Strategies for Designing Diarylethenes as Efficient Nonlinear Optical

Subscriber access provided by TAIPEI MEDICAL COLLEGE

Article

Strategies for Designing Diarylethenes as Efficient Nonlinear Optical Switches Kathy J. Chen, Adèle D. Laurent, and Denis Jacquemin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp412071e • Publication Date (Web): 03 Feb 2014 Downloaded from http://pubs.acs.org on February 11, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

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

The Journal of Physical Chemistry

Strategies for Designing Diarylethenes as Efficient Nonlinear Optical Switches Kathy J. Chen,† Adèle D. Laurent,† and Denis Jacquemin∗,†,‡ Laboratoire CEISAM - UMR CNRS 6230, Université de Nantes, 2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France, and Institut Universitaire de France, 103, bd Saint-Michel, F-75005 Paris Cedex 05, France. E-mail: [email protected]

∗ To

whom correspondence should be addressed Nantes ‡ IUF, Paris † CEISAM,

1 ACS Paragon Plus Environment


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

Abstract In this theoretical first principles study, we systematically investigate the second-order nonlinear optical (NLO) properties of a large panel of dithienylethene (DTE) derivatives in order to identify design strategies towards i) large absolute first hyperpolarizibilities (βtot ) and ii) large contrasts between the NLO responses of the open and closed forms (βratio ). Our results show that applying the traditional push-pull organic chromophore design strategy to individual diarylethenes yielded substantial βtot values (100 to 180×10−30 esu); counterintuitively, however, larger βtot responses did not necessarily correlate with larger contrasts between the the closed and open forms (βratio ∼4). For the class of inverse-DTE, we observed both smaller open

closed >β NLO responses and a reversal of the usual βtot tot

trend. On the other hand, multi-

switchable systems comprising several DTE units were shown to be capable of exhibiting both large βtot (100 to 500 × 10−30 esu) and large βratio >10 due to the supra-additive contribution of each successive ring cyclization to the βtot of the molecule. These findings pave the way for the rational design of diarylethenes towards NLO applications.

Keywords: Diarylethene, Dithienylethene, Photochrome, NLO Switch, Second-Order NLO, Hyperpolarizability, Density Functional Theory.

Introduction The field of nonlinear optics (NLO) has proliferated in recent decades, particularly with the discovery of organic molecules engineered for large non-linear optical responses. In addition to being compatible with current semiconductor technologies, organic NLO molecules possess ultrafast response times and extended π-electron distributions that are easily perturbed by electromagnetic fields – properties that are highly attractive for photonic applications such as electro-optic switching or frequency doubling. 1,2 Various strategies have been adopted to develop compounds with NLO properties that can be modulated by means of redox reactions, protonation or deprotonation processes, or by photochromism. 3–5 The groups of Castet, Champagne, Pozzo and coworkers have


Page 2 of 31

Page 3 of 31

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


made significant inroads in the field of NLO switching, focusing mainly on acido-triggered molecular switches with a spiro structure, 6–10 as well as enol-keto tautomerism-based switches, 11 and dihyroazulene-vinylheptafulvene switches; 12 these findings have been reviewed in a recent Account. 13 In the same line, Garza and Scuseria have recently modeled the NLO switching activity of fulgides. 14 Indeed, photochromic compounds – spiropyans, diarylethenes, stilbenes, fulgides and many others – make particularly elegant candidates for NLO switches due to their photoinduced isomerization. 15,16 Among the multitude of photochromic molecules, diarylethenes (DAE) and their thiophene-containing subset, dithienylethenes (DTE), constitute highly promising candidates due to their photostable, fatigue-resistant, and high-yield interconversion between their open and closed isomers. 17,18 DAEs are known to isomerize into a well-conjugated closed form upon irradiation at ultraviolet (UV) frequencies, whereas light in the visible range interconverts the molecule back into its poorly conjugated open form (see Scheme 1). This isomerization corresponds with marked changes in the physical properties of two forms, not the least of which include different absorption spectra, electrochemical properties, and NLO responses. 19–21 Moreover, multiple DAE units may be coupled into a single system, giving rise to multiswitchable molecules that are the subject of intense investigations for their potential application as logical storage units. 22–26

UV visible S

S

S

S

Scheme 1: Open (left) and closed (right) forms of photochromic DAEs.

A current goal of efficient NLO design is the identification of photochromes possessing large second-order responses (βtot ) and distinct optical signatures between individual pisomers. 27–30 However, because of the difficulty of synthesizing DAEs, and especially those with asymmetric end-group functionality, experimental NLO data for push-pull DAE systems have been limited. 31,32 Electrical field induced second harmonic generation (EFISH) measurements of DTE



