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Theoretical investigation on radical anion promoted electrocyclization in photochromes
Journal of Molecular Graphics and Modelling 97 (2020) 107550
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Journal of Molecular Graphics and Modelling journal homepage: www.elsevier.com/locate/JMGM
Theoretical investigation on radical anion promoted electrocyclization in photochromes Nadia Bibi, Naveen Kosar, Khurshid Ayub, Tariq Mahmood* Department of Chemistry, COMSATS University Islamabad, Abbottabad Campus, Abbottabad, 22060, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2019 Received in revised form 24 January 2020 Accepted 24 January 2020 Available online 27 January 2020
Rapid electrocyclization is proposed under radical anionic conditions in organic photochromes. DFT calculations have been performed to investigate the radical anion mediated electrocyclization in different organic photochromes. Furthermore, the activation barriers under radical anionic conditions are compared with those in neutral and radical cationic conditions. The nuclear independent chemical shift (NICS(0)) and synchronicity calculations have been performed for the confirmation of concerted nature and aromatic character of transition states, respectively. The activation barrier for thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) under radical anionic conditions is very lower (DH ¼ 5.92 kcal/mol, DG ¼ 6.97 kcal/mol) than under neutral conditions, but higher than that in radical cationic conditions (DH ¼ 3.13 kcal/mol, DG ¼ 4.0 kcal/mol). Similarly, the other prominent classes of photochromes; dithienylethene (DH ¼ 20.12 kcal/mol, DG ¼ 21.55 kcal/mol) and vinylheptafulvene (DH ¼ 23.72 kcal/mol, DG ¼ 24.82 kcal/mol) have shown decreased activation barrier under radical anionic condition. However, activation barrier of fulgide under radical anionic conditions is not different than those under neutral and radical cationic conditions. Synchronicity and NICS(0) values for organic photochromes also show significant changes under radical anionic conditions. © 2020 Elsevier Inc. All rights reserved.
Keywords: Density functional theory Photochromes Electrocyclization Radical anion
1. Introduction Photochromism is the reversible isomerization of chemical species upon exposure to light, where both the isomers differ in absorption spectra, refractive index, dielectric constant, geometries and electronic structures [1,2]. This remarkable phenomenon was determined factually by Markwald et al., during the study of tetrachloro-1,2-keto-naphthalenone and benzo-1-naphthyrodine under light [3]. Reversibility is the prime criterion in naming this process, although some photochromes undergo irreversible reactions. This field has grown rapidly during the past decade to discover and improve photoswitchable materials [4,5] for many applications, for example; memory devices [6], magnetic switches [7,8], electronics, non-linear optical devices [9], liquid crystals [10e12], metal cation extraction [13], and encoder-decoder [14]. Other applications of photochromism include the indication of molecular transition by observing photochromic shifts [15]. The significant factors that enable the use of photochromes in different
* Corresponding author. E-mail address: [email protected] (T. Mahmood). https://doi.org/10.1016/j.jmgm.2020.107550 1093-3263/© 2020 Elsevier Inc. All rights reserved.
applications are; the ease of synthesis, high sensitivity, rapid response, fatigue resistance [16], high thermal stability [17], high quantum yield [7], large polarizibility variation, non-destructive readout [18] and reactivity in solid state [19]. Many photoswitching molecules are categorized into different classes; these may include organic and inorganic photochromes [20,21]. Some of the photochromes with organic nature are diarylethenes, vinylheptafulvenes, fulgides, chromenes, spiropyrane and spirooxazone while inorganic photochromes include zinc halide and silver chloride. The organic photochromes are the topic of active research since last three decades [22e24]. Organic photochromes have gained special attention in different applications due to their nondestructive readout, fatigue resistivity, detectability and thermal stability [25]. The photogenerated species can be reverted back to initial state either photochemically (P-type) or thermally (T-type) [26]. Azobenzenes, spirooxazines, spiropyrans, naphthopyrans, benzopyrans are typical T-type photochromic compounds, while fulgides and diarylethenes with heterocyclic ring are stable P-type photochromes [27]. They are further classified as positive and negative photochromes. Positive photoswitches involve the conversion from colorless A isomer to the colored isomer B i.e. lmax(A) is smaller than lmax(B) or vice versa. The nature of application
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dictates the mode of isomerization in photochromes [28]. For example, switch application [29] requires quick interconversion between two isomers, whereas memory application requires high thermal stabilities of each isomer [30]. Photochromism proceeds through different chemical processes. Some of the most important processes are; electrocyclization [31,32], cis-trans isomerism [32], intramolecular group transfers [33], intramolecular hydrogen transfer [33] and pericyclic reactions [34]. Among all the processes, electrocyclization is an excellent strategy for the synthesis of many photochromic molecules that follows the rules of orbital symmetry conservation, originally proposed by Roald Hoffmann and R.B.Woodward [35]. Depending upon the number of p-electrons and experimental conditions (photochemical or thermal), the electrocyclization follows either conrotatory or disrotatory mechanism [36]. For example, the electrocyclization (thermal reversion) in cyclophanediene (CPD) involves the conrotatory mechanism with activation barrier of 20.36 kcal/mol [37]. Electrocyclization reactions of dihydropyrens and dithienylethene have shown the lowest activation barrier for thermal reversion under neutral conditions, as compared to the other classes [38]. Mitchell et al., reported that activation barrier is affected by extent of conjugation, electronic density and nature of group at ring closing termini [36,39e41]. Moreover, the substitution effect for electrocyclization under neutral conditions has also been investigated to reveal the variation in activation energies. The results revealed that methyl substituted CPD had the lowest activation barrier (DH ¼ 20.36 kcal/mol) as compared to nitrile substituted CPD (DH ¼ 25e27 kcal/mol) under same conditions at the same position [42]. It is experimentally proved that nitrile substituted cyclophanedienes are more stable than methyl substituted CPD with half-life of 30 years at room temperature [43]. Ayub et al., also demonstrated theoretically as well as experimentally that nature of substituent at closing termini affects the activation barrier [44]. Robinson et al., reported that the thermal return in DHP is accelerated by incorporating radical stabilizing groups at position of high spin density [45]. The various experimental and theoretical data from literature have divulged that the formation of radical cation accelerates the rate of electrocyclization in some photochromes. Similarly, Staykov and coworkers reported that removing one electron from the atoms present at ring closing termini lowers the activation barrier from open to closed form significantly [46]. Mahmood et al., reported acceleration of electrocyclization in photochromism for different classes such as fulgides, CPD-DHP, dithienylethene and VHF-DHA under radical cationic conditions [47] and revealed that the radical cation lowers the activation barrier for electrocyclization compared to neutral. For example, a low activation barrier (DH ¼ 3.13 kcal/mol, DG ¼ 4.01 kcal/mol) and high reaction energy (DH ¼ 28.48 kcal/ mol, DG ¼ 27.04 kcal/mol) is observed for radical cation mediated thermal reversion of cyclophanediene in comparison to thermal return under neutral conditions (DH ¼ 20.60 kcal/mol, DG ¼ 20.98 kcal/mol). Mahmood et al., also reported that, contrary to the other photochromic compounds, the thermal return of VHFDHA and fulgides is not affected with the formation of radical cation. Although, acceleration of electrocyclization has been investigated under radical cationic and neutral conditions however, electrocyclization under radical anionic conditions has not been explored. In the current study, we have discussed the acceleration of electrocyclization in organic photochromes under radical anionic conditions. 2. Computational methods All the theoretical calculations for different classes of
photochromes under anionic conditions are performed on Gaussian 09 [48]. The optimized structures are visualized through GaussView 5.0 [49]. B3LYP/6-31G(d,p) and M06-2X/6-311þG(d,p) level of theories are used for the optimization of geometries. M06-2X is a reliable method, to accurately estimate the activation barrier for electrocyclization reactions [50,51]. Moreover, the transition states are located through Berny algorithm and confirmed by frequency analysis (one imaginary frequency). The same level of theory is used to calculate the synchronicity. Synchronicity is mathematically expressed as (Eq. (1)) [47].
