Resolving the Singlet Excited State Manifold of Benzophenone by First

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Spectroscopy and Excited States

Resolving the singlet excited states manifold of benzophenone by first-principles simulations and ultrafast spectroscopy Javier Segarra-Martí, Elena E. Zvereva, Marco Marazzi, Johanna Brazard, Elise Dumont, Xavier Assfeld, Stefan Haacke, Marco Garavelli, Antonio Monari, Jérémie Léonard, and Ivan Rivalta J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.7b01208 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Journal of Chemical Theory and Computation

Resolving the singlet excited states manifold of benzophenone by first-principles simulations and ultrafast spectroscopy Javier Segarra-Martí,*ab Elena Zvereva,cd Marco Marazzi,*ce Johanna Brazard,f Elise Dumont,a Xavier Assfeld,b Stefan Haacke,f Marco Garavelli,g Antonio Monari,b Jérémie Léonard,f and Ivan Rivalta*a a

Université de Lyon, École Normale Supérieure de Lyon, CNRS, Université Claude Bernard

Lyon 1, Laboratoire de Chimie UMR 5182, F-69342, Lyon, France b

Present address: Imperial College London, Department of Chemistry, SW7 2AZ London, UK

c

Université de Lorraine and CNRS, LPCT UMR 7019. Nancy, 54000 France

d

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre, Russian

Academy of Sciences, Arbuzov str. 8, 420088 Kazan, Russia. e

Present address: Departamento de Química, Centro de Investigación en Síntesis Química

(CISQ), Universidad de La Rioja, Madre de Dios, 53, 26006 Logroño, Spain f

Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg

and Labex NIE, UMR 7504, F-67000 Strasbourg, France g

Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del

Risorgimento 4, I-40136 Bologna, Italy

*

E-mail: [email protected]; [email protected] ; ivan.rivalta@ens-

lyon.fr KEYWORDS: Benzophenone, ultrafast electronic spectroscopy, excited state absorption, wavefunction methods, time-dependent density functional theory.

ACS Paragon Plus Environment

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ABSTRACT Accurate characterization of the high-lying excited state manifolds of organic molecules is of fundamental importance for the interpretation of the rich response detected in time-resolved nonlinear electronic spectroscopies. Here, we have characterized the singlet excited states manifold of benzophenone (BP), a versatile organic photoinitiator and a well-known DNA photosensitizer. Benchmarks of various multiconfigurational/multireference (RASSCF/PT2) and time-dependent density functional theory (TD-DFT) approaches allowed assignments of experimental linear absorption signals of BP in the ultraviolet (UV) region, with unprecedented characterization of ground state absorptions in the far UV. Experimental transient absorption spectra obtained by UV-Vis pump-probe spectroscopy at very short time-delays are shown to be directly comparable to theoretical estimates of excited state absorptions (from the low-lying nOπ* and ππ* singlet states) in the Franck-Condon region. Multireference computations provided reliable interpretation of the PP spectra, with TD-DFT results yielding a fair agreement as long as electronic transitions featuring double excitations contributions are not involved. These results lay the groundwork for further computational studies and interpretation of experimental nonlinear electronic spectra of benzophenone in more complex systems, such as BP/DNA adducts.

1. INTRODUCTION Benzophenone (BP) is an extremely versatile organic molecule known for its use in applications ranging from inks, imaging and clear coatings for the printing and packaging industry,1 to therapeutic purposes.2-5 Its most known attribute is related to its electronic structure, and particularly to the featured long-lived lowest-lying triplet state that can be easily generated at long wavelengths (>320 nm).6 This makes of BP an ideal photoinitiator,7 as it was proven in photodynamic therapy: BP acts as a non-covalently bound photosensitizer, producing several photo-lesions to DNA.4 Nevertheless, routine experimental characterization techniques found extreme difficulties to investigate BP/DNA binding modes,8 and only recent theoretical works have shown the possibility of two stable interacting modes: the minor groove mode and the less common double insertion mode, characterized by the simultaneous displacement of a full nucleobase pair from the Watson-Crick pairing.2,

3, 9, 10

In this context, state-of-the-art optical

nonlinear spectroscopic techniques in conjunction with theoretical modeling could play a crucial


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role in uncovering the non-covalent BP binding to DNA and unequivocally disentangling particular arrangements/intercalations. However, little is known about the excited states manifold of BP that can be probed by advanced optical spectroscopies, while DNA/RNA nucleobases electronic structures have been widely investigated.11-13 Ultrafast pump-probe electronic spectroscopy has been over the last two decades the favored tool to track photoinduced phenomena with high temporal resolution (of the order of femtoseconds), providing invaluable insights into the photophysics of several molecular systems in complex environments.11,

14-16

Standard pump-probe optical technique involves two delayed

laser pulses (pump and probe) yielding a third-order nonlinear response of the sample. For a pump pulse centered at a given frequency (or wavelength) and a probe pulse incoming after a certain time delay (namely population or waiting time) the nonlinear signals are gathered in a “monodimensional” (1D) spectrum, reporting the signal intensities as function of the probe frequencies (or wavelengths). The wealth of information contained in the third-order nonlinear response is partially hidden in a 1D spectrum due to the absence of spectral resolution along the pump pulse frequency. Two-dimensional electronic spectroscopy (2DES) techniques17-20 overcome this limitation by employing multi-pulse sequences that allow recording the nonlinear response (for a given population time) in a 2D map, where the signals are reported as function of both pump and probe frequencies. 2DES spectra, thus, contain direct information on electronic couplings and energy/electron processes (appearing as off-diagonal signals in a 2D map) and can be very powerful in characterizing chromophore arrangements and their photophysics in multichromophoric systems.20-23 Time-resolved (PP and 2DES) optical spectroscopies involving pulses in the ultraviolet (UV) region have taken longer to be developed as compared to those in the visible (Vis) spectral range, due to the need to employ more advanced optical technologies in this higher-energy regime.24, 25 Experimental 2DUV spectra are just appearing in the literature,26, 27

indicating a new route for resolving structural arrangements in multichromophoric systems

involving UV-active units, such as BP and nucleobases in BP/DNA complexes, in the near future. Due to the large amount of information provided in nonlinear 1D or 2D experiments, which can often span energy windows over 9 eV broad and featuring more than a hundred electronic excited states,28 the interpretation of the spectra obtained strongly relies on theoretical models that aid in their interpretation. These range from parameterized model Hamiltonians,29-32


