Computational Study of the Mechanism of the Photochemical and

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Computational Study of the Mechanism of the Photochemical and Thermal Ring Opening/Closure Reactions and Solvent Dependence in Spirooxazines Pedro Javier Castro, Isabel Gómez, Maurizio Cossi, and Mar Reguero J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp3025045 • Publication Date (Web): 18 Jun 2012 Downloaded from http://pubs.acs.org on June 24, 2012

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The Journal of Physical Chemistry

Computational Study of the Mechanism of the Photochemical and Thermal Ring Opening/Closure Reactions and Solvent Dependence in Spirooxazines Pedro J. Castro,a Isabel Gómez,a Maurizio Cossi,b and Mar Reguero*,a a

Department de Química Física I Inorgànica.Universitat Rovira i Virgili. C. Marcel·lí Domingo s/n, Tarragona-43007 (Spain).

b

Dipartimento di Scienze e Innovazione Tecnologie (DISTA). Università Del Piemonte Orientale. Via T. Michel 11, I-15100, Alessandria (Italy) e-mail: [email protected]

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) TITLE RUNNING HEAD: Photochromism of spiroxazines

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ABSTRACT

The spirooxazine/merocyanine couple constitutes a photochromic system that can change from the colourless spirooxazine to the intensely coloured merocyanine by thermal or photochemical activation by a reaction that opens the spiro ring of the oxazine. The mechanisms of the ring opening/closure reactions that interconnect these two isomers have been elucidated by means of a computational study. First, we have used the CASSCF/CASPT2 method to determine in detail these mechanisms in gas phase for a small model that contains the photoactive part of the whole system. We have found that the state of spirooxazine excited by the initial absorption changes gradually to a lower excited state that is involved in a conical intersection that connects it with the ground state of merocyanine. The same conical intersection is involved in the backwards photochemical reaction. Secondly, using a larger model that includes all the heteroatoms of the system and using the DFT (B3LYP) method, we have studied the influence of a solvent environment on the thermal equilibrium between the open and closed species. It has been observed experimentally that the thermal equilibrium between these forms is practically unaltered by polar aprotic solvents, but it can be displaced towards the coloured form in mixtures of polar protic and aprotic solvents, even if the first one is found in a very small proportion. To reproduce the experimental environments, we have taken into account the long-range effect of the polar aprotic solvent considering it a polarizable continuum, as done in the PCM method, and the short-range effect of the protic solvent including some explicit water molecules in the cluster studied at the atomic level. The results obtained are in good agreement with the experimental observations and explain the reason of this peculiar behaviour.

KEYWORDS: Ab initio calculations, photochemistry, excited states, potential energy surfaces, photochromism.


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INTRODUCTION Photochromism is a phenomenon where a reversible photoinduced reaction leads to a product with different characteristics on the visible spectrum than the original reactant. It has attracted much attention (as a token see reviews of references1-3) because of the wide range of possible applications in molecular devices such as optical memories, data storage devices, metal detectors, etc.3-5 A necessary characteristic of a photochromic system to arise commercial interest is the fatigue resistance. Spiropyrans (SP) and spirooxazines (SO) (Scheme 1) are prototypes of photochromic compounds. Both are composed of two heterocyclic nearly planar moieties linked by a tetrahedral spiro-carbon which prevents conjugation between the two π-electron systems. As a result, the spiro-compounds are almost colourless since the lowest electronic transition of the molecule occurs in the near UV region. When these colourless compounds are irradiated with ultraviolet light they isomerize to merocyanine (MC). The photoreactivity of these molecules is due to the cleavage of the C–O spiro bond under UV irradiation and subsequent rotation about C–C bond (Scheme 2). The merocyanine is then an open structure that absorbs in the visible region so it is intensely coloured. The SO and MC forms can coexist in equilibrium, but without external stimulus, the equilibrium is in most cases shifted towards the closed form so in general these solutions are colourless. In these cases the MC formed by irradiation reverts back to the more stable isomer (SO) by a thermal process or by a photochemical one absorbing visible light. Under continuous UV irradiation, a new equilibrium is established where the MC isomers in general predominate, so the system is coloured. When irradiation stops, though, the solution returns again slowly to the original equilibrium. Although the spiropyran family has been more profusely studied, spirooxazines are more promising from the commercial point of view because their photoresistance is greater due to the stabilization induced by the nitrogen atom of the oxazine ring.1, 6 The equilibrium between SO and MC can be shifted by substituents in the indoline or naphthooxazine heterocycles that comprise the spirooxazines. For example aromatic rings connected to the oxazine ring shift the equilibrium towards the SO form4 while electron-donating groups in the indoline moiety shift it 3 ACS Paragon Plus Environment


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towards the MC form.7 The equilibrium is also sensitive to the temperature and polarity of the solvent. Regarding in particular the solvent effect, the scarce information available refers mostly to spiropyran systems, while little has been published about spirooxazines. For spiropyrans, it has been found that different substituents can gauge the effect of the solvent polarity on the reactivity: some members of a family can be practically insensitive to the medium polarity and have very similar ratio of closed (colorless) and open (colored) conformations in apolar and polar solvents while for others, e.g. those bearing electron withdrawing groups on the pyran ring, the open form is more abundant in polar environments.7,8 Recent kinetic and spectroscopic studies have suggested a similar behaviour for spirooxazines. Moreover, the spirooxazine/merocyanine systems exhibit a very interesting feature related to the H-bonding solute-solvent interactions: binary mixtures of polar aprotic and protic solvents (like acetonitrile/water mixtures) are able to shift the equilibrium between open and closed forms and hence modify the colour and the spectral properties of the solution.9 By understanding the details of the reaction mechanism and the cause of the influence of the solvent on the reactivity, it could be possible to tune the photochemistry of these photochromic compounds, and this is the motivation of this study. We have focused our attention on the family of the spiroindolinonaphthooxazines, in particular on the compound showed in Scheme 2, a member of the less studied but more commercially interesting family of oxazines. We have used high level ab initio methods to investigate the mechanism of the photochemical and thermal spirooxazine/merocyanine interconversion and the influence of the solvent on this process. We have modelled both long-range solvent effects and short-range specific solute-solvent interactions to try to reproduce the effect on the thermal equilibrium, not only of the aprotic polar solvents, but also of binary mixtures of polar aprotic and protic solvents. But the computational requirements of some of the ab initio methods used here make prohibitive the study of the full system, and therefore models have been used to represent the moiety of interest. A smaller model (Model-1) showed in Scheme 3a, has been used to study the mechanism of the thermal and photochemical processes of the SO/MC interconversion in vacuum and in a continuum solvent. Justification of the adequacy of the use of this model is included in the Conclusions section, in view of ACS Paragon Plus Environment

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the results obtained. This part of the work has been carried out with wavefunction-based ab-initio methods (CASSCF/CASPT2). To modelize the short-range effects of the solvent, explicit interactions of polar-solvent molecules with the heteroatoms of the system must be taken into account. For this reason we have used a larger model (Model-2) where all the heteroatoms of the active site of the photochromic moiety are included (Scheme 3b), together with explicit water molecules representing a polar H-bonding solvent. The large size of this system (complete model plus explicit solvent molecules) and the need of studying only the ground state reaction, made advisable the use of DFT methods, less computationally demanding, for this part of the study.