Pn Sy ¼ 1
i¼1
jdBi dBAV j dBAV
2n 2
(1)
In the above equation, n represents the number of bonds concerned and dBi represents dissimilarity in bond index at TS. The dBi and dBAV in equation (1) can be calculated from equations (2) and (3).
dBi ¼
R BTS i Bi
(2)
BPi BRi
dBAV ¼ n1
n X
dBi
(3)
i¼1
Furthermore, natural bond orbital method is used to calculate the Wiberg bond indices. Wiberg bond index is the measurement of the number of electrons of an atom A involved in a covalent bonding with another atom B. The index was originally proposed by Wiberg in 1968 [52]. The absolute magnetic shielding at the center of the ring i.e. aromaticity of transition state is confirmed by performing nuclear independent chemical shift (NICS) analysis. The NICS(0) simulations are performed by placing the ghost atom in the centroid of electronic cloud. Frontier molecular orbitals analysis is also performed to study the distribution of singly occupied molecular orbitals (SOMO) at same method.
3. Results and discussion 3.1. Radical anionic mediated thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) The transition state of radical anion isomerization is located, to study the activation barrier for thermal return of CPD to DHP. The optimized geometries of transition state, CPD and DHP are represented in Fig. 1. The activation barrier, overall energy of reaction and other variables such as synchronicity, NICS(0) values and symmetry of CPD-DHP are summarized in Table 1. The numbering system for overall discussion of all classes of photochromes is given in Fig. 2 for maintaining uniformity in discussion among open, transition state and closed isomers of the respective class. Analysis of the optimized structures under radical anionic conditions reveals a notable change in the bond length during isomerization from CPD (open form) to DHP (closed form). The bond length of C8eC16 (ring closing atoms) in optimized structure of CPD is 2.60 Å which is noticeably decreased to 2.00 Å in the TS and then to 1.56 Å in the closed form (DHP). This considerable difference of bond length indicates the isomerization of cyclophanediene to DHP. Similarly, the prominent dihedral angles (C3eC2eC1eC15 and C6eC7eC8eC3) in CPD are 0.67 and 25.00 , respectively. The dihedral angles are changed to 0.09 and 18.90 , in the transition state and 7.00 and 22.00 in the DHP, respectively. This alteration in torsion angles indicates planarization during isomerization. The
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Fig. 1. The optimized geometries of CPD, TS and DHP under radical anionic condition. The grey and white color balls represent carbon and hydrogen atoms, respectively.
Table 1 Enthalpies of activation (DHact in kcal/mol), Gibbs free energies of activation (DGact in kcal/mol), enthalpies of reaction (DHR in kcal/mol), Gibbs free energies of reaction (DGR in kcal/mol), Synchronicities (Sy), (NICS(0), in ppm) and transition states symmetries (Symm) of different photochromes classes under radical anionic condition at M06-2X/6311þG(d,p). S$NO.
Photochromes
Radical anion
DHact
DGact
DHR
DGR
Sy
NICS(0)
Symm
1 2 3 4
CPD-DHP Dithienylethene (open-closed) VHF-DHA Fulgides (open-closed)
5.94 20.12 23.72 29.76
6.97 21.55 24.82 30.44
26.02 7.92 9.76 13.92
24.71 5.64 11.09 12.67
1.00 1.00 0.87 1.00
5.31 71.78 2.70 2.40
C1 C1 C1 C1
Fig. 2. The general numbering scheme for discussion.
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electro-cyclizing p -electronic cloud (Fig. 3). The NICS(0) value is 5.31 ppm for TS of CPD-DHP thermal return (under radical anionic condition) which confirmed the antiaromatic character of TS. The observation of SOMO orbitals revealed that the p cloud is concentrated on the ring-closing terminus in the TS. The SOMO surfaces of CPD, TS and DHP are given in Fig. 4.
3.2. Radical anion mediated electrocyclization of dithienylethene
Fig. 3. Localization of ghost atom in the centroid of electrocyclic termini.
enthalpy of activation for CPD-DHP under radical anionic condition is 5.94 kcal/mol (DG ¼ 6.97 kcal/mol) which is 14.08 kcal/mol lower than that under neutral conditions (DH ¼ 20.60 kcal/mol, DG ¼ 20.98 kcal/mol) [43] whereas it is 2.78 kcal/mol higher than that of electrocyclization in radical cation (DH ¼ 3.13 kcal/mol, DG ¼ 4.01 kcal/mol) [47], respectively. The transition state located under radical anionic conditions possesses the C1 point group of symmetry, (different from the TS for radical cation (Cs). Whereas the transition state under neutral condition has C2h symmetry. The overall reaction enthalpy for radical anion mediated isomerization is DH ¼ 26.02 kcal/mol, (DG ¼ 24.71 kcal/mol). The enthalpy of reaction under neutral and radical cationic conditions are DH ¼ 22.25 kcal/mol (DG ¼ 21.13 kcal/mol) and DH ¼ 29.00 kcal/mol (DG ¼ 27.04 kcal/mol), respectively. The results reflect radical anionic mediated isomerization of CPD-DHP is more exothermic as compared to that under neutral conditions (DH ¼ 22.25 kcal/mol, DG ¼ 21.13 kcal/mol). However, it is less exothermic as compared with radical cation system DH ¼ 28.48 kcal/mol (DG ¼ 27.04 kcal/mol). Moreover, synchronicity is calculated to determine that whether TS is completely synchronous (lying in the center) or not. The synchronicity value under radical anionic conditions is 1.00, which reflects perfectly synchronized TS that lies in the middle. The synchronicity of radical cationic CPD-DHP isomerization is asynchronous with the synchronicity value of 0.67. Nuclear independent chemical shift indicates the aromatic behavior of transition state in the reaction. Aromaticity is an important parameter in highly conjugated cyclic systems, and it has strong correlation with NICS which is theoretically calculated. To explore the aromaticity of transition state for CPD-DHP electrocyclization under radical anionic conditions, NICS(0) is calculated via putting the ghost atom in the core of the
Activation barrier for dithienylethene isomerization under radical anionic conditions is also investigated at the same level of theory (M06-2X/6-311þG(d,p)). The optimized geometries (open form, closed form and TS) under radical anionic conditions are given in Fig. 5. The optimized geometries under radical anionic conditions have shown noticeable alteration in the bond distance among ring closing atoms (C1eC2). C1eC2 bond distance in the open form is 3.60 Å. The same bond length is 2.00 Å in the transition state and 1.50 Å in closed form of dithienylethene. The decreasing bond length between terminal atoms is the effectual indicator of electrocyclization of dithienylethene. Similarly, the important dihedral angle (C10eC11eC12eC3) in open form under radical anionic conditions is 23.80 which significantly decreases to 4.43 in TS and increases again to 7.80 in closed isomer. This change in dihedral angle brings the electrocyclic termini into a suitable orientation for isomerization. Enthalpy of activation for ring closing of dithienylethene under radical anionic condition is DH ¼ 20.12 kcal/mol (DG ¼ 21.55 kcal/ mol) which is quite less as compared to that under neutral (DH ¼ 48.13 kcal/mol (DG ¼ 50.28 kcal/mol)) conditions. However, the enthalpy of activation is higher as compared to isomerization under radical cationic conditions (DH ¼ 19.44 kcal/mol, DG ¼ 22.29 kcal/mol)) at B3LYP/6-31G(d,p) method [47]. The overall enthalpy of this reaction is DH ¼ 7.92 kcal/mol (DG ¼ 5.64 kcal/mol), which shows that the isomerization is exothermic. All the transition states (under radical cation, radical anion and neutral conditions) have C1 symmetry and follow conrotatory motion (radical cation, radical anion). The TS of dithienylethene is completely synchronized in the case of radical anion with synchronicity value of 1.00. Whereas the corresponding TS under radical cation is highly asynchronous (0.51) [47]. NICS(0) calculations of the TS are performed at M06-2X/6-311þG(d,p) level of theory (Fig. 6), which has shown the highly aromatic (than radical cation) character of the dithienylethene with value of 71.78 ppm. The SOMO of all isomers of dithienylethene (radical anion) are
Fig. 4. SOMO orbitals of CPD, TS and DHP (radical anion).
