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How Metals Can Help Multiphotochromism: An Ab Initio Study Arnaud Fihey, and Denis Jacquemin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03324 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 4, 2016

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The Journal of Physical Chemistry

How Metals Can Help Multiphotochromism: an Ab Initio Study Arnaud FIHEY∗,† and Denis JACQUEMIN∗,†,‡ †Chimie Et Interdisciplinarité, Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS no. 6230, BP 92208, Université de Nantes, 2, Rue de la Houssinière, 44322 Nantes, Cedex 3, France. ‡Institut Universitaire de France, 1 rue Descartes, F-75005 Paris Cedex 05, France. E-mail: [email protected]; [email protected] Phone: +32 (0) 2 76 64 51 73; +32 (0) 2 51 12 55 64

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Abstract Dimers of dithienylethenes containing a metallic center in their bridging unit often exhibit a full and stepwise photochromism and their photoactivity typically surpasses the majority of their metal-free counterparts that are often plagued by the impossibility to switch all units. In this work, we investigate five different photochromic dimers with a relativistic Spin-Orbit Time-Dependent Density Functional Theory (SO-TD-DFT) method to explore the possible sources of this enhanced photoactivity. In particular the potential intersystem crossing (ISC) from an intense singlet state in the UV region to a triplet state presenting the necessary topology to trigger the ring closing of the switch is unravelled in several cases. The efficiency of this crossing is found dependent on both the nature of the metallic center and the structure of the dithienylethene ligand but, in general, improves the probability for the second ring closing starting from the partially switched dimer. This work provides the first theoretical evidences of the presence of strong relativistic effects yielding ISC to a “photochromic” triplet state in these complex systems. This allows for the rationalization of several experimental outcomes.

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Introduction Photochromism allows one to control the state of the matter at the molecular level, 1,2 and it has been used to build a wide variety of light-responsive intelligent systems in several booming fields, such as optoelectronics and molecular logic gates. 3–5 Among the photochromic systems based on the interconversion between an open and a closed isomer, dithienylethene (DTE) derivatives certainly stand as one of the most widely used families of organic switches, notably because of their thermal stability, high fatigue resistance and particularly rapid (ps) photochemical reactions. 6–10 During the last decade, numerous efforts have been achieved to use these organic molecules as building blocks in complex supramolecular switches potentially exhibiting emerging functionalities. 11 However the outcomes have often been disappointing. Indeed, when several DTE units are linked together through a conjugated and/or short organic bridge, so that a strong π electronic communication between the different entities is established, only partial photochromism is typically observed, i.e, it is not possible to photoswitch all the DTE units, but only a fraction of them. 12,13 In parallel, the insertion of organic photochromes in organometallic complexes has been proven useful both to control non-linear optical properties 14 and to combine the switching abilities of the DTEs with the redox properties of the metal. 15,16 Interestingly, when two photochromic DTEs are attached to a metallic center, the resulting multiswitch is, in most cases, completely operational: it shows a full –all DTEs can be closed– and stepwise –intermediate closed-open isomers can be isolated– photochromism (see Scheme 1). These organometallic systems obviously contrast with the corresponding metal-free dimer, in which, as stated above, the doubly-closed isomer is often not formed. This statement is well illustrated by comparing the ethynyl-based 1 and 2 in Scheme 2: only the later displays full photochromism, as demonstrated by 1 H NMR measurements. 17 Successful examples of DTE multimers based on a metallic center have been developed by Chen et al., 17,18 where the DTE is attached to the metal through an ethynyl linker (2), and also by the groups of Guerchais, 19 Rigaut, 16 and Abru˜ na 20 who used bipyridine rings substituting the DTEs to bind the metal. Similar strategies have been 3



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used with several noble and transition metals to build gold, 18,21 platinium, 22 ruthenium, 18 zinc 19 and copper 23 organometallic assemblies bearing several DTE switches (see Scheme 2 for representative examples). Heteronuclear complexes combining a ruthenium and two iron metallic centers have also been successfully synthesized and remained fully photoactive. 24 Interestingly, when the two photochromes are attached to a gold atom through the aromatic bridge maintaining together the two thiophene rings in the open isomer (3 in Scheme 2), the resulting multiswitch is not fully active as only one DTE closes under UV irradiation. 21 This hints for rather complex structure-property relationships in this type of multiphotochromes. F2