COMPUTATIONAL DETAILS The ground state and the lower singlet excited states of the species involved in both the thermal and photochemical reactions of Model-1 have been studied carrying out multiconfigurational self consistent field (MCSCF) calculations of the complete active space (CAS) SCF class10 using the d-polarized splitvalence basis set 6-31G(d).11 To investigate the spirooxazine ring-opening reaction coordinate we have used an active space with 12 electrons and 11 orbitals. This active space includes the six benzenoid orbitals, the π/π* orbitals of the N-C double bond, the σ/σ* orbitals of the C-O single bond, and the orbital of the non-bonding pair of the oxygen atom which is conjugated with the aromatic ring (considered as a pO in the SP form but a nO in the MC isomer). Some calculations have been performed with an active space of 14 electrons and 12 orbitals, increased by adding the non-bonding pair orbital of the nitrogen atom. The wavefunctions obtained allow the exploration of the potential energy surfaces (PES) of the singlet ground state, hereafter designated by S0, and of the singlet excited states generated by excitations of the types (π−π*), (pO−π*), (pO−σ*) and (nN−π*). Full geometry optimizations were performed with CASSCF(12,11) without any symmetry constraint to locate the critical points of interest on the above mentioned PES. Numerical frequency calculations were performed to determine the nature of the stationary points. Intrinsic reaction coordinates (IRCs)12 were also computed to determine the pathways linking critical structures (stationary points and conical intersections) when necessary. ACS Paragon Plus Environment

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To incorporate the effect of the dynamic valence-electron correlation on the relative energies of the lower excited states, we performed second-order multiconfigurational perturbation theory calculations (CASPT2).13 These single point energies were calculated at the CASSCF(12,11)/6-31G(d) optimized geometries and all valence electrons were correlated. All CASPT2 computations were performed using the completed Fock matrix in the definition of the zero-order Hamiltonian, together with an imaginary level shift of 0.2 in order to prevent the incorporation of intruder states.14 To allow the possible mixing between the reference state and one or more CASSCF states of the secondary space, we have used the Multi State CASPT2 approach (MS-CASPT2).15 It uses a multidimensional reference space and constructs an effective Hamiltonian which is computed perturbatively, allowing the CASSCF states to interact via non diagonal terms. The eigenvalues of the effective Hamiltonian are the MS-CASPT2 energies while the eigenvectors give the Perturbed Modified CAS Configuration Interaction (PM-CASCI) functions (linear combinations of CASSCF states). Conical intersections were optimized16 using state-averaged orbitals, and the orbital rotation derivative correction (which is usually small) to the gradient was not computed. Owing to the fact that the environment can change the shift of the merocyanine-spirooxazine equilibrium, we have also studied Model-1 (Scheme 3a) in acetonitrile. Long range solvent effects were included at the CASSCF/CASPT2 level with the conductor-like version of the polarizable continuum model (C-PCM)17 where the solvent is treated as an infinite continuum dielectric, while the solute is embedded in a molecular cavity obtained as interlocking spheres centred on each nucleus. The solute cavity was built with the standard United Atom Topological Model, UAH0 procedure, assigning atomic radii on the basis of the chemical connectivity, and including the hydrogens in the same spheres as the heavy atoms they are bonded to. Vertical excitations were computed with the non-equilibrium version of PCM, to account for the part of the solvation which cannot equilibrate in such a fast event like the change of electronic state by absorption of light; on the other hand, the S1 potential energy surface was computed in equilibrium with the solvent.


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In order to study the effect of the binary mixtures of protic and aprotic solvents on the thermal SO/MC equilibrium, we had to take into account the possible short-range interactions of the protic solvent with the heteroatoms of the photoreactive moiety. For this reason a second study was developed on the larger Model-2 (Scheme 3b). This model also allows studying the possible isomers of the real system. The specific solute/protic-solvent interactions (H-bonds), were taken into account including some specific water molecules in the studied system, that was calculated as a cluster or ‘supermolecule’ immersed in a continuum aprotic polar medium (acetonitrile). The ground state structures of Model-2 were fully optimized at the DFT level using the B3LYP18 functional and the 6-31G basis set. The B3LYP and CASSCF calculations were carried out with the Gaussian 09 system of programs,19 while the CASPT2 computations were performed with the MOLCAS 7.0 program package.20

RESULTS AND DISCUSSION Gas Phase Mechanism The opening/closure reaction mechanism was investigated at the CASSCF/CASPT2 level in the gas phase using the smaller Model-1. While the thermal reaction occurs exclusively through the ground state potential energy surface (PES), to study the photochemical reaction it is necessary to explore the PES of the low-lying excited states, that in this case are four, of (π−π*), (pO-π*), (pO-σ*) and (nN- π*) character. The obtained geometries for the main critical points of the reaction mechanisms are shown in the paper, while secondary structures are collected in the Supplementary Information. The energetics corresponds always to CASPT2 results obtained at CASSCF geometries, unless otherwise stated.

