F. Novel Structural Modifications Associated with the Highly Efficient Internal Conversion of 2-(2′ Hydroxyphenyl)benzotriazole Ultraviolet Stabilizers. J. Am. Chem. Soc. 1997, 119, 5445–5446. (36) Paterson, M. J.; Robb, M. A.; Blancafort, L.; DeBellis, A. D. Theoretical Study of Benzotriazole UV Photostability: Ultrafast Deactivation Through Coupled Proton and Electron Transfer Triggered by a Charge-Transfer State. J. Am. Chem. Soc. 2004, 126, 2912–2922. (37) Sobolewski, A. L.; Domcke, W.; Hättig, C. Photophysics of Organic Photostabilizers. Ab Initio Study of the Excited-State Deactivation Mechanisms of 2-(2′ Hydroxyphenyl)benzotriazole. J. Phys. Chem. A. 2006, 110, 6301–6306. (38) Paul, B. K.; Guchhait, N. A Computational Insight into the Photophysics of a Potent UV Absorber Tinuvin P: Critical Evaluation of the Role of Charge Transfer Interaction and Topological Properties of the Intramolecular Hydrogen Bonding. Comp. Theor. Chem. 2011, 966, 250–258. (39) Sobolewski, A. J.; Domcke, W. In Conical Intersections: Electronic Structure, Dynamics and Spectroscopy; Domcke, W., Yarkony, D. R., Köppel, H., Eds.; World Scientific

24


Page 24 of 42

Page 25 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Publishing: Singapore, 2011; Vol. 17; Chapter Efficient Excited-State Deactivation in Organic Chromophores and Biologically Relevant Molecules: Role of Electron and Proton Transfer Processes, pp 51–82. (40) Riedle, E.; Bradler, M.; Wenninger, M.; Sailer, C. F.; Pugliesi, I. Electronic Transient Spectroscopy from the Deep UV to the NIR: Unambiguous Disentanglement of Complex Processes. Faraday Discuss. 2013, 163, 139–158. (41) Lochbrunner, S.;

Wurzer, A. J.;

Riedle, E. Microscopic Mechanism of Ul-

trafast Excited-State Intramolecular Proton Transfer:

A 30-fs Study of 2-(2′ -

Hydroxyphenyl)benzothiazole. J. Phys. Chem. A 2003, 107, 10580. (42) Sekikawa, T.; Schalk, O.; Wu, G.; Boguslavskiy, A. E.; Stolow, A. Initial Processes of Proton Transfer in Salicylideneaniline Studied by Time-Resolved Photoelectron Spectroscopy. J. Phys. Chem. A 2013, 117, 2971–2979. (43) Lee, J.; Kim, C. H.; Joo, T. Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction. J. Phys. Chem. A 2013, 117, 1400–1405. (44) Barbatti, M.; Aquino, A. J. A.; Lischka, H.; Schriever, C.; Lochbrunner, S.; Riedle, E. Ultrafast Internal Conversion Pathway and Mechanism in 2-(2′ Hydroxyphenyl)benzothiazole: A Case Study for Excited-State Intramolecular Proton Transfer Systems. Phys. Chem. Chem. Phys. 2009, 11, 1406–1415. (45) Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. Intramolecular Proton Transfer and Excited-State Relaxation in 2-(2-Hydroxyphenyl)benzothiazole. J. Am. Chem. Soc. 1980, 102, 5631–5635. (46) Mohammed, O. F.; Luber, S.; Batista, V. S.; Nibbering, E. T. J. Ultrafast Branching of Reaction Pathways in 2-(2′ -Hydroxyphenyl)benzothiazole in Polar Acetonitrile Solution. J. Phys. Chem. A 2011, 115, 7550–7558.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(47) Mart´ınez, T. J.; Ben-Nun, M.; Levine, R. D. Multi-Electronic-State Molecular Dynamics: A Wave Function Approach with Applications. J. Phys. Chem. 1996, 100, 7884–7895. (48) Ben-Nun, M.; Mart´ınez, T. J. Nonadiabatic Molecular Dynamics: Validation of the Multiple Spawning Method for a Multidimensional Problem. J. Chem. Phys. 1998, 108, 7244–7257. (49) Pijeau, S.; Hohenstein, E. G. Improved Complete Active Space Configuration Interaction Energies with a Simple Correction from Density Functional Theory. J. Chem. Theory Comput. 2017, 13, 1130–1146. (50) Gidopoulos, N.; Theophilou, A. Hartree-Fock Equations Determining the Optimum Set of Spin Orbitals for the Expansion of Excited States. Philos. Mag. B 1993, 69, 1067–1074. (51) Gidopoulos, N. I.; Papaconstantinou, P. G.; Gross, E. K. U. Ensemble-Hartree-Fock Scheme for Excited States. The Optimized Effective Potential Method. Physica B 2002, 318, 328–332. (52) Granucci, G.; Toniolo, A. Molecular Gradients for Semiempirical CI Wavefunctions with Floating Occupation Molecular Orbitals. Chem. Phys. Lett. 2000, 325, 79–85. (53) Slav´ıˇcek, P.; Mart´ınez, T. J. Ab Initio Floating Occupation Molecular Orbital-Complete Active Space Configuration Interaction: An Efficient Approximation to CASSCF. J. Chem. Phys. 2010, 132, 234102. (54) Wesolowski, T. A.; Warshel, A. Frozen Density Functional Approach for Ab Initio Calculations of Solvated Molecules. J. Phys. Chem. 1993, 97, 8050–8053. (55) Manby, F. R.; Stella, M.; Goodpaster, J. D.; Miller, T. F. A Simple, Exact DensityFunctional-Theory Embedding Scheme. J. Chem. Theory Comput. 2012, 8, 2564–2568. 26