F2 F2

F2

F2

F2 F2

F2

F2

F2

F2

F2

F2

S

M open-open (oo)

S

S

F2 F2

F2

F2

S

S

UV

UV S

F2

Visible

S

S

M S

S

closed-open (co)

Visible

S

M S doubly closed (cc)

Scheme 1: Full photochromism in complex of DTEs. M represents the metallic coordination center. In short, the partial photochromism in metal complexes is more an exception than a rule, 11 and this enhanced photoactivity in the presence of an heavy element remains to be explained for DTE dimers. We note that for DTE monomers there have already been several studies addressing the importance of a metal. The first explanation for an increased photochromic activity was provided by De Cola et al. 25 and later confirmed by Scandola and coworkers 26 who proposed that DTE derivatives can cyclize through a photoactive triplet state rather than a singlet state in some specific cases. To support this idea the later group investigated a single DTE linked to a Ru coordination center and used both timeresolved spectroscopy and theoretical methods. 26 In the case of a Pt complex bearing one photochromic unit and a polypyridine moiety, the role of a photoactive triplet has also been proposed as an explanation for the feasibility of the ring closing under irradiation with a particularly large wavelength. 27 Moreover, using steady-state and nanosecond transient absorption spectroscopies, a triplet pathway has also been unravelled for a metal-free DTE 4


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F2

F2 F2

S

F2

F2

F2

S

S

S

1

F2 F2

F2 F2

F2

+

F2

S

N

N

S

Au N

N S

Au

S

S

S

S

S

3

2 F2

F2 F2

F2 C4H9

S

C4H9 C4H9 P Pt

S

F2

F2

S

S

P C4H9

C4H9 C4H9 4

F2 F2 N S

S

F2 F2

F2 PH2P PPH2 Ru PH2P PPH2

F2 N

S

S

5

Scheme 2: Dimers of DTE investigated. monomer in presence of triplet sensitizers. 28,29 In all cases, the singlet to triplet InterSystem Crossing (ISC) is of course facilitated by the intrinsically large spin-orbit coupling (SOC) induced by the metal, or by the presence of a sensitizer. In the experimental study of dimer 4, this concept has also been transposed to a DTE dimer and Robert et al. who suggested that “rapid intersystem crossing likely populates the triplet manifold of DTEs”, granting efficient photochromism, 22 though evidences of this mechanism were not provided. For investigating excited-state phenomena, theoretical tools are often useful if not necessary. If the photoreactivity of the isolated DTE core has been thoroughly explored thanks to 5



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multiconfigurational wavefunction methods, 30,31 the theoretical studies of the ground-state structures of the DTE multimers, that are large π-conjugated systems, have been exclusively conducted with the help of Density Functional Theory (DFT) methods. Time-Dependent DFT (TD-DFT) 32,33 has logically been applied to describe the excited states, to simulate the absorption spectra, and to predict the cyclization reactions in DTE dyads and triads. 34–36 For the large metal complexes investigated here (Scheme 2), wavefunction methods are also beyond reach, and SOC-TD-DFT is the method of choice to access the excited states properties. Such approach has been applied in a wide variety of metal complexes with a focus often set on the evaluation of Metal-to-Ligand Charge Transfer (MLCT) bands. 37,38 For such transitions exhibiting a large metallic character, simplified methodologies have also been used, i.e., one can rely on the sole consideration of the atomic coefficients of the heavy metal atom in the molecular orbitals populated in the electronic transitions of interest to estimate the SOCs. 39 The number of previous ab initio works performed for metal-containing DTE multimers is limited, 14,23,40 and most of these studies focused on their non-linear optical properties rather than their photoactivity. 14,23 Nevertheless, in a previous scalar TD-DFT study, 40 it has been shown that the analysis of the singlet excited states alone predict falsely a limited photochromism in the platinium-based DTE dimer 4, and a potential mechanism involving a photoactive triplet was evoked, though no spin-orbit calculations were performed to support this statement. The goal of the present contribution is to clarify the role of the metal on the full photochromsim and to investigate the potential pathways leading to a photoactive triplet state. To this end, the spin-orbit relativistic optical properties of four organometallic DTE dimers and one organic dimer (see Scheme 2) are detailed in the following, with consecutive emphasises on: i) the determination of the singlet states suited for ring closing and their evolution when the dimer is a metallic complex or a pure organic compound; ii) the modification of those singlet states when considering spin-orbit mixed excitations; and iii) the possible ISC after the vertical excitation allowing to populate a “photochromic” triplet state from