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Fig. 5. Optimized geometries of radical anion (open form, transition state (TS) and closed form) of dithienylethene.
Fig. 6. TS of dithienylethene having ghost atom.
given in SI. 1. SOMO investigation confirmed the conrotatory motion of atom at ring closing terminals in the transition state. 3.3. Radical anionic mediated electrocyclization of vinylheptafulvene to dihydroazulene Dihydroazulene (DHA) is T-type photochrome for which closed isomer is more stable and reverts back thermally to vinylheptafulvene (VHF). Both the isomers differ in the absorption spectrum in UVevisible region. The significant change in bond length is observed when VHF elecrocyclizes to dihydroazulene under radical anionic conditions. Under radical anionic condition, the C1eC10 bond distance between the ring-closing atoms is 3.45 Å in the reactant (VHF), and it decreases to 2.04 Å in TS and 1.59 Å in product (DHA). The important dihedral angles (C2eC3eC4eC10 and C1eC2eC3eC4 and) in VHF are 17.63 and 12.09 , respectively. The relevant torsion angles are 5.51 and 20.72 in the transition state, respecttively and 7.82 and 7.10 in the closed form, respectively. Variation in the dihedral angles brings electrocyclic termini (C1 and C10) into a favourable orientation for electrocycling. The TS of VHFDHA reflects the disrotatory movement of functional groups attached to the ring closing atoms and it has C1 point group of symmetry. Thermal isomerization of VHF to DHA follows
Fig. 8. TS of VHF-DHA having ghost atom.
disrotatory (Woodward-Hoffmann allowed movement) pathway. The enthalpy change for radical anion mediated thermal reversion of VHF-DHA is DH ¼ 23.72 kcal/mol (DG ¼ 24.82 kcal/mol) at M062X/6-311þG(d,p) method which is smaller than the reported value for electrocyclization under radical cation (DH ¼ 28.03 kcal/mol, DG ¼ 29.52 kcal/mol) [47] and neutral pathway (DH ¼ 28.39 kcal/ mol, DG ¼ 28.98 kcal/mol) at B3LYP/6-31G(d,p) method [53,54]. Contrary to radical cation the activation barrier for VHF to DHA are significantly affected (decreased) under radical anion. Furthermore, the energy of the reaction in the case of radical anionic condition is DH ¼ 9.76 kcal/mol (DG ¼ 11.09 kcal/mol) which is low as compared to that for radical cation DH ¼ 15.08 kcal/mol (DG ¼ 16.48 kcal/mol) [47]. The reaction energy under radical cationic condition is high compared to neutral conditions DH ¼ 3.11 kcal/mol (DG ¼ 4.03 kcal/mol), which shows that the reaction (radical anion as compared to neutral) is endothermic. The calculated synchronicity value (0.87) shows that the transition state for isomerization of VHF-DHA is synchronous unlike that for radical cation system (0.68). NICS(0) value for thermal electrocyclization of VHF-DHA is 2.70 ppm which indicates that the transition state with radical anion is non-aromatic, while the TS for VHF-DHA with radical cation is aromatic (NICS(0) ¼ 8.86 ppm) (see Fig. 8). Fig. 7
Fig. 7. The optimized structures of VHF, TS and DHA.
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Fig. 9. The optimized geometries of open, TS and closed form of fulgides.
3.5. Radical anionic electrocyclization at B3LYP/6-31G(d,p) method
Fig. 10. TS of fulgide having ghost atom.
shows the optimized structures of VHF, TS and DHA and SI. 2 shows the SOMO surfaces of VHF-DHA under anionic conditions.
3.4. Radical anionic mediated electrocyclization of fulgide The impact of radical anion on the activation energy of fulgide electrocyclization is also examined. During switching, the variation in the bond distances of ring closing termini atoms is noticed. For example, the C3eC4 bond length is 3.34 Å for the open form. The same bond distance decreased to 2.36 Å in TS, which is finally reduced to 1.50 Å in the closed form (see Fig. 9). The torsion angles such as C7eC8eC9eC2, C4eC7eC8eC9 C3eC2eC9eC8 in the open form are 6.83 , 11.6 and 6.8 . The respective angles are changed to 6.70 , 10.25 and 26.16 in the TS, which bring the ring closing atoms in proper orientation for isomerization. The enthalpy of activation (DH) for the thermal switching of fulgide under anionic condition is 29.76 kcal/mol (DG ¼ 30.44 kcal/mol) which is higher than the values already discussed under neutral (DH ¼ 25.71 kcal/ mol, DG ¼ 26.89 kcal/mol) and cationic conditions (DH ¼ 25.09 kcal/mol, DG ¼ 26.02 kcal/mol) at B3LYP/6-31G(d,p) method [47]. The complete enthalpy of the reaction with radical anion is 13.92 kcal/mol (DG ¼ 12.67 kcal/mol), which shows that the reaction is highly exothermic when compared to that under neutral condition (DH ¼ 7.88 kcal/mol). However, to reaction under radical anionic condition is less exothermic as compared to that under cationic conditions (DH ¼ 19.62 kcal/mol). Transition state of fulgide under anionic condition has C1 symmetry. The computed synchronicity value (Sy ¼ 1) revealed that the transition state is perfectly synchronous. The synchronicity of radical anion mediated TS is similar to that of neutral (Sy ¼ 1.00) but significantly high than that of radical cation (Sy ¼ 0.66). The NICS(0) of thermal anionic TS value is 2.40 ppm which represents the non-aromatic nature of TS (Fig. 10). The NICS(0) value (2.40 ppm) under radical anionic condition is dissimilar to previously reported value (3.20 ppm) of radical cationic condition [45]. SI. 3 shows the SOMO of open, TS and closed isomers of fulgide (radical anion).