Page 26 of 42

Page 27 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(56) Coe, J. D.; Mart´ınez, T. J. Ab Initio Multiple Spawning Dynamics of Excited State Intramolecular Proton Transfer: The Role of Spectroscopically Dark States. Mol. Phys. 2008, 106, 537–545. (57) Hack, M. D.; Wensmann, A. M.; Truhlar, D. J.; Ben-Nun, M.; Mart´ınez, T. J. Comparison of Full Multiple Spawning, Trajectory Surface Hopping, and Converged Quantum Mechanics for Electronically Nonadiabatic Dynamics. J. Chem. Phys. 2001, 115, 1172– 1186. (58) Hohenstein, E. G.; Bouduban, M. E. F.; Song, C.; Luehr, N.; Ufimtsev, I. S.; Mart´ınez, T. J. Analytic First Derivatives of Floating Occupation Molecular OrbitalComplete Active Space Configuration Interaction on Graphical Processing Units. J. Chem. Phys. 2015, 143, 014111. (59) Hohenstein, E. G. Analytic Formulation of Derivative Coupling Vectors for Complete Active Space Configuration Interaction Wavefunctions with Floating Occupation Molecular Orbitals. J. Chem. Phys. 2016, 145, 174110. (60) Ufimtsev, I. S.; Mart´ınez, T. J. Quantum Chemistry on Graphical Processing Units. 1. Strategies for Two-Electron Integral Evaluation. J. Chem. Theory Comput. 2008, 4, 222–231. (61) Ufimtsev, I. S.; Mart´ınez, T. J. Quantum Chemistry on Graphical Processing Units. 2. Direct Self-Consistent-Field Implementation. J. Chem. Theory Comput. 2009, 5, 1004–1015. (62) Ufimtsev, I. S.; Mart´ınez, T. J. Quantum Chemistry on Graphical Processing Units. 3. Analytical Energy Gradients, Geometry Optimization, and First Principles Molecular Dynamics. J. Chem. Theory Comput. 2009, 5, 2619–2628. (63) Luehr, N.; Ufimtsev, I. S.; Mart´ınez, T. J. Dynamic Precision for Electron Repulsion

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Integral Evaluation on Graphical Processing Units (GPUs). J. Chem. Theory Comput. 2011, 7, 949–954. (64) Titov, A. V.; Ufimtsev, I. S.; Luehr, N.; Mart´ınez, T. J. Generating Efficient Quantum Chemistry Codes for Novel Architectures. J. Chem. Theory Comput. 2013, 9, 213–221. (65) Coe, J. D.; Levine, B. G.; Mart´ınez, T. J. Ab Initio Molecular Dynamics of ExcitedState Intramolecular Proton Transfer Using Multireference Perturbation Theory. J. Phys. Chem. A 2007, 111, 11302–11310. (66) Aquino, A. J. A.; Plasser, F.; Barbatti, M.; Lischka, H. Ultrafast Excited-State Proton Transfer Processes: Energy Surfaces and on-the-Fly Dynamics Simulations. Croat. Chem. Acta 2009, 82, 105–114. (67) Schriever, C.; Lochbrunner, S.; Ofial, A. R.; Riedle, E. The Origin of Ultrafast Proton Transfer: Multidimensional Wave Packet Motion Vs. Tunneling. Chem. Phys. Lett. 2011, 503, 61. (68) The “spawning” geometries in an AIMS simulation are roughly analogous to “hopping” geometries in surface-hopping dynamics. (69) Domcke, W. In Conical Intersections: Electronic Structure, Dynamics and Spectroscopy; Domcke, W., Yarkony, D. R., Köppel, H., Eds.; World Scientific Publishing: Singapore, 2004; Vol. 15; Chapter Generic Aspects of the Dynamics at Conical Intersections: Internal Conversion, Vibrational Relaxation and Photoisomerization, pp 395–427. (70) Lenz, K.; Pfeiffer, M.; Lau, A.; Elsaesser, T. Resonance Raman and Femtosecond Absorption Studies of Vibrational Relaxation Initiated by Ultrafast Intramolecular Proton Transfer. Chem. Phys. Lett. 1994, 229, 340–346.

