Toward Spatiotemporally Controlled Synthesis of Photoresponsive

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Towards Spatiotemporally Controlled Synthesis of Photoresponsive Polymers: Computational Design of Azobenzene-Containing Monomers for Light-Mediated ROMP Qunfei Zhou, Ishan Fursule, Brad J. Berron, and Matthew J Beck J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05807 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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The Journal of Physical Chemistry

Towards Spatiotemporally Controlled Synthesis of Photoresponsive Polymers: Computational Design of Azobenzene-Containing Monomers for Light-Mediated ROMP Qunfei Zhou,†,‡ Ishan Fursule,† Brad J. Berron,∗,† and Matthew J. Beck∗,†,‡ Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY, and Center for Computational Sciences, University of Kentucky, Lexington, KY E-mail: [email protected]; [email protected] Phone: (859)-257-2791; (859)257-0039. Fax: (859)323-1929

To whom correspondence should be addressed CME, University of Kentucky ‡ CCS, University of Kentucky

∗

†

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Abstract Density functional theory calculations have been used to identify the optimum design for a novel, light-responsive ring monomer expected to allow spatial and temporal control of ring opening metathesis polymerization (ROMP) via light-mediated changes in ring strain energy. The monomer design leverages ring-shaped molecules composed of 4,4’-diaminoazobenzene (ABn) closed by alkene-α, ω-dioic acid linkers. The atomic geometries, formation enthalpies and ring strain energies of azobenzene (AB)-containing rings with various length linkers have been calculated. The AB(2,2) monomer is identified to have optimal properties for light-mediated ROMP, including high thermodynamic stability, low ring strain energy (RSE) with cis AB, and high RSE with trans AB. Time-dependent DFT calculations have been used to explore the photoisomerization mechanism of isolated AB and AB-containing rings, and calculations show that trans-to-cis and cis-to-trans photoisomerization of the optimal AB(2,2) ring molecule can be achieved with monochromatic green and blue light, respectively. The AB(2,2) monomer identified here is expected to allow precise, reversible, spatial and temporal light-mediated control of ROMP through AB photoisomerization, and to have promising potential applications in the fabrication of patterned and/or responsive AB-containing polymer materials.

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Introduction Photo-responsive materials including azobenzene (AB) and its derivatives have a wide range of potential applications as photo switches in photopharmacology, 1–3 molecular motors in drug delivery, nano-scale machines, 4–6 and in molecular and optoelectronic devices. 7–10 These attractive properties of AB-containing materials are all due to the photo-induced isomerization of azobenzene from a trans conformation under visible light to a cis conformation under UV light, and vice versa. 11,12 As highlighted in Fig. 1, the principal effect of trans-to-cis photoisomerization of, e.g., 4,4’-diaminoazobenzene (ABn), is to reduce the end to end length of the molecule by about 28%. AB photoisomerization is rapid and reversible, observable both at the molecular- and macroscale, and has been associated with significant alterations in physical and chemical properties 13 of AB-containing materials. 4,14–16 The incorporation of AB into polymers results in light-responsive macroscale prop-

NH2

erties that are of interest for a range of appli-

NH2

cations in optical devices, sensors, information displays and storage systems.

17–19

UV 11.89Å

For

N N

example, the wettibility of polymer films can

visible

N N

8.56Å

NH2

be modulated through the reversible changes

NH2

of dipole moment and surface alignment of

Figure 1: Photoisomerization of trans (left) and cis (right) 4,4’-diamino-azobenzene functional groups induced by photoisomer(ABn), respectively. ization of trans and cis AB. 20–22 In addition, the photo-induced dynamical properties of AB-containing liquid crystal polymers suggest they may be used as artificial muscles. 13,16,23,24 However, previous studied responsive polymer materials are limited to two-dimensional films



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with defect-free thickness that are usually beyond the penetration of UV light. 13,25 Separately, ring-opening metathesis polymerization (ROMP) is a powerful and widely used method for the controlled synthesis of polymer materials and films. 26–32 ROMP is a type of chain-growth polymerization based on olefin metathesis where cyclic monomers (“ring” monomers) open at a carbon-carbon double bond in the presence of a catalyst, and link together to form growing chains. Polymerization, as opposed to re-closing of the ring monomer, is driven by the existence of a non-zero ring strain energy (RSE). 29,31 For large enough RSE, ROMP is preferential to reclosing in the presence of a catalyst, as an open configuration allows the release of RSE present in a closed ring. Too small RSEs limit polymerization, as open configuration are increasingly likely to re-close instead of polymerizing (see Fig. 2). Therefore, RSE represents a driving force for ROMP which could be harnessed to activate or deactivate ROMP if it were possible to control or modulate RSE. To the knowledge of the authors, no such control of strain-promoted reactions has been previously described. Here we use quantum chemical calculations to design novel AB-containing monomers in which RSE (and therefore ROMP) can be controlled by the reversible cis-trans photoisomerization of AB. The monomers designed here are expected to enable light-

SE R w lo

hig hR SE RO MP

n

controllable ROMP, specifically, to allow the appli-

Figure 2: A brief sketch of ROMP. cation of different wavelengths of light to activate When RSE is too small, the cyclic monomer tends to be closed where or deactivate ROMP by modulating RSE. Lightno polymer forms. The right branch controllable ROMP will not only enable a remote, non- shows an example of ROMP when there are high enough RSE in the invasive and instantaneous temporal control of ROMP cyclic monomer. beyond reliance on the exhaustion of added catalyst or the addition of a termination reagent, 29,33–35 but also, though the use of optical masks and filters, allow coincident spatial control of ROMP, allow3 ACS Paragon Plus Environment

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ing unprecedented ability to fabricate complex, patterned, photo-responsive polymers.