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like the Frenkel excitonic Hamiltonian and related methods including electrostatic fluctuations,33, 34

which make use of parameters obtained from gas-phase ab initio quantum-chemical

computations on monomeric species, to computational protocols based on a sum-over-states (SOS) approach35 where the ab initio calculations are carried out in dimeric/multimeric hybrid quantum mechanics/molecular mechanics (QM/MM) schemes and then coupled with non-linear response theory to properly account for inter-monomer and monomer-solvent strong interactions.22, 36-39 It is important also to note that the electronic excited states involved in these experiments span all kinds of different excitations, ranging from charge transfer, to localized/delocalized excitations and/or doubly excited states, meaning that the method employed for their characterization should be able to describe their energies and transition dipole moments on an even footing. For this reason, multiconfigurational methods such as those based on complete/restricted active space self consistent field40, 41 and second-order perturbation theory (CASPT2/RASPT2)42 are often preferred for simulating third-order optical nonlinear spectra to other computationally cheaper and more popular approaches like time-dependent density functional theory (TD-DFT).43 This is based on the reported difficulties faced by TD-DFT on several of the required fronts,44 namely: the lack of a robust framework to account for doubly excited states45 even though plenty of novel ideas are emerging to solve this particular issue,46-49 the strong dependence of errors in the excitation energies estimates on the particular choice of functional, type of excitation and system under study,50, 51 and the singularities found in the linear response treatment when computing transition dipole moments among different electronic excited states.52 In this work, both ground (linear) and excited (nonlinear) state vertical absorptions arising from the ground state equilibrium geometry of BP are computed, by employing various of multiconfigurational CASPT2/RASPT2 schemes and TD-DFT approaches. The electronic excitations computed in vacuum are thoroughly analyzed and compared with the experimental spectra obtained in various solvents. A detailed analysis of the active space dependence in RASSCF approaches,53 the functional dependence in TD-DFT, and of the influence of basis set contractions is carried out. A thorough comparison between multiconfigurational wave function methods and TD-DFT is then carried out to ascertain if the latter suffices to describe the main excited state absorption (ESA) signals featuring in 1D pump-probe and 2D spectra of BP.


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2. COMPUTATIONAL & EXPERIMENTAL DETAILS 2.1 Computational details All calculations were performed in the gas phase. The possible influence of surrounding media to the computed vertical excitation energy (i.e., absorption wavelength) and oscillator strength (f) was neglected, as inclusion of weakly interacting aprotic/apolar solvent molecules (like hexane or cyclohexane) within continuum solvation models induces just a small (4−10 nm) bathochromic shift of the entire linear absorption spectrum,54-56 and the experimental data reported here indicate, in general, small solvatochromic shifts in the spectral regions investigated. The ground state (S0) geometry of BP (see Figure S1 in the SI) was optimized at the second-order Møller-Plesset perturbation theory (MP2)57-61 level employing the 6-31G** basis set62, 63 with the Gaussian09 package.64 The obtained stationary point was further corroborated as a minimum by frequency calculations. On top of the BP equilibrium geometry, vertical transitions from the S0 state (S0¦Sn, with n>0) and from the Sn excited states (Sn¦Sm, with n=1-3 and m>n) have been computed at different levels of theory, without imposing symmetry. These vertical transitions are representative of the excitations accessible in the Franck-Condon region (at the ground state equilibrium) and have been here thus compared directly with linear absorption (S0¦Sn transitions) and time-resolved (Sn¦Sm transitions) experimental spectra, i.e. PP spectra collected at zero delay time. 2.1.1. Multiconfigurational wavefunction methods. Multiconfigurational

complete/restricted

active

space

self-consistent

field

(CASSCF65/RASSCF66) computations were carried out employing different excitation schemes: i) a CAS scheme comprising all π bonding and anti-bonding orbitals plus the carbonyl lone pair, i.e. CAS (16, 15), which could be presently employed only in state-average procedures involving just few excited states (due to high computational demands); ii) a fully RAS1/RAS3 scheme, depicted in Figure 1 in orange, where all occupied and unoccupied valence π orbitals plus the carbonyl lone pair were considered in the RAS1 and RAS3 subspaces, respectively, allowing the permutation of four, five, and six holes/electrons, and giving rise to the RAS(4,8|0,0|4,7), RAS(5,8|0,0|5,7) and RAS(6,8|0,0|6,7) active spaces, respectively; and iii) a mixed RAS1/RAS2/RAS3 scheme (RAS2 subspace depicted in green in Figure 1) yielding the RAS(4,6|4,3|4,6) active space where the orbitals included in the RAS2 are treated as a complete


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active subspace. An equal weights state averaging procedure was carried out comprising the lowest 100 roots in the computation to ensure the presence of all relevant states in the UV/Vis window of interest (for pump-probe spectra) in the RAS1/RAS3 schemes (ii), whereas up to 60 roots could be employed in the RAS1/RAS2/RAS3 computations (iii) due to the high computational cost. Atomic Natural Orbital basis sets with a large contraction (ANO-L)67, 68 were employed throughout in their double-ζ- (VDZP), triple-ζ (VTZP) and quadruple-ζ (VQZP) polarized contractions (see SI). All multiconfigurational wave function computations were carried out employing the MOLCAS 8 package.69 Other computational details are given in the SI.

Figure 1. BP molecular orbitals in the full CAS(16,15) active space and schematic representation of the different restricted active spaces (RAS) tackled in the present work. Molecular orbitals in the green background are those included in the RAS2 subspace of the RAS(4,6|4,3|4,6) scheme where orbitals in the orange background denote those remaining in the RAS1 (top panel, occupied orbitals) and RAS3 (bottom panel, virtual orbitals) subspaces. RAS(X,8|0,0|X,7) active spaces involve all 15 orbitals depicted with empty RAS2 subspace and with X (equal to 4, 5 or 6) denoting the number of holes/electrons in the RAS1/RAS3 subspaces. Orbital labeling is given to facilitate excited state assignment, with nO denoting the oxygen lone pair of the carbonyl group, H and L denoting HOMO and LUMO benzene-like frontier molecular orbitals and B1 and B2 subscripts used to differentiate among equivalent orbitals delocalized over the two benzenic moieties and displaying different orbital phases.