Thermal Reaction In order to study the thermal reaction, we obtained the ground state potential energy surface along the reaction path that connects reactants and products (geometries in Figure 1). For the MC product two conformations exist, corresponding to the trans and cis isomers. These stationary structures show very similar geometries with strongly localized C=C double bonds and a slightly polarized C=O double bond. ACS Paragon Plus Environment

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The colourless SO reactant was found to be 27.6 kcal mol-1 more stable than the trans-MC, while this isomer was 9.5 kcal mol-1 more stable than the cis-MC. A transition state connecting the SO reactant with the product cis-MC was located with a O−C2 distance of 2.063 Å (Figure 1). At CASPT2 level this barrier is 40.8 kcal mol-1 high, precluding the thermal ring-opening reaction in usual conditions. On the other hand, the barrier for the opposite reaction is only 3.8 kcal mol-1 above the cis-MC minimum. These results are in good agreement with the experimental observation that in general the MC structures evolve to the SO on a time scale of seconds to minutes in non-polar solvents, even at low temperatures, while the direct reaction is only possible after an initial photoexcitation. No transition state for the trans/cis isomerization could be located. To clarify the topology of the surface in this region, we calculated the profile between the trans-MC and the cis-MC minima at the CASPT2 level (Figure S1). At this level, the cis-MC minimum has been shifted to a different geometry from the one optimized at CASSCF level, with and energy 8.9 kcal mol-1 above the trans-MC isomer. The path from cis-MC to trans-MC seems to be an almost barrierless process, so the final product of the ring-opening reaction will be the trans-MC isomer. Nevertheless, the small barrier found for the trans MC to cis-MC isomerization —9.1 kcal.mol-1— suggests that in some merocyanine derivatives this barrier can be so low that the cis-MC minimum could be partially populated at the equilibrium. These results can only be compared with other computational results obtained with DFT methods for larger systems,21 which provide similar minimum energy differences although larger barriers due to the larger steric impediments of the systems studied with DFT. Photochemical Reaction In order to study the mechanism of the photochemical SO ↔ MC interconversion, we first have to analyse the excited states involved in the reaction. From a schematic VB point of view, the opening reaction will be favoured by excitations from the pO orbital perpendicular to the ring plane, to the σ*O-C2 orbital. The first condition will allow the formation of the πO-C10 bond while the second will favour the breaking of the σO-C2 bond. This excited state will correlate with the ground state of the open MC form 8 ACS Paragon Plus Environment

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(the electronic distribution so obtained will correspond to the one of the MC ground state). On the other hand, in MC the excited state corresponding to an excitation from the nO orbital to the π*O-C10 orbital will favour the closure reaction, given that this excited state correlates with the SO ground state. So a conical intersection between the PES corresponding to the ground states of the SO and MC species is expected. In fact, this is the main feature found in the mechanism of a similar reaction for a spiro model system studied by our group some years ago22 and in a more recent work23 and also found for a napthoxazine studied at semiempirical and DFT methods.24 The first steps in our study of the photochemical reaction must be then to locate energetically the excited states involved in the reaction, to determine the states that can be populated by the initial excitation and to find how these states are related with the excited states that favour the forth and back reactions. Opening reaction Vertical excitations from the ground state closed species in Model-1 were calculated together with the corresponding oscillator strengths of the S0→Sn transitions at the CASSCF(14,12)/6-31G(d) and CASPT2/6-31G(d)//CASSCF(14, 12)/6-31G(d) levels. One of the most important points in the elucidation of the mechanism of an excited state reaction is to determine the character of the excited states involved in the reaction. For this it is necessary to analyse the orbitals involved in the excitations that describe those states. The molecular orbitals of interest for the lower states of SO are shown in Figure 2. Given that the concept of HOMO and LUMO is not meaningful in a CAS treatment, we just keep a numerical labelling. Orbital 33 corresponds mainly to the lone pair of the N atom (nN orbital) while orbitals 35 and 34 are formed mainly by π orbitals, but show a certain contribution of pO orbital. Orbital 36, 37 and 38 are mainly formed by π* orbitals, but orbitals 36 and 38 show also a contribution of the σ* orbital. Based on these orbitals, we analysed the character of the states of SO at the CASSCF (denoted SiSCF) and at the MS-CASPT2 (denoted SiMS) levels, because the inclusion of the interaction of CASSCF states through dynamic correlation (in the MS-CASPT2 treatment) can change not only the energy of the states, but also the character of the wavefunctions that describe them. The description of the four lowest 9 ACS Paragon Plus Environment


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excited states at MS-CASPT2 level is collected in Table 1. For the MS-CASPT2 calculations, a reference CASSCF function including the 10 lowest singlet states in the state average was used. The strong interaction of the CASSCF functions that leads to the change in the nature of the states at the MSCASPT2 level is made evident by the composition of the PM-CAS-CI functions shown in the Supplementary Information (Table S1). Given the nature of these states, we expect that states S1MS and S3MS could be involved in the opening reaction of the spiro ring. Table 1 shows the vertical energies, oscillator strengths (f) and excitations that characterize the nine lowest states of SO. These results show that the probability of population of the state S1MS is high, with an absorption energy of 95.3 kcal.mol-1 for the colourless SO. It is well-known that the spirooxazine absorbs in the near UV region. The experimental values of the absorption energies of different spirooxazines are around 70 kcal mol-1,6b which is lower than the values obtained in this study. However, it must be taken into account that we are comparing the values of a large system with the results of a model system, so a quantitative agreement can not be expected. The next step of our study is to explore the PES of the lowest excited states at the CASSCF level. We first looked for the most crucial point of the mechanism of the photochemical spiro-ring opening, that is the conical intersection between the S0 and S1 excited states. This CI was located at the geometry shown in Figure 3, that also shows the vectors that expand the branching space. As expected, the character of the S1 state at this point can be described mainly by a (pO−σ*) excitation. The two degeneracy-lifting coordinates imply mainly a change in the O–C2 bond distance. In fact, both coordinates are essentially parallel, describing, like in the case of the spiropyran we studied before,[22] a monodimensional branching space. Following the only coordinate of this space in one direction, the O– C2 distance increases, yielding the MC open species, while following the opposite direction the O–C2 bond is shorted, driving the system to the SO structure in the S0 surface. To know in detail the ringopening mechanism, the path from the FC zone to this CI must be determined, to correlate the states characterized at the FC zone with the ones involved in the CI located, and to determine if there is any barrier along this path. ACS Paragon Plus Environment