N

O

N

O

m

m

N

N N

N

n

n

O N

O N

(a) (b)

Figure 3: The proposed cyclic monomers consists of ABn and alkene-α, ω-dioic acid linkers. The cyclic monomer is denoted as AB(m, n) where m and n is the number of methylene groups at the two sides of the C=C double bond. cis AB(m, n) (a) and trans AB(m, n) (b) refer to monomers with ABn in cis and trans conformation. The conformation of the alkene linker shown here is in trans conformation. However, with AB(m, n) of different values of (m, n), it can be either trans or cis depending on which one is more thermodynamically stable. To identify monomers that enable light controlled ROMP, we have considered ring monomers composed of 4,4’-diaminoazobenzene (ABn, Fig. 1) combined with alkene-α, ωdioic acid “linkers” [denoted L(m, n)] that connect the two amino groups of the ABn to form an AB-containing ring [denoted AB(m, n)], as shown in Fig. 3. The length of the linker is specified by the number of methylene groups—m and n—on either side of the C=C double bond. AB(m,n) ring monomers are expected to exhibit non-trivial ring strain energies that vary with linker length (that is, different values of m and n). For suitable linker lengths, the RSE of AB(m,n) ring molecules with AB in the cis conformation is expected to differ significantly from the RSE of the same AB(m,n) molecule with AB in the trans conformation. This difference enables intentional control of RSE via photoisomerization of AB. In this way, the activation and deactivation—effectively the rate—of catalyst-induced ROMP can be directly and elegantly controlled by exposure of suitable AB(m, n) monomers to different wavelengths of light. In order to determine the optimal linker lengths maximizing photo-mediated control of 4 ACS Paragon Plus Environment


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ROMP, cyclic molecules AB(1,1), AB(1,2), AB(2,2), AB(2,3), AB(3,3), AB(3,4), AB(4,4), AB(5,5) have been investigated in this study. Among these molecules, the monomer for optimum light-mediated control of ROMP is found to be AB(2,2), which exhibits high stability (that is, low formation enthalpy), very low RSE with AB in the cis conformation, and very high RSE with AB in the trans conformation. Calculations show that photoisomerization of open AB(2,2) molecules proceeds similar to that of isolated ABn, but slightly different for closed AB(2,2) monomers. Meanwhile, the wavelengths of light triggering photoisomerization for AB-containing molecules are different from that for isolated ABn.

Computational Details All calculations have been performed using density-functional theory (DFT) as implemented in the Gaussian09 package. 36 The B3LYP functional 37 and 6-31G∗ basis set 38 were employed based on numerous previous studies of AB and AB-derivatives demonstrating excellent agreement between computed and measured results. 39–43 Full geometry optimization was performed for each of the molecule. Harmonic vibrational frequency calculations were used to obtain enthalpy and zero-point energy corrections at 298 K. Solvation effects of dichloromethane (CH2 Cl2 , DCM, except as noted below), a widely used organic solvent that could dissolve azobenzene-containing molecules, were accounted for by using the integral equation formalism variant of polarizable continuum model (IEFPCM), where the solute is placed in a cavity created via a set of overlapping spheres within a continuous medium (the solvent) with dielectric constant ǫr . 44,45 The properties of a range of relevant molecules were considered in seeking an optimal AB-containing ring monomer. Eight linkers with (m, n) values of (1,1), (1,2), (2,2), (2,3), (3,3), (3,4), (4,4), (5,5) were considered, along with their corresponding AB(m, n) molecules. The closed AB(m, n) rings are depicted in Fig. 3, and are “open” when the C=C double


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bond in the attached linker is broken, as shown in Scheme 2. When working with AB(m, n) monomers or isolated linkers in an open configuration, dangling C bonds are passivated with methylene groups at each end (that is, –C* groups are passivated to form –C=CH2 ). This passivation approach mimics ROMP, where open molecules will bind to nearby open molecules for C=C attachment bonds (Fig. 2). NH2

m

Cl O

N

L

H N

CH2

)

+ HCl + H C 2

N

n

Cl NH2

m

N

+

N

O

O

n

CH2

CH2

O

N H

L n)

ABn

AB

,n)

Scheme 1: Constitution of cis AB(m, n) ring monomers (same applies for trans ring monomers) from ABn and half linkers, L(m) and L(n). This is used for calculation of formation enthalpy for AB(m, n) rings. To compare the thermodynamic stability of the ring monomers, formation enthalpies were calculated for open and closed AB(m, n) rings. Reference energies for formation enthalpy calculations are those corresponding to ABn, isolated linkers, and other relevant small molecular, e.g. HCl, H2 C=CH2 . To model formation enthalpies likely relevant to possible AB(m,n) synthesis pathways, we chose reference molecules for the linker portion of AB(m, n) monomers as CH2 CH(CH2 )m COCl “half” linkers [denoted L(m)/L(n)], which attach at opposite ends of ABn and close at C=C to yield AB(m, n) ring molecules, see Scheme 1. This reaction produces HCl and C2 H4 as byproducts. As noted above, open linkers are passivated with methylene groups to mimic polymerized chains, and therefore enthalpy changes due to ring closing must account for two CH2 groups, which we reference as H2 C=CH2 molecules. The formation enthalpy can be calculated for open [oAB(m, n)] molecules as:

∆Hf [oAB(m, n)] = H[oAB(m, n)] + 2H[HCl] − H[ABn] − H[L(m)] − H[L(n)]


(1)


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and for closed [cAB(m, n)] ring molecules as:

∆Hf [cAB(m, n)] = H[cAB(m, n)]+2H[HCl]+H[H2 C = CH2 ]−H[ABn]−H[L(m)]−H[L(n)] (2) O H N