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If not specified, RASPT2 calculations refer to the RASPT2(5,8|0,0|5,7)/ANO-L-VTZP scheme, which is assigned as the preferential computational scheme in this work. Molecular orbitals (MOs) of π-type are labeled using single benzene-like molecule highest occupied (HOMO, H) and lowest unoccupied (LUMO, L) molecular orbitals as reference,70,

71

and

adopting the B1 and B2 subscripts to indicate analogous orbitals delocalized over the two benzenic moieties and displaying different orbital phases. This labeling is preferred here despite the HOMO orbital of BP is the oxygen lone pair of the carbonyl group (nO). The state assignment at RASSCF (or CASSCF) level is based on the leading configuration state function (CSF) of a given state, as determined by its weight to the overall multiconfigurational wave function. Two or more CSFs are included when they feature similar weights on the same state for completeness. 2.1.2. TD-DFT calculations. To test the adequacy of the Time-Dependent Density Functional Theory (TD-DFT)72-75, linear absorption spectrum of BP in the optimized ground state geometry was calculated with the Gaussian09 program.64 The lowest-lying 100 singlet states were taken into account and the excitation energies of the S1, S2, S3, S4, S9 and S15 excited states referred to S0 were straightforwardly compared with RASPT2(5,8|0,0|5,7)/ANO-L-VTZP reference values (Figures 2-3). Several hybrid functionals expected to produce good spectroscopic parameters54,

76

have

been used: i) Becke Three-Parameter Hybrid Functionals: B3PW91,77-79 B3P86 and B3LYP;77, 80, 81 ii) Functionals Including Dispersion or with τ-Dependent Gradient-Corrected Correlation and Long-Range-Corrected Functionals: APFD,82 wB97XD,83 CAMB3LYP84, M062X85 and TPSSh;86-88 iii) Other Hybrid Functionals: X3LYP,89 B98,90, 91 PBE092 and BHandHLYP.93 The TPSSh, B3-group, PBE0 and BHandHLYP functionals were taken as they heavily differ in the amount of non-local Fock-exchange included (10%, 20%, 25% vs. 50 %), which is known to influence excitation energies and excited state properties rather strongly.76 CAMB3LYP was taken as it better predicts excitation energies of charge-transfer (CT) character. Influence from the inclusion of dispersion corrections can be concluded from the APFD, wB97XD and M062X functionals.


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The augmented Pople's Triple-ζ basis set 6-311++G** was used,94 since inclusion of polarization and diffuse functions are important for excited state simulations of a such small system as benzophenone.54, 55 This was evidenced by comparing such basis set with the 6-31G**. To describe the excited state absorption signals, the quadratic response functions formalism95, 96 within the DFT framework was used as implemented in the TeraChem code.97-99 2.2 Experimental details Benzophenone (BP), hexane, cyclohexane and methanol spectroscopy grade solvents from Sigma-Aldrich were used without further purification. The molar extinction coefficients were determined using a UV/Vis Perkin Elmer Lambda 950 spectrophotometer, adopting a 10-mm quartz cell and after solubilizing a precise amount of benzophenone in the various solvents. The ultrafast pump-probe setup (PP) was previously described elsewhere.100-102 In short, the output of a commercial 5 kHz amplified Ti:sapphire laser (Pulsar, Amplitude Technology, delivering 40fs, 0.5-mJ, 800-nm pulses) was split into two. One beam pumped a commercial collinear optical parametric amplifier (Topas, Light conversion) delivering tunable IR pulses, which are subsequently frequency-converted to 333 or 293 nm, in order to excite BP in two different initial electronic states. The second 800-nm beam was focused on a moving CaF2 plate to generate a UV-Vis supercontinuum, used as a broadband, chirped probe beam. The probe beam was split into a sample and a reference beam. Both reference and sample probe beams were detected at 220 Hz using a spectrometer and a CCD camera, allowing the detection of the 330-660 nm spectral range of the supercontinuum pulse. The reference beam spectrum is used to normalize the spectrum of the probe beam transmitted by the sample, in order to attenuate the contribution of probe intensity fluctuations on the overall experimental noise floor. Pump and probe beams were focused and overlapped spatially in a fused silica cuvette of thickness 0.5mm, in which the BP solutions were circulated with a peristaltic pump. Their relative polarizations were set at the magic angle (54.7°) to measure population kinetics under isotropic conditions. By chopping the pump beam at 110Hz, the absorption spectra of the unexcited and excited samples are recorded alternatively and the differential absorption spectra (ΔA) are calculated, as a function of the time delay between pump and probe pulses. The pump beam intensity was maintained low enough that the ΔA signal varies linearly with the pump light power.


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For each BP sample, the non-linear signal from the solvent was recorded independently in the pure solvent and subtracted to the BP data. The instrument response function (IRF) was 60 fs, estimated from the width of the instantaneous solvent Raman signal. All ΔA spectra shown in this paper are corrected (by post-processing) from the effect of the chirp of the probe beam, characterized by the time-dependence of the solvent non-linear signal. As a result, the zero timedelay is defined with an error bar of 20 fs throughout the covered spectral window. The transient spectra recorded after excitation at 293 nm were obtained by appending two datasets covering two adjacent probing spectral windows below and above 366 nm. 3. RESULTS 3.1 Linear absorption The experimental linear absorption spectrum of BP in hexane is reported in Figure 2, featuring three different spectral regions in the UV region: one in the near UV (NUV) spanning the 300-380 nm range and showing a very weak single band with absorption maximum (λmax) at ~346 nm; a second broad band spanning the 225-300 nm range (and λmax=248 nm) in the middle UV (MUV), featuring a clear shoulder at around 285 nm; finally a high-energy very intense band in the far UV (FUV) at λ0.4 eV. For the PBE0 functional, with a larger amount of non-local Fock-exchange (25% vs. 20% of the


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B3-group), the deviation for S4(ππ*) decreases to ~0.3 eV. APFD, B98 and X3LYP functionals follow the general trend with quite accurate S1(nOπ*) and strong deviations observed for S4(ππ*). On the other hand, BH and HLYP with 50% of non-local Fock-exchange and wB97XD, as in the case of CAM-B3LYP, give the highest discrepancies with the RASPT2 reference for the first excited state (from 0.29 to 0.53 eV) but reproduce nicely S4(ππ*) excitation energy (within 0.11 eV). Concerning S2 and S3, a behavior similar to S1 is overall found. Especially, among all tested functionals, TD-M062X gives the best agreement with the RASPT2 reference for both S1(nOπ*) and S4(ππ*) excited states (within ~0.12 eV). Nevertheless, this functional is highly inaccurate to reproduce S0¦S2,3 states (ca. 0.7 eV deviations).

