2-Dimensional Nonlinear Optical Switching Materials: Molecular

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

2-Dimensional Nonlinear Optical Switching Materials: Molecular Engineering Towards High Nonlinear Optical Contrasts Marc Hänsel, Christoph Barta, Clemens Rietze, Manuel Utecht, Karola Rueck-Braun, Peter Saalfrank, and Petra Tegeder J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08212 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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The Journal of Physical Chemistry

2-Dimensional Nonlinear Optical Switching Materials: Molecular Engineering Towards High Nonlinear Optical Contrasts Marc Hänsel,† Christoph Barta,‡ Clemens Rietze,¶ Manuel Utecht,¶ Karola Rück-Braun,‡ Peter Saalfrank,¶ and Petra Tegeder∗,† †Physikalisch-Chemisches Institut, Ruprecht-Karls-Universit¨ at Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ‡Institut f¨ ur Chemie, Organische Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany ¶Institut f¨ ur Chemie, Theoretische Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany E-mail: [email protected] Phone: +49 (0) 6221 548475 Abstract Combining photochromism and nonlinear optical (NLO) properties of molecular switches-functionalized self-assembled monolayers (SAMs) represents a promising concept towards novel photonic and optoelectronic devices. Using second harmonic generation, density-functional theory and correlated wave function methods we studied the switching abilities as well as the NLO contrasts between different molecular states of various fulgimide-containing SAMs on Si(111). Controlled variations of the linker systems as well as of the fulgimides enabled us to demonstrate very efficient reversible

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photoinduced ring-opening/closure reactions between the open and closed forms of the fulgimides. Thus, effective cross sections on the order of 10−18 cm−2 are observed. Moreover, the reversible switching is accompanied by pronounced NLO contrasts up to 32 %. Further molecular engineering of the photochromic switches and the linker systems may even increase the NLO contrast upon switching.

Introduction Photochromic molecules can be reversibly switched by light between at least two molecular states that show different adsorption spectra. 1,2 Relevant examples are azobenzenes, diarylethenes, fulgides, spiropyrans, and fulgimides. They undergo large modifications in their electronic properties upon switching, for instance in the molecular first- and also second-order hyperpolarizabilities. Thus, combining photoinduced switching abilities with nonlinear optical (NLO) properties facilitates the formation of switchable second-order NLO materials. 3–6 These materials are promising for applications in optoelectronic and photonic devices, e.g., optical data storage and chemical sensors. 7–16 The unambiguous advantage of devices based on second-order NLO properties arises from symmetry constraints. The second-order NLO response of a centrosymmetric sample is confined to its symmetry breaking surface (interface). Hence, very high NLO signal contrasts are expected even for monolayers and buried interfaces since no bulk contributions appear. It has been shown for photochromic dyes incorporated in polymers or Langmuir-Blodgett films and for organic photochromic crystals that the NLO response can be switched via reversible photochemical reactions. 4,17–20 However, in these systems switching-induced irreversible changes in the initial alignment of the dyes have been found resulting in a significant reduction of the number of switching cycles. An approach to avoid this problem is the creation of two-dimensional layered functional units by immobilization and aligning molecular switches at planar solid surfaces, which has been recently demonstrated. 5,6 Thereby the incorporation of molecular switches into self-assembled monolayers (SAMs) immobilized on Si and Au, respectively, led to photoswitchable surfaces, 2