We have also investigated the activation energies for thermal reversion of organic photochromes under radical anionic conditions at B3LYP/6-31G(d,p) level of theory. In the case of CPD/DHP conversion, the activation energy measured under radical anionic conditions at B3LYP method is lower (DH ¼ 4.52 kcal/mol and DG ¼ 6.80 kcal/mol) than that of M06-2X method (DH ¼ 5.94 kcal/ mol and DG ¼ 6.97 kcal/mol). The activation enthalpy for electrocyclization of dithienylethene (under radical anionic condition) at B3LYP is 22.80 kcal/mol (DG ¼ 25.60 kcal/mol), which is higher in comparison to the values at M06-2X (DH ¼ 20.12 kcal/mol and DG ¼ 21.55 kcal/mol). The activation energy values (DH ¼ 27.65 kcal/mol and DG ¼ 29.56) at B3LYP level of theory for thermal reversion of VHF-DHA (under radical anionic condition) are almost 4e5 kcal/mol higher compared than the values at M062X method (DH ¼ 23.72 kcal/mol and DG ¼ 24.82). However, the activation energy for thermal return of fulgide under radical anion condition shows a difference of 1 kcal/mol between B3LYP (DH ¼ 28.34 kcal/mol and DG ¼ 29.47 kcal/mol) and M06-2X (DH ¼ 29.76 kcal/mol and DG ¼ 30.44 kcal/mol)). Moreover, reaction energies calculated at B3LYP method are less exothermic as compared to M06-2X method (except radical anion thermal switching of CPD to DHP which is more endothermic at B3LYP method). All the photochromes have shown similar synchronicity values at both level of theories.
4. Conclusions We have investigated the activation enthalpies for thermal reversion of different photochromes under radical anionic conditions at M06-2X/6-311þG(d,p) level of theory. Among all the studied photochromes, dihydropyrene, dithienylethene and vinylheptafulvenes have shown the acceleration of electrocyclization with radical anion, while the activation barrier measured for fulgides under radical anion (DH ¼ 29.76 kcal/mol (DG ¼ 30.44 kcal/ mol) is high as compared to that under neutral (DH ¼ 25.71 kcal/ mol, DG ¼ 26.89 kcal/mol) and cationic conditions (DH ¼ 25.09 kcal/mol, DG ¼ 26.02 kcal/mol). In the case of CPDDHP the activation energy measured with radical anion (DH ¼ 5.94 kcal/mol, DG ¼ 6.97 kcal/mol) is very small as compared to the neutral (DH ¼ 20.60 kcal/mol, DG ¼ 20.98 kcal/mol), but it is a little bit high as compared to radical cation DH ¼ 3.13 kcal/mol, DG ¼ 4.01 kcal/mol). The CPD-DHP has shown the perfectly synchronized transition state (Sy ¼ 1) with aromatic characteristics during radical anion thermal switching. The activation energy for thermal reversion of dithienylethene (under radical anion) is (DH ¼ 20.12 kcal/mol, DG ¼ 21.55 kcal/mol), which is lower in
N. Bibi et al. / Journal of Molecular Graphics and Modelling 97 (2020) 107550
comparison to the neutral (DH ¼ 48.13 kcal/mol, DG ¼ 50.28 kcal/ mol) but high compared to the radical cation (DH ¼ 19.44 kcal/mol, DG ¼ 22.29 kcal/mol), respectively. The synchronicity value of 1 has shown perfectly synchronous transition state of dithienylethene under same conditions. Aromatic nature of radical anion transition state of dithienylethene is confirmed from the high NICS(0) value. Under radical anionic conditions, the activation energy for the switching of dihydropyrene and dithienylethene is significantly reduced and their results are consistent with the results of radical cationic condition. The activation energy in VHF-DHA thermal reversion with radical anion is DH ¼ 23.72 kcal/mol (DG ¼ 24.82 kcal/mol) which is smaller to already reported value under radical cationic conditions H ¼ 28.03 kcal/mol (DG ¼ 29.52 kcal/mol) and neutral conditions H ¼ 28.39 kcal/mol (DG ¼ 28.98 kcal/mol). The reported synchronicity value (0.87) indicate that the transition state for isomerization of VHF-DHA is asynchronous; whereas NICS(0) value (2.70 ppm) indicates that the TS are nonaromatic with radical anion. It indicates that radical anion formation has effect on the activation energy of thermal return of fulgide. The activation energy for thermal switching of fulgides with radical anion is DH ¼ 29.76 kcal/mol (DG ¼ 30.44 kcal/ mol), which is higher in comparison to neutral DH ¼ 25.71 kcal/mol (DG ¼ 26.89 kcal/mol) and radical cation DH ¼ 25.09 kcal/mol (DG ¼ 26.02 kcal/mol). The synchronicity is 1, which indicates that the TS is perfectly synchronous with non-aromatic nature. The impact of radical anion on electrocyclization in photochromism is in progress for other photochromes. These photoswitches are used in rigid polymer matrix using radical polymerization and bridged imidazole dimers [55]. Acknowledgements The authors acknowledge the Higher Education Commission of Pakistan (Grant No. 3013) and COMSATS University, Abbottabad Campus. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmgm.2020.107550. References [1] M. Irie, Diarylethenes for memories and switches, Chem. Rev. 100 (2000) 1685e1716. [2] Y. Yokoyama, Fulgides for memories and switches, Chem. Rev. 100 (2000) 1717e1740. [3] Colour- and optically changing smart materials, in: Smart Mater., Birkh€ auser Basel, Basel, n.d.: pp. 72e97. [4] M.S.M. Rawat, S. Mal, P. Singh, Photochromism in anils - a review, Open Chem. J. 2 (2015) 7e19. [5] G. Berkovic, V. Krongauz, V. Weiss, Spiropyrans and spirooxazines for memories and switches, Chem. Rev. 100 (2000) 1741e1754. [6] F.M. Raymo, M. Tomasulo, Electron and energy transfer modulation with photochromic switches, Chem. Soc. Rev. 34 (2005) 327. [7] R.H. Mitchell, C. Bohne, Y. Wang, S. Bandyopadhyay, C.B. Wozniak, Multistate p switches: synthesis and photochemistry of a molecule containing three switchable annelated dihydropyrene units, J. Org. Chem. 71 (2006) 327e336. [8] K. Matsuda, M. Irie, A diarylethene with two nitronyl nitroxides: photoswitching of intramolecular magnetic interaction, J. Am. Chem. Soc. 122 (2000) 7195e7201. [9] F. Dietz, G. Olbrich, S. Karabunarliev, N. Tyutyulkov, Photoswitching of dipole moments, charge-transfer and spectroscopic properties, Chem. Phys. Lett. 379 (2003) 11e19. [10] J.A. Delaire, K. Nakatani, Linear and nonlinear optical properties of photochromic molecules and materials, Chem. Rev. 100 (2000) 1817e1846. [11] M. Banghart, A. Mourot, D. Fortin, J. Yao, R. Kramer, D. Trauner, Photochromic blockers of voltage-gated potassium channels, Angew. Chem. Int. Ed. 48 (2009) 9097e9101. [12] A.S. Matharu, S. Jeeva, P.S. Ramanujam, Liquid crystals for holographic optical data storage, Chem. Soc. Rev. 36 (2007) 1868.