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a dipole-allowed singlet state. The dependences on the nature of the metal and the linking position of the DTE on the coordination center are investigated through the consideration of structurally different complexes. To the best of our knowledge, this is the first attempt to rationalize the photactivity of multiphotochromic complexes combining the widely used TDDFT/virtual molecular orbital approach 34,41 with a SOC analysis to pinpoint spin-forbidden mechanisms.

Computational Details The geometries of the different structures have been fully optimized using the PBE0 hybrid functional, 42 combined with the 6-311G(d,p) atomic basis set for the light atoms and the LANL2DZ pseudopotential-type basis set for the metallic centers. Frequency calculations were next performed at the same level of theory to ascertain the minimal nature of the ground-state geometries. Solvent effects (dichloromethane) were included in both steps with the help of the Polarizable Continuum Model (PCM). 43 The Gaussian09 package 44 was used for these calculations. In a subsequent stage, spin-orbit relativistic TD-DFT calculations have been performed with the ADF-2014 software. 45,46 For each dimers 30 singlet and 30 triplets have been typically computed, at the B3LYP 47 /TZP (all-electrons) level. The relativistic effects have been included through the use of the ZORA Hamiltonian, 48 and spin-orbit effects have been taken into account using a perturbative approach during the TD-DFT calculations to obtain spin-orbit excitations mixing the spin-free scalar relativistic singlets and triplets. 49 Solvent effects (dichloromethane) have also been included during the TD-DFT calculations using the COSMO continuum method. 50 Such protocol has already been proven to accurately describe the optical properties of metallic complexes of different natures. 39,51 The spin-orbit integrals between singlet Sn and triplets Tm states presented below have been calculated as follows using the matrix element of the SO Hamiltonian HSO ; 52,53

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hSn |HSO | Tm i =

(

X

(Re2 hSn |HSO | Tm,α i + Im2 hSn |HSO | Tm,α i)

α=x,y,z

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)1/2

(1)

where Re and Im are the real and imaginary part of the matrix, and α denotes the Cartesian sublevels of the triplets. For Pt (4) and Ru (5) complexes the alkyl chains binding the phosphorous atoms (see Scheme 2) have been replaced by hydrogen atoms during the excited states calculations in order to keep the calculations tractable. Those ligands are optically transparent in the investigated wavelength range and are not expected to impact significantly the TD-DFT results.

Results and Discussions We investigated the five experimentally known dimers represented in Scheme 2. In 2 17 and 3 21 the two DTEs are linked to a gold atom, and 4 and 5 are respectively platinium 22 and ruthenium 18 complexes. Compounds 2, 4 and 5 use an ethynyl termination to bind the metal and are fully active photochromic systems. In contrast, the two DTEs in 3 are linked to the metal through their bridge. In this system, only one of the two photochromes closes, leading to 3-co. As a reference, the results on the partially active metal-free dimer 1 17 are also presented. For each compound two isomers have been investigated: the openopen and the closed-open forms. A full description of all electronic transitions in the UVVisible region is not the scope of this work, and all the results presented in the following focus on the feasibility of the two subsequent photochemical reactions (oo→co and co→cc). In particular the competition between the three following mechanisms is discussed: i) the vertical population of a singlet state triggering cyclization; ii) the population of a triplet state that also allows this reaction, via an ISC; and iii) the possible interconversion (IC), leading to a fall from a high-lying singlet/triplet onto a lower singlet/triplet state that permits (or not) photochromism. Indeed, in DTE derivatives, the photocyclization is typically a 10−12 - 10−15 8


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s process 54 and so is IC (10−11 - 10−14 s timescale 55 ). Both phenomena can compete, though, when they are both possible no quantification of the priority between the two mechanisms is possible at the TD-DFT level. Concerning ISC in DTEs, no timescale has been reported yet but experimental evidences of a triplet pathway for cyclization 25,26 implies that it can occur prior to, or simultaneously with, both ring closing and IC. Given these facts, we will first consider the competition between cyclization from a singlet state and IC mechanism, and next the ISC to a triplet state that is considered accessible when a sufficient SOC is present. Lastly we account for cyclization from a triplet state reached by ISC and IC to a lower triplet state.