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which showed two NLO states. 5,6 Here, we utilized flat Si(111) surfaces functionalized with SAMs containing fulgimides. The switching process in fulgimides involve a reversible photostimulated ring-opening/closure reaction between the closed form (C) and the open isomer (E) as depicted in Fig. 1a). Additionally, an E/Z-isomerization can occur. In the liquid phase, fulgimides exhibit various favorable properties such as high thermal stabilities, high conversion rates between the open and closed forms, high quantum efficiencies, and good fatigue resistance. 21,22 Fulgimides possess large first- and second-order molecular hyperpolarizabilities, which depend on the isomeric form. In particular, the second-order molecular hyperpolarizability is highly influenced by the delocalized π-electron system created by the ring-closure reaction. 23 Using second harmonic generation (SHG) enables to study the second-order NLO response of the fulgimide-functionalized SAM/Si(111) system and accordingly the switching state as well as the NLO contrast between the states. SHG probes the second-order nonlinear susceptibility χ(2) of the sample. For a centrosymmetric substrate, the second-order NLO response solely originates from the symmetry-breaking interface. 24–27 Therefore the absence of an interfering bulk-contribution to the SHG signal results in a very high NLO contrast between the respective switching states of molecular switch-functionalized SAMs. In the case of a fulgimide-functionalized SAM on Si(111) a NLO contrast of around 17% has been measured, 5 whereas for an azobenzene-containing SAM on gold a value of 16% has been detected between the two different switching states. 6 Our present study demonstrates photoinduced reversible switching between the two molecular states, closed and open forms, towards high NLO contrasts using controlled chemical variations of the linker systems as well as of the fulgimides in fulgimide-functionalized SAMs on Si(111). Specifically, three different indolylfulgimide switching units with alkyne linkers will be considered, abbreviated as 3-IF-Al, 2-IF-Al and 2-IF-Sp-Al in what follows (see Fig. 1c)-e)). By means of SHG, supported by density-functional theory (DFT) and correlated wave function calculations, we find reversible switching in all investigated systems.

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In all cases the NLO contrasts between the open and closed forms are above 20%. For a special combination of the polarization of the incoming and outgoing light even a value of 32% for one particular system is observed. Moreover, the switching efficiencies are very high, possessing values known for fulgimides in solution. Thus, indicating an efficient decoupling of the switches from both the substrate and the neighboring molecules.

Experimental Section Sample preparation. The silicon substrates were functionalized using an on-surface synthesis approach (for the reaction scheme see electronic supplementary information, ESI). In the first step the Si(111) surface was cleaned for 30 min at 100◦ C in a piranha solution (H2 SO4 /H2 O2 ) and afterwards etched with a 40 % ammonium fluoride solution for 15 min at room temperature. This led to a fully H-terminated Si(111) surface. In the next step the alkyne monolayer was grafted via a thermal reaction of the Si(111) surface with 1,8-nonadiyne at 170◦ C for 3 h under protective gas atmosphere (N2 ). 28 The fulgimides functionalized with a phenyl ring carrying an iodine moiety at the nitrogen of the imide part were attached to the alkyne monolayer by a Sonogashira reaction (5 mM fulgimide, 10 mol% Pd(PPh3 )4 and 10 mol% CuI in toluene, DMF, Et3 N solution for 18 h at room temperature). The fulgimides were synthesized following the approach of Rentzepis et al. 29,30 Afterwards the samples were cleaned and stored under argon until they were transferred into the SHG chamber. X-ray reflectivity studies revealed a surface coverage of 68±5 % with the alkyne linker molecules. The coverage of fulgimides on top of the alkyne layer is about 14 %, i.e., a switch is attached to every 7th alkyne molecule. 31 SHG measurement. The SHG experiments were carried out using a 300 kHz regenerative amplified Ti-sapphire laser system, which generates ca. 50 fs pulses at a wavelength of 800 nm. 5,6,32,33 The beam was focused onto the sample under an incident angle of 45◦ with respect to the surface normal. The SHG signal (400 nm) was focused into a monochro-

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mator and detected by a photomultiplier tube. The analogous signal was converted by a counting unit and acquired using a PCI counting device. The polarization of the probe beam was controlled in front and behind of the sample by a half-wave plate combined with a Glan-Thompson polarizer. Measuring the photoisomerization abilities of the fulgimidefunctionalized SAMs via the NLO response as well as the NLO contrast (see Fig. 2) the applied polarization combination was p-in/all-out. P-polarization leads to perpendicular and a parallel component of the light compared to the sample, whereas the s-polarized light only has a parallel component (for details see below). All experiments were performed under inert gas (N2 ) and at room temperature. Illumination experiments. The samples were illuminated using two diodes at a wavelength of λ = 530 nm and λ = 365 nm, respectively, to induce the ring-closure (365 nm) or the ring-opening reaction (530 nm). Quantum chemical calculations support the effective absorption behaviour of closed and open forms of the three molecules under study in these wavelengths regimes (cf. ESI, Sec.S7.4). To quantify the switching processes effective cross sections were measured. Due to the thermal instability of the closed forms in 3-IF-Al/Si(111) and 2-IF-Al/Si(111) the thermal decay was considered in the cross section calculations. In the case of two competing processes (thermal and illumination) leading to the open isomer, the cumulative time (measured under illumination) becomes: 1 1 1 = + τc τi τt