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Mitchell, Calculation driven synthesis of an excellent dihydropyrene negative photochrome and its photochemical properties, J. Am. Chem. Soc. 133 (2011) 4040e4045. [37] M. Munakata, L.P. Wu, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, K. Furuichi, Reversible photochromism of a crystalline dithienylethene copper(I) polymer, J. Am. Chem. Soc. 118 (1996) 3305e3306. [38] N. Khan, N.S. Sheikh, A.F. Khan, R. Ludwig, T. Mahmood, W. Rehman, Y.S.S. AlFaiyz, K. Ayub, Towards thermally stable cyclophanediene-dihydropyrene photoswitches, J. Mol. Model. 21 (2015) 148. [39] M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Photochromism of diarylethene molecules and crystals: memories, switches, and actuators, Chem. Rev. 114 (2014) 12174e12277. [40] R.H. Mitchell, P. Zhang, D.J. Berg, R.V. Williams, An experimental estimate of the relative aromaticity of the cyclooctatetraene dianion by fusion to dimethyldihydropyrene, Chem. Commun. 48 (2012) 8144. [41] C. Bohne, R.H. 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Contents lists available at ScienceDirect
Journal of Molecular Graphics and Modelling journal homepage: www.elsevier.com/locate/JMGM
Theoretical investigation on radical anion promoted electrocyclization in photochromes Nadia Bibi, Naveen Kosar, Khurshid Ayub, Tariq Mahmood* Department of Chemistry, COMSATS University Islamabad, Abbottabad Campus, Abbottabad, 22060, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2019 Received in revised form 24 January 2020 Accepted 24 January 2020 Available online 27 January 2020
Rapid electrocyclization is proposed under radical anionic conditions in organic photochromes. DFT calculations have been performed to investigate the radical anion mediated electrocyclization in different organic photochromes. Furthermore, the activation barriers under radical anionic conditions are compared with those in neutral and radical cationic conditions. The nuclear independent chemical shift (NICS(0)) and synchronicity calculations have been performed for the confirmation of concerted nature and aromatic character of transition states, respectively. The activation barrier for thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) under radical anionic conditions is very lower (DH ¼ 5.92 kcal/mol, DG ¼ 6.97 kcal/mol) than under neutral conditions, but higher than that in radical cationic conditions (DH ¼ 3.13 kcal/mol, DG ¼ 4.0 kcal/mol). Similarly, the other prominent classes of photochromes; dithienylethene (DH ¼ 20.12 kcal/mol, DG ¼ 21.55 kcal/mol) and vinylheptafulvene (DH ¼ 23.72 kcal/mol, DG ¼ 24.82 kcal/mol) have shown decreased activation barrier under radical anionic condition. However, activation barrier of fulgide under radical anionic conditions is not different than those under neutral and radical cationic conditions. Synchronicity and NICS(0) values for organic photochromes also show significant changes under radical anionic conditions. © 2020 Elsevier Inc. All rights reserved.
Keywords: Density functional theory Photochromes Electrocyclization Radical anion
1. Introduction Photochromism is the reversible isomerization of chemical species upon exposure to light, where both the isomers differ in absorption spectra, refractive index, dielectric constant, geometries and electronic structures [1,2]. This remarkable phenomenon was determined factually by Markwald et al., during the study of tetrachloro-1,2-keto-naphthalenone and benzo-1-naphthyrodine under light [3]. Reversibility is the prime criterion in naming this process, although some photochromes undergo irreversible reactions. This field has grown rapidly during the past decade to discover and improve photoswitchable materials [4,5] for many applications, for example; memory devices [6], magnetic switches [7,8], electronics, non-linear optical devices [9], liquid crystals [10e12], metal cation extraction [13], and encoder-decoder [14]. Other applications of photochromism include the indication of molecular transition by observing photochromic shifts [15]. The significant factors that enable the use of photochromes in different
* Corresponding author. E-mail address: [email protected] (T. Mahmood). https://doi.org/10.1016/j.jmgm.2020.107550 1093-3263/© 2020 Elsevier Inc. All rights reserved.
applications are; the ease of synthesis, high sensitivity, rapid response, fatigue resistance [16], high thermal stability [17], high quantum yield [7], large polarizibility variation, non-destructive readout [18] and reactivity in solid state [19]. Many photoswitching molecules are categorized into different classes; these may include organic and inorganic photochromes [20,21]. Some of the photochromes with organic nature are diarylethenes, vinylheptafulvenes, fulgides, chromenes, spiropyrane and spirooxazone while inorganic photochromes include zinc halide and silver chloride. The organic photochromes are the topic of active research since last three decades [22e24]. Organic photochromes have gained special attention in different applications due to their nondestructive readout, fatigue resistivity, detectability and thermal stability [25]. The photogenerated species can be reverted back to initial state either photochemically (P-type) or thermally (T-type) [26]. Azobenzenes, spirooxazines, spiropyrans, naphthopyrans, benzopyrans are typical T-type photochromic compounds, while fulgides and diarylethenes with heterocyclic ring are stable P-type photochromes [27]. They are further classified as positive and negative photochromes. Positive photoswitches involve the conversion from colorless A isomer to the colored isomer B i.e. lmax(A) is smaller than lmax(B) or vice versa. The nature of application
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dictates the mode of isomerization in photochromes [28]. For example, switch application [29] requires quick interconversion between two isomers, whereas memory application requires high thermal stabilities of each isomer [30]. Photochromism proceeds through different chemical processes. Some of the most important processes are; electrocyclization [31,32], cis-trans isomerism [32], intramolecular group transfers [33], intramolecular hydrogen transfer [33] and pericyclic reactions [34]. Among all the processes, electrocyclization is an excellent strategy for the synthesis of many photochromic molecules that follows the rules of orbital symmetry conservation, originally proposed by Roald Hoffmann and R.B.Woodward [35]. Depending upon the number of p-electrons and experimental conditions (photochemical or thermal), the electrocyclization follows either conrotatory or disrotatory mechanism [36]. For example, the electrocyclization (thermal reversion) in cyclophanediene (CPD) involves the conrotatory mechanism with activation barrier of 20.36 kcal/mol [37]. Electrocyclization reactions of dihydropyrens and dithienylethene have shown the lowest activation barrier for thermal reversion under neutral conditions, as compared to the other classes [38]. Mitchell et al., reported that activation barrier is affected by extent of conjugation, electronic density and nature of group at ring closing termini [36,39e41]. Moreover, the substitution effect for electrocyclization under neutral conditions has also been investigated to reveal the variation in activation energies. The results revealed that methyl substituted CPD had the lowest activation barrier (DH ¼ 20.36 kcal/mol) as compared to nitrile substituted CPD (DH ¼ 25e27 kcal/mol) under same conditions at the same position [42]. It is experimentally proved that nitrile substituted cyclophanedienes are more stable than methyl substituted CPD with half-life of 30 years at room temperature [43]. Ayub et al., also demonstrated theoretically as well as experimentally that nature of substituent at closing termini affects the activation barrier [44]. Robinson et al., reported that the thermal return in DHP is accelerated by incorporating radical stabilizing groups at position of high spin density [45]. The various experimental and theoretical data from literature have divulged that the formation of radical cation accelerates the rate of electrocyclization in some photochromes. Similarly, Staykov and coworkers reported that removing one electron from the atoms present at ring closing termini lowers the activation barrier from open to closed form significantly [46]. Mahmood et al., reported acceleration of electrocyclization in photochromism for different classes such as fulgides, CPD-DHP, dithienylethene and VHF-DHA under radical cationic conditions [47] and revealed that the radical cation lowers the activation barrier for electrocyclization compared to neutral. For example, a low activation barrier (DH ¼ 3.13 kcal/mol, DG ¼ 4.01 kcal/mol) and high reaction energy (DH ¼ 28.48 kcal/ mol, DG ¼ 27.04 kcal/mol) is observed for radical cation mediated thermal reversion of cyclophanediene in comparison to thermal return under neutral conditions (DH ¼ 20.60 kcal/mol, DG ¼ 20.98 kcal/mol). Mahmood et al., also reported that, contrary to the other photochromic compounds, the thermal return of VHFDHA and fulgides is not affected with the formation of radical cation. Although, acceleration of electrocyclization has been investigated under radical cationic and neutral conditions however, electrocyclization under radical anionic conditions has not been explored. In the current study, we have discussed the acceleration of electrocyclization in organic photochromes under radical anionic conditions. 2. Computational methods All the theoretical calculations for different classes of
photochromes under anionic conditions are performed on Gaussian 09 [48]. The optimized structures are visualized through GaussView 5.0 [49]. B3LYP/6-31G(d,p) and M06-2X/6-311þG(d,p) level of theories are used for the optimization of geometries. M06-2X is a reliable method, to accurately estimate the activation barrier for electrocyclization reactions [50,51]. Moreover, the transition states are located through Berny algorithm and confirmed by frequency analysis (one imaginary frequency). The same level of theory is used to calculate the synchronicity. Synchronicity is mathematically expressed as (Eq. (1)) [47].