E

O

NH

m

CH2

m

N

+ H2C

N

CH2

-E

N

= RSE

N

n O N H

NH

n

CH2

O

Scheme 2: Homodesmotic equation for the RSE calculation. Ring strain energies for closed AB(m, n) monomers were computed using a relative approach introduced by previous studies which showed good agreement with experimentally measured RSEs. 46–48 This approach is based on a homodesmotic equation from a reaction scheme 47 with the closed ring as reactant and a strain-free open ring as the product, balanced with small alkane or alkene equivalents to keep the number of each type of carbon-carbon bonds (e.g., C-H, C-C, C=C) equal in reactants and products. For the ROMP reaction, the RSE is released by opening the closed monomers–that is, breaking the C=C bond. Thus in this study, the ring strain energy related to ROMP, is calculated according to Scheme 2 where a H2 C=CH2 reference molecule was used to ensure equivalent numbers of C=C double bonds in both reactants and products. Potential energy curves (PEC) allow investigation of the pathways of photoisomerization of AB and azobenzene derivatives. PECs for photoisomerization of a subset of AB(m, n) monomers were investigated using the method reported by Crecca and Roitberg, 42 previously used to study AB. PECs for previously identified AB photoisomerization pathways in the electronic ground state, S0 , were generated by rotating the C1-N=N-C2 (CNNC) dihedral angle (see Fig. 1) in AB between -40.0◦ and 220.0◦ for the rotation pathway, and rotating


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the N=N-C1 (NNC1) or N=N-C2 (NNC2) angle (see Fig. 1) between 80.0◦ to 180.0◦ for the inversion pathway, both at 10.0◦ intervals. During PEC calculations, atoms other than those in the rotating angles were fully relaxed to their low-energy configuraitons. Time-dependent DFT (TDDFT), as implemented in Gaussian 09 was used to explore the optical excitation energies and electronically excited photoisomerization pathways of AB(m, n) monomers. A number of previous calculations of the excitation energies of AB and AB derivatives have been reported based on different theories. Calculations employing TDDFT, 42 complete active space self-consistent field (CASSCF) 49,50 and perturbative iterative selection (CIPSI) 51 have all been reported, and while all methods have been demonstrated to deviate from experimental measurements, it has been shown that TDDFT generally predicts excitation energies that are below, but within 10% of experimental results. 42 Therefore TDDFT has been used in this study to calculate vertical excitation electronic energies of AB(m, n) monomers to determine the wavelengths necessary to trigger photoisomerization. PECs for electrically excited monomers in the S1 and S2 states were generated by calculating the singlet vertical excitation energies for every point in the ground-state PEC using TDDFT 52 in the level of B3LYP/6-31G∗ .

Results and Discussion Functional AB-containing “ring” monomers that enable light-activated ROMP must (a) be stable in a closed-ring configuration that (b) exhibits low RSE with AB in the cis conformation, but also high RSE with AB in the trans conformation. In addition, (c) photoisomerization of AB must still be possible and controllable when AB is in, at a minimum, the closed ring configuration. In seeking to satisfy these design requirements we have used quantum chemical calculations to survey the geometries and properties of isolated AB and alkene-α, ωdioic acid chloride linkers, including L(1,1), L(1,2), L(2,2), L(2,3), L(3,3), L(3,4), L(4,4) and



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L(5,5), as well as open and closed AB(m, n) ring monomers containing the same linkers. Once optimal AB-containing monomers satisfying criteria (a) and (b) were identified, the monomer’s photoisomerization mechanisms were explored.

Properties of isolated AB and ABn molecules Table 1: Optimized geometries of trans and cis isomers of AB and ABn considering solvation effect of DCM, comparing with previous experimental results. angle/degree trans

cis

AB ABn AB exp.a AB ABn AB exp.b

∠CNNC

∠NNC

180.0 179.9 180.0 9.9 12.5 0.0

115.0 115.4 114.1 124.1 124.9 121.9

distance/Å

∆ H∗

d(N = N ) d(C − N )

(eV)

1.26 1.27 1.25 1.25 1.26 1.25

1.42 1.41 1.43 1.44 1.43 1.45

0.58 0.67 0.58c

enthalpies here are relative to the thermodynamically more stable trans isomers.a. Ref; 53 b. Ref; 54 c. Ref. 55 ∗

Before considering complete AB(m, n) monomers, the structure and properties of isolated AB, and ABn molecules were studied. Table 1 reports computed structural parameters and relative enthalpies for relaxed (minimum energy) cis and trans AB and ABn structures. Results are presented for calculations of molecules in DCM, and agree well with both previous experimental results for trans AB crystals 53 and cis AB in benzene 54 (see Table 1), as well as previous theoretical calculations. 42,56–59 We have also calculated gas phase relative enthalpy values (see Table S1 in supporting information) which are about 0.05 eV larger than the DCM-solvated values for both AB and ABn. Note that the enthalpy differences in Table 1 are given for the higher enthalpy isomer of each molecule, which is, in this case, the cis conformation. The calculated enthalpy of cis 9 ACS Paragon Plus Environment

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AB is 0.64 eV higher (in gas phase) and 0.58 eV higher (in DCM) than that of trans AB. This is very close to previously reported experimental values of 0.58 eV obtained for AB in solid phase. 55 For ABn, the enthalpy differences between cis and trans are 0.72 eV and 0.67 eV in gas phase and DCM solvent, respectively, both a little larger than for AB itself. The DFT optimized geometries and lengths of isolated linkers with different numbers of methylene groups—m and n values—with solvation effects of DCM are shown in figure S1 and S2 in supporting information. Table 2: Bond angles (in degree) of the optimized closed and open AB(m, n) ring molecules with different linker sizes. trans and cis refer to conformation of AB. NNC1 and NNC2 are two of the NNC angles as shown in Fig. 1. ∆max is the range over which each bond angle varies among the AB(m, n) molecules. closed-cis AB(m, n) CNNC NNC1 NNC2

open-cis

closed-trans

open-trans

CNNC NNC

CNNC NNC1 NNC2

CNNC NNC

AB(1,1) AB(1,2) AB(2,2) AB(2,3) AB(3,3) AB(3,4) AB(4,4) AB(5,5)