28


Page 28 of 42

Page 29 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(71) Pfeiffer, M.; Lenz, K.; Lau, A.; Elsaesser, T. Resonance Raman Studies of Heterocyclic Aromatic Compounds Showing Ultrafast Intramolecular Proton Transfer. J. Raman Spec. 1995, 26, 607–615. (72) Kim, C. H.; Joo, T. Coherent Excited State Intramolecular Proton Transfer Probed by Time-Resolved Fluorescence. Phys. Chem. Chem. Phys. 2009, 11, 10266. (73) Lochbrunner, S.; Schultz, T.; Schmitt, M.; Shaffer, J. P.; Zgierski, M. Z.; Stolow, A. Dynamics of Excited-State Proton Transfer Systems via Time-Resolved Photoelectron Spectroscopy. J. Chem. Phys. 2001, 114, 2519–2522. (74) Rini, M.; Kummrow, A.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. Femtosecond Mid-Infrared Spectroscopy of Condensed Phase Hydrogen-Bonded Systems as a Probe of Structural Dynamics. Faraday Discuss. 2002, 122, 27–40. (75) Rini, M.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. Ultrafast Vibrational Relaxation Processes Induced by Intramolecular Excited State Hydrogen Transfer. Chem. Phys. Lett. 2003, 374, 13–19. (76) Aquino, A. J. A.; Lischka, H.; Hättig, C. Excited-State Intramolecular Proton Transfer: A Survey of TDDFT and RI-CC2 Excited-State Potential Energy Surfaces. J. Phys. Chem. A 2005, 109, 3201–3208. (77) Tuna, D.; Lefrancois, D.; Wola´ nski, L.; Gozem, S.; Schapiro, I.; Andruniów, T.; Dreuw, A.; Olivucci, M. Assessment of Approximate Coupled-Cluster and AlgebraicDiagrammatic-Construction Methods for Ground- and Excited-State Reaction Paths and the Conical-Intersection Seam of a Retinal-Chromophore Model. J. Chem. Theory Comput. 2015, 11, 5758–5781. (78) Luber, S.; Adamczyk, K.; Nibbering, E. T. J.; Batista, V. S. Photoinduced Proton Coupled Electron Transfer in 2-(2′ -Hydroxyphenyl)benzothiazole. J. Phys. Chem. A 2013, 117, 5269–5279. 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(79) Ben-Nun, M.; Mart´ınez, T. J. A Multiple Spawning Approach to Tunneling Dynamics. J. Chem. Phys. 2000, 112, 6113–6121.

30


Page 30 of 42

Page 31 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1: Geometric parameters (defined in Fig. 2) relevant to the photochemistry of TINUVIN-P at several geometries. Geometry (symmetry) S0 minimum (Cs ) S1 keto (Cs ) MECI 1 (C1 ) MECI 2 (C1 ) MECI 3 (C1 )

ROH (˚ A) 0.991 1.803 1.981 2.229 3.276

RNH (˚ A) 1.758 1.031 1.038 1.031 1.019

Dihedral angle (◦ ) 0.0 0.0 70.7 81.0 88.0

31


Pyramidalization angle (◦ ) 0.0 0.0 44.2 21.6 17.7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Page 32 of 42

Page 33 of 42





Page 34 of 42

Page 35 of 42





Page 36 of 42

Page 37 of 42





Page 38 of 42

Page 39 of 42





Page 40 of 42

Page 41 of 42





Page 42 of 42

Excited-State Dynamics of a Benzotriazole Photostabilizer: 2-(2

Recommend Documents