Figure 3. S0¦Sn (with n = 1, 2, 3, 4, 9, 15) transition energies differences between several TDDFT functionals (employing the 6-311++G** basis set) and the RASPT2(5,8|0,0|5,7)/ANO-LVTZP computations, with positive and negative values indicating TD-DFT energies being blue and red shifts, respectively. Mean absolute errors (MAE) are also shown for each functional, including all considered states and neglecting S4, MAE(-S4), to highlight its specific impact.


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Considering also the bright high-lying S9 and S15 states, the overall trends reported in Fig. 3 show that most functionals (B3PW91, B3BP86, B3LYP, APFD, X3LYP, B98 and PBE0) provide mean absolute errors (MAE) below 0.27 eV, which decrease to ~0.22 eV if the S0¦S4 excitation is not accounted for, indicating how these functionals describe reasonably well all excited states (up to ca. 7.0 eV from S0) but S4. Interestingly, the four functionals (M062X, wB97XD, CAM-B3LYP, BH and HLYP) that partially misbehave for S1 and clearly misbehave for S2, S3, S9 and S15, are those that best reproduce the S0¦S4 excitations energy computed at the RASPT2 level, with deviations smaller than 0.15 eV. This could be due to the fact that the S0¦S4 vertical transition corresponds to a pure ππ*-type transition at RASPT2 level, while at TD-DFT it results in a different (depending on the functional) mixture of ππ*, nOπ* and σσ* character. The same applies, even if at a lower extent, to the high lying S0¦S9 and S0¦S15 vertical transitions. Table 1. Linear absorption of BP. Comparison between experimental absorption maxima in hexane (in eV [nm]) and vertical excitations energies (in eV, with corresponding wavelengths given in nanometers within brackets) calculated at various levels of theory.a Oscillator strengths are given in parentheses. For transition energies beyond 5 eV, only the brightest states (i.e. f > 0.1) are reported. States b Assignment c Exp. λMAX (eV [nm]) CASPT2(16,15)/ ANO-L-VDZP d CASPT2(16,15)/ ANO-L-VDZP e RASPT2(4,8|0,0|4,7)/ ANO-L-VTZP f RASPT2(5,8|0,0|5,7)/ ANO-L-VTZP f RASPT2 (6,8|0,0|6,7)/ ANO-L-VTZP f RASPT2(4,6|4,3|4,6)/ ANO-L-VTZP f CAM-B3LYP/ 6-31G** CAM-B3LYP/ 6-311++G** B3LYP/

S1 (nOπ*) nO→ L 3.58 [346]

S2 (ππ*) HB1→ L

S3 (ππ*) HB2→ L

4.35 [285]

S4 (ππ*) H-1B1→ L

S9 (ππ*) H-1B2→ L+1B1

S15 (ππ*) HB1→ L+1B1

5.00 [248]

≈6.10 [203]

>6.40 [194]

-

-

3.66 [339] (0.001) 3.48 [356] (0.001) 3.42 [363] (0.001) 3.46 [358] (0.001)

4.33 [286] (0.003) 4.29 [289] (0.003) 4.31 [288] (0.004) 4.33 [286] (0.003)

4.43 [280] (0.001) 4.31 [288] (0.001) 4.34 [286] (0.001) 4.36 [284] (0.001)

5.39 [230] (0.150) 5.34 [232] (0.165) 5.00 [248] (0.288) 5.11 [243] (0.245)

-

-

6.06 [205] (0.127) 5.97 [208] (0.186)

6.48 [191] (1.158) 6.34 [196] (1.284)

3.45 [359] (0.001)

4.35 [285] (0.003)

4.38 [283] (0.001)

5.26 [236] (0.200)

5.80 [214] (0.247)

6.14 [202] (1.376)

3.37 [368] (0.001) 3.75 [331] (0.001) 3.76 [330] (0.001) 3.52 [352]

4.28 [290] (0.003) 5.08 [244] (0.017) 4.95 [251] (0.020) 4.58 [271]

4.31 [288] (0.001) 5.14 [241] (0.040) 5.02 [247] (0.043) 4.66 [266]

5.09 [244] (0.246) 5.30 [234] (0.349) 5.15 [241] (0.354) 4.77 [260]

5.90 [210] (0.188) 6.64 [187] (0.169) 6.38 [194] (0.230) 6.25 [198]

6.27 [198] (1.324) 7.23 [172] (0.839) 6.86 [181] (0.633) 6.83 [182]


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a

6-31G** (0.001) (0.013) B3LYP/ 3.51 [353] 4.47 [277] 6-311++G** (0.001) (0.015) Full tables with both bright and dark states within

(0.048) (0.235) (0.160) (0.862) 4.56 [272] 4.65 [267] 6.08 [204] 6.60 [188] (0.055) (0.238) (0.148) (0.656) the first 6-7 eV are given in Table S1 in the SI; b State labeling

and transitions assignment are given with respect to RASPT2(5,8|0,0|5,7)/ANO-L-VTZP level, which is used as the reference RASPT2 level due to its qualitative and quantitative agreement with the available experimental evidence as shown in Figure 2; c orbitals are depicted in Fig. 1; d Data from ref. 103; e This work, averaging over 8 states; f This work, averaging over 100 states.

Table 1 displays in details experimental absorption maxima, theoretical assignments of the S0¦Sn vertical transitions in terms of the molecular orbitals involved in the electronic transitions and transition energies computed at different levels of theory. These range from a CASPT2 approach, only affordable for the lowest-lying states, to several RASPT2 approaches with 4, 5 or 6 holes/electrons in their RAS1/RAS3 subspaces, a mixed RAS1/RAS2/RAS3 scheme with a RAS2(4,3) space and TD-DFT computations employing the B3LYP (TD-B3LYP) and CAM-B3LYP (TD-CAMB3LYP) functionals. We have selected these two functionals as representatives of the two opposite behaviors described previously (see Figure 3), considering that they were recently shown to provide a balanced description of both ground and low-lying excited state properties in the Franck−Condon region for similar systems.104 The influence of the basis set adopted is also reported in Table 1. In the following sections, the results reported in Figure 2 and Table 1 will be discussed for three different spectral windows, i.e. those of the experimental BP linear absorption spectrum (NUV, MUV and FUV). 3.1.1 NUV spectral window The NUV weak band at ~350 nm (3.58 eV) is attributed to the unique electronic excited state appearing around this energy, namely the lowest-lying S1 (nOπ*) state. All levels of theory agree in predicting the low intensity of the S0¦S1 transition (f=0.001), as expected for symmetry forbidden n¦π* transitions with π –symmetry partially distorted in the ground state equilibrium geometry (see Fig. S1 in the SI), and consistent with the low extinction coefficient observed experimentally (Figure 2 inset). The S0¦S1 transition energy is computed close to the experimental λmax, within a tenth of an eV by most methods except for the RASPT2(4,6|4,3|4,6) and TD-CAMB3LYP computations yielding differences of –0.21 and +0.17 eV, respectively. It