8.3 9.4 11.3 10.8 8.6 11.8 10.7 10.5

120.3 121.3 123.2 121.8 120.0 124.1 123.7 123.5

120.3 121.5 123.2 123.2 120.0 124.2 124.8 124.1

11.6 11.7 11.6 11.5 11.6 11.6 11.6 11.6

124.4 124.3 124.3 124.4 124.5 124.6 124.4 124.2

138.2 144.6 151.3 151.2 159.7 162.9 165.8 169.3

113.6 112.4 113.8 114.0 113.8 114.4 114.6 114.3

113.6 115.4 114.1 114.4 114.7 114.7 114.9 115.2

179.7 179.7 179.7 179.7 179.7 179.7 179.5 179.5

115.1 115.1 115.1 115.0 115.0 115.0 115.0 115.0

∆max

3.5

4.1

4.8

0.2

0.4

31.1

2.2

1.8

0.2

0.1

Structure and Stability of AB(m,n) rings The optimized geometries of closed and open AB(m, n) molecules with AB in both cis and trans conformations are shown in Fig. 4 for each L(m, n) with m=n (as above structures were optimized with DCM as the solvent). Ring molecules with L(m, n) for m 6= n are shown in Figure S3 of the supporting information. Geometric parameters for all considered ring molecules are reported in Table 2. The AB units in open AB(m, n) molecules, both 10 ACS Paragon Plus Environment


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with trans AB, and decrease with smaller linker lengths from 169.3◦ to 138.2◦ , where ∆max is as large as 31.1◦ . It can also be observed from Fig. 4 that trans AB units in closed rings are not planar and are increasingly non-planar with decreasing linker lengths. For the closed AB(m, n) rings with cis AB, CNNC and NNC bond angles vary for different length linkers, analogous to the increased non-planarity of trans AB in closed rings. Smaller CNNC angles are observed for closed-cis AB(1,1) and AB(1,2) since L(1,1) and L(1,2) are themselves shorter than cis ABn, which is therefore forced to bend closed more so than isolated cis AB or ABn. The length of L(2,2) is very similar to that of cis ABn, as shown in Figure S2, and the CNNC angle for closed AB(2,2) with cis AB is very similar to that of isolated AB, 9.9◦ (Table 1). Longer linker lengths result in relatively larger CNNC angles in closed AB(m, n) ring molecules with cis AB, except for AB(3,3) where the linker configuration seems to drive the ABn amino end-groups towards each other, resulting in a smaller than expected CNNC angle. Different NNC1 and NNC2 angles are present among the closed rings, likely resulting from details of the linker chain C-C backbone arrangement. In certain cases these internal effects result in asymmetric NNC1 and NNC2 values, while highly symmetric geometries connecting the N=N (at AB) and C=C (at the linker) bonds yield equal NNC1 and NNC2 angles and are observed for the closed rings AB(1,1) (cis and trans AB), AB(2,2) (cis AB) and AB(3,3) (cis AB), see Fig. 4. As seen in Fig. 4, some of the alkene linkers in geometry optimized AB(M, n) molecules exhibit trans alkene conformations while others exhibit cis alkene conformations. The structures in Fig. 4 are optimized geometries in minimum energy configurations. We have studied the structures and energies of AB(m, n) molecules in local minimum energy configurations with the alkene in alternative conformations to those shown in Fig. 4. For example, the minimum-energy geometries of closed AB(2,2) have trans alkene as shown in Fig. 4. Closed AB(2,2) with cis alkene linkers have also been calculated and are reported in Figure S5 of the supporting information, and the enthalpies of molecules in these alternative configurations 12 ACS Paragon Plus Environment


are included in Table S3. As a result, closed AB(2,2) with a cis alkene have higher enthalpy than the trans alkene configurations. Table S3 highlights that the influence of alkene conformation on entalpy is small compared to the effect of the conformation of AB. That is, while alkene conformations other than those reported in Fig. 4 can likely be observed at finite temperature, alkene conformation changes will not alter the AB-conformation-driven trends in formation energy and RSE. Therefore, in the following we adopt a naming convention for AB(m, n) ring molecules that only specifies the AB conformation—that is, cis AB(m, n) denotes that the molecule has AB in the cis conformation, regardless of the alkene conformation. cis AB(m,n) 0

formation enthalpy (kcal/mol)

formation enthalpy (kcal/mol)

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(1,1) (1,2) (2,2) (2,3) (3,3) (3,4) (4,4) (5,5)

-5

-10

-15

close open

(a)

close open

20 10 0

(1,1) (1,2) (2,2) (2,3) (3,3) (3,4) (4,4) (5,5)

-10 -20

trans AB(m,n)

(b)

Figure 5: Enthalpy of formation for closed and open cis AB(m, n)(a) and trans AB(m, n)(b).

To assess stability of AB-containing rings–that is, effectively criterion (a) from above–the enthalpy of formation of the AB(m, n) ring monomers was calculated in both open and closed ring configurations. As noted above, we use the enthalpies of ABn and half linker precursors as reference energies (see Scheme 1 and Eq. 2). While other reaction precursors may be used to synthesize AB(m, n) rings, we employ this particular energy reference primarily to assess relative stability among different rings. Although quantitative prediction of free energy changes for the opening/closing reaction are not possible due to kinetic and entropic effects that are beyond the scope of the 13 ACS Paragon Plus Environment

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present work, comparison of formation enthalpies of optimized closed versus open AB(m, n) molecules provides important insight into the relative stability and potential ease of synthesis of the various ring molecules, as demonstrated by previous studies on other molecular systems. 60–63 Formation enthalpies calculated according to Eq. 1 for open AB(m, n) molecules and Eq. 2 for closed AB(m, n) rings are reported in Fig. 5. Solvent (DCM) effects have been included in these calculations. enthalpy difference (kcal/mol)

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


15

AB(m,n)

more stable trans

10 5 0

(1,1) (1,2) (2,2) (2,3) (3,3) (3,4) (4,4) (5,5)

-5 -10

open close

-15

cis

more stable

-20

Figure 6: The enthalpy differences between trans and cis AB(m, n), ∆H=H[cis AB(m, n)]H[trans AB(m, n)].