First electrocyclization Analysis of the singlet states Let us first detail here the nature of the singlet states obtained for the different dimers in their open-open form, along with a study of the molecular orbital populated by UV absorption. We aim to assess the possibility to reach the mixed closed-open isomer. In the following, we consider as feasible the electrocyclization of a DTE if a dipole-allowed transition with a significant oscillator strength populates a virtual orbital possessing a bonding contribution between the carbon atoms involved in the C-C bond formation. These transitions are considered as the first step of the photoreactivity, leading eventually to the ring closing of the DTE, and are refered as “photochromic transitions” in the following. For additional discussions on this methodology, that has often been shown suitable to predict the reactivity of DTEs, we redirect the interest reader to previous works. 34,41 For all dimers, the absorption spectra of the doubly-open isomers present a low-lying intense electronic transition involving a virtual orbital that possesses the photochromic topology (see Table 1). These photochromic virtual orbitals are represented in Figure 1. One can observe significant differences between the photochromic orbitals of the metal-free 1-oo and

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the different complexes. First, while the LUMO is mainly localized on the organic bridge for 1-oo (but is still bonding between the reactive carbon atoms, see the SI), the two lowest virtual orbitals of 2-oo, 4-oo and 5-oo present the hallmark shape and are similar to the LUMO+1 and LUMO+2 of 1-oo. For the three metal complexes the LUMO and LUMO+1 are almost purely found on one or the other DTE, when they are half-shared upon both units for 1-oo, indicating a higher conjugation in this latter case. In 3-oo the outcome is different due to the linkage of the two DTEs through their imidazole-like moiety, and the LUMO and LUMO+1 are actually located on the metallic bridge, and only the LUMO+2 and LUMO+3 can be viewed as photochromic orbitals with the expected bonding topology between the reactive carbon atoms. Table 1 details the computed electronic transitions that populate these photochromic orbitals for the different dimers. The first and intense transition (f=1.76) of 1-oo is found at 402 nm and mainly corresponds to a photochromic HOMO→LUMO transition that can lead to the ring-closure of one of the DTE unit in the dimer. For 2-oo, the two photochromic orbitals LUMO and LUMO+1 are accessed through the intense S0 →S3 transition at 358 nm, the two lower-lying transitions being also photochromic transitions but with trifling oscillator strengths (they are detailed in Table S1 in the SI) . Moreover, the S0 → S7 , close in energy, can also lead to the direct ring closing of one photochrome. The situation is similar for both 4-oo and 5-oo complexes, as intense transitions found in the 350-400 nm region significantly populate the photochromic LUMO/LUMO+1 and can ultimately lead to the first isomerization. For 2-oo, 4-oo and 5-oo, the photochromic reaction is additionally not impeded by an IC to the lowest S1 as, in all three cases, this weakly-dipole allowed singlet state also possesses the needed photochromic topology (Table S1 in the SI). The structurally different 3-oo presents two first intense transitions S0 → S3 and S0 → S4 , that are nearly degenerated at 282 nm (f =0.13) and 281 nm (f =0.12), and respectively involve the photochromic LUMO+2 and LUMO+3 orbitals. The combination of those two photochromic transitions is then likely to trigger the first cyclization. We underline that the