(1)

where τc denotes the cumulative decay under illumination, τt the thermal decay, and τi the part introduced by the illumination, i.e., the photoinduced ring-opening reaction. Solved for the illumination part, equ. 1 becomes: τi =

τc · τt τt − τc

(2)

Based on the decay times τi and the photon dose np on the sample surface per time interval 5



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the effective cross sections (σ) were calculated via: σ=

1 τi · np

(3)

A photon flux of fU V = 2.07·1016 photons s−1 cm−2 was used during illumination with UV light (365 nm) and a photon flux of fvis = 2.79·1016 photons s−1 cm−2 during light exposure with visible photons (530 nm). Quantum chemical calculations. The SHG intensity scales quadratically with the second-order nonlinear susceptibility χ(2) or the first hyperpolarizability tensor β, which is the molecular equivalent of χ(2) . Hence, it is necessary to gain insight into the components of this tensor for all three isomers of the three different molecular systems. DFT as well as correlated wave function methods were used to calculate the molecular hyperpolarizabilities of all here investigated systems. As a first step a structure optimization of the fulgimides including the alkyne linker was performed with hybrid-DFT based on the B3LYP/6-31G∗ level. Thereby we chosen a molecular model in which the anchoring unit (Si-CH=CH(CH2 )5 -) was replaced by ethyl, -CH2 –CH3 to reduce calculating time. Note, it has been shown that the influence of a longer alkane chain has a vanishing influence on the electronic structure. 34 Optimization was done for all three switches, in their closed (C) and two possible open forms, Z and E (see Fig.1 below). In a second step, the dynamic (frequency dependent) hyperpolarizabilities βijk (−2ω; ω; ω) were calculated at the MP2 (Møller-Plesset perturbation theory at the seond order) level with an extended basis set, 6-311++G∗∗ at the B3LYP/631G∗ geometries based on the static ones by a well-established method of Champagne et al. 35,36 The frequency-dependent calculations were conducted at a frequency of ω =

c 2πλ

with

λ = 800 nm corresponding to the laser wavelength used in the SHG experiments. Details of the procedure are described in the Supporting Information, Sec.S7, where also the calculation of other properties of the switches is reported. The quantum chemical determination of SHG responses requires precise knowledge of the adsorption geometries which are unknown for

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the systems under study. An elaborate possibility to eliminate this problem would be to sample all possible molecular conformations and calculate for all the NLO response. 37 Here, instead, we estimate the NLO response from some simpler, approximate measures. The first of these are averaged hyperpolarizabilities β0 , which are defined as: 38 2 1/2 βx + βy2 + βz2 1 = βiii + (βijj + βjij + βjji ) 3 j=i

β0 =

(4)

βi

(5)

The components βi (where i =x,y,z with respect to the surface) define also a “hyperpolarizability vector”, β = (βx , βy , βz ) given the (p-probe, or p-in) character of the SHG experiments conducted here and the observed dominance of the βzzz tensor element (see below and ESI), in particular βz and also βzzz itself are other valid, approximate measures for estimating NLO responses for the present setups. A further useful measure are frequency dispersion factors Fzzz (ω) of the latter element, as introduced in Ref., 15 defined as the ratio of dynamical and static hyperpolarizability components βzzz , i.e., Fzzz (ω) =

βzzz (−2ω; ω, ω) βzzz (0; 0, 0)

.