For cis AB(m, n) ring molecules formation enthalpies are all negative (exothermic) for both open and closed AB(m, n) [Fig. 5(a)], indicating high thermodynamic stability. However, for trans AB(m, n) [Fig. 5(b)], the formation enthalpies of the closed-trans AB(m, n) are all positive (endothermic), but decrease as linker lengths become larger. This makes sense because short linkers can be seen bending the trans AB (see Fig. 4), indicating high strain energy that increases the overall energy of the molecule–an effect that would decrease with longer linkers, as observed here. This indicates that closed-cis AB(m, n) ring molecules are more stable than equivalent closed-trans AB(m, n) ring molecules. In contrast to some closed ring molecules, both isolated AB and ABn, the thermodynamically more stable conformation is the trans isomer. This is also the case for open AB(m, n) molecules, see Fig. 6 which shows the relative enthalpies of trans and cis AB(m, n) ring molecules in open and closed forms. Positive values indicate that trans AB(m, n) con14 ACS Paragon Plus Environment


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formations are thermodynamically more stable, while negative values indicate cis AB(m, n) conformations are more stable. For closed ring molecules, the relative stability of the trans and cis AB(m, n) isomers can be reversed, with cis AB(m, n) isomers becoming thermodynamically more stable when linkers are shorter than L(3,4). This, of course, is just a restatement of the fact that sufficiently short linkers introduce non-trivial ring strain energy into closed-trans rings. In combination these results point to two conclusions. First, closed AB(m, n) ring molecules are minimum-energy structures, and are stable against spontaneous decomposition. Second, open AB(m, n) molecules are more stable in trans conformations, similar to isolated AB and ABn molecules, while certain closed rings are more stable in the cis conformation. This second point implies that smaller closed-trans rings will exhibit a thermodynamic driving force for transformation to closed-cis conformations, and therefore that small [linkers shorter than L(3,4)] closed rings may not be shelf stable as trans isomers but as cis isomers.

Ring Strain Energies Ring strain energies, the driving force behind ROMP, can be extracted from enthalpies of AB(m, n) rings in closed versus open forms. The RSE for the closed-trans and cis AB(m, n) rings was calculated according to Scheme 2 and is reported in Fig. 7. The RSE of closedtrans AB(m, n) rings is large (∼50 kcal/mol) for the small ring AB(1,1), and decreases monotonically with increasing linker length. The RSE of closed-cis AB(m, n) rings oscillates with increasing linker length in a narrow energy window between about 6 to 11 kcal/mol. Overall, cis AB(m, n) rings have smaller RSE than trans AB(m, n) rings for all except the largest considered one, AB(5,5) ring molecule. The RSE trends in Fig. 7 arise directly from the structural and thermodynamic data presented above. Effectively, for small linkers, closing trans ring molecules requires substan15 ACS Paragon Plus Environment

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ring strain energy (kcal/mol)

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50

RSE_cis RSE_trans

40 30 20 10 0

(1,1) (1,2) (2,2) (2,3) (3,3) (3,4) (4,4) (5,5)

closed AB(m,n)

Figure 7: Calculated ring strain energies for the closed-trans and closed-cis AB(m, n).

tial bending of the AB (and linker) portions of the ring. For shorter linkers, e.g. shorter than L(2,3), the linkers will also be stretched to bridge the ends of the longer ABn unit (see Fig. 4). Longer linkers can arrange their C-C bond conformations to better accommodate both trans and cis ABn, reducing bending (that is, stretched or compressed bond lengths or angles) along the ring. Closed-cis rings exhibit low ring strain energies for at least two reasons. First, in contrast to trans ABn, the cis ABn has already closed about 120 degrees (sum of the supplementary angles of NNC1 and NNC2) of the ring. In addition, cis ABn is shorter than all but the two shortest linkers, implying that linkers larger than L(1,2) need not be stretched to close the ring. Finally we note that linkers substantially longer than either cis or trans ABn tend not to “force open” the ABn, rather they internally reconfigure their C-C backbones at no or very little energy cost. We are now in a position to identify whether any of the considered ring molecules satisfy both criteria (a) and (b), above, and, if so, which monomer is optimal for enabling lightmediated ROMP. Fundamentally, we seek a ring monomer that is thermodynamically stable and synthesizable in the less-strained cis conformation, and exihibits a substantial difference 16 ACS Paragon Plus Environment


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in RSE between trans and cis conformations. Based on these criteria, we note that all cis AB(m, n) ring monomers are thermodynamically stable, but that the closed AB(2,2) ring has a small RSE of 6.6 kcal/mol in cis conformation and a large difference in RSE between cis and trans isomers. It has been reported that the minimum required RSE for surface-initiated ROMP carried out in solution is 13.3 kcal/mol. 64 The RSE of cis AB(2,2) is lower than this threshold value, while trans AB(2,2) has a much higher RSE. This implies that for an AB(2,2) ring monomer ROMP will be inactive when the monomer is in the cis conformation, but will be active when in the trans conformation. Therefore, if the AB unit in AB(2,2) rings can be photoisomerized similarly to isolated AB and ABn, then AB(2,2) rings will allow ROMP to be activated or deactivated simply through controlled exposure to light. We also note that AB(1,1), AB(1,2) and AB(2,3) have large differences in RSE between their cis and trans conformations. In these cases, though, the computed cis RSE is ∼10 kcal/mol, which is close to the critical RSE for driving ROMP. Therefore it may not be possible to deactivate ROMP simply by photoisomerization to the cis conformation of these ring monomers. In addition, the symmetric AB(3,3) and AB(4,4) rings also have very low RSE in cis conformations, but exhibit much smaller differences in RSE betweeen cis and trans conformations compared to AB(2,2). Therefore, AB(2,2) would be expected to exhibit better photo-control of ROMP kinetics than either AB(3,3) or AB(4,4). In combination, the present results clearly identify AB(2,2) as the optimal AB-containing ring monomer for enabling light-actuated ROMP.