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computed blueshift of the first intense transitions (below 300 nm) compared to the other dimers (between 350 and 400 nm) is perfectly in the line with the UV-Visible measurements. 17,18,21,22 In 3-oo, the S1 is located on the metallic bridge, and IC from the S3 /S4 to this lower (by 0.26 eV) singlet state would yield to a non-photoreactive situation. This indicates that IC in 3-oo is probably slower than cyclization or ISC (see below). Table 1: Computed singlet states (Sn ) involving the photochromic orbital for the doubly open isomers, along with the corresponding wavelength and oscillator strength. The percentage of photochromic orbital (phot. MO) involved in the transitions is also given. Dimer 1-oo 2-oo

photochromic MO LUMO LUMO/LUMO+1

3-oo

LUMO+2/LUMO+3

4-oo

LUMO/LUMO+1

5-oo

LUMO/LUMO+1

Sn S1 S3 S7 S3 S4 S2 S3 S5 S8

Wavelength (nm) (f ) 402 (1.76) 358 (0.45) 342 (0.52) 282 (0.13) 281 (0.12) 403 (0.11) 364 (1.15) 357 (0.33) 362 (0.38)

Amount of phot. MO populated 84% 87% 75% 96% 96% 97% 29% 70% 87%

For all the dimers, purely organic or relying on an organometallic linker, it is clear that a low-lying electronic transition promotes efficiently an electron towards the photochromic orbital that can induce the ring closing of one DTE. The electronic structure of the open DTE within the complexes is hardly modified, the photochromic transitions clearly remain mostly Ligand-to-Ligand (LL) transitions and do not evolve into clear Metal-to-Ligand (MLCT) or Ligand-to-Metal (LMCT) charge transfer transition (see the SI for additional analysis on the photochromic transitions). The presence of such photactive singlet states may nevertheless be tuned by the SOC effects, a point investigated below.

Spin-orbit coupling and triplet state population To illustrate the impact of the SOC on the computed excited states, Figure 2 presents both the scalar and SOC TD-DFT sticks spectra determined for the different open-open dyads

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Table 2: SOC values between intense singlet states (Sn ) and closest triplet states (Tm ) for the doubly open forms. The oscillator strengths (singlet only) and energies of each state are also given. Sn in the UV (f, E (eV/nm))

Tm (E (eV/nm))

1-oo

S1 (1.76, 3.08/402)

2-oo

S3 (0.45, 3.46/358)

T2 (2.70/460) T3 (2.79/445) T7 (3.21/386) T8 (3.21/386) T7 (3.21/386) T8 (3.21/386) T9 (3.50/354) T10 (3.51/353) T7 (4.04/307) T8 (4.05/306) T20 (4.75/261) T7 (3.25/381) T8 (3.27/379) T7 (3.25/381) T8 (3.27/379) T9 (3.42/363) T10 (3.45/360) T8 (3.02/410) T9 (3.08/402) T13 (3.21/387) T14 (3.39/366) T8 (3.02/410) T9 (3.08/402) T13 (3.21/387) T14 (3.39/366)

S5 (1.03, 3.56/348)

3-oo

S1 (0.73, 4.14/299)

4-oo

S13 (0.45, 4.81/258) S3 (1.15, 3.40/364) S5 (0.33, 3.47/357)

5-oo

S8 (0.38, 3.43/362)

S12 (1.23, 3.51/354)

D

E b −1 Sn H SO Tm (cm ) 7.75 0.83 4.14 14.1 4.40 17.2 7.98 44.9 3.99 3.86 29.3 20.2 9.10 49.0 29.5 3.53 1.14 32.2 29.5 5.21 6.50 27.2 1.76 1.69 3.06

organic 1-oo, with possible connections between S1 and either the photochromic T2 or T3 . Those values, below 10 cm−1 are typical of organic chromophores, 56 and may be sufficient to provoke ISC, but not very efficiently. In contrast, the SOC are significantly increased in all the doubly open metallic complexes, where at least one coupling attains ca. 30 cm−1 . For instance in 2-oo, S5 is well coupled to the photochromic T10 (44.9 cm−1 ) that is also very close in term of energy, and the S5 → T10 ISC is thus likely to occur. Important coupling are also found between S13 and T20 (29.3 cm−1 ) in 3-oo, S5 and T7 (49.0 cm−1 ) in 4-oo and between S8 and T8 (32.2 cm−1 ) in 5-oo. Regarding potential subsequent IC mechanisms within the triplet manyfold following this ISC, it appears that all the T1 in the 14