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

poled films, for instance, showed that DTE functionalized with -OMe and -CN end-groups exhibit βSHG ratios of 2.1 to 3.5 between their closed and open forms. 33 More commonly studied are organometallic DAE systems such as DTE-Zn(II)-DTE 34,35 and bipyridine Ir(III)-DTE2 , 36 both of which have experimentally exhibited both large NLO responses [β (closed) ∼ 1000 − 2000 × 10−30 esu] and large contrasts (βratio >20) arising from the intense intra-ligand charge transfer mediated by the electron-accepting metal fragment; however, these feats are signatures of the metal centers and not of the isolated DTE moieties in themselves. 37–39 Due to the cost of organic synthesis, theoretical tools are indispensable for identifying design strategies to adapt push-pull DAEs for NLO applications. It is known that β is greatest when a large transition moment is present between the ground and excited states, i.e., when a large photoinduced charge-transfer takes place. 40 Consequently, traditional push-pull organic chromophores consisting of electron-donating and accepting moieties separated by a π-bridge often exhibit large β coefficients. 41 In DAEs, the NLO responses can be modulated by increasing the π-backbone length or by modifying electron-withdrawing and electron-donating substituents in order to enhance the polarity difference and π-asymmetry in the chromophore. 42–44 The optical properties of DAEs are also highly dependent on the nature and position of substituents attached to the photochrome; for example, Del Zoppo et al. have shown that the second-order hyperpolarizabilities of functionalized DTEs varied from 0.9 to 8.2×10−34 esu depending on the end-group. 21 Recent theoretical studies on functionalized Pt(II)-DTE and DTE-polyoxometalate systems with a variety of substituents have focused on obtaining large betas exceeding 100×10−30 esu; however, it is worth noting that the βratio remained small in these systems, with contrasts ranging from a one to four-fold difference between the open and closed forms. 45,46 We also remark that the vast majority of theoretical studies investigating such photochromic systems utilized density functional theory (DFT) methods, which afford a reasonable balance of computational time, system-size, and accuracy in calculating spectroscopic and optical properties. In their study employing DFT and a range-separated hybrid functional to investigate the NLO properties of photochromic fulgides, Garcia et al. concluded that the ωB97XD/6-31G(d,p) methodology yielded spectroscopic and op-


Page 4 of 31

Page 5 of 31

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


tical properties in qualitative agreement with experiments. 14 In this work, we apply several structural design strategies to DAEs in order to investigate how different heteroatoms, substituents, and structures modulate the contrast of second-order NLO properties in their open and closed forms. Our investigation largely focuses on perfluoro-DTE (groups I, II and III, see Scheme 2) since previous experimental and theoretical works have shown that they have many advantages over other DAE architectures, including greater photostability and reversible electrocyclization. 17 Nonetheless, DAEs with a variety of bridging moieties (group IV) are also considered, as well as multiswitchable DAEs in all their open/closed isomeric combinations (group V) for a total of 70 monoswitchable and 10 multiswitchable systems; the raw data for all systems may be found in the Supporting Information (SI). Owing to their prevalence and ease-of-synthesis, we choose p-phenyl-CN and p-phenyl-NMe2 end-groups as model substituents for our push-pull systems, in which the dimethylamine group acts as the electron donor and the nitrile as the acceptor. For this panel of push-pull DAE/DTE-derived photochromes, we carry out a density functional theory investigation of their NLO properties. To our knowledge, this work presents, to date, the largest set of second-order NLO data for DAEs compiled for the purpose of proposing paths towards the rational design of high-contrast NLO switches.

Computational Methods Open and closed forms of each chromophore were optimized using the parameter-free exchangecorrelation PBE0 functional. 47,48 Vibrational frequency calculations were carried out in order to obtain zero-point energy corrections, Gibbs free energies, and to confirm that minimal groundstate structures have been reached. For geometry optimizations and frequency calculations, we employed the triple-ζ polarized 6-311G(d,p) atomic basis set, which had been shown to be sufficient for reproducing accurate ground-state structural parameters for DAEs. 49 Tightened residual force threshold of 1 × 10−5 a.u. and energy threshold of 1 × 10−9 a.u. were imposed. For each system, static and frequency-dependent polarizability and first-order hyperpolarizability tensors have



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

F2 C CF2

F2C

F2C

CF2

Me

S

F2 C

F2 C

F2C

R1

Page 6 of 31

R3

S

Me

R2

S

R1

Me

S

S

R4

I

R1

R2

S

R2

Me

II (CF2)2 (CF2)4 (CH2)3

CF2

III F2 C

F2 C F2C

F2C

CF2

CF2

R2

R1 Me

S

R1

Me

Me

S

Me

R2

O

R1

IV-a,b,c

O

Me

S

R2

S

Me

IV-d

IV-e R3

O

O

H N

O

O

O NH

O

Me

Me

S

R1

S

Me

S

R1

R2

IV-f

Me

S

Me

R2

IV-g

S Me

S

Me

R1 F2 C

CN

S

O

Me

Me

Me O

CF2

F2C

O

Me

S

S

Me

R2

V-a

Me

F2C

F2 C CF2

CF2

Me

S

F2C

S

S R2

F2 C

Me

S

Me

R2

CF2

Me

S

R1

S

Me

S

V-e

F2 C F2C

F2 C CF2

F2C

Me

Me

F2 C CF2

F2C

Me C

S

Me

F2C

V-d

R1

S

C

S

Me

CF2

V-c CF2

Me C

R1

Me

F2 C

F2C

F2 C

Me

R1

R2

V-b

F2 C

F2C

O S

R2

R2

IV-h

R1

R1 NC

S

R1

S

Me

C

C S

C

S

Me

CF2

S

Me

S

R2

V-f Scheme 2: Representation of investigated DTE structures (closed isomer)