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Of the lowest three excited states in the Franck Condon geometry at the CASSCF level, the S2SCF state, of (nN-π*) character, is not expected to be involved this opening reaction, so to simplify the exploratory calculations, an active space (12,11) was used where the nN orbital was excluded from the (14,12) active space. Doing so the calculations are less computer-time demanding and the S2SCF state will not be described in the results so obtained. In this way we have determined the gradients of the PES of the S1SCF and S3SCF states at the FC zone (force vectors shown in Figure S2 of the Supplementary Information) and the relaxation paths followed. For both states the gradients at the FC region involve an elongation of the N-C3 bond and a shortening of the O-C10 bond. Following this gradient on the S1SCF PES, the relaxation leads to a minimum (shown in Figure 3) 87.7 kcal mol-1 more energetic than the ground-state SO minimum. The path from this S1 minimum to the S1 /S0 CI will be characterize mainly by the stretching of the O–C2 bond. Along this path we located a transition state, with a O–C2 bond distance of 1.617 Å (Figure 3) and only 3.8 kcal mol-1 higher in energy than the S1 minimum. Relaxation on the S3SCF PES from the FC region decreases continuously the energy of this state. At some point (that we have not determined), this surface must cross the S2SCF PES (so the S2SCF and S3SCF states interconvert), leading eventually through a barrierless path to a conical intersection with the S1SCF state. This is located 102.6 kcal mol-1 above the ground state SO minimum, at a geometry shown in Figure S3 of the Supplementary Information. Comparison of this geometry with that of the TS located on S1 suggests that the last topological feature is generated by the avoided crossing that surrounds every CI, and creates an adiabatic path on the S1 PES connecting the electronic configurations of the two states involved in the conical intersection. In studies of mechanism of photochemical reactions with the CASSCF/CASPT2 protocol, the exploration of the PES is performed in general at the CASSCF level because the CASSCF wavefunctions are usually good approximations of the PM-CAS-CI wavefunctions. Unfortunately this is not the case here, but the size of the system and the development of the computational resources do not allow the exploration of the PES at the CASPT2 level. The initial exploration at the CASSCF level that ACS Paragon Plus Environment

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we have performed first can be consequently only a rough approximation, so we must refine our previous results at the CASPT2 level. The energy of the S1/S0 CI (optimized at the CASSCF level) recalculated at the MS-CASPT2(14,12) level, places this CI 57.3 kcal mol-1 (average value) above the S0 minimum. The energy difference between S0 and S1 states is small (only 1.0 kcal mol-1), what indicates that the CASSCF description is a good approximation, in this area of the PES, for the description of the CASPT2 surfaces. To have a global image of the evolution of the lowest energy excited states at the MS-CASPT2 level along the first steps of the excited state ring-opening reaction, we calculated the profile of the 9 lowest states along the interpolated path from the FC region to the S2SCF/S1SCF CI and then to the S1SCF/S0SCF CI. The energy profiles obtained are shown in the Figure 4. This figure includes also, for comparison, the energy profiles obtained at the CASSCF(14,12)/6-31G* level. Comparing both descriptions, we observe that the S2/S1 crossing found at CASCSF level does not appear at the CASPT2 level. This is due, from the computational point of view, to the fact that the states described at the CASPT2 level are the ones resulting after the interaction of CASSCF states. In this particular case, the S2/S1 CI seems to give place, in its surroundings, to a strongly avoided crossing, so in the adiabatic description given by the MSCASPT2 wavefunctions, the change in the electronic configuration is very smooth along the path without any crossing of states. This change is produced not only by the weight of the CSF of the PMCAS-CI eigenfunctions, but also by the composition of the molecular active orbitals, as is shown in Figure 5 for some chosen points of the paths of Figure 4. From the electronic point of view, as the O-C2 distance increases, the 1(pO−σ*) CSF is stabilised, so the energy of the states with a relevant contribution of this configuration decreases. From a diabatic point of view the energy of the state labelled S4MS at the Franck Condon region decreases in energy along the path towards the S1/S0 CI, becoming one of the states involved in the crossing, where it can be described almost exclusively by the(pO−σ*) excitation. The CASPT2 description, then, shows that the first step of the ring opening reaction occurs along the PES of the S1 state, which is the one populated by the initial excitation, along a barrierless path of ACS Paragon Plus Environment

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elongation of the O-C2 bond that leads directly to a CI with the ground state. The relaxation on the ground-state surface reached by decay at this crossing can lead to revertion to the reactant closed isomer (following the gradient difference vector when leaving the degeneracy) but this path, although possible, is less probable. The inertia will lead the system preferentially, following the derivative coupling vector of the branching space, to the S0 minimum of the cis isomer of merocianine (cis-MC). The subsequent evolution of the reaction will be the same as in the thermal opening reaction, leading finally to the transMC product. Closure reaction The closure reaction takes place at irradiation of the products of the opening reaction, which will be the cis- and trans-MC isomers in the proportion given by the thermodynamic equilibrium between the corresponding minima established along the ground state surface. To account for the initial excitation of these species, we calculated first the vertical excitation energies to the first three excited states of cisand trans-MC. Table 2 summarizes the results obtained, using as reference energy that of the S0 transMC minimum. For MC, the characters of the S1SCF, S2SCF and S3SCF states are 1(nO−π*), 1(nN−π*) and 1

(π−π*) respectively. The character of these states is preserved in the MS-CASPT2 results, so in this

part of the study is not necessary to distinguish between CASSCF and CASPT2 states. According to the oscillator strength, the state most populated by the photoexcitation is the third singlet excited state.

The search for the minima corresponding to the cis- and trans- species of MC in the different states gave the following results. The 1(π−π*) state cis-MC and trans-ME minima and the connecting cis/trans isomerization transition state were located (Figure S4 of Supporting Information), but they laid on the second excited state PES. It means that a crossing between the S2 and S3 surfaces must occur between the FC region and the minima. For the 1(nN−π*) no minimum was located, while for the 1(nO−π*) state we located, on the S1 surface, the trans-MC minimum and a TS state to the region of the cis-MC (Figure S5 of Supporting Information), but the flatness of the surface in this latter area did not allow to locate a ACS Paragon Plus Environment

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proper minimum for the cis-MC species. At the MS-CASPT2 level, the trans-MC isomer on S2-(π−π*) state lies at 73.1 kcal mol-1 above the S0 minimum whereas the S1-1(nO−π*) state lies at 43.0 kcal mol-1. The computed barrier from the trans-MC to the cis-MC structure for S2-(π−π*) is 18.7 kcal mol-1 while it is 8.0 kcal mol-1 for S1. Since the state pumped with the initial excitation is the S3 state and the state involved in the photochemical reaction is the S1, the first part of the reaction path after light absorption must be nonadiabatic. We have already reported the results of the ring closure mechanism of similar Merocyanine-to-Spiropyran system, the 6-(2-propenyliden)cyclohexadienone.22 We showed that irrespective of the conformation of the reactant, the absorption of the initial excitation promotes the system to the S2 excited state that has a (π−π*) character. An equilibrium between the trans- and cis-MC isomers could be established on this potential energy surface, but a very efficient nonadiabatic path to the S1 state, that has a (nO−π*) character, is open all the way along the isomerization path. The seam of a sloped S2/S1 conical intersection runs parallel to this path at accessible energies, so the radiationless decay can take place at any rotational angle and the internal conversion to the S1-(nO−π*) state will be very fast. Once on the S1 surface, again the trans-cis equilibrium may be established, but from the cis minimum, a low-barrier path leads to a S1/S0 conical intersection. The initial movement of the nuclei will lead the system through the CI to the ground state of the 2H-benzopyran product in a very fast second step of the reaction. For the oxazine system studied here, we expect a similar scenario, with some differences due to the presence of the S2 (nN−π*) in the FC region. When searching, at the CASSCF level, where the S3SCF/S2SCF and S2SCF/S1SCF radiationless decays occur, we found two conical intersections in the vicinity of the trans-MC minima. The first one corresponds to a peacked conical intersection between the 1