(6)

Finally, two other measures are the so-called Electric Field Induced Second Harmonic Generation (EFISHG) hyperpolarizability βEFISHG (−2ω, ω, ω) =

3 μi βi 5 i μ

,

(7)

and the Hyper Rayleigh Scattering (HRS) hyperpolarizability 2 1/2 2 βHRS (−2ω, ω, ω) = βZZZ + βXZZ

,

(8)

respectively (see ESI and Ref. 35 ). In Eq.(7), the dipole moment μ and its components μi were

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calculated on the MP2/6-311++G∗∗ level, using B3LYP/6-31G∗ geometries. The quantities 2 2 and βXZZ in Eq.(8) are functions of all tensor elements of β (see ESI and Ref. 35 ). βZZZ

(a)

(b)

hn

open-forms N

H

O N R

O

O

N R

nm

(5

HN

65

S V

(3

VI

closed-form

O

E-form

)

30

nm

)

Z-form

2w

w

H

UV (365 nm)

SHG

probe

N

UV (365 nm)

O

U

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

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N

O

N H

O

O N R

O

C-form

(c)

OH O

NH

9 OH

9

Si (111)

(d)

(e)

H N

N

N

H

N

H

5 H

H

O

O

5

N

O

H

H

5

5 H

H

5

O O

O

H

H

5

5

H

H

Si (111)

Si (111)

3-indolylfulgimide + alkyne linker (3-IF-Al)

3-indolylfulgimide + amide linker (3-IF-Am)

N

H

5

5

H

Si (111)

2-indolylfulgimide alkyne linker (2-IF-Al)

2-indolylfulgimide + spacer + alkyne linker (2-IF-Sp-Al)

Figure 1: (a) Photochromism in fulgimide upon illumination with UV (λ = 365 nm) and visible (VIS) (λ = 530 nm) light. Fulgimides perform a reversible reaction between the closed (C) and a mixture E/Z-isomers of the open form. (b) Scheme of the SAM containing fulgimide moiety, immobilized via an amide (Am) linker studied in Ref., 5 called here 3IF-Am. SHG is used to study the switching state via the NLO response. c)-e) Fulgimide containing SAMs on Si(111) investigated in the present study. (c) 3-indolylfulgimide (3-IFAl), (d) 2-indolylfulgimide (2-IF-Al), and (e) 2-indolylfulgimide with a –CH2 -spacer group (2-IF-Sp-Al) connected via an alkyne (Al) linker to the Si substrate.

Results and Discussion Based on our previous study in which we used an amide (Am) linker and a 3-indolylfulgimide (3-IF-Am, see Fig. 1b) to successfully follow the approach of utilizing functionalized pho8


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toswitchable Si surfaces to form a 2-dimensional NLO switching material, 5 we here aim towards high NLO contrasts between the open and closed form of fulgimide containing SAMs on Si(111). To follow this task we varied the linker system as well as the molecular switch (see Fig. 1b)-e)). Going from the amide to the alkyne (Al) linker opens the way to enhance the stability of the SAM towards oxidation and hydrolysis of remaining Si-H bonds, because of the high coverage (65%). 39 We investigate the influence of changing the switch, i.e., using a 3-indolylfulgimide (3-IF-Al) versus a 2-indolylfulgimide (2-IF-Al) on switching efficiency and the NLO contrast. In addition, we introduced a -CH2 - spacer unit in order to interrupt the electronic interaction between the closed form of the fulgimide and the phenyl ring of the linker system (2-IF-Sp-Al). To induce the ring-opening/closure reaction we illuminated a mixture of the open Z and E forms with UV light (365 nm) to form the closed C isomer, while exposure with visible light (530 nm) stimulates the back-reaction (C to E conversion). The switching state is probed by SHG via the NLO response. Photoinduced switching of the NLO response. Figure 2 shows the SHG signal amplitude as a function of illumination steps. Large dependencies of second-order NLO response upon light exposure with UV and VIS photons, respectively, are identified for all systems. We assign the signal changes to the photoinduced switching between the two photostationary states (PSSs), the open (E/Z) and the closed forms (C), in each of the three systems. For the 3-IF-Al/Si(111) interface the signal amplitude changes reversibly by 25±2 % between the two PSSs. The 2-IF-Al/Si(111) possesses a NLO contrast of 20±2 %, while for the 2-IF-Sp-Al/Si(111) interface a value of 22±3 % is measured. Hence, changing the the switch from 3-IF to 2-IF lead to a contrast decrease of around 5 %. The introduction of a –CH2 -spacer increases the contrast slightly (around 2 %). For comparison, in the case of the 3-IF-Am/Si(111) (see Fig. 1b) a contrast of 18±2 % has been observed. 5 Thus, using an alkyne linker which connects the fulgimide via a phenyl ring to the silicon substrate instead of an amide linker leads to an increase of the NLO contrast between the open and closed states. However, we note that the closed forms of 3-IF-Al and 2-IF -Al are kinetically unstable. The