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is noteworthy that vertical excitation energies do not provide an exact direct comparison to absorption maxima nor they provide information on spectral lineshapes.105-107 Indeed, the use of more elaborate methods could take into account dynamics effects or vibronic coupling, nevertheless such a study is out of the scope of the present contribution. Here, transition energies and oscillator strengths are used to assign experimental bands largely separated in the UV spectrum (with absorption maxima energy gaps > 0.5 eV) and to provide a basic comparison between the different computational approaches adopted. 3.1.2 MUV spectral window Moving to the MUV region, a shoulder at 285 nm (4.35 eV) is observed experimentally being here assigned to the almost degenerate S2 (ππ*) and S3 (ππ*) states, arising from benzenictype H¦L transitions, HB1¦L and HB2¦L, respectively, (cf. Figure 1). As expected, all levels of theory indicate larger oscillator strengths to these π¦π* transitions with respect to the n¦π* transition associated to the S1(nOπ*) state, in agreement with experimental extinction coefficients shown in Figure 2. CASPT2/RASPT2 computations indicate that transitions to the S3 and S1 states have similar (small) oscillator strengths while the S0¦S2 transition has a three-fold larger f (ca. 0.003). TD-DFT results, instead, give higher oscillator strengths (i.e. transition dipole moments) for the S0¦S3 transition (f≈0.04-0.05) with respect to S0¦S2 (f≈0.01-0.02) that are in turn much larger than those of the S0¦S1 transition. Previous CASPT2(16,15)/ANO-L-VDZP computations predicted an S2-S3 energy gap of ca. 0.1 eV and S0¦S2,S3 transition energies within 0.08 eV from the experimental absorption maximum at ca. 4.35 eV (285 nm).103 Removing symmetry constraints and employing 8 state-averaged excited states, i.e. SA8, our SA8CASPT2(16,15)/ANO-L-VDZP calculations (hereafter SA8-CASPT2) provide smaller S2-S3 energy gap (ca. 0.02 eV) and similar transition energies, within 0.06 eV from the absorption maximum. All RASPT2 computations (using SA100) agree with the SA8-CASPT2 results. TDB3LYP and TD-CAMB3LYP computations yield S2-S3 energy gaps of 0.01) ESAs found at the RASPT2(5,8|0,0|5,7)/ANO-L-VTZP level (green sticks) are reported, except for S1-ESA2 (f ≈ 0.001) which is depicted for direct comparison with TDDFT computations, i.e. B3LYP/6311++G** (blue sticks) and CAM-B3LYP/6-311++G** (red sticks).


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The experimental PP spectrum of BP shows a very intense band at wavelengths below 380 nm (in the NUV probing region) associated to very bright high-energy Sn state(s) lying more than 3.26 eV above S1, and a broad band in the visible (500-600 nm), featuring two close peaks at ca. 540 and 570 nm (2.30 and 2.17 eV, respectively), related to states at around 2.0-2.5 eV from S1. RASPT2(5,8|0,0|5,7)/ANO-L-VTZP computations predict two types of S1¦Sn bright transitions in the 350-670 nm probing window, featuring single electronic excitations from the oxygen lone pair to the L+1 benzenic-like π*-orbitals (nO→L+1) in the lower energy region and accompanied by double excitations of the type nO,HB2=>L (or nO,HB2=>L+1B1) at higher energies. The computed permanent dipole moments (μ) of these Sn states (see Table S2 in the SI) indicate that the bright S1¦Sn vertical transitions are associated to Sn(nOπ*) states with μ close to that of S1 (μ ≈ 0.9 Debye) and rather different from S0 (μ ≈ 2.8 Debye). A minor solvatochromic shift has to be expected for these ESAs signals, in agreement with experimental evidences. Low energy S1¦Sn transitions falling in the near-IR, being associated to Sn states with ππ* nature, are found to be dark in vacuum at all multireference and at TD-CAMB3LYP levels while TD-B3LYP predicts slightly bright transitions (see Figure S5 in the SI). It is worth noting that solvation increases the brightness of these near-IR transitions, as reported elsewhere.115 As reported in Table 2, five bright S1¦Sn vertical transitions are found in the broad energy range from 1.8 to 4.5 eV above the S1 state energy when employing both multireference and TD-DFT methods: single excitations are associated to the first two ESAs, namely S1-ESA1 and S1-ESA2, while double excitations contribute to three high-lying ESAs, namely S1-ESA3-5. In particular, at the RASPT2(5,8|0,0|5,7)/ANO-L-VTZP level, the S1-ESA1 is found at 1.98 eV (626 nm) with a sizable oscillator strength (f≈0.03) while the S1-ESA2 is computed at 2.41 eV (515 nm) as a dark state, falling in the 2.0-2.5 eV (from S1) energy range of the experimental broad band. These results are consistently found at all RASPT2 levels (as reported in Table 2), suggesting that the S1-ESA1 is mainly contributing to the experimental band in the Vis. The double-peaked asymmetric shape observed for this band is possibly due to vibronic effects, not accounted for in this work. Notably, TD-DFT results in this energy window provide two bright ESAs that involve the same nO→L+1 single excitations found at RASPT2 levels. However, at both TD-DFT levels S1-ESA1 and S1-ESA2 are found closer to each other, with TD-B3LYP and TD-CAM-B3LYP transition energies being significantly shifted, in the red and the blue, respectively, compared to the RASPT2 reference. TD-CAMB3LYP seems to provide better


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results than TD-B3LYP when comparing with the experimental PP spectrum in this region, however, both functionals predict S1-ESA2 signal to be stronger than S1-ESA1, in contrast with experimental band shape and RASPT2 results. Table 2. Excited states absorptions of BP originating from the S1 state, i.e. S1-ESAs, at the FC region. Comparison between different RASPT2/ANO-L-VTZP schemes and B3LYP/CAMB3LYP (6-31G**/6-311++G**) levels of theory. Energies are given in eV and corresponding wavelengths are reported in nanometers within brackets. Oscillator strengths are given in parentheses. The first four brightest (i.e. f> 0.01) ESAs at the RASPT2 levels are reported.