Pn Sy ¼ 1
i¼1
jdBi dBAV j dBAV
2n 2
(1)
In the above equation, n represents the number of bonds concerned and dBi represents dissimilarity in bond index at TS. The dBi and dBAV in equation (1) can be calculated from equations (2) and (3).
dBi ¼
R BTS i Bi
(2)
BPi BRi
dBAV ¼ n1
n X
dBi
(3)
i¼1
Furthermore, natural bond orbital method is used to calculate the Wiberg bond indices. Wiberg bond index is the measurement of the number of electrons of an atom A involved in a covalent bonding with another atom B. The index was originally proposed by Wiberg in 1968 [52]. The absolute magnetic shielding at the center of the ring i.e. aromaticity of transition state is confirmed by performing nuclear independent chemical shift (NICS) analysis. The NICS(0) simulations are performed by placing the ghost atom in the centroid of electronic cloud. Frontier molecular orbitals analysis is also performed to study the distribution of singly occupied molecular orbitals (SOMO) at same method.
3. Results and discussion 3.1. Radical anionic mediated thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) The transition state of radical anion isomerization is located, to study the activation barrier for thermal return of CPD to DHP. The optimized geometries of transition state, CPD and DHP are represented in Fig. 1. The activation barrier, overall energy of reaction and other variables such as synchronicity, NICS(0) values and symmetry of CPD-DHP are summarized in Table 1. The numbering system for overall discussion of all classes of photochromes is given in Fig. 2 for maintaining uniformity in discussion among open, transition state and closed isomers of the respective class. Analysis of the optimized structures under radical anionic conditions reveals a notable change in the bond length during isomerization from CPD (open form) to DHP (closed form). The bond length of C8eC16 (ring closing atoms) in optimized structure of CPD is 2.60 Å which is noticeably decreased to 2.00 Å in the TS and then to 1.56 Å in the closed form (DHP). This considerable difference of bond length indicates the isomerization of cyclophanediene to DHP. Similarly, the prominent dihedral angles (C3eC2eC1eC15 and C6eC7eC8eC3) in CPD are 0.67 and 25.00 , respectively. The dihedral angles are changed to 0.09 and 18.90 , in the transition state and 7.00 and 22.00 in the DHP, respectively. This alteration in torsion angles indicates planarization during isomerization. The
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3
Fig. 1. The optimized geometries of CPD, TS and DHP under radical anionic condition. The grey and white color balls represent carbon and hydrogen atoms, respectively.
Table 1 Enthalpies of activation (DHact in kcal/mol), Gibbs free energies of activation (DGact in kcal/mol), enthalpies of reaction (DHR in kcal/mol), Gibbs free energies of reaction (DGR in kcal/mol), Synchronicities (Sy), (NICS(0), in ppm) and transition states symmetries (Symm) of different photochromes classes under radical anionic condition at M06-2X/6311þG(d,p). S$NO.
Photochromes
Radical anion
DHact
DGact
DHR
DGR
Sy
NICS(0)
Symm
1 2 3 4
CPD-DHP Dithienylethene (open-closed) VHF-DHA Fulgides (open-closed)
5.94 20.12 23.72 29.76
6.97 21.55 24.82 30.44
26.02 7.92 9.76 13.92
24.71 5.64 11.09 12.67
1.00 1.00 0.87 1.00
5.31 71.78 2.70 2.40
C1 C1 C1 C1
Fig. 2. The general numbering scheme for discussion.
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electro-cyclizing p -electronic cloud (Fig. 3). The NICS(0) value is 5.31 ppm for TS of CPD-DHP thermal return (under radical anionic condition) which confirmed the antiaromatic character of TS. The observation of SOMO orbitals revealed that the p cloud is concentrated on the ring-closing terminus in the TS. The SOMO surfaces of CPD, TS and DHP are given in Fig. 4.
3.2. Radical anion mediated electrocyclization of dithienylethene
Fig. 3. Localization of ghost atom in the centroid of electrocyclic termini.
enthalpy of activation for CPD-DHP under radical anionic condition is 5.94 kcal/mol (DG ¼ 6.97 kcal/mol) which is 14.08 kcal/mol lower than that under neutral conditions (DH ¼ 20.60 kcal/mol, DG ¼ 20.98 kcal/mol) [43] whereas it is 2.78 kcal/mol higher than that of electrocyclization in radical cation (DH ¼ 3.13 kcal/mol, DG ¼ 4.01 kcal/mol) [47], respectively. The transition state located under radical anionic conditions possesses the C1 point group of symmetry, (different from the TS for radical cation (Cs). Whereas the transition state under neutral condition has C2h symmetry. The overall reaction enthalpy for radical anion mediated isomerization is DH ¼ 26.02 kcal/mol, (DG ¼ 24.71 kcal/mol). The enthalpy of reaction under neutral and radical cationic conditions are DH ¼ 22.25 kcal/mol (DG ¼ 21.13 kcal/mol) and DH ¼ 29.00 kcal/mol (DG ¼ 27.04 kcal/mol), respectively. The results reflect radical anionic mediated isomerization of CPD-DHP is more exothermic as compared to that under neutral conditions (DH ¼ 22.25 kcal/mol, DG ¼ 21.13 kcal/mol). However, it is less exothermic as compared with radical cation system DH ¼ 28.48 kcal/mol (DG ¼ 27.04 kcal/mol). Moreover, synchronicity is calculated to determine that whether TS is completely synchronous (lying in the center) or not. The synchronicity value under radical anionic conditions is 1.00, which reflects perfectly synchronized TS that lies in the middle. The synchronicity of radical cationic CPD-DHP isomerization is asynchronous with the synchronicity value of 0.67. Nuclear independent chemical shift indicates the aromatic behavior of transition state in the reaction. Aromaticity is an important parameter in highly conjugated cyclic systems, and it has strong correlation with NICS which is theoretically calculated. To explore the aromaticity of transition state for CPD-DHP electrocyclization under radical anionic conditions, NICS(0) is calculated via putting the ghost atom in the core of the
Activation barrier for dithienylethene isomerization under radical anionic conditions is also investigated at the same level of theory (M06-2X/6-311þG(d,p)). The optimized geometries (open form, closed form and TS) under radical anionic conditions are given in Fig. 5. The optimized geometries under radical anionic conditions have shown noticeable alteration in the bond distance among ring closing atoms (C1eC2). C1eC2 bond distance in the open form is 3.60 Å. The same bond length is 2.00 Å in the transition state and 1.50 Å in closed form of dithienylethene. The decreasing bond length between terminal atoms is the effectual indicator of electrocyclization of dithienylethene. Similarly, the important dihedral angle (C10eC11eC12eC3) in open form under radical anionic conditions is 23.80 which significantly decreases to 4.43 in TS and increases again to 7.80 in closed isomer. This change in dihedral angle brings the electrocyclic termini into a suitable orientation for isomerization. Enthalpy of activation for ring closing of dithienylethene under radical anionic condition is DH ¼ 20.12 kcal/mol (DG ¼ 21.55 kcal/ mol) which is quite less as compared to that under neutral (DH ¼ 48.13 kcal/mol (DG ¼ 50.28 kcal/mol)) conditions. However, the enthalpy of activation is higher as compared to isomerization under radical cationic conditions (DH ¼ 19.44 kcal/mol, DG ¼ 22.29 kcal/mol)) at B3LYP/6-31G(d,p) method [47]. The overall enthalpy of this reaction is DH ¼ 7.92 kcal/mol (DG ¼ 5.64 kcal/mol), which shows that the isomerization is exothermic. All the transition states (under radical cation, radical anion and neutral conditions) have C1 symmetry and follow conrotatory motion (radical cation, radical anion). The TS of dithienylethene is completely synchronized in the case of radical anion with synchronicity value of 1.00. Whereas the corresponding TS under radical cation is highly asynchronous (0.51) [47]. NICS(0) calculations of the TS are performed at M06-2X/6-311þG(d,p) level of theory (Fig. 6), which has shown the highly aromatic (than radical cation) character of the dithienylethene with value of 71.78 ppm. The SOMO of all isomers of dithienylethene (radical anion) are
Fig. 4. SOMO orbitals of CPD, TS and DHP (radical anion).