(π−π*) and 1(nN−π*) states (S3/S2 CI). Going through it, the system keeps the same electronic

configuration (1(π−π*)) when it changes from S3 to S2. The second conical intersection is a sloped one between the 1(π−π*) and 1(nO−π*) states (S2/S1 CI), located 10 kcal mol-1 above the trans-1(π−π*) minimum, but around 20 kcal mol-1 below the excitation energy. Given that the minimum of the ACS Paragon Plus Environment

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(nO−π*) state is more stable than those of the 1(π−π*), this CI opens an accessible channel for internal

conversion from the 1(π−π*) state to the 1(nO−π*) state that will be fast and efficient. No conical intersection between the 1(nN−π*) and 1(nO−π*) states could be located. The geometries of the minimum energy points of these intersections optimized at the CASSCF (12,11)/6-32G(d) level and the coordinates of the branching space, the gradient difference and the derivative coupling vectors, are shown in Figure S6 of Supplementary Information. In both cases, the two degeneracy-lifting coordinates were almost parallel, including only skeletal deformations, so the CI is monodimensional, resembling very much the ones found in the 6-(2-propenyliden)cyclohexadienone model system.22 To determine how the system evolves after the initial excitation and confirm the features outlined by the CASSCF results, the profiles of the S0, S1, S2 and S3 surfaces between the Franck-Condon region and the 1(π−π*) trans-MC minimum were calculated at the MS-CASPT2//CASSCF level by linear interpolation. The results are shown in Figure 6. We can see that the relaxation to the 1(π−π*) trans-MC minimum is a direct process. The most important feature is that the 1(π−π*) → 1(nO−π*) internal conversion can take place very efficiently because the 1(nN−π*) state becomes quickly the third excited state in the reaction path, so the S1(nO−π*) will be populated directly from the S2(π−π*). Thus, it is proved that the 1(nN−π*) state does not play any important role in this photochemical reaction, neither in the closure nor in the opening process, and the 1

(π−π*) state rapidly decays to the S1 state which is actually the reactive state. From the S1 trans-MC isomer the system can easily overcome the cis/trans transition state, only 8,0

kcal mol-1 high and, following the transition vector,

decay rapidly to the peaked S0/S1 conical

intersection of the opening reaction, through which the photochemical closure can also easily occur due to the inertia of the movement of the nuclei. The system will leave the degeneracy very rapidly towards the valley of the product, because the excited state continues directly along the ground-state potential energy surface avoiding excited state equilibration.


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A cartoon showing all the features of the PES explained in this section is shown in Figure 7, where the critical points located at CASSCF level appear highlighted together with the CASSCF energies. Mechanism in acetonitrile To calculate the effect of a bulk polar solvent on the reaction mechanism we included the long range effects of the solvent through the PCM model as described in reference17. To reproduce the experimental conditions we chose acetonitrile as solvent, characterized by a dielectric constant of 26.6 at room temperature. Like in the gas phase, we used the smallest model (Model-1) for this set of calculations. These results are collected in Table 3. All the structures involved in the reaction mechanism were reoptimized at the CASSCF level (and the ground state geometries also at DFT level, see next section), except the CI geometry, as this type of calculation is not implemented in the commercial version of the Gaussian-09 package. The energies of the CASSCF optimized structures were recalculated, in the solvent environment, at the CASPT2 level. As can be seen from the results in Table 3, the qualitative description of the thermal reaction does not change when the solvent is taken into account: the SO isomer is still more stable than the trans-MC one, by 29.2 and 25.7 kcal mol-1 at the CASPT2 and DFT levels respectively, and the trans-MC structure is again the most stable MC isomer, while the shallow minimum corresponding to the cis geometry is higher in energy. The barrier for the ring opening reaction, SO → MC, is still too high so it makes the thermal reaction only possible at high temperatures. Also in these calculations the effect of the dynamical electron correlation seems to be quite important since all CASSCF results differ appreciably from the CASPT2. These ones, though, are in a remarkably good agreement with the DFT results. It is noteworthy that the thermal reaction path is affected only marginally by solvent effects, even if the total solvation energies are not negligible at all (they are, for example, 8.5 kcal mol-1 and 5.4 kcal mol-1 for SO and trans-MC respectively), in agreement with the sizable computed dipole moments (i.e. 1.4, 2.2 and 2.1 Debye for SO, trans-MC and cis-MC isomers, respectively). Actually, the inspection of the SO and MC structures shows that the electronegative oxygen and nitrogen atoms, responsible for the dipoles, always keep in the same relative positions, while the reaction coordinate corresponds mainly to ACS Paragon Plus Environment

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the rotation of a less polar carbon tail. It is not surprising then that the reaction profile is not very affected by the medium polarity. Special consideration deserves the calculation on the conical intersection structure. As stated before, the geometry cannot be reoptimized with the PCM methodology, so the results given in Table 3 correspond to CASSCF and CASPT2 single point calculations at the CASSCF geometry obtained in gas phase. On top of this handicap, it cannot be taken for granted that the solvent is equilibrated with the excited state electronic distribution when the systems goes through the S0/S1−CI, but the theoretical methodology can only calculate the effect of the solvent in the two extremes cases, the non-equilibrium and the equilibrium time-regimes. The option adopted in our study was the first one to be coherent with the computations of the rest of the critical points. At CASSCF level, the relative energies of the S0 and S1 states, that should be degenerate, are 47.9 and 50.6 kcal mol-1. At the single-state CASPT2 level the results are similar, giving a ground state-(nO−π*) state energy gap of only 3 kcal mol-1, but when the multi-state CASPT2 approach is used the energy difference enlarges up to 12 kcal mol-1. These results indicate that a strong interaction exists between those two states, what means that at the CASPT2 level the geometry of the gas phase CASSCF conical intersection lies on an avoided crossing region. This is not a surprising result, because around a CI there is a region of avoided crossing. All in all, these results confirm that the CI between the S0 and S1 states still exists when dynamical electron correlation is taken into account. As a whole, the data reported in this section clearly show that the long range solvent effects have a very little influence on Model-1 thermal and photochemical reactivity. As mentioned in the Introduction, there are experimental evidences that solvent polarity may or may not affect the reactivity of spirooxazine/merocyanine couples depending on the ring substituents, and our results confirm that the “reactive nucleus”, modelled in the small Model-1, is not sensitive to polar environments, although the various species have non negligible solvation energies, as repeatedly pointed out. Effect of addition of a protic solvent