9


5

5 H

5

H

Si (111)

0.9

25±2%

0.8

E/ZPSS 0.7

2-IF-Al N H O N

O

H

H

5

5

5

H

H

SHG Intensity (normalized)

5

(b)

N H

O O

N

H

5 H

H

5 H

Si (111)

5

20

CPSS

0.95

20±2%

0.90 0.85

E/ZPSS

0.80 0

2-IF-Sp-Al

10 15 Illumination Steps

1.00

Si (111)

(c)

VIS (530 nm)

H

H

VIS (530 nm)

O

UV (365 nm)

N

CPSS

1.0

UV (365 nm)

O

5


20

1.05

CPSS

1.00 0.95

22±3%

0.90 0.85 0.80

VIS (530 nm)

H N

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UV (365 nm)

(a) 3-IF-Al


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E/ZPSS

0.75 0

5


20

Figure 2: Changes in the SHG signal amplitude as a function of light exposures at different wavelengths of λ = 365 and λ = 530 nm, respectively, for a) the 3-IF-Al/Si(111), b) the 2IFAl/Si(111), and c) the 2-IF-Sp-Al/Si(111). In all systems light-induced reversible changes in the NLO interfacial response due to the ring-opening/closure process is found. The values are referenced to the SHG intensities of the closed forms. ring-opening reaction occurred at room temperature within τ = 238±74 s in 3-IF-Al and τ = 302±30 s in 2-IF-Al (see ESI, Sec. S4). On the other hand the implementation of the CH2 group in 2-IF-Sp-Al results in a thermal stability of both the open and closed forms. Hence, a significant activation barrier exists to avoid the thermally activated reverse reaction. We suggest that the CH2 -group decouples the fulgimide electronically from the phenyl ring and the alkyne group leading to a thermally stable system. Note, that the 3-IF-Am/Si(111) is also thermally stable. 5 Concluding, the utilization of the alkyne linker with a connecting phenyl ring leads to an increase in the NLO-contrast compared to the amide linker, but to a 10


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destabilization of the closed state. In contrast, the 2-IF-Sp-Al/Si(111) is stable and possesses a 4% higher NLO contrast compared to the 3-IF-Am/Si(111) interface. Our calculations (see ESI, Table S5) suggest that all switches (3-IF-Am, 3-IF-Al, 2-IF-Al, 2-IF-Sp-Al) are kinetically stable with respect to a thermal ring opening reaction, C → E. There we find also no large differences between the four molecules. It should be noticed, though, that the additional CH2 -group(s) present in 2-IF-Sp-Al (and 3-IF-Am) and not in 3-IF-Al and 2-IF-Al, make the former conformationally more flexible leading to different intermolecular interactions which were not considered here. Tentatively, therefore, we attribute the reduced lifetimes of 3-IF-Al and 2-IF-Al to altered intermolecular interactions. Calculated hyperpolarizabilities and polarization-dependent SHG. The hyperpolarizabilities and approximate SHG measures were calculated along the lines described in the experimental section. Thus, we first optimized the structures for the open and closed fulgimides (see experimental section) using hybrid DFT on the B3LYP/6-31+G∗ level. Distinct minima for the closed and the open E-form and Z-form were found. Figure 3 depicts the low-energy structures of all systems. For fulgimides in general, the most noticeable difference in the electronic structure between the open and closed forms is the degree of delocalization of the π-electron system. While the π-electron system is delocalized over the four conjugated rings of the fulgimide moiety in the closed state, it is interrupted in the open forms due to the torsion between the indole (2,3–benzopyrrole) and imide (pyrrole-2,5-dione) planes.