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series of dimers are photochromic triplet (see Table S3 in the SI), and a “fall” from any of the previously pinpointed Tn to T1 would still be favorable for photocyclization. In short, the influence of the metal clearly appears in all the complexes. Even if the singlet and triplet states of interest are mainly localized on the ligand part so that the UVVis spectra is only slightly modified by mixed spin-orbit states, the presence of the heavy atom allows for a great enhancement of the SOCs. Globally in all the metal complexes at least one possible ISC path to a photochromic triplet can be found in addition to the already demonstrated efficiency of the photochromic singlet. For all dimers except 3-oo, this ISC is nevertheless clearly in competition with a very likely IC to the photochromic S1 analyzed in the previous subsection, as the singlet states are energetically close to each other in the UV region though a definitive assessment of the relative speeds of these two processes cannot be given at this stage. In short, the first ring closing is accessible, which agrees with the experimental observation of the formation of the closed-open mixed isomers in all the investigated dimers. 17,18,21,22

Second electrocyclization Analysis of the singlet states We investigate now the mixed dimers where one DTE is closed and the second remains open: 1-co to 5-co. Table 3 lists the calculated electronic transitions presenting a large contribution from a photochromic orbital centered on the open switch. Such molecular orbital is the LUMO+1 for the metal-free dimer as well as for 2-co, 4-co and 5-co. Indeed, as can be seen in Figure 3, the LUMO for those dimers is located on the most conjugated part, the closed DTE, which is the expected behavior in mixed closed-open DTE dyads. 41 As for the open-open isomer, while the LUMO+1 of 1-co is still partially localized on the closed form due to the strong π-conjugation brought by the ethynyl bridge between the two subunits, the presence of the metallic center leads in the complexes to photochromic virtual orbitals that are solely located on the open DTE. This is a first qualitative explanation of 15



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the superior photochromic activity for the metal-containing structures. The case of 3-co is again different, as the LUMO is located on the closed DTE, but the LUMO+1 is centered on the gold atom, making the LUMO+2 the first orbital displaying a photochromic topology. For the organic dimer 1-co, the experimentally observed limited photochromism, 17 i.e, the impossibility to form the doubly closed isomer 1-cc in solution, is in agreement with the TD-DFT results summarized in Table 3 and the analysis of the involved virtual molecular orbitals. The S0 → S14 transition involving the LUMO+1 photochromic orbital in a nonnegligible amount is found at 337 nm, but: i) it is not very intense with an oscillator strength of 0.11; and ii) it involves only partially (at 37%) this orbital; iii) exciting the system to such high excited states is likely to yield internal conversion rather than photochromism, as many excited-states of similar energies are present in that region. In brief, irradiating in the UV does not lead to any efficient state able to trigger the second ring closure to form 1-cc. Considering now the singlets of the different complexes of DTEs, one can see in Table 3 that, in contrast with 1-co, it is possible to find in the metal-containing structures significantly dipole-allowed singlet states populating the photochromic orbital (LUMO+1 or LUMO+2) with strongly dominating contributions (> 80%). In the case of 2-co, the relatively intense S0 → S7 transition (f =0.27) at 388 nm, can initiate the second cyclization observed experimentally. 17 For 3-co the S0 → S17 is non-negligible but relatively weak (f =0.11) and is also too high in energy if one considers the experimental irradiation wavelength (300 nm), 21 so that it cannot be accessed in the conditions of the measurments. This could explain the loss of the second-ring closure for this specific dimer. 21 The S0 → S8 photochromic transition of 4-co is found at 388 nm and is likely to be activated by UV irradiation but it presents a weak oscillator strength (0.06). A more intense second S0 → S25 photochromic transition is found but it involves only partially the LUMO+1 (37%) and is located at too high energy to be populated with the experimental irradiation wavelength (365 nm). On the basis of the singlet states, it seems unlikely to form 4-cc, and this apparently contrasts with the experimental evidences. 22 5-co behaves similarly though the S0 →