Me

S

R2

Page 7 of 31

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


been calculated at the ωB97X/6-311++G(d,p) level of theory – that is, we have utilized a functional with correct asymptotic behavior, which is known to be crucial for obtaining accurate NLO data. 50–53 Because extensive theoretical work has been carried out on DTE in previous years, it has been established that this methodology accurately reproduces geometries and spectroscopic properties of DAE and DTE molecules; hence, further evaluation of basis sets and exchange-correlation functionals for our molecules would be superfluous. 20,30,49 The dipole polarizabilities (α) and total hyperpolarizabilities corresponding to the static β (0; 0, 0), electro-optical Pockel’s effect β EOPE (−ω; ω, 0), and second harmonic generation β SHG (−2ω; ω, ω) tensors were obtained analytically from the optimized structures using a coupled-perturbed KohnSham (CPKS) approach. The static second-order hyperpolarizabilities, hereafter denoted as βtot , were calculated via the generic expression, 2,54 1 1 βtot = (βx2 + βy2 + βz2 ) 2 5

where βi = ∑(βi j j + β ji j + β j ji ),

(1)

j

where i and j are the index of the Cartesian axis. The frequency-dependent values of β EOPE and β SHG have also been calculated analytically at 1907 nm (0.65 eV). As the static and dynamic hyperpolarizabilities follow similar chemical trends, our discussion will be principally in terms of the total static β (0; 0, 0), simply designated as βtot . The frequency-dependent hyperpolarizabilities can be found in the SI along with the experimentally-relevant scalar components of β projected along the parallel and perpendicular directions to the dipole moment. NLO data are presented in units of 10−30 cm5 esu−1 . Results have been principally presented in the gas phase; although we note that most experimental studies have been carried out in condensed phases. Because solvent effects may be relevant for accurate predictions of optical properties of organic dyes, 54 we have evaluated the effect of bulk solvent on the NLO properties using the polarizable continuum model (PCM). 55 The PCM model is an implicit solvation model and, as such, it allows fast estimates of environmental effects but does not explicitly consider specific solute-solvent interactions, such as hydrogen bonds or ion



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

Page 8 of 31

pairing. Since the lasers used in NLO experiments excite electron density at femtosecond speeds, such excited phenomena are typically modeled using a non-equilibrium (neq) solvated model, in which only the electronic contributions of the solvent are instantaneously equilibrated to the excited solute charge distribution. However, continual exposure of the system to the laser would imply a solvent fully adapted to the excited solute charge, suggesting an equilibrium (eq) solvation condition. 56 For that reason, the NLO data for several molecules have been presented in the framework of both the equilibrium and non-equilibrium solvation models, considering both n-hexane and ethanol. All calculations were carried out using the Gaussian09 suite. 57

Results We have investigated eighty molecules that have been grouped into five categories labeled I−V (see Scheme 2). In the first category are the DTEs with a traditional 1,2-bis-(3-thienyl)-perfluorocyclopentene bridge and two substituents, R1 and R2 , on position 5 of the thiophenes. 58,59 The DTEs in group II bear additional substituents R3 and R4 on the reactive carbons at position 2 of each thiophene. 60 Because the substituents at R3 and R4 do not participate in the π-conjugated path of the closed form, this group of molecules should allow us to restrict substituent effects to the open form. Group III comprises the class of inverse-DTEs, in which inverted position 5 of the sulfurs in the thiophenes decrease the delocalization in the closed form. 61,62 In group IV, we investigate the NLO properties of DAEs with bridging architectures other than the perfluorocyclopentene bridge. 20 Lastly, multiswitchable DAEs are categorized in group V, which includes dimers and trimers of DTEs. 30 We first begin our discussion with a concise treatment of solvent effects (see Table 1 and Table 2), then we present the results of the monoswitches in the second Section (see Table 3), followed by the results observed in the multiswitchable systems in the third Section (see Table 4).


Page 9 of 31

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


Solvent effects and frequency dispersion As stated above, we have modeled implicit solvent effects for three prototypical molecules: Id, I-h, I-i. From the results tabulated in Table 1 and Table 2, it can be clearly seen that, as expected, the static α and βtot values are equal in both the equilibrium and non-equilibrium limits. Once the external field of λ = 1907 nm is applied, a large difference between the two solvation models can be observed in ethanol; both α and β are significantly larger in equilibrium than in the non-equilibrium mode in this polar solvent. By contrast, those of hexane differs negligibly between the two models due to the small dielectric constant, and indeed, we observe that the hyperpolarizabilities for both solvation models are equal within numerical accuracy. For both hexane and ethanol, polarizabilities and hyperpolarizabilities significantly increase in the solvated phase compared to the gas phase environments – the increase is more marked in the polar ethanol solvent than in n-hexane, as the polar environment stabilizes the charge-separated resonance form and decreases the gap energy. In the case of the closed isomer of compound I-d, for instance, the static gas phase properties and the static solvated properties change as 516 to 719 a.u. for α and from 112 to 312×10−30 esu for βtot in ethanol; but only from 516 to 583 a.u. for α and from 112 to 174×10−30 esu for βtot in n-hexane. Lastly, we remark on how the values of βtot vary among the three molecules: in hexane, the trend of I-dcoo in Figure 3); this is on account of the electron-accepting character of the DTE bridge participating in the charge-transfer in the closed form. We underline that these large NLO ratio variations between the simply and multiply closed isomers are even more striking when considering that for similar systems, the observed λmax increases by 200 − 300 nm after the initial ring closure, but varies only minimally upon closure of further DTE units. 23,24