Photoabsorption of Azobenzene-Containing Monomers While AB(2,2) ring monomers exhibit thermodynamic and structural properties that satisfy criterion (a) and (b) for ring monomers that will enable light-actuated ROMP, it must also be determined whether such rings retain the favorable photoisomerization capabilities of 17 ACS Paragon Plus Environment

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isolated AB molecules. This is simply criterion (c) from above: is photoisomerization of AB(2,2) possible and controllabe? To assess this we first consider the absorption spectra of AB(2,2) rings. The UV-Vis absorption spectra of AB(2,2) molecules can be derived from TDDFT-based calculations of the electronic excitation energies. Electron excitation energies along with oscillator strengths,

f,

70000

for iso-

60000

(a)

AB ABn open AB(2,2) closed AB(2,2)

trans

lated AB, ABn, open and closed AB(2,2)

50000

monomers have been obtained from TDDFT

30000

12000

20000

8000

40000

absorption

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calculations and are reported in Table 3. Calculated UV-Vis spectra are shown in Fig. 8. Previous experimental studies have reported that the S0 → S1 and S0 → S2 excitation energies for trans AB are 2.79

16000

4000

10000 0 14000 12000 10000 8000 6000 4000 2000 0 200

0 200 400 600 800

(b)

and 3.95 65 eV, respectively. For cis AB,

cis

AB ABn open AB(2,2) closed AB(2,2)

300

400

500

600

700

wavelength (nm)

the first and second excitation energies are Figure 8: TDDFT calculated UV-Visble absorption spectra for trans and cis AB, ABn, 2.82 and 4.77 65 eV, respectively. All the ex- open and closed AB(2,2) monomers. For trans citation energies calculated by TDDFT will isomers, only closed-trans AB(2,2) has two absorption peaks, as seen in the inset. be systematically smaller than experimental measurements as observed here for AB, an effect previously ascribed to charge transfer, e.g. charge transfer between the central azo-group and the phenyl-moieties for S2 excitations of cis AB, which can not be addressed in TDDFT calculations. 66 Despite this, the qualitative trends in TDDFT excitation energies are considered to be accurate. 66 Comparison of excitation energies among isolated AB, ABn, open and closed AB(2,2) monomers shows that while detailed excitation energies vary by up to ∼0.2 eV (∼0.7 eV) for the S0 → S1 (S0 → S2 ) excitations, the ring monomer excitation energies are generally bounded by excitation energies for AB versus ABn. This implies that the effects on optical 18 ACS Paragon Plus Environment


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Table 3: Excitation energies and oscillator strengths f for AB, ABn, open and closed trans and cis AB(2,2) monomers. structures trans

cis

AB ABn open AB(2,2) close AB(2,2) AB ABn open AB(2,2) close AB(2,2)

S0 → S1 (n → π ∗ )

S0 → S2 (π → π ∗ )

E/eV

f1

E/eV

f2

2.58 2.75 2.61 2.38 2.61 2.43 2.45 2.53

0.0000 0.0002 0.0003 0.0733 0.0529 0.0979 0.1042 0.0877

3.60 3.00 3.05 3.22 3.99 3.66 3.71 3.67

0.9047 1.1925 1.5442 0.5050 0.0787 0.2246 0.2990 0.1573

and photoisomerization properties of the AB unit by its presence within both open and closed AB(2,2) monomers are no larger than the effects of altering the end group of isolated AB. As ABn retains the favorable optical absorption and photoisomerization properties of AB, this is promising for the behavior of AB(2,2) rings. Interestingly, variations in oscillator strength are larger, suggesting that details of charge redistribution upon excitation may differ among isolated AB and AB(2,2) rings. Moving beyond excitation energies, Fig. 8 reports TDDFT computed UV-Vis absorption spectra. Fig. 8 shows that there is a single dominant absorption peak for AB, ABn, open and closed AB(2,2) monomers with AB in the trans conformation. This peak corresponds to the S0 → S2 (π → π ∗ ) excitation. This is because the n → π ∗ transition is forbidden by symmetry, and is consistent with the (effectively) zero oscillator strengths reported in Table 3. However, the AB unit in closed-trans AB(2,2) is no longer planar leading to a breaking of symmetry and a significantly larger (though still relatively small) oscillator strength of 0.0733 for the n → π ∗ excitation. Thus upon careful inspection (see inset), there is a second lower-intensity peak (the intensity is larger than that of n → π ∗ peak of cis AB, see Fig. 8(b)) at larger wavelength of about 521 nm, as seen in the inset of Fig. 8 (a), 19 ACS Paragon Plus Environment

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indicating that n → π ∗ excitation is available for closed-trans AB(2,2). In contrast, all the cis conformations exhibit two distinct absorption peaks corresponding to S0 → S1 (at larger wavelength) and S0 → S2 (at smaller wavelength), respectively. It is also noteworthy that, like for ABn, absorption peaks for AB(2,2) rings are redshifted relative to peaks for AB. This effect is greater for trans conformations, and is large enough to move the S0 → S2 absorption peaks for trans AB(2,2) into visible wavelengths. This may enable visible-light-based applications of both ring monomers and ringbased ROMP polymers. Finally, we note that successful light-mediated ROMP requires that cis-to-trans and trans-to-cis photoexcitation energies are distinct (that is, occurring at different wavelengths). As can be seen, both open and closed AB(2,2) monomers satisfy this requirement, and exhibit even larger differences in trans versus cis absorbed wavelengths than isolated AB molecules.