Secondly, we calculated the dynamical hyperpolarizabilities (see experimental section and ESI), the averaged hyperpolarizabilities β0 , diagonal components of the β tensors, components of the hyperpolarizability vectors β, and HRS as well as EFISHG hyperpolarizabilities, using MP2/6-311++G∗∗. Further, we computed the frequency dispersion factors Fzzz (ω) as defined in Eq.(6) on the same level of theory. Selected values are shown in Table 1, others can be found in the ESI (Tabs. S2 and S3). For comparison, in the ESI we also give the corresponding information for the reference compound 3-IF-Am taken from Ref. 5 We should

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Figure 3: Structures and “hyperpolarizability vectors” β = (βx , βy , βz ) (arrows) of the isomers of the three investigated systems a)-c). Closed (C) and open (E and Z) fulgimides studied in this work, optimized at the B3LYP/6-31G∗ level of theory. On this level, E is the most stable form followed by Z and C (see ESI, Tab.S1). The coordinate system is chosen such that the atoms C1 and N2 (see Fig. (a), left panel) form the z-axis, and atoms C1, N2, and C3 lie in the (xz)-plane. Nitrogen atoms are blue, oxygen red, carbon grey, and hydrogen light-grey. The “hyperpolarizability vectors” are arbitrarily originating in the center of mass of the respective molecule, and some of them are scaled.

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note that the molecular coordinate system was chosen as indicated in Fig.3, consistent with the choice made in Ref. 5 We also note that the molecular z-axis defined in this way will coincide only approximately with the z-axis relative to the surface (i.e., the surface normal, in particular for 2-IF-Sp-Al, the molecular and surface z-axis are expected to differ), and that these quantities depend on the choice of the coordinate system. Therefore, fully quantitative conclusions should not be drawn from our analysis. Table 1: Selected dynamical hyperpolarizability tensor elements βiii (−2ω; ω, ω) and approximate derived SHG measures, for the three compounds studied in this work, isomers C, E and Z in each case. Calculations were done with scaled MP2/6-311++G∗∗ at optimized B3LYP/6-31G∗ geometries. All values are in atomic units, where 1 a.u. for β is 8.641×10−33 esu or 3.62 ×10−42 m4 V−1 .

form C E Z C E Z C E Z

Parameter β0 βz βy βx βHRS βEFISHG Fzzz (ω) 3-IF-Al -65178 49 1385 98635 -94578 8941 26534 38829 -149960 13.0 -1787 696 -912 2042 -1877 528 -605 1031 -3673 3.1 744 -912 2262 3654 980 -2931 1950 1908 -3372 2.0 2-IF-Al -25303 -109 253 25593 -25006 -106 5450 11095 -45816 5.1 -2835 2168 -62 9407 -6955 6184 1370 5342 -6651 2.0 4463 34 20561 21561 269 151 21559 9285 8588 10.9 2-IF-Sp-Al -20697 -350 254 21029 -20464 -1580 4577 9205 -37456 6.2 -4675 1738 -108 8695 -6979 5095 967 4234 -7819 2.4 -62 471 16499 17617 740 -561 17593 7729 9605 0.2 βzzz

βyyy

βxxx

From the table we note the following: When switching from closed (C) to open (E, Z) forms, the SHG markers which we consider most relevant for the “p-in” experiments of Fig.2, namely βzzz , β0 , βz , βHRS , and βEFISHG decrease usually considerably in magnitude, for all molecules. As in Ref., 5 we tentatively interpret this as being indicative of a stronger SHG signal of C as compared to E/Z, in agreement with experiment. More specifically, for the C form, βzzz and βz are by far the largest tensor and vector elements, respectively, for all three molecules; upon switching to E and Z. βzzz and βz become smaller and in fact, for the Z-forms, other elements (such as βxxx or βx ) can dominate. This is due to the fact 13