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S17 photochromic transition at 344 nm is slightly more intense (f =0.10) and involves the LUMO+1 up to 85%, i. e., the situation is more favorable. Looking at the singlet states only, the variety of conclusions regarding the photoactivity of the organic or organometallic mixed closed-open dimers presented in this Section perfectly illustrates the well-known bottleneck of this second ring cyclization. Here, depending on the nature of the metal and the attachment position of the DTE to the coordination center, the photochromic singlet may be found weak, too high in energy or involving only partially the photochromic orbital, leading to a non favorable scenarios for photcyclization. One should keep in mind that IC processes can still occur following the vertical light absorption. In this UV region where a high density of excited states is found, it is non-trivial to predict specific pathways for such extended compounds. Nevertheless, in all cases, if the IC leads to the lowest S1 located on the closed DTE, this would make the second isomerization impossible. To be able to explain the experimental outcomes, that cannot be fully rationalized with the spin-free TD-DFT results, we consider in the following the SOC effects on the optical properties of the closed-open series. Table 3: Computed singlet states (Sn ) involving the photochromic orbital for the closed-open isomers, see Table 1 for more details. complex 1-co 2-co 3-co 4-co 5-co

photochromic MO L+1 L+1 L+2 L+1 L+1

Sn S14 S7 S17 S8 S25 S17

Wavelength (f ) 337 (0.11) 388 (0.27) 280 (0.11) 388 (0.06) 302 (1.13) 344 (0.10)

amount of phot. MO populated 37% 83% 94% 97% 37% 85%

Spin-orbit coupling and triplet state population As for the open-open dimers, sticks spectra in Figure 4 reveals that the inclusion of the SOC in the TD-DFT calculations does not lead to significant changes in the vertical transitions of the different closed-open complexes, only the intensity of a few transitions being slightly modified. As for the open-open isomers, we undertake the investigation of the coupling 17




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Table 4: SOC values between intense singlet states (Sn ) and closest triplet states (Tm ) for the closed-open forms. The oscillator strengths (singlet only) and energies of each state are also given. Dimer 1-co

2-co

3-co

4-co

5-co

Sn in the UV (f,E (eV/nm)) S6 (0.26, 2.96/419)

Tm (E (eV, nm))

T4 (2.57/482) T6 (2.75/451) T7 (2.90/427) S10 (0.34, 3.49/355) T7 (2.90/427) T10 (3.14/395) T13 (3.30/375) S6 (0.22, 3.14/394) T4 (2.62/473) T6 (2.88/430) T7 (2.96/419) S7 (0.27, 3.20/388) T4 (2.62/473) T6 (2.88/430) T7 (2.96/419) S8 (0.40, 3.22/385) T4 (2.62/473) T6 (2.88/430) T7 (2.96/419) S10 (0.54, 3.53/351) T4 (2.62/473) T6 (2.88/430) T7 (2.96/419) T12 (3.28/378) S9 (0.12, 3.88/320) T6 (3.32/373) T7 (3.44/361) T10 (3.62/343) S12 (0.72, 4.16/298) T6 (3.32/373) T7 (3.44/361) T10 (3.62/343) T16 (4.07/305) S6 (0.43, 3.08/402) T4 (2.60/477) T6 (2.88/431) T7 (2.93/423) S7 (0.27, 3.16/392) T4 (2.60/477) T6 (2.88/431) T7 (2.93/423) S9 (0.33, 3.29/376) T4 (2.60/477) T6 (2.88/431) T7 (2.93/423) T13 (3.27/379) S8 (0.42, 3.10/399) T5 (2.42/512) T7 (2.77/448) T8 (2.83/438) S9 (0.22, 3.13/396) T5 (2.42/512) T7 (2.77/448) T8 (2.83/438) S13 (0.15, 3.48/356) T5 (2.42/512) T7 (2.77/448) 19 T8 (2.83/438) ACS Paragon Plus Environment T12 (3.22/385) T19 (3.43/362)

D

E b −1 Sn H SO Tm (cm ) 1.46 0.94 1.56 0.43 1.46 2.13 129 32.2 23.1 2.58 7.26 34.7 44.2 4.86 16.6 7.77 3.49 7.76 9.36 1.69 3.02 0.04 6.12 23.5 0.13 5.57 76.9 25.4 21.9 52.9 26.0 32.3 9.26 8.78 96.7 2.98 14.7 2.73 0.80 5.24 1.04 0.76 6.16 4.77 1.85 0.91 8.65