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As stated in the Introduction, one of the most intriguing features of the photochromism of spirooxazines is related to the role of protic solvents in shifting the equilibrium between open and closed forms. This equilibrium can be easily monitored by absorption spectroscopy, since the closed isomer absorbs in the UV region, while the open form exhibits an absorption peak in the visible range. In polar aprotic solvents, the equilibrium is markedly shifted towards SO, and the resulting solution is almost colourless, depending the actual amount of coloured open form on the nature of the substituents. When acetonitrile/water mixtures are used, though, the solution becomes much more coloured. It can be completely bleached by visible light irradiation, but slowly reverts to the initial colour when the light is turned off. This behaviour has been interpreted as follows: protic solvents, even if they are present in a small proportion, stabilize the MC isomer relative to the SO one, shifting the thermal equilibrium towards the coloured MC form. Irradiation causes, like in the gas phase or in aprotic solvents, the photoisomerization between the closed and open forms through the mechanisms (forwards and backwards) investigated in the previous sections, decreasing the proportion of the coloured form while the irradiation is present. When the photoexcitation stops, the system is restored to the initial conditions through the slower thermal isomerization. To investigate this hypothesis theoretically, only the ground state potential energy surface needs to be studied in the presence of the protic solvent molecules. The fact that the photochemical reaction does not change when water is mixed with acetonitrile indicates that the reaction path on the excited state potential energy surfaces is not affected by protic solvents. Regarding the model to be used in this part of our study, however, we need one that contains all the heteroatoms present in the reactive moiety of the real system, because all of them could be involved in hydrogen bonds with water molecules and affect the reactivity. For this reason we used a larger model (Model-2 in Figure 3) in this part of our work. But as mentioned before, the short range effects of a protic solvent (H bonds) cannot be reproduced by the PCM method, so we also need to include explicit water molecules in the part of the system studied at the atomic level. In a preliminary study up to three water molecules were included in the calculations to allow for interactions with all the heteroatoms of Model-2. It was observed that only ACS Paragon Plus Environment

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one H bond kept formed, but the second water molecule still changed the relative energies of the species studied, and not so the third one. Consequently, we included only up to two water molecules in the subsequent calculations. The enlargement of the size of the system to be studied and the fact that only the ground state was to be investigated, made advisable the use of DFT methods in these calculations. In fact, the aim of this part of the study is only to analyse trends, no to obtain quantitative results. Anyway, to check the suitability of this methodology to study this kind of problem, we reproduced the study of the ground state reaction for Model-1 reported in the previous section at the DFT level, reoptimizing the geometries of the critical points to obtain the DFT energies. The results obtained, included in Table 3, show a remarkably good agreement with the CASPT2 results, giving a reliable ground for the subsequent study. For Model-2, unlike for the simpler Model-1 used above, different isomers can coexist. In Figure 6 we show the three merocyanine bonds around which a cis/trans isomerism is possible, giving rise to eight isomers labelled as LMN, with L, M, N= C (cis) or T (trans). Out of them, structures LMT cannot undergo the closure reaction and they are known to be less stable that the LMC isomers,[23-26] so only the four LMC structures shown in Figure 6 will be considered in the following. From them, TTC and CTC are the isomers considered in previous experimental and computational works as the stable MC forms in actual photochromic systems (a fair compendium of previous works is given in page 2758 of reference2). We have optimized these isomers at the B3LYP/6-31G level for Model-2, in gas phase and in acetonitrile, without and with one or two explicit water molecules. Geometries of the main structures with one and two water molecules can be found in Figure S6 of Supplementary Information. In all environments and both for R=H and R=CH3 TTC and CTC are more stable than TCC and CCC, although the relative stability for the first two isomers oscillates slightly, in agreement with previous experimental and theoretical results for spyrooxazines and spiropyrans.23,

26

The less stable

isomer is the CCC one due to the steric repulsion produced by this conformation. In the case of Model-2 (R=CH3) with two water molecules, even, this species was not found because the ring directly closed. Surprising is the stability of the CCC isomer for R=H in the gas phase, only 2.2 kcal mol-1 less stable ACS Paragon Plus Environment

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than the TTC one. This anomalous result is due to the proximity in this configuration of the amino H and the O atoms (only 1.53Å far apart) that allows forming an intramolecular H-bond. This interaction is an artefact of the simplified model (Model-2, R=H), which disappears when the nitrogen bears a methyl substituent, like in the actual spiroindolinonaphthooxazine. For this reason we limited the following study to the more realistic Model-2 with R=CH3. The most interesting result concerns the closed form energies (last column of Table 4): the SO form is favoured with respect to TTC by 9.6 kcal mol-1 in gas phase, energy difference that decreases to 5.0 when the system is in a polar solvent. When we represent the mixture of protic and aprotic solvents by adding water molecules to the model, the relative stability of the SO form decreases even more: it is almost isoenergetic with the TTC isomer when we include one water molecule, and it becomes clearly less stable than the TTC and CTC isomers when two water molecules are taken into account. This result is in agreement with the trend observed experimentally and commented above. The reason is probably the difference on the partial charge of the O atom, which is larger in the open forms than in the closed one (Mulliken charges in acetonitrile: -0.63 and -0.63 of TTC and CTC v.s. -0.58 for SO) giving place to stronger H-bonds in the first isomers, stabilising them relative to the closed form. The conclusion is that the experimental behaviour in protic/aprotic solvent mixtures can be reproduced by the calculations provided the model is large enough to include all the electronic effects on the heteroatoms.