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

Page 20 of 31

Table 4: Relative free energies (kcal/mol) and static βtot (10−30 esu) for multiswitchable DAEs in all closed (c) and open (o) isomers

Molecule

R1

R2

Isomer

V-a

Ph-CN

Ph-NMe2

V-b

Ph-CN

Ph-NMe2

V-c

Ph-CN

Ph-NMe2

V-d1

H

H

V-d2

Ph-CN

Ph-NMe2

V-e

Ph-CN

Ph-NMe2

V-f1

H

H

V-f2

ph

ph

V-f3

phCN

phNMe2

cc oc oo cc oo cc co oo cc co oc oo cc co oc oo cc co oc oo ccc cco coc coo oco ooo ccc cco coc coo oco ooo ccc cco coc coo occ oco ooc ooo

†

∆G†

βtot

βratio †

-3.0 -10.4 – 11.2 – 22.5 8.2 – 27.5 13.4 13.9 – 20.7 11.4 8.9 – 19.2 11.8 9.4 – 39.9 26.0 27.0 13.2 12.6 – 35.2 24.5 22.4 10.0 11.9 – 33.6 24.9 21.4 12.2 21.6 12.6 9.6 –

116.2 23.4 7.7 96.8 2.1 126.4 76.1 21.8 3.1 29.4 30.1 0.9 309.5 86.9 104.0 25.7 207.0 83.6 87.1 25.5 15.3 78.4 12.8 33.4 24.1 1.3 33.0 34.1 14.2 21.1 26.2 1.5 555.5 181.8 169.0 83.6 292.3 57.2 107.8 27.2

15.1 3.0 – 45.4 – 5.8 3.5 – 3.4 31.7 32.5 – 12.0 3.4 4.0 – 8.1 3.3 3.4 – 11.8 60.6 9.9 25.8 18.6 – 21.8 22.5 9.3 13.9 17.3 – 20.4 6.7 6.2 3.1 10.8 2.1 4.0 –

Relative to the fully open isomer


Page 21 of 31

600

20.4

β (10-‐30 esu)

500 400 12.0

10.7

300

6.7 6.2

200 5.2

3.4

100 0

c

o

4.0

4.0

cc co oc oo

3.1

2.1

ccc occ cco coc ooc coo oco ooo

Isomer

Figure 3: βtot of closed (c) and open (o) isomers of monomeric, dimeric, and trimeric DTE (from left to right: I-d, V-d2, V-f3). Units are labeled from the electron-accepting terminus to the electron-donating terminus. Values above each bar indicate the βratio of the isomers compared to the fully-open form.

40.0 35.0 ΔG (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


30.0 25.0 20.0 15.0 10.0 5.0 0.0

c

cc

co oc

ccc occ cco coc ooc coo oco

Isomer Figure 4: ∆G of closed (c) and open (o) isomers of monomeric, dimeric, and trimeric DTE (from left to right: I-d, V-d2, V-f3). Energies relative to the corresponding fully-open isomers.



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

Conclusions Understanding structure-property relationships is of paramount importance for proposing rule-ofthumb guidelines for rational molecule design. Ideally, NLO switches should possess not only high hyperpolarizabilities (βtot ) but also large NLO contrasts between the two switchable forms (βratio ). We have investigated the effect of four principal chemical modifications – push-pull character, π-conjugation, bridging type, and number of switching units – for a series of 80 diarylethene derivatives. Our DFT study has shown that strategies for designing traditional push-pull chromophores for large hyperpolarizabilities do not necessarily entail ideal properties for high-contrast NLO switching because of the unexceptional, albeit significant, contrasts (βratio < 10) between the βtot closed and open forms of the monomers. For the DAE monomers that we have investigated, extending the π-conjugation and enhancing the push-pull charge-transfer character significantly improves the overall βtot values; however, due to the fact that the push-pull effect is present in both forms, optimizing for large βtot does not necessarily yield large values of βratio . Of the pushpull compounds in group I, we found that molecules I-d, I-h, I-i exhibit large NLO responses (βtot >100×10−30 esu) and moderate βratio of 4 to 6; these results that demonstrate that even these relatively simple DTE monoswitches already possess the capability for NLO switching. For the class of inverse-DTE (group III), we found that these molecules exhibited not only a reversal of open

closed >β the βtot tot

trend observed for the other groups, but also a markedly reduced maximal NLO

response. The surest strategy to build high-contrast NLO-switchable DTE chromophores proved to be combining individual switches into multiswitchable systems such that the switching units lie along the entire push-pull conjugated path. We observe that closure of each additional switch unit contributes a supra-additive increase to both βtot and βratio . Whereas few monomers exceeded βtot =24×10−30 esu, the hyperpolarizabilities of the fully closed dimer and trimers were in excess of 100×10−30 esu, reaching upwards 500×10−30 esu for V-f. The latter compound attained a βratio =20 between the fully opened and fully closed forms, that is, greater than the experimental βratio found in metal-based multi-DTE tetramers and hexamers. 29,34 However, experiments have 22 ACS Paragon Plus Environment

Page 22 of 31

Page 23 of 31

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


affirmed that for linearly-linked switches, regulation of the isomerism of specific units becomes increasingly more cumbersome the longer the multiswitchable chain. 23,25 In conclusion, we have shown that the structure and substituents of push-pull DAEs can be exploited to modulate the second-order NLO response, but only to a certain extent because the most valuable systems are those that are more difficult to control. We hope that the insights and caveats that we have presented in this study will be exploited for rational design of organic NLO switches in the future.