Assignment b Exp. (eV [nm])

S1-ESA1

S1-ESA2

S1-ESA3

nO → L+1B1

nO→ L+1B2

nO→ L+1B2 nO,HB2⇒ L c

2.0-2.5 [500-600]

S1-ESA4

S1-ESA5

nO→ L+1B1 nO→ L+1B1 nO,HB2⇒ L c nO,HB2⇒ L+1B1 c > 3.26 [< 380]

RASPT2(4,8|0,0|4,7)

1.88 [659] (0.029)

2.32 [534] (0.000) a

3.45 [359] (0.119)

3.50 [354] (0.302)

3.48 [356] (0.111)

RASPT2(5,8|0,0|5,7)

1.98 [626] (0.028)

2.41 [515] (0.001) a

3.42 [363] (0.118)

3.97 [312] (0.209)

4.20 [295] (0.097)

RASPT2(6,8|0,0|6,7)

2.07 [599] (0.021)

2.47 [502] (0.001) a

3.39 [366] (0.141)

3.75 [331] (0.275)

4.44 [279] (0.066)

2.10 [590] 2.53 [490] 3.44 [360] (0.024) (0.001) a (0.108) 2.33 [533] 2.45 [507] 2.62 d [473] CAM-B3LYP (0.029) (0.091) (0.037) 1.68 [740] 1.85 [670] 3.17 d [391] B3LYP (0.056) (0.108) (0.002) a S1-ESA2 being added for comparison with TD-DFT results;

3.64 [341] (0.323) 3.99d [311] (0.043) 3.18d [390] (0.048)

4.17 [297] (0.109) 4.08d [304] (0.103) 3.22d [385] (0.018)

RASPT2(4,6|4,3|4,6)

b

orbitals are depicted in Fig. 1.

c

The doubly excited configuration is only featured at the RASPT2 levels of theory.

d

These transitions already feature sizable contributions from σ orbitals.

It is worth noting that very few ESAs (S1¦Sn) appear in the very broad probing range, which may seem to contrast the broad experimental bands registered. This is partially due to computing electronic excitations in the gas phase and from considering solely equilibrium geometries, where only the dipole-allowed (S1)nOπ*¦(Sn)nOπ* have sizable associated oscillator strengths thus leading to very few transitions given the system only features a single oxygen lone pair orbital that limits the number of such transitions. Nevertheless, we have shown in another work that the inclusion of the solvent,115 which breaks symmetry of the gas-phase geometry,


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results into partially allowed (S1)nOπ*¦(Sn)ππ* transitions providing a few more signals in the low-energy region that partially account for the experimental spectral broadening while displaying the S1-ESAs signals here described as the main spectroscopic fingerprints. Three very intense high-energy ESA signals, namely S1-ESA3, S1-ESA4 and S1-ESA5, are predicted by the RASPT2(5,8|0,0|5,7)/ANO-L-VTZP computations in the NUV probing window (at 300-360 nm) in quite nice agreement with the intense band rising below 380 nm observed experimentally. However, these S1¦Sn transitions feature strong doubly excited characters and (except for S1-ESA3) much stronger fluctuations are observed for their transition energies while increasing the number of holes/electrons in the active space, yielding up to 1.0 eV difference for S1-ESA5 by going from RASPT2(4,8|0,0|4,7) to RASPT2(6,8|0,0|6,7) schemes. This highlights more pronounced active space dependence when dealing with double excitations, and particularly high-energy states, as opposed to previous RASPT2 benchmarks focused on linear absorption and where up to four holes/electrons suffice for their proper characterization.53 However, it is worth noting that whereas the energetic positions may significantly vary, the estimated oscillator strengths are in reasonable agreement for all RASPT2 levels employed. As shown in Table 2, these S1-ESA3-5 states feature mainly nO¦L+1 transitions that are strongly mixed with double excitations, making their characterization by TD-DFT methodologies rather challenging. This can be readily seen by observing the pronounced differences for the lowest energy NUV ESA, namely the S1-ESA3, both in terms of energy and oscillator strength, featured by the different TD-DFT approaches with respect to RASPT2. S1-ESA3 TD-CAMB3LYP and TD-B3LYP transition energies are 0.8 and 0.2 eV red-shifted, respectively, with respect to RASPT2(5,8|0,0|5,7) values, while the various RAS schemes provide rather similar values (around 3.4 eV). The oscillator strength associated to the S1-ESA3 transition is also quite reduced from ca. 0.1 in RASPT2 to < 0.04 at TD-DFT levels. Upon close inspection of the orbitals associated to the TD-DFT electronic transitions, one can observe non-negligible contributions arising from σ orbitals, with σ¦π* transitions unexpected at these energies and rather dark (due to orbital symmetry) providing an interpretation for the discrepancy with respect to the RASPT2 results. In order to possibly reduce the computational cost, we have investigated the basis set effect on the S1-ESA signals. Whereas plenty of information is available in the literature on the basis set effect to ground state vertical excitation energies, very little is known as to how it


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affects excited state absorptions.116 We found that quantitative agreement can be observed for the different basis sets within the RASPT2(5,8|0,0|5,7) reference (see Figure S5 in the SI), particularly in the Vis window. TD-DFT transition energies are more largely affected by using 631G** instead of 6-311++G** basis set, leading to systematic red-shifts and evident differences in the high-energy NUV range (Figure S5b in the SI). In conclusion, we observed that ESAs computed at the RASPT2 level provide consistent assignments of the NUV and Vis bands recorded in the experimental PP spectrum of BP at zero time delay. Also TD-DFT results can be reasonably compared with RASPT2 calculations for ESAs in the Vis and become more problematic at higher energies (in the NUV), where double excitations start to contribute significantly to the high-lying excited states.