CONCLUSION The study of the mechanism of the thermal and phtochemical ring opening/clousure reactions in spirooxazines has been performed using ab initio computations in model compounds. The use of the smaller model, Model-1, to study the mechanism of the open reaction is clearly justified, given that it is well known that the spectra of spirooxazines are the sum of the spectra of the individual indoline and oxazine moieties (neglecting the charge transfer states, that are not involved in the reactivity studied here), so it is evident that each part of the system hardly has any influence on the photochemical ACS Paragon Plus Environment

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properties of the other part. The justification of the use of Model-1 in the closure reaction is based on previous theoretical studies on the absorption of merocyanine open forms,[25] that show that their lowest excited states do not involve transitions from the lone pair of the nitrogen of the indoline moiety, so our study can be limited to the oxazine part. This limitation will obviously induce quantitative changes in excitation energies, but qualitative changes can be safely be though little probable. The study of the thermal reaction along the ground state explains the preference for the colourless species in gas phase and aprotic solvents, given the larger stability of the closed SO form in these environments. Regarding the photochemical reaction, we have shown that this mechanism is similar to that on spiropyrans, in spite of the existence in spirooxazines of a 1(nN−π*) excited state of low energy (that was no present in spiropyrans) because this state is not involved in the reaction mechanism. We have also shown that the characterization of the excited states involved in the first stages of the photochemical opening reaction, very important to describe correctly the evolution of the system, can only be properly done when the electron correlation effect is included in the calculations, in this case through the MMCASPT2 method. The description of an adiabatic PES defined all along the reaction path by a unique electronic configuration appears not to be a good picture in this case, where a strongly avoided crossing induces a very gradual change of electronic configuration. Regarding the closure reaction, the key point of the initial steps of the reaction mechanism is the type of the conical intersections that the relaxation path of the excited state populated initially (1(π−π*) state) goes through. The first CI is a peaked one, so the system changes from S3 to S2 without changing the electronic configuration. The second one is a sloped one, where the system migrates from the minimum of the S2(π−π*) state to the minimum of the S1(nO−π*) state. In both, the closure and opening mechanisms, the reactant and the product valleys are connected by a peaked conical intersection between the S0 and S1 states, that is involved in both, the direct and the reverse photochemical reactions.


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We have also analysed the influence of the environment on the photochemical and thermal reactions, trying to modelize the most intriguing experimental observations. We tried to explain first why the photochemical reaction was affected only in some cases by polar solvents. Using Model-1, which includes only the photoactive part of the spirooxazine, we have shown than the profiles of the PES of the ground and excited states along the reaction path are not modified by the presence of a polar solvent. This fact is not due to a negligible value of the solvation energy, but to an almost constant value of this energy all along the reaction path. The presence of substituents in different derivatives can modify this behaviour due to a differential interaction between some substituents and the solvent. In these cases a solvent polarity influence on the photochemical reaction could be observed. The second intriguing experimental observation that we have studied is the influence of binary mixtures of polar aprotic and protic solvents on the thermal equilibrium, even in the cases where the protic solvent is present in very small amounts. For this part of the study we have used the more economic DFT computational method, given that we had to use a larger model and take into account explicit water molecules to account for the H-bonds possibly formed and thanks to the fact that only the ground state is involved in the thermal equilibrium. We have shown that in fact the H-bonds formed with the protic solvent molecules are responsible of the preferential stabilization of the open MC form relative to the closed SO form, so the equilibrium in the ground state is displaced towards the MC coloured form in the presence of protic solvents. The good agreement of the conclusions derived from our theoretical results with experimental observations supports the adequacy and quality of our computational methodology.

Acknowledgements


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Financial support has been provided by the Spanish Ministerio de Ciencia e Innovación (Projects HI04-337 and CTQ2011-23140) and the Generalitat de Catalunya (Project 2009SGR462 and Xarxa d’R+D+I en Química Teòrica i Computacional, XRQTC) Supporting Information Available

Composition of the PM-CAS-CI functions in terms of CASSCF states at the SO ground state geometry. Energy path on the S0 potential energy surface from trans-MC to cis-MC. Geometries of secondary critical points mentioned in the main text. This information is available free of charge via the Internet at http://pubs.acs.org

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a) Tomasulo, M.; Deniz, E.; Sortino, S.; Raymo, F. M. Photochem. Phtobiol. Sci. 2010, 9, 136-

140. b) Coelho, P. J.; Carvalho, L. M.; Moura, J. C. V. P.; Raposo, M. M. M. Dyes Pigm. 2009, 82, 130-133. c) Szacilowski, K. Chem. Rev. 2008, 108, 3481-3548. d) Tan, B. H.; Yoshio, M.; Ichikawa, T.; Mukai, T.; Ohno, H.; Kato, T. Chem Comm. 2006, 45, 4703-4705. [6]

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Phys. Org. Chem 2005, 18, 315-320. [10] Roos, B. O. Adv. Chem. Phys. 1987, 69, 399-445. [11] a) Herhe, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257-2261. b) Harihan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213-222. [12] a) Fukui, K. Acc. Chem. Res. 1981, 14, 363; b) Schmidt, M. W.; Gordon, M. S.; Dupuis, M. J. Am. Chem. Soc. 1985, 107, 2585. c) Gonzalez, C.; Schlegel, B. J. Chem. Phys. 1989, 90, 2154. d) Gonzalez, C.; Schlegel, B. J. Phys. Chem. 1990, 94, 5523. [13] a) Anderson, K.; Malmqvist, P. A.; Roos, B. O.; Sadlej, A. J.; Wolinski, K. J. Phys. Chem. 1990, 94, 5483. b) Anderson, K.; Malmqvist, P. A.; Roos, B. O. J. Chem. Phys. 1992, 96, 1218. [14] a) Roos, B. O.; Andersson, K. Chem. Phys. Letters 1995, 245, 215. b) Roos, B. O.; Andersson, K.; Fülscher, M. P.; Serrano-Andrés, L.; Pierloot, K.; Merchán, M.; Molina, V. J. Mol. Struct. Theochem 1996, 388, 257. [15] Finley, J.; Malmqvist, P. A.; Roos, B. O.; Serrano-Andrés, L. Chem. Phys. Lett. 1998, 288, 299306. [16] a) Celani, P.; Robb, M. A.; Garavelli, M.; Bernardi, F.; Olivucci, M. Chem. Phys. Lett. 1995, 243, 1-8. b) Garavelli, M.; Celani, P.; Fato, M.; Bearpark, M. J.; Smith, B. R.; Olivucci, M.; Robb, M. A. J. Phys. Chem. A 1997, 101, 2023-2032. [17] a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comp. Chem. 2003, 24, 669.