Photoisomerization Mechanisms in AB(2,2) rings While excitation energies and UV-Vis spectra of AB(2,2) rings are qualitatively similar to those of isolated AB molecules, the ring structures do quantitatively modify excitation energy values (and absorption peak positions) and oscillator strengths. This leaves open the possibility that though light absorption by AB(2,2) is similar to that by isolated AB, absorption (and therefore electron excitation) may not trigger photoisomerization. In addition, even if AB(2,2) monomers undergo photoisomerization, do they follow isomerization mechanisms similar to those for isolated AB? To address these questions, potential energy curves (PEC) along the rotation and inversion photoisomerization pathways have been calculated in the S0 , S1 and S2 states. These pathways are calculated using the approach introduced by Crecca and Roitberg, 42 which they applied to study the photoisomerization mechanism of isolated AB and ABn. We have calculated the PECs for the open and closed monomers as well as for isolated AB and ABn 20 ACS Paragon Plus Environment


in order to compare with previous results and to verify our calculations. -18593

-33253

-15583 -15584

S0 S1 S2

-18595

-31114

S0 S1 S2

-33254

Energy (eV)

Energy (eV)

-15582

Energy (eV)

-18594

S0 S1 S2

-18596 -18597

-33255

-31115

Energy (eV)

-15581

-33256 -33257

S0 S1 S2

-31116 -31117

-15585 -18598

-15586 -50

0

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-31118

-33258

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200

0

50

100

150

-50

200

0

(a) AB

50

100

150

-50

200

0

CNNC (degree)

CNNC (degree)

CNNC (degree)

(b) ABn

50

100

150

200

CNNC (degree)

(c) open AB(2,2)

(d) closed AB(2,2)

Figure 9: Potential energy curves for (a) AB, (b) ABn, (c) open and (d) closed AB(2,2) along the rotation pathway. There are two minimums for each PEC at ground state S0 . The left minimum corresponds to cis isomers and right minimum is the trans isomers.

-31112

-33251

-18592

S0 S1 S2

-18593

Energy (eV)

-15581

S0 S1 S2

-15582 -15583 -15584

S0 S1 S2

-33252 -33253

-18594

Energy (eV)

-15580

-18595 -18596 -18597

-31114

-33254 -33255 -33256 -33257

-15585

-18598

-15586

-18599

S0 S1 S2

-31113

Energy (eV)

-15579

Energy (eV)

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-31115 -31116 -31117 -31118

-33258

100

150

200

NNC (degree)

(a) AB

250

300

-31119 100

150

200

250

300

NNC (degree)

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150

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250

NNC (degree)

(b) ABn

(c) open AB(2,2)

300

100

150

200

250

300

NNC (degree)

(d) closed AB(2,2)

Figure 10: Potential energy curves for (a) AB, (b) ABn, (c) open and (d) closed AB(2,2) along the inversion pathway. The left minimum at S0 corresponds to cis isomers and right minimum is the trans isomers. For the photoisomerization of unsubstituted AB, a conical intersection has been found between S0 and S1 along the rotation pathway at a CNNC dihedral angle of about 90◦ according to both CASSCF 41 and constrained TDDFT 56 calculations. The TDDFT-calculated PECs for isolated AB and ABn along the rotation [see Fig. 9(a) and (b)] and inversion [see Fig. 10(a) and (b)] pathways are very similar to previous studies. The PECs along the rotation pathway for open and closed AB(2,2) monomers [see Fig. 9(c) and (d)], are both similar to that of isolated AB and ABn (except an inverse relative stability of cis and trans conformations for closed AB(2,2) monomer), indicating that photoisomerization along the rotation pathway is also available for cis-to-trans isomerization of open and closed AB(2,2) 21 ACS Paragon Plus Environment

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monomers. For open AB(2,2) monomer, the PECs along the inversion pathway is very similar to that of isolated ABn [see Fig. 10(b) and (c)], that the S0 → S2 excitation ends up to near the S2 minimum, where the S1 -S2 energy gap is very small, so that fast relaxation from S2 → S1 can be expected. Therefore, though n → π ∗ transtion is symmetry forbidden for trans ABn and open AB(2,2) monomer, S2 → S1 relaxation after S0 → S2 excitation allows for trans-to-cis photoisomerization following the rotation pathway at S1 state. For isolated AB, there are no intersection between S1 and S2 for inversion pathway, as shown in Fig. 10(a), where concerted-inversion pathway is proposed for trans-to-cis isomerization. 42,67–69 On the contrary, for closed-trans AB(2,2), there is no intersection between S1 and S2 along the inversion pathway, similar to that for isolated AB (see Fig. 10). However, the n → π ∗ excitation is not forbidden by symmetry due to the non-planar structure of the AB unit. Therefore, the rotation pathway at S1 for trans-to-cis isomerization is available for closed-trans AB(2,2) monomers: after n → π ∗ excitation, the geometry of trans AB(2,2) relaxes at S1 surface from about 150◦ to 90◦ where S1 -to-S0 transition occurs, following with further geometry relaxation at S0 surfaces to ground states of either cis (in this case, transto-cis photoisomerization achieved) or trans (in this case, this trans AB(2,2) at S0 can be excited to S