Acknowledgement K.J.C. thanks the European Research Council (ERC) for the Ph.D. grant (Marches no◦ 278845). A.D.L thanks the Institut de Chimie of the C.N.R.S. for financial support. D.J. acknowledges both the European Research Council for the starting grant (Marches no◦ 278845) and the Région des Pays de la Loire for the recrutement sur poste stratégique. Allocations of computation time from the GENCI-CINES/IDRIS (c2013085117), the CCIPL (Centre de Calcul Intensif des Pays de Loire), and the CEISAM’s Troy cluster are gratefully acknowledged.

Supporting Information Available Energies, static and frequency-dependent α and β values for all isomers of all molecules. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Green, K. A.; Cifuentes, M. P.; Samoc, M.; Humphrey, M. G. Metal Alkynyl Complexes as Switchable NLO Systems. Coord. Chem. Rev. 2011, 255, 2530–2541. (2) Kurtz, H.; Dudis, D. Quantum Mechanical Methods for Predicting Nonlinear Optical Properties. Rev. Comput. Chem. 1998, 12, 241–279. (3) Guan, W.; Liu, C. G.; Song, P.; Yang, G. C.; Su, Z. M. Quantum Chemical Study of Redox-Switchable SecondOrder Optical Nonlinearity in Keggin-Type Organoimido Derivative [PW11O39(ReNC6H5)]n− (n=2–4). Theor. Chem. Acc. 2009, 122, 265–273.



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

(4) Asselberghs, I.; Hennrich, G.; Clays, K. Proton-Triggered Octopolar NLO Chromophores. J. Phys. Chem. A 2006, 110, 6271–6275. (5) Canary, J. W. Redox-Triggered Chiroptical Molecular Switches. Chem. Soc. Rev. 2009, 38, 747–756. (6) Sanguinet, L.; Pozzo, J.-L.; Guillaume, M.; Champagne, B.; Castet, F.; Ducasse, L.; Maury, E.; Soulié, J.; Mançois, F.; Adamietz, F. et al. Acidoswitchable NLO-phores: Benzimidazolo[2,3-b]oxazolidines. J. Phys. Chem. B 2006, 110, 10672–10682. (7) Mançois, F.; Sanguinet, L.; Pozzo, J.-L.; Guillaume, M.; Champagne, B.; Rodriguez, V.; Adamietz, F.; Ducasse, L.; Castet, F. Acido-Triggered Nonlinear Optical Switches: Benzazolo-oxazolidines. J. Phys. Chem. B 2007, 111, 9795–9802. (8) Plaquet, A.; Guillaume, M.; Champagne, B.; Castet, F.; Ducasse, L.; Pozzo, J.-L.; Rodriguez, V. In Silico Optimization of Merocyanine-Spiropyran Compounds as Second-Order Nonlinear Optical Molecular Switches. Phys. Chem. Chem. Phys. 2008, 10, 6223–6232. (9) Mançois, F.; Pozzo, J.-L.; Pan, J.; Adamietz, F.; Rodriguez, V.; Ducasse, L.; Castet, F.; Plaquet, A.; Champagne, B. Two-Way Molecular Switches with Large Nonlinear Optical Contrast. Chem. Eur. J. 2009, 15, 2560– 2571. (10) Champagne, B.; Plaquet, A.; Pozzo, J.-L.; Rodriguez, V.; Castet, F. Nonlinear Optical Molecular Switches as Selective Cation Sensors. J. Am. Chem. Soc. 2012, 134, 8101–8103. (11) Bogdan, E.; Plaquet, A.; Antonov, L.; Rodriguez, V.; Ducasse, L.; Champagne, B.; Castet, F. Solvent Effects on the Second-Order Nonlinear Optical Responses in the Keto-Enol Equilibrium of a 2-Hydroxy-1-Naphthaldehyde Derivative. J. Phys. Chem. C 2010, 114, 12760–12768. (12) Plaquet, A.; Champagne, B.; Castet, F.; Ducasse, L.; Bogdan, E.; Rodriguez, V.; Pozzo, J.-L. Theoretical Investigation of the Dynamic First Hyperpolarizability of DHA–VHF Molecular Switches. New J. Chem. 2009, 33, 1349–1356. (13) Castet, F.; Rodriguez, V.; Pozzo, J.-L.; Ducasse, L.; Plaquet, A.; Champagne, B. Design and Characterization of Molecular Nonlinear Optical Switches. Acc. Chem. Res. 2013, 46, 2656–2665. (14) Garza, A. J.; Osman, O. I.; Wazzan, N. a.; Khan, S. B.; Scuseria, G. E.; Asiri, A. M. Photochromic and Nonlinear Optical Properties of Fulgides: A Density Functional Theory Study. Comput. Theor. Chem. 2013, 1022, 82–85. (15) Yokoyama, Y. Fulgides for Memories and Switches. Chem. Rev. 2000, 100, 1717–1740.