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Fig. 5. Optimized geometries of radical anion (open form, transition state (TS) and closed form) of dithienylethene.
Fig. 6. TS of dithienylethene having ghost atom.
given in SI. 1. SOMO investigation confirmed the conrotatory motion of atom at ring closing terminals in the transition state. 3.3. Radical anionic mediated electrocyclization of vinylheptafulvene to dihydroazulene Dihydroazulene (DHA) is T-type photochrome for which closed isomer is more stable and reverts back thermally to vinylheptafulvene (VHF). Both the isomers differ in the absorption spectrum in UVevisible region. The significant change in bond length is observed when VHF elecrocyclizes to dihydroazulene under radical anionic conditions. Under radical anionic condition, the C1eC10 bond distance between the ring-closing atoms is 3.45 Å in the reactant (VHF), and it decreases to 2.04 Å in TS and 1.59 Å in product (DHA). The important dihedral angles (C2eC3eC4eC10 and C1eC2eC3eC4 and) in VHF are 17.63 and 12.09 , respectively. The relevant torsion angles are 5.51 and 20.72 in the transition state, respecttively and 7.82 and 7.10 in the closed form, respectively. Variation in the dihedral angles brings electrocyclic termini (C1 and C10) into a favourable orientation for electrocycling. The TS of VHFDHA reflects the disrotatory movement of functional groups attached to the ring closing atoms and it has C1 point group of symmetry. Thermal isomerization of VHF to DHA follows
Fig. 8. TS of VHF-DHA having ghost atom.
disrotatory (Woodward-Hoffmann allowed movement) pathway. The enthalpy change for radical anion mediated thermal reversion of VHF-DHA is DH ¼ 23.72 kcal/mol (DG ¼ 24.82 kcal/mol) at M062X/6-311þG(d,p) method which is smaller than the reported value for electrocyclization under radical cation (DH ¼ 28.03 kcal/mol, DG ¼ 29.52 kcal/mol) [47] and neutral pathway (DH ¼ 28.39 kcal/ mol, DG ¼ 28.98 kcal/mol) at B3LYP/6-31G(d,p) method [53,54]. Contrary to radical cation the activation barrier for VHF to DHA are significantly affected (decreased) under radical anion. Furthermore, the energy of the reaction in the case of radical anionic condition is DH ¼ 9.76 kcal/mol (DG ¼ 11.09 kcal/mol) which is low as compared to that for radical cation DH ¼ 15.08 kcal/mol (DG ¼ 16.48 kcal/mol) [47]. The reaction energy under radical cationic condition is high compared to neutral conditions DH ¼ 3.11 kcal/mol (DG ¼ 4.03 kcal/mol), which shows that the reaction (radical anion as compared to neutral) is endothermic. The calculated synchronicity value (0.87) shows that the transition state for isomerization of VHF-DHA is synchronous unlike that for radical cation system (0.68). NICS(0) value for thermal electrocyclization of VHF-DHA is 2.70 ppm which indicates that the transition state with radical anion is non-aromatic, while the TS for VHF-DHA with radical cation is aromatic (NICS(0) ¼ 8.86 ppm) (see Fig. 8). Fig. 7
Fig. 7. The optimized structures of VHF, TS and DHA.
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Fig. 9. The optimized geometries of open, TS and closed form of fulgides.
3.5. Radical anionic electrocyclization at B3LYP/6-31G(d,p) method
Fig. 10. TS of fulgide having ghost atom.
shows the optimized structures of VHF, TS and DHA and SI. 2 shows the SOMO surfaces of VHF-DHA under anionic conditions.
3.4. Radical anionic mediated electrocyclization of fulgide The impact of radical anion on the activation energy of fulgide electrocyclization is also examined. During switching, the variation in the bond distances of ring closing termini atoms is noticed. For example, the C3eC4 bond length is 3.34 Å for the open form. The same bond distance decreased to 2.36 Å in TS, which is finally reduced to 1.50 Å in the closed form (see Fig. 9). The torsion angles such as C7eC8eC9eC2, C4eC7eC8eC9 C3eC2eC9eC8 in the open form are 6.83 , 11.6 and 6.8 . The respective angles are changed to 6.70 , 10.25 and 26.16 in the TS, which bring the ring closing atoms in proper orientation for isomerization. The enthalpy of activation (DH) for the thermal switching of fulgide under anionic condition is 29.76 kcal/mol (DG ¼ 30.44 kcal/mol) which is higher than the values already discussed under neutral (DH ¼ 25.71 kcal/ mol, DG ¼ 26.89 kcal/mol) and cationic conditions (DH ¼ 25.09 kcal/mol, DG ¼ 26.02 kcal/mol) at B3LYP/6-31G(d,p) method [47]. The complete enthalpy of the reaction with radical anion is 13.92 kcal/mol (DG ¼ 12.67 kcal/mol), which shows that the reaction is highly exothermic when compared to that under neutral condition (DH ¼ 7.88 kcal/mol). However, to reaction under radical anionic condition is less exothermic as compared to that under cationic conditions (DH ¼ 19.62 kcal/mol). Transition state of fulgide under anionic condition has C1 symmetry. The computed synchronicity value (Sy ¼ 1) revealed that the transition state is perfectly synchronous. The synchronicity of radical anion mediated TS is similar to that of neutral (Sy ¼ 1.00) but significantly high than that of radical cation (Sy ¼ 0.66). The NICS(0) of thermal anionic TS value is 2.40 ppm which represents the non-aromatic nature of TS (Fig. 10). The NICS(0) value (2.40 ppm) under radical anionic condition is dissimilar to previously reported value (3.20 ppm) of radical cationic condition [45]. SI. 3 shows the SOMO of open, TS and closed isomers of fulgide (radical anion).