3.2.2 Excited state absorptions arising from low-lying ππ* states Upon excitation at 293 nm of BP in methanol, i.e. in the S2 and/or S3 low-lying (ππ*) excited states, an ultrafast internal conversion (IC) to the S1 state is observed on the sub-200fs as evidenced by the 200-fs spectrum shown in Figure 5, which is nearly identical to the early time spectrum observed upon direct excitation in S1 (see Figure 4, see also Figure S6 and S8 in the SI). This IC thus occurs in methanol solution on the same time scale as previously observed in gas phase by photoelectron spectroscopy.117 Of central relevance here, however, the early-time transient spectrum (around 0 fs) has a distinct shape which indicates that the experimental time resolution is sufficient to observe the ESA from these (ππ*) S2 and/or S3 states, before this ultrafast IC occurs. The 0-fs experimental PP spectrum after excitation at 293 nm is positive at all wavelengths, with two main bands, one in the Vis (around 530-630 nm) and one very broad in the NUV (at wavelengths 0.01) ESAs found are labeled. The dashed black line is the transient spectrum observed 200 fs after 293 nm excitation and is the characteristic transient absorption spectrum of the S1 state. The analysis of the computed S2-ESAs reveals two main low-energy transitions, one (out of the experimental probing window) lying at 689 nm (1.80 eV) and associated to an Sm excited state dominated by the HB2→L single excitation, named S2-ESA1, and a second S2¦Sm transition (S2-ESA2) associated to a similar Sm(ππ*) state, having significant contributions from the H1B2→L+1B2 excitation, as indicated in Table 3. The S2¦Sm transition energy associated to the S2ESA2 fits quite well the position of the Vis band of the 0-fs experimental transient absorption


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spectrum reported in Figure 5. Several S2¦Sm transitions in the 350-500 nm region (larger in number but less bright than S1¦Sm transitions) and two bright (f > 0.01) S2-ESAs between 400 and 500 nm (i.e. 2.48-3.10 eV), i.e. S2-ESA3 and S2-ESA4, are predicted by multireference computations. This result suggests a broadening of the NUV band towards longer wavelengths (accompanied by intensity attenuation) with respect to S1-ESAs signals, in very good agreement with experimental data (Figures 5 and S8). Experimental PP spectra show indeed analogous NUV band broadening (and weakening) when comparing the spectrum after excitation at 293 nm at zero time delay (i.e. S2-ESAs signals) with the spectrum at 200 fs (Figure 5), which is nearly identical to the 0-fs spectrum observed upon direct excitation in S1 (i.e. S1-ESAs signals, see Figures 4 and S6). Considering the different nature of the S1(nOπ*) and S2(ππ*) states and the higher number of occupied π orbitals (i.e. seven, see Figure 1) with respect to the one nO orbital of BP, it has to be expected that symmetry allowed S2(ππ*)¦Sm(ππ*) transitions are more numerous than S1(nOπ*)¦Sm(nOπ*) transitions. By looking at the assignments of intense ESA signals in Table 2 and Table 3 it is clear, indeed, that all S1(nOπ*)¦Sm bright transitions involve Sm states with nOπ* character while the most intense S2-ESAs signals (S2-ESA1-7) are associated to S2(ππ*)¦Sm(ππ*) transitions. Notably, RASTP2 calculations predict that the density of S2ESAs signals start increasing only at energies > 2.5 eV (below 500 nm) while only one ESA signal is found in the Vis (i.e. S2-ESA2), providing an interpretation of experimental PP spectra. The assignment of S2-ESA signals and the S2(ππ*)¦Sm(ππ*) transition energies computed at different RASPT2 levels are reported in Table 3. S2-ESA1 and S2-ESA2 are described mainly by single excitations, the former shifting towards higher energies by increasing the holes/electrons in the RAS space and the latter being less dependent on it. Starting from S2ESA3 we observed large discrepancies in the RASPT2(4,8|0,0|4,7) results with respect to higher levels with concomitant appearance of contributions from double excitations, dominating in S2ESA4 and S2-ESA6. Overall, the RASPT2(5,8|0,0|5,7) level provides rather similar results to RASPT2(6,8|0,0|6,7) scheme, except for S2-ESA1 for which a deviation of 0.25 eV is found. Partially filling the RAS2 space, i.e. adopting the RASPT2(4,6|4,3|4,6) scheme, it is not yielding dramatic variations while increasing so much the computational cost that high-lying S2-ESAs could not be computed within the first 100 states. Basis set effects analogous to S1-ESAs have been found. The characterization of the S2-ESAs could be also achieved by employing TD-DFT quadratic response using the 6-31++G** basis set. In general, the results indicate a blue shift, as


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compared to RASPT2, even though TD-B3LYP shows a good agreement for S2-ESA4 and especially S2-ESA3 (see Table S3 in the SI). Table 3. Excited states absorptions of BP originating from the S2(ππ*) state, i.e. S2-ESAs, at the FC region. Comparison between different RASPT2/ANO-L-VTZP schemes. Energies are given in eV and corresponding wavelengths are reported in nanometers within brackets. Oscillator strengths are given in parentheses. The first eight brightest (i.e. f > 0.01) ESAs at the RASPT2 levels are reported. Exp. (eV [nm])

Assignment a

RASPT2 (4,8|0,0|4,7)

RASPT2 (5,8|0,0|5,7)/

RASPT2 (6,8|0,0|6,7)

RASPT2 (4,6|4,3|4,6)

HB2→ L

1.46 [849] (0.034)

1.80 [689] (0.019)

2.05 [605] (0.035)

1.70 [729] (0.025)

HB2→ L H-1B2→ L+1B2

2.11 [588] (0.030)

2.17 [571] (0.026)

2.06 [602] (0.011)

2.12 [585] (0.024)

2.17 [571] (0.007)

2.64 [470] (0.012)

2.63 [471] (0.007)

2.63 [471] (0.009)

3.65 [340] (0.019)

2.93 [423] (0.012)

2.78 [446] (0.017)

3.07 [404] (0.011)

S2-ESA1 S2-ESA2

1.97-2.34 [530-630]

S2-ESA3

S2-ESA4

a

2.88-3.75 [330-430]

HB2→ L H-1B2→ L+1B1 HB2,H-1B2⇒ L,L+2B1 nO,HB2⇒ L,L+2B1 nO,HB1⇒ L,L+1B2 nO,HB1⇒ L,L+1B1

S2-ESA5

H-1B1→ L+2B1 H-1B2→ L+1B2

4.01 [309] (0.008)

3.42 [363] (0.016)

3.28 [378] (0.012)

3.63 [342] (0.015)