1

again under same light). The cis-to-trans photoisomerization can occur in

the same way after n → π ∗ excitation for cis AB(2,2). However, n → π ∗ excitation energies (or light wavelengths) for cis and trans AB(2,2) are different, see Fig. 8, indicating the possibility for independent cis-to-trans and trans-to-cis photoisomerization by using blue and green light, respectively. It can also be seen in Fig. 9(d) that the S1 PEC has a large gradient with respect to configuration, indicating fast structural relaxation can be achieved at S1 surface, which leads to faster trans-to-cis photoisomerizatioin for AB(2,2) than for AB. In summary, cis-to-trans photoisomerization can be achieved following the energy curve at S1 after n → π ∗ excitation along the rotation pathway for all studied isolated AB, ABn 22 ACS Paragon Plus Environment


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molecules, as well as open and closed AB(2,2) molecules. trans-to-cis photoisomerization can be achieved after S0 → S2 excitation for both ABn and open AB(2,2) molecule. The light wavelengths for cis-to-trans and trans-to-cis photoisomerization of open AB(2,2) molecule are different, 506 nm and 407 nm, respectively. For closed AB(2,2) ring molecules, cis-totrans photoisomerization can be achieved under 489 nm (blue) light that induces the n → π ∗ excitation for cis AB(2,2). Due to the presence of n → π ∗ transition for trans AB(2,2), it is possible to reverse the isomerization and return to the cis conformation under 521 nm (green) light. Therefore, the light wavelengths for cis-to-trans and trans-to-cis photoisomerization of both open and closed AB(2,2) molecules are similar but distinct, allowing for separate control of the cis-to-trans and trans-to-cis processes—and therefore RSE modulation in closed AB(2,2) leading to activation/deactivation of ROMP–with monochromatic blue versus green light.

Summary and Conclusion We have successfully designed a new type of azobenzene-containing monomer allowing for light-controllable fabrication of photo-responsive polymers through light-mediated ROMP by modulating RSE from photoisomerization of azobenzene. Based on investigation of stability, RSE and photoisomerization mechanism, the optimal monomer for light-mediated ROMP was determined to be the AB(2,2) ring molecule. Closed AB(2,2) ring monomers have low RSE (6.6 kcal/mol) and high stability in the cis conformation and high RSE (35.2 kcal/mol) in the trans conformation. The wavelengths for trans-to-cis and cis-to-trans photoisomerization of both open and closed AB(2,2) monomers are distinguishable so that reversible photoisomerization is possible and controllable using different wavelengths of light. The photoisomerization mechanism for open AB(2,2) monomers is similar to that for isolated ABn, with cis-to-trans isomerization triggered at the wavelengths of 506 nm after S0 → S1


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excitation, and trans-to-cis isomerization triggered at 407 nm after S0 → S2 excitation. For closed AB(2,2) monomer, both cis-to-trans and trans-to-cis photoisomerization can occur along the rotation pathway after S0 → S1 excitation, but triggered at light wavelengths of 489 nm and 521 nm, respectively. The NMR chemical shifts and UV-Vis spectra calculated for open and closed AB(2,2) monomers are provided as a reference for future experimental studies. This study of modulating the RSE by the photoisomerization of AB-groups in ringshaped monomers opens up new applications for both ROMP synthesis and ring-shaped AB-containing compounds because RSE plays an important role in a number of reactions in organic chemistry. The large difference in RSE for closed AB(2,2) monomer allows for control of ROMP using light of different wavelengths. When the monomer is in cis conformation, the RSE is very small so that ROMP is inactive with no polymers formed. The cis-to-trans photoisomerization results in highly-strained trans monomers readily to open up and form polymers (ROMP activated). After shining green light, trans-to-cis isomerization would occur and the closed monomers will remain closed so that ROMP become inactive again. This reversible isomerizaiton allows for instantaneous control of activation/deactivation of ROMP, and thus the thickness of the films. By applying different light at different regions or by blocking light at certain regions, patterning is enabled. Therefore, by using this new type of monomer, temporal and spatial control of ROMP are available. Beyond ROMP, RSE has also been found to promote click-reactions, such as azidealkyne or Diels-Alder 70 cycloaddition where those bioorthogonal reactions occur with strained cycloalkynes or cycloalkenes instead of using the conventionally toxic Cu(I) catalyst. 71–75 Higher-strained bioorthogonal molecules are desired for those click-reactions to enhance the reaction kinetics. However, the synthesis of high-strained molecules is difficult and the yield is low. With the ability to modulate RSE with light, the azobenzene-containing molecule designed in this work is promising in the application of click-chemistry: the ring molecule 24 ACS Paragon Plus Environment


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can be more easily synthesized in the low-strained and more stable cis conformation. Upon exposure to light of suitable wavelengths, the high-strained trans conformation can be obtained to activate the click reaction. This avoids difficulty in synthesizing high-strained ring-molecules and would allow for temporal and spatial light-mediation of click reactions used in drug delivery, in-vitro and in-vivo imaging or labeling.

Acknowledgement This work and all the authors were supported by National Science Fundation (Grant No. CMMI 1334403). We also thank Center for Computational Sciences of University of Kentucky for the computation resources.

Supporting Information Available Geometric parameters of trans and cis AB and ABn in gas phase from DFT calculations. Geometries of linkers and linker lengths comparing with the length of trans and cis ABn. Geometries of open and closed AB(m, n) ring molecules with m 6= n. Molecular Orbital (HOMO and LUMO) of closed AB(2,2) monomers. Local-minimum energy geometries and enthalpies of cis and trans AB(2,2) with different alkene conformations as discussed in manuscript. NMR chemical shifts for open and closed cis/trans AB(2,2) monomers. Atomic coordinates and energies of AB, ABn and AB(m, n) molecules.

This material is available

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

References (1) Velema, W. A.; Szymanski, W.; Feringa, B. L. Photopharmacology: beyond proof of principle. J. Am. Chem. Soc. 2014, 136, 2178–2191.


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Toward Spatiotemporally Controlled Synthesis of Photoresponsive

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