We have also investigated the activation energies for thermal reversion of organic photochromes under radical anionic conditions at B3LYP/6-31G(d,p) level of theory. In the case of CPD/DHP conversion, the activation energy measured under radical anionic conditions at B3LYP method is lower (DH ¼ 4.52 kcal/mol and DG ¼ 6.80 kcal/mol) than that of M06-2X method (DH ¼ 5.94 kcal/ mol and DG ¼ 6.97 kcal/mol). The activation enthalpy for electrocyclization of dithienylethene (under radical anionic condition) at B3LYP is 22.80 kcal/mol (DG ¼ 25.60 kcal/mol), which is higher in comparison to the values at M06-2X (DH ¼ 20.12 kcal/mol and DG ¼ 21.55 kcal/mol). The activation energy values (DH ¼ 27.65 kcal/mol and DG ¼ 29.56) at B3LYP level of theory for thermal reversion of VHF-DHA (under radical anionic condition) are almost 4e5 kcal/mol higher compared than the values at M062X method (DH ¼ 23.72 kcal/mol and DG ¼ 24.82). However, the activation energy for thermal return of fulgide under radical anion condition shows a difference of 1 kcal/mol between B3LYP (DH ¼ 28.34 kcal/mol and DG ¼ 29.47 kcal/mol) and M06-2X (DH ¼ 29.76 kcal/mol and DG ¼ 30.44 kcal/mol)). Moreover, reaction energies calculated at B3LYP method are less exothermic as compared to M06-2X method (except radical anion thermal switching of CPD to DHP which is more endothermic at B3LYP method). All the photochromes have shown similar synchronicity values at both level of theories.
4. Conclusions We have investigated the activation enthalpies for thermal reversion of different photochromes under radical anionic conditions at M06-2X/6-311þG(d,p) level of theory. Among all the studied photochromes, dihydropyrene, dithienylethene and vinylheptafulvenes have shown the acceleration of electrocyclization with radical anion, while the activation barrier measured for fulgides under radical anion (DH ¼ 29.76 kcal/mol (DG ¼ 30.44 kcal/ mol) is high as compared to that under neutral (DH ¼ 25.71 kcal/ mol, DG ¼ 26.89 kcal/mol) and cationic conditions (DH ¼ 25.09 kcal/mol, DG ¼ 26.02 kcal/mol). In the case of CPDDHP the activation energy measured with radical anion (DH ¼ 5.94 kcal/mol, DG ¼ 6.97 kcal/mol) is very small as compared to the neutral (DH ¼ 20.60 kcal/mol, DG ¼ 20.98 kcal/mol), but it is a little bit high as compared to radical cation DH ¼ 3.13 kcal/mol, DG ¼ 4.01 kcal/mol). The CPD-DHP has shown the perfectly synchronized transition state (Sy ¼ 1) with aromatic characteristics during radical anion thermal switching. The activation energy for thermal reversion of dithienylethene (under radical anion) is (DH ¼ 20.12 kcal/mol, DG ¼ 21.55 kcal/mol), which is lower in
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comparison to the neutral (DH ¼ 48.13 kcal/mol, DG ¼ 50.28 kcal/ mol) but high compared to the radical cation (DH ¼ 19.44 kcal/mol, DG ¼ 22.29 kcal/mol), respectively. The synchronicity value of 1 has shown perfectly synchronous transition state of dithienylethene under same conditions. Aromatic nature of radical anion transition state of dithienylethene is confirmed from the high NICS(0) value. Under radical anionic conditions, the activation energy for the switching of dihydropyrene and dithienylethene is significantly reduced and their results are consistent with the results of radical cationic condition. The activation energy in VHF-DHA thermal reversion with radical anion is DH ¼ 23.72 kcal/mol (DG ¼ 24.82 kcal/mol) which is smaller to already reported value under radical cationic conditions H ¼ 28.03 kcal/mol (DG ¼ 29.52 kcal/mol) and neutral conditions H ¼ 28.39 kcal/mol (DG ¼ 28.98 kcal/mol). The reported synchronicity value (0.87) indicate that the transition state for isomerization of VHF-DHA is asynchronous; whereas NICS(0) value (2.70 ppm) indicates that the TS are nonaromatic with radical anion. It indicates that radical anion formation has effect on the activation energy of thermal return of fulgide. The activation energy for thermal switching of fulgides with radical anion is DH ¼ 29.76 kcal/mol (DG ¼ 30.44 kcal/ mol), which is higher in comparison to neutral DH ¼ 25.71 kcal/mol (DG ¼ 26.89 kcal/mol) and radical cation DH ¼ 25.09 kcal/mol (DG ¼ 26.02 kcal/mol). The synchronicity is 1, which indicates that the TS is perfectly synchronous with non-aromatic nature. The impact of radical anion on electrocyclization in photochromism is in progress for other photochromes. These photoswitches are used in rigid polymer matrix using radical polymerization and bridged imidazole dimers [55]. Acknowledgements The authors acknowledge the Higher Education Commission of Pakistan (Grant No. 3013) and COMSATS University, Abbottabad Campus. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmgm.2020.107550. References [1] M. Irie, Diarylethenes for memories and switches, Chem. Rev. 100 (2000) 1685e1716. [2] Y. Yokoyama, Fulgides for memories and switches, Chem. Rev. 100 (2000) 1717e1740. [3] Colour- and optically changing smart materials, in: Smart Mater., Birkh€ auser Basel, Basel, n.d.: pp. 72e97. [4] M.S.M. Rawat, S. Mal, P. Singh, Photochromism in anils - a review, Open Chem. J. 2 (2015) 7e19. [5] G. Berkovic, V. Krongauz, V. Weiss, Spiropyrans and spirooxazines for memories and switches, Chem. Rev. 100 (2000) 1741e1754. [6] F.M. Raymo, M. Tomasulo, Electron and energy transfer modulation with photochromic switches, Chem. Soc. Rev. 34 (2005) 327. [7] R.H. Mitchell, C. Bohne, Y. Wang, S. Bandyopadhyay, C.B. Wozniak, Multistate p switches: synthesis and photochemistry of a molecule containing three switchable annelated dihydropyrene units, J. Org. Chem. 71 (2006) 327e336. [8] K. Matsuda, M. Irie, A diarylethene with two nitronyl nitroxides: photoswitching of intramolecular magnetic interaction, J. Am. Chem. Soc. 122 (2000) 7195e7201. [9] F. Dietz, G. Olbrich, S. Karabunarliev, N. Tyutyulkov, Photoswitching of dipole moments, charge-transfer and spectroscopic properties, Chem. Phys. Lett. 379 (2003) 11e19. [10] J.A. Delaire, K. Nakatani, Linear and nonlinear optical properties of photochromic molecules and materials, Chem. Rev. 100 (2000) 1817e1846. [11] M. Banghart, A. Mourot, D. Fortin, J. Yao, R. Kramer, D. Trauner, Photochromic blockers of voltage-gated potassium channels, Angew. Chem. Int. Ed. 48 (2009) 9097e9101. [12] A.S. Matharu, S. Jeeva, P.S. Ramanujam, Liquid crystals for holographic optical data storage, Chem. Soc. Rev. 36 (2007) 1868.
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