Page 24 of 31

Page 25 of 31

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


(16) Feringa, B. L.; van Delden, R. a.; Koumura, N.; Geertsema, E. M. Chiroptical Molecular Switches. Chem. Rev. 2000, 100, 1789–1816. (17) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. Inflammation, Growth Regulatory Molecules & Atherosclerosis. J. Cell. Biochem. 1992, 50, 1–30. (18) Delaire, J. A.; Nakatani, K. Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials. Chem. Rev. 2000, 100, 1817–1846. (19) Kim, E.; Kim, M.; Kim, K. Diarylethenes with Intramolecular Donor–Acceptor Structures for Photo-Induced Electrochemical Change. Tetrahedron 2006, 62, 6814–6821. (20) Maurel, F.; Perrier, A.; Perpète, E. A.; Jacquemin, D. A Theoretical Study of the Perfluoro-Diarylethenes Electronic Spectra. J. Photochem. Photobiol. A Chem. 2008, 199, 211–223. (21) Del Zoppo, M.; Lucotti, A.; Bertarelli, C.; Zerbi, G. A New Class of Molecules with Large, Switchable Vibrational Non-Linear Optical Responses: Photochromic Diarylethene Systems. Vib. Spectrosc. 2007, 43, 249–253. (22) Irie, M. Diarylethenes for Memories and Switches. Chem. Rev. 2000, 100, 1685–1716. (23) Perrier, A.; Maurel, F.; Jacquemin, D. Single Molecule Multiphotochromism with Diarylethenes. Acc. Chem. Res. 2012, 45, 1173–1182. (24) Jacquemin, D.; Perpète, E. A.; Maurel, F.; Perrier, A. Ab Initio Investigation of the Electronic Properties of Coupled Dithienylethenes. J. Phys. Chem. Lett. 2010, 1, 434–438. (25) Kaieda, T.; Kobatake, S.; Miyasaka, H.; Murakami, M.; Iwai, N.; Nagata, Y.; Itaya, A.; Irie, M. Efficient Photocyclization of Dithienylethene Dimer, Trimer, and Tetramer: Quantum Yield and Reaction Dynamics. J. Am. Chem. Soc. 2002, 124, 2015–2024. (26) Higashiguchi, K.; Matsuda, K.; Asano, Y.; Murakami, A.; Nakamura, S.; Irie, M. Photochromism of Dithienylethenes Containing Fluorinated Thiophene Rings. Eur. J. Org. Chem. 2005, 2005, 91–97. (27) Nitadori, H.; Ordronneau, L.; Boixel, J.; Jacquemin, D.; Boucekkine, A.; Singh, A.; Akita, M.; Ledoux, I.; Guerchais, V.; Le Bozec, H. Photoswitching of the Second-Order Nonlinearity of a Tetrahedral Octupolar Multi DTE-Based Copper(I) Complex. Chem. Commun. 2012, 48, 10395–10397. (28) Cho, M. J.; Choi, D. H.; Sullivan, P. a.; Akelaitis, A. J.; Dalton, L. R. Recent Progress in Second-Order Nonlinear Optical Polymers and Dendrimers. Prog. Polym. Sci. 2008, 33, 1013–1058.



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

(29) Ordronneau, L.; Aubert, V.; Guerchais, V.; Boucekkine, A.; Le Bozec, H.; Singh, A.; Ledoux, I.; Jacquemin, D. The First Hexadithienylethene-Substituted Tris(bipyridine)metal Complexes as Quadratic NLO Photoswitches: Combined Experimental and DFT Studies. Chem. Eur. J. 2013, 19, 5845–9. (30) Jacquemin, D.; Perpète, E. A.; Maurel, F.; Perrier, A. Doubly Closing or Not? Theoretical Analysis for Coupled Photochromes. J. Phys. Chem. C 2010, 114, 9489–9497. (31) Sevez, G.; Pozzo, J.-L. Toward Multi-Addressable Molecular Systems: Efficient Synthesis and Photochromic Performance of Unsymmetrical Bisthienylethenes. Dyes Pigm. 2011, 89, 246–253. (32) Tian, H.; Yang, S. Recent Progresses on Diarylethene Based Photochromic Switches. Chem. Soc. Rev. 2004, 33, 85–97. (33) Marinotto, D.; Castagna, R. Photoswitching of the Second Harmonic Generation from Poled Phenyl-Substituted Dithienylethene Thin Films and EFISH Measurements. J. Phys. Chem. C 2011, 115, 20425–20432. (34) Aubert, V.; Guerchais, V.; Ishow, E.; Hoang-Thi, K.; Ledoux, I.; Nakatani, K.; Le Bozec, H. Efficient Photoswitching of the Nonlinear Optical Properties of Dipolar Photochromic Zinc(II) Complexes. Angew. Chemie 2008, 120, 587–590. (35) Ordronneau, L.; Aubert, V.; Métivier, R.; Ishow, E.; Boixel, J.; Nakatani, K.; Ibersiene, F.; Hammoutène, D.; Boucekkine, A.; Le Bozec, H. et al. Tunable Double Photochromism of a Family of Bis-DTE Bipyridine Ligands and their Dipolar Zn Complexes. Phys. Chem. Chem. Phys. 2012, 14, 2599–605. (36) Aubert, V.; Ordronneau, L.; Escadeillas, M.; Williams, J. a. G.; Boucekkine, A.; Coulaud, E.; Dragonetti, C.; Righetto, S.; Roberto, D.; Ugo, R. et al. Linear and Nonlinear Optical Properties of Cationic Bipyridyl Iridium(III) Complexes: Tunable and Photoswitchable? Inorg. Chem. 2011, 50, 5027–5038. (37) Le Bozec, H.; Guerchais, V. Photochromic Bipyridyl Metal Complexes: Photoregulation of the Nonlinear Optical and/or Luminescent Properties. Comptes Rendus Chim. 2013, 16, 1172–1182. (38) Guerchais, V.; Ordronneau, L.; Le Bozec, H. Recent Developments in the Field of Metal Complexes Containing Photochromic Ligands: Modulation of Linear and Nonlinear Optical Properties. Coord. Chem. Rev. 2010, 254, 2533–2545. (39) Ordronneau, L.; Nitadori, H.; Ledoux, I.; Singh, A.; Williams, J. a. G.; Akita, M.; Guerchais, V.; Le Bozec, H. Photochromic Metal Complexes: Photoregulation of Both the Nonlinear Optical and Luminescent Properties. Inorg. Chem. 2012, 51, 5627–5636.