S2-ESA6

HB2,H-1B2⇒ L

4.42 [281] (0.090)

4.22 [294] (0.011)

4.28 [290] (0.022)

-

S2-ESA7

HB2→ L+2B2 H-2→ L+1B1 H-1B1,HB1⇒ L,L+2B1

4.48 [277] (0.019)

4.42 [281] (0.024)

4.38 [283] (0.013)

-

Orbitals are depicted in Fig. 1

Analogously to S2(ππ*), the degenerate S3(ππ*) has also been considered for possible contributions to the PP spectrum, i.e. S3-ESAs, after excitation in the NUV. However, the S3ESAs signals are generally very similar to the S2 ones while S3(ππ*)¦Sm(ππ*) transitions have smaller oscillator strengths (see Table S3 in the SI). Considering that, as explained above, by using pump pulse at 293 nm we expect to populate more S2 than S3 and that the S3-ESAs are even less intense than the S2 ones, we could conclude that the S3(ππ*) state would contribute much less than S2(ππ*) to this experimental PP spectrum.


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4. CONCLUSIONS Assignment of linear absorption UV bands of BP is obtained by computing the S0¦Sn vertical transitions at the gas phase ground state equilibrium geometry. Experimental absorption spectra have been collected in polar protic (methanol) and non-polar aprotic solvents (hexane and cyclohexane) up to the FUV spectral region (λ1) computed at the RASPT2(5,8|0,0|5,7)/ANO-LVTZP level. Notably, RASPT2 computations predict S1¦Sm transition energies and oscillator strengths, i.e. S1-ESAs, in nice agreement with the PP experimental spectra at very early time delays. The comparison of RASPT2 and TD-DFT results is fairly good up to the Vis probing window, while S1-ESAs in the NUV have strong double excitations contributions that cannot be yet predicted by TD-DFT. In order to further assess the reliability of multireference and TD-DFT computations in simulating the nonlinear electronic spectra of BP, we have compared experimental PP spectra at zero waiting time, after excitation in the MUV (in the red tail at 293 nm), with ESA signals arising from S2(ππ*)¦Sm transitions (with m>2). RASPT2 results allowed assignments of the nonlinear signals that yield a very broad band in the NUV and a band in the Vis. These results provide benchmarks for multireference and TD-DFT computations of high-lying singlet excited states of BP that are of particular relevance for further experimental and computational studies involving time-resolved nonlinear electronic spectroscopy of BP in extended media and/or in complex systems. For instance, the outcome of this work in conjunction with previous characterizations of excited state manifold of DNA/RNA nucleobases would lay the groundwork for simulations and interpretation of nonlinear 2D spectra that could discriminate between various BP/DNA complexes. ASSOCIATED CONTENT Supporting Information The Supporting Information contains the following: (1) further computational details, (2) Cartesian coordinates, and (3) additional results.


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Funding The authors thank the Agence National de la Recherche project FEMTO-2DNA (ANR-15-CE29-0010). M. G. acknowledges support by the European Research Council Advanced Grant STRATUS (ERC-2011-AdG No. 291198). ACKNOWLEDGMENT Supports from Universities of Lorraine and Strasbourg, as well as ENS Lyon and CNRS are gratefully acknowledged. I.R. gratefully acknowledges the support of ENSL “Fonds Recherche, MI-LOURD-FR15” and the use of HPC resources of the “Pôle Scientifique de Modélisation Numérique” (PSMN) at the ENS-Lyon, France. REFERENCES 1. Snedeker, S. M., Benzophenone Uv-Photoinitiators Used in Food Packaging: Potential for Human Exposure and Health Risk Considerations. In Toxicants in Food Packaging and Household Plastics: Exposure and Health Risks to Consumers, Snedeker, S. M., Ed. Springer London: London, 2014; pp 151-176. 2. Marazzi, M.; Wibowo, M.; Gattuso, H.; Dumont, E.; Roca-Sanjuan, D.; Monari, A., Hydrogen Abstraction by Photoexcited Benzophenone: Consequences for DNA Photosensitization. Phys. Chem. Chem. Phys. 2016, 18, 7829-7836. 3. Dumont, E.; Wibowo, M.; Roca-Sanjuán, D.; Garavelli, M.; Assfeld, X.; Monari, A., Resolving the Benzophenone DNA-Photosensitization Mechanism at Qm/Mm Level. J. Phys. Chem. Lett. 2015, 6, 576-580. 4. Cuquerella, M. C.; Lhiaubet-Vallet, V.; Cadet, J.; Miranda, M. A., Benzophenone Photosensitized DNA Damage. Acc. Chem. Res. 2012, 45, 1558-1570. 5. Boscá, F.; Miranda, M. A., New Trends in Photobiology (Invited Review) Photosensitizing Drugs Containing the Benzophenone Chromophore. J. Photochem. Photobiol. B: Biol. 1998, 43, 1-26. 6. Galrdy, L. E.; Craig, L. C.; Printz, M. P., Benzophenone Triplet: A New Probe of Biological Ligand-Receptor Interactions. Nature 1973, 242, 127-128. 7. McTiernan, C. D.; Alarcon, E. I.; Hallett-Tapley, G. L.; Murillo-Lopez, J.; Arratia-Perez, R.; Netto-Ferreira, J. C.; Scaiano, J. C., Electron Transfer from the Benzophenone Triplet Excited State Directs the Photochemical Synthesis of Gold Nanoparticles. Photochem. Photobiol. Sci. 2014, 13, 149153. 8. Zeglis, B. M.; Pierre, V. C.; Barton, J. K., Metallo-Intercalators and Metallo-Insertors. Chem. Commun. 2007, 4565-4579. 9. Gattuso, H.; Dumont, E.; Chipot, C.; Monari, A.; Dehez, F., Thermodynamics of DNA: Sensitizer Recognition. Characterizing Binding Motifs with All-Atom Simulations. Phys. Chem. Chem. Phys. 2016, 18, 33180-33186. 10. Dumont, E.; Monari, A., Benzophenone and DNA: Evidence for a Double Insertion Mode and Its Spectral Signature. J. Phys. Chem. Lett. 2013, 4, 4119-4124. 11. Crespo-Hernandez, C. E.; Cohen, B.; Hare, P. M.; Kohler, B., Ultrafast Excited-State Dynamics in Nucleic Acids. Chem. Rev. 2004, 104, 1977-2019.


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Resolving the Singlet Excited State Manifold of Benzophenone by First

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