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Table 1. Character of the 9 lowest excited states, energies (∆E, kcal mol-1) relative to the SO ground state minima (-697.370 a. u.) and oscillator strengths (f) at the Frank Condon geometry for Model-1 obtained at the MS-CASPT2//CASSCF(14,12) level in gas phase. State

f

∆ECASPT2

S1MS

1

(π-π*), 1(pO−σ*)

95.3

0.100

S2MS

1

(nN−π*), 1(π−π*)

109.6

0.041

119.2

0.025

S3MS

1

1

1

(nN−π*), (π−π*), (pO−π*)

S4MS

1

(π−π*), 1(pO−π*)

141.4

0.135

S5MS

1

(π−π*), 1(pO−π*)

154.4

0.308

S6MS

1

1

159.3

0.020

(π−π*)

178.8

0.081

(nN−π*)

185.8

0.003

1

189.2

0.064

(π−π*), (pO−σ*) 1

S7MS

1

S8MS S9MS

(π−π*)

Table 2. Absorption energies (∆E, kcal mol-1) and oscillator strengths (f) for the two MC isomers in gas phase obtained using a (14,12) active space and a S0-S3 state average. Trans-MC State

S0

∆Ea

∆Eb

CASSCF

Cis-MC f

∆Ea

∆Eb

CASPT2

CASSCF

CASPT2

0.0

0.0

9.5

9.9

f

S1

1

(nO−π*) 71.1

47.6

0.000

80.0

54.4

0.00

S2

1

(nN−π*) 92.9

74.0

0.000

94.1

74.1

0.00

1

83.3

0.32

111.3

91.9

0.23

S3 a b

(π−π*) 101.9

Referred to −436.406527 u. a. Referred to −436.612738 u. a.


26

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Table 3. CASSCF(12,11), CASPT2 and DFT (B3LYP) relative energies (in kcal mol-1) for Model-1 in acetonitrile for ground and (nO−π*) states.

CASSCFa

CASPT2b DFT[c]

SO

TSopen cis-MC trn/cis-TS trans-MC

S0

0.0

49.9

29.8

barrierless

16.1

S1

86.5

90.6

flat

59.9

51.7

S0

0.0

43.8

36.8

---

29.2

S1

68.5

64.3

---

56.4

48.4

S0

0.0

40.5

32.8

barrierless

25.7

a

Referred to −436.42292 a. u.

b

Referred to –437.61823 a. u.

c

Referred to –438.98254 a. u.

Table 4. Relative energies of some isomers of Model-2 with R=H and R=CH3 in different environments. Energies obtained at the B3LYP/6-31G level.

R=CH3

TTC

CTC

TCC

CCC

SO

Gas phase

0.0

1.5

8.0

13.3

-9.6

Acetonitrile

0.0

1.4

7.9

12.7

-5.0

+ 1 H2O

0.0

-2.1

4.8

9.5

0.5/0.9a

+ 2 H2O

0.0

2.5

---b

---b

6.0/6.1a

R=H

TTC

CTC

TCC

CCC

SO

Gas phase

0.0

-1.5

7.5

2.2

-11.15

Acetonitrile

0.0

0.3

7.4

7.6

-4.7

a Molecule non-planar; two isomers b No minimum found: the ring closes


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Page 28 of 33

Scheme 1. Photochromic moiety in spiropyrans and spirooxazines. Scheme 2. Photochromic reaction in spirooxazines. Scheme 3. Model systems to study the phtochromism of spiroozaxines. (a) Model-1. (b) Model-2 (R=H, CH3). Figure 1. Geometries of the open and closed minima and transition state on the S0 potential energy surface of the thermal ring-opening reaction. Geometries optimized at the CASSCF(12,11)/6-31G(d) level; bond distances in Å. Figure 2. Molecular orbitals involved in the description of the excitations of the first excited states of SO of Model-1, obtained at the CASPT2(14,12)/6-31G(d) level. Figure 3. Geometries of the S1 minimum, S1 transition state of the ring-openning reaction and. S1/S0 conical intersection ((v1) derivative coupling vector; (v2) gradient difference vector). Geometry optimizations performed at the CASSCF(12,11)/6-31G(d) level. All bond distances are in Å. Figure 4. Energy profile of the PES of the lowest 9 states fo SO, interpolated from the FC region to the S2/S1 CI and to the S1/S0 CI. a) CASSCF(12,11)/6-31G(d) level; b) CASPT2(12,11)/6-31G(d) level. Figure 5. Molecular orbitals 35 and 36 at some selected geometries along the path interpolated from the FC region to the S2/S1 CI and to the S1/S0 CI. Figure 6. Energy profiles of the S0, S1, S2 and S3 potential energy surfaces along the path interpolated between Franck Condon trans-MC geometry to 1(π-π*) trans-MC minimum. (a) CASSCF(14,12)/631G(d) results; (b) CASPT2(14,12)/6-31G(d) results. Figure 7. CASSCF(14,12)/6-31G(d) energy profile of the potential energy surfaces of the lowest energy states along the reaction close spirooxazine to trans-merocyanine. Figure 8. Isomers of the MC due to the configuration of the three bonds around which a cis/trans isomerism is possible. Only the ones able to yield closure reaction are shown.


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Scheme 1

Scheme 2

Scheme 3

Figure 1

2.063

1.391

1.262 1.448

1.328

1.455 1.342

1.362 1.444 1.437

1.361

closure TS


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Page 30 of 33

Figure 2

Figure 3

Figure 4


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Figure 5 PM-CAS-CI WAVEFUNCTION ALONG THE PATH FC

2

51% ϕ35 → ϕ36

62% ϕ35 → ϕ36

17% ϕ34 → ϕ36

7% ϕ34 → ϕ36

S2MS/S1MS

4

5

6

S1 MS

64% ϕ35 → ϕ36

61% ϕ35 → ϕ36

65% ϕ35 → ϕ36

68% ϕ35 → ϕ36

63% ϕ35 → ϕ36

/S0MS

Figure 6

Figure 7 1

1

( - *) (po- *)

( - *)

1

103 S3 / S2

131

1 (nN- *) 93 1 (no- *) 71

S2 / S1

1

(nN- *)

1

( - *)

18.7 115

S2 / S1

Flat region

101

3.8

8.0 (po- *)

S1 / S0

1

Barrierless

0.2

47.5

9.1

N

N

N

N

O

O

O

O

SO

opening/closure TS

cis-MC

trans-MC


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Figure 8

CTC

TCC

TTC

CCC


32

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TABLE OF CONTENTS


33

Computational Study of the Mechanism of the Photochemical and

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