Page 26 of 31

Page 27 of 31

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


(40) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bredas, J. L. Electric Field Modulated Nonlinear Optical Properties of Donor-Acceptor Polyenes: Sum-Over-States Investigation of the Relationship between Molecular Polarizabilities (α, β , and γ) and Bond Length Alternation. J. Am. Chem. Soc. 1994, 116, 10703–10714. (41) Di Bella, S.; Fragalà, I.; Marks, T. J.; Ratner, M. A. Large Second-Order Optical Nonlinearities in Open-Shell Chromophores. Planar Metal Complexes and Organic Radical Ion Aggregates. J. Am. Chem. Soc. 1996, 118, 12747–12751. (42) Sliwa, M.; Létard, S.; Malfant, I.; Nierlich, M.; Lacroix, P. G.; Asahi, T.; Masuhara, H.; Yu, P.; Nakatani, K. Design, Synthesis, Structural and Nonlinear Optical Properties of Photochromic Crystals: Toward Reversible Molecular Switches. Chem. Mater. 2005, 17, 4727–4735. (43) Patel, P. D.; Masunov, A. E. Theoretical Study of Photochromic Compounds. 1. Bond Length Alternation and Absorption Spectra for the Open and Closed Forms of 29 Diarylethene Derivatives. J. Phys. Chem. A 2009, 113, 8409–8414. (44) Marder, S. R. Organic Nonlinear Optical Materials: Where We Have Been and Where We Are Going. Chem. Commun. 2006, 131–134. (45) Zhang, M.-Y.; Wang, C.-H.; Wang, W.-Y.; Ma, N.-N.; Sun, S.-L.; Qiu, Y.-Q. Strategy for Enhancing SecondOrder Nonlinear Optical Properties of the Pt(II) Dithienylethene Complexes: Substituent Effect, π-Conjugated Influence, and Photoisomerization Switch. J. Phys. Chem. A 2013, 117, 12497–12510. (46) Ma, T.-Y.; Ma, N. N.; Yan, L.-K.; Zhang, T.; Su, Z.-M. Theoretical Exploration of PhotoisomerizationSwitchable Second-Order Nonlinear Optical Responses of Two-Dimendional Λ- and W-Shaped Polyoxometalate Derivatives of Dithienylperfluorocyclopentene. J. Phys. Chem. A 2013, 117, 10783–10789. (47) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods Without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158–6170. (48) Ernzerhof, M.; Scuseria, G. Assessment of the Perdew–Burke–Ernzerhof Exchange-Correlation Functional. J. Chem. Phys. 1999, 110, 5029–5036. (49) Jacquemin, D.; Perpète, E. A. Ab Initio Calculations of the Colour of Closed-Ring Diarylethenes: TD-DFT Estimates for Molecular Switches. Chem. Phys. Lett. 2006, 429, 147–152. (50) Jacquemin, D.; Perpète, E. A.; Medved’, M.; Scalmani, G.; Frisch, M. J.; Kobayashi, R.; Adamo, C. First Hyperpolarizability of Polymethineimine with Long-Range Corrected Functionals. J. Chem. Phys. 2007, 126, 191108(1–4).



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

(51) Rudberg, E.; Sałek, P.; Helgaker, T.; Agren, H. Calculations of Two-Photon Charge-Transfer Excitations Using Coulomb-Attenuated Density-Functional Theory. J. Chem. Phys. 2005, 123, 184108(1–7). (52) Sekino, H.; Maeda, Y.; Kamiya, M.; Hirao, K. Polarizability and Second Hyperpolarizability Evaluation of Long Molecules by the Density Functional Theory with Long-Range Correction. J. Chem. Phys. 2007, 126, 014107(1– 6). (53) Iikura, H.; Tsuneda, T.; Yanai, T.; Hirao, K. A Long-Range Correction Scheme for Generalized-GradientApproximation Exchange Functionals. J. Chem. Phys. 2001, 115, 3540–3544. (54) Murugan, N. A.; Kongsted, J.; Rinkevicius, Z.; Agren, H. Breakdown of the First Hyperpolarizability/BondLength Alternation Parameter Relationship. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 16453–16458. (55) Tomasi, J.; Mennucci, B.; Cancès, E. The IEF Version of the PCM Solvation Method: an Overview of a New Method Addressed to Study Molecular Solutes at the QM Ab Initio Level. J. Mol. Struct. THEOCHEM 1999, 464, 211–226. (56) Kim, H. J.; Hynes, J. T. Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. III. Quantum Theory. J. Chem. Phys. 1992, 96, 5088–5110. (57) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09 Revision D.01. Gaussian Inc. Wallingford CT 2009. (58) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Light-Triggered Electrical and Optical Switching Devices. J. Chem. Soc. Chem. Commun. 1993, 1439–1442. (59) Higashiguchi, K.; Matsuda, K.; Asano, Y.; Murakami, A.; Nakamura, S.; Irie, M. Photochromism of Dithienylethenes Containing Fluorinated Thiophene Rings. Eur. J. Org. Chem. 2005, 2005, 91–97. (60) Tatsumi, Y.; Kitai, J.-i.; Uchida, W.; Ogata, K.; Nakamura, S.; Uchida, K. Photochromism of 1,2-Bis(2thienyl)perfluorocyclopentene Derivatives: Substituent Effect on the Reactive Carbon Atoms. J. Phys. Chem. A 2012, 116, 10973–10979. (61) Norsten, T.; Peters, A. Reversible [7]-Thiahelicene Formation Using a 1, 2-Dithienylcyclopentene Photochrome. J. Am. Chem. Soc. 2001, 123, 7447–7448. (62) Irie, M.; Sakemura, K. Photochromism of Dithienylethenes with Electron-Donating Substituents. J. Org. Chem. 1995, 60, 8305–8309.