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therefore not clearly counterbalanced by an alternative ISC pathway. Interestingly, for the platinium based complex 4, a large coupling value of 76.9 cm−1 is found when considering the S6 and T4 states, and this clearly indicates a possible triplet cyclization pathway allowing to correct the previous spin-free conclusions and actually explain the experimental formation of 4-cc. Additionally, the coupling between S9 and T7 leads to a large SOC integral as well (96.7 cm−1 ), reinforcing this conclusion. For the Ru-based dimer 5-co, quite low SOC values are found between the intense singlet states in the UV, S8 , S9 and S13 , and any of the E D b −1 photochromic triplets. Even though the S8 H SO T5 reaches 14.7 cm , that may not be

sufficient to significantly access the photochromic T5 as the energy difference between the

two states is ca. 0.70 eV. Therefore a triplet mechanism for cyclization is not found and only the singlet path is probably efficient in 5-co. This explains well the quite low quantum yield measured for the second cyclization (0.16) of this compound. 18

Conclusions We presented a spin-orbit relativistic TD-DFT analysis on a series of dimers of DTEs connected either through an organic bridge or a metallic coordination center. The sole consideration of the presence singlet states populating a virtual molecular orbital displaying a photochromic topology allows to explain the first photocyclization in all derivatives. This fits the experimental formation of the mixed closed-open form that is systematically observed. The addition of the SO effects does not significantly modify the vertical electronic transitions mixing singlets and triplets spin-free states, but the computation of the SOC integrals between singlet and triplet states reveals potential additional pathways for cyclization through the population of a triplet state bearing the correct topology for the C-C bond formation. This is triggered by an ISC from a significantly dipole-allowed singlet state in the UV region and adds, for every metallic complex, a secondary pathway to trigger the cyclization of the first DTE. Worth notifying is the fact that, starting from higher singlet states, this ISC is

21



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in most cases competing with likely IC to the lowest singlet state, that is still favorable for the isomerization, leading to the same photoproducts. For the study of the feasibility of the second cyclization, the variety of conclusions reflects the versatility of these complex multiswitches. The study of the excited states in the UV region reveals a non-efficient population of the correct photochromic singlet-state for the metal-free 1-co, and the consideration of the SO effects does not add any ISC pathway to a photoactive triplet, in agreement with the absence of 1-cc in solution. Concerning the metallic-based closed-open dimers, the SOC-TD-DFT results for the two gold dimers 2-co and 3-co, lead to opposite photochemical behaviors. If in 2-co both the population of a photochromic singlet state and the access to a photochromic triplet state through an ISC can efficiently triggered the cyclization, none of those two patterns are found likely in 3co, proving the influence of the binding position to the metal on the optical properties. In parralel, the spin-free consideration of the singlets for the platinium-based compound (dimer 4) leads to incomplete or incorrect prediction, and only the addition of the SO effects that reveals a large coupling between a singlet state in the UV region irradiated and a photoactive triplet state permits to reach the correct conclusions. Finally for a Ru-based complex, ISC is found to be not very efficient and only the access to dipole-allowed singlet in the UV region can explain the experimental formation of 5-cc with a low quantum yield. All of these SOC-TD-DFT conclusions on metal-containing dimers match experiments, and we have thus demonstrated for the first time, that the consideration of the SOC effect is essential to predict and rationalize photochromism in metallic multiswitches. Nevertheless, we mention that other phenomena can also be incriminated for the partial photochromism in dimers of DTEs, and in particular excited state energy transfer (EET) form the excited open form to the closed form. 57 This EET mechanism is currently being investigated in our group.

22


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Acknowledgement A. F. thank the European Research Council (ERC, Marches - 278845) for supporting his postdoctoral grant. D. J. acknowledges the ERC and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches – 278845) and a recrutement sur poste stratégique, respectively. This research used resources of CCIPL (Centre de Calcul Intensif des Pays de Loire), CINES (Centre Informatique National de l’Enseignement Suprieur ) (grant x2016085117) and a local Troy cluster.

Supporting Information Available Additional TD-DFT analysis for open-open and closed-open dimers, LUMO of 1-oo with a low isocontour value, convoluted UV-Visible spectra, complete list of the photochromic triplet states.

This material is available free of charge via the Internet at http://pubs.

acs.org/.

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