Page 28 of 31

Page 29 of 31

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


(63) Cammi, R.; Mennucci, B.; Tomasi, J. On the Calculation of Local Field Factors for Microscopic Static Hyperpolarizabilities of Molecules in Solution with the Aid of Quantum-Mechanical Methods. J. Phys. Chem. A 1998, 102, 870–875. (64) Willetts, A.; Rice, J. E.; Burland, D. M.; Shelton, D. P. Problems in the Comparison of Theoretical and Experimental Hyperpolarizabilities. J. Chem. Phys. 1992, 97, 7590–7599. (65) Nakamura, S.; Kobayashi, T.; Takata, A.; Uchida, K.; Asano, Y.; Murakami, A.; Goldberg, A.; Guillaumont, D.; Yokojima, S.; Kobatake, S. et al. Quantum Yields and Potential Energy Surfaces: a Theoretical Study. J. Phys. Org. Chem. 2007, 20, 821–829. (66) Yuan, K.; Boixel, J.; Le Bozec, H.; Boucekkine, A.; Doucet, H.; Guerchais, V.; Jacquemin, D. Perfluorocyclohexene Bridges in Inverse DiArylEthenes: Synthesis through Pd-Catalysed C-H Bond Activation, Experimental and Theoretical Studies on their Photoreactivity. Chem. Commun. 2013, 49, 7896–7898. (67) Hania, P. R.; Telesca, R.; Lucas, L. N.; Pugzlys, A.; van Esch, J.; Feringa, B. L.; Snijders, J. G.; Duppen, K. An Optical and Theoretical Investigation of the Ultrafast Dynamics of a Bisthienylethene-Based Photochromic Switch. J. Phys. Chem. A 2002, 106, 8498–8507. (68) Chen, Y.; Zeng, D. X.; Fan, M. G. Synthesis and Photochromic Properties of Functional Diarylethene with a 2,5-Dihydrothiophene Bridging Unit. Org. Lett. 2003, 5, 1435–1437. (69) Krayushkin, M. M.; Pashchenko, D. V.; Lichitskii, B. V.; Valova, T. M.; Strokach, Y. P.; Barachevskii, V. A. Synthesis and Properties of Dihetaryl-Substituted Furanones. Synthesis of Photochromic Dithienylethenes Containing a Furanone Bridging Fragment. Russ. J. Org. Chem. 2006, 42, 1816–1821. (70) Irie, M.; Sayo, K. Solvent Effects on the Photochromic Reactions of Diarylethene Derivatives. J. Phys. Chem. 1992, 96, 7671–7674. (71) Krayushkin, M.; Shirinyan, V. Photochromic Dihetarylethenes: XVII. New Synthesis of Photochromic NAlkyldithienylmaleimides. Russ. J. Org. Chem. 2002, 38, 1335–1338. (72) Krayushkin, M.; Ivanov, S. Photochromic Dihetarylethenes. 7. Synthesis of Bis(thienylazoles), Photochromic Analogs of Diarylethenes. Russ. Chem. Bull. 2001, 50, 116–121. (73) Kobatake, S.; Irie, M. Synthesis and Photochromic Reactivity of a Diarylethene Dimer Linked by a Phenyl Group. Tetrahedron 2003, 59, 8359–8364.



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

(74) Yam, V.; Lee, J.; Ko, C.; Zhu, N. Photochromic Diarylethene-Containing Ionic Liquids and N-Heterocyclic Carbenes. J. Am. Chem. Soc. 2009, 131, 912–913. (75) Kobatake, S.; Kuma, S.; Irie, M. Single-Crystalline Photochromism of Diarylethene Dimers Bridged by a Spiro Structure. J. Phys. Org. Chem. 2007, 20, 960–967.


Page 30 of 31

Page 31 of 31

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


Graphical Abstract

Hyperpolarizabilities (β) of Diarylethene switches

multiswitches

monoswitches

A

I O

A

D

larger β smaller β

small contrast

high contrast


I O

X

I O

D

Strategies for Designing Diarylethenes as Efficient Nonlinear Optical

Recommend Documents