that for the Z-form, the indole and imide moiety move out of the z-direction into the xyplane. Furthermore, the frequency dispersion F (ω) of the hyperpolarizability is a meaningful measure, especially its zzz-component as defined in Eq.(6), because βzzz contributes the most to the hyperpolarizability. The frequency dispersion is usually highest for the Cform (between a factor of 5 to 13 dynamical increase of the hyperpolarizability), while the dispersion is rather small for the E and Z fulgimdes. An exceptional case is the Z-species of 2-IF-Al, for which also a large dispersion factor of ∼ 11 is found. Considering the SHG measures βzzz , β0 , βz , βHRS , and βEFISHG and also the dispersion factor Fzzz , we may define rough “SHG signal contrasts” as ratio β(C)/β(E/Z) (or F (C)/F (E/Z)). These ratios (not shown) immediately suggest that 3-IF-Al is expected to have a larger contrast than 2-IF-Al and 2-IF-Sp-Al. The latter two have similar β(C)/β(E/Z) ratios, however, indicating that the -CH2 - bridge has no large effect on the contrast. These last two findings are also in agreement with the experimental observations of Fig.2, according to which the contrast for 3-IF-Al is around 25%, but only around 20-22% for 2-IF-Al and 2-IF-Sp-Al, respectively. Finally, in the ESI (Table S3) we compare the mentioned β-values with those obtained for 3-IF-Am in Ref. 5 It is found that the SHG measures of 3-IF-Am are smaller than for 3-IF-Al, and that the contrast is larger for the latter than for the former – again in agreement with experiment. However, the larger contrast of 2-IF-Al and 2-IF-Sp-Al compared to 3-IF-Am, cannot be confirmed by this kind of analysis. As suggested elsewhere, 40,41 a large NLO response can be accompanied by a large redshift in the excitation spectrum of the molecule, which then also affects the contrast. Since excitation spectra are often easier to compute than NLO properties, this could be an efficient method, e.g., for prescreening NLO properties and their contrast. In fact, from Fig. S10 of the ESI, where excitation spectra are shown on the TD-B3LYP/6-31G∗ level of theory, we find for the C-fulgimides studied in this work the following lowest-energy absorption wavelengths: λ(3-IF-Al)=577 nm, λ(2-IF-Al)=515 nm, and λ(2-IF-Sp-Al)=492 nm. Again, this nicely reflects the various NLO measures, e.g., β0 (3-IF-Al)> β0 (2-IF-Al)≈ β0 (2-IF-Sp-

14


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Al). Upon switching to Z or E forms, the lowest-energy absorption peaks are shifted to higher energies (smaller wavelengths). For instance, the wavelength shift is 180 nm for 3-IF-Al, 81 nm for 2-IF-Al, and 73 nm for 2-IF-SpAl. This could be interpreted as a measure for the contrast in NLO response by switching, which is also 3-IF-Al > 2-IF-Al ≈ 2-IF-Sp-Al. While these indirect measures, the lowest-energy absorption and their shift upon switching may indeed be useful, unfortunately the contrast for 3-IF-Am molecule of Ref. 5 (which is lower than for the other three studied here) cannot be explained in this way either: The shift of the lowest-energy absorption peak is large, 162 nm, similar to 3-IF-Al. In summary, as discussed in our previous publications 5,6 and in the ESI, a quantitative comparison between the calculated molecular first hyperpolarizabilities and the measured macroscopic second-order nonlinear susceptibility and their contrast is difficult. The same seems true for analysis based on spectral shifts. From the experimental point of view, the signal from the remaining Si–H and Si-linker sides contributes to the SHG signal intensities in an unknown way. This background is detected for both PSSs. Thus, it reduces the apparent NLO contrast. In addition, phase shifts could play a role, 42,43 but they are expected to be small for low film thicknesses, as for the films studied here. Concerning the theoretical considerations, apart from the restrictions mentioned in the ESI (notably the single-molecule model), influences of the substrate may be non-negligible as discussed in Ref. 44 It has been shown that surface-induced effects are negligible for rising dimensions of the linker systems and that only chromophores with an ionic character exhibit an impact. Since the distance between the silicon substrate and the fulgimides in the SAM investigated in the present study is rather large and the fulgimides possess non ionic character, we assume not a significant influence of the silicon substrate on the calculated hyperpolarizabilities. To gain a deeper understanding of the influence of the molecular orientation on the NLO contrast and to probe the different elements of the second-order susceptibility tensor we performed polarization-dependent SHG measurements. Thereby we measured the signal level and the NLO contrast of both photochromic states for different polarization combinations of

15



(a)

w s-in p-in

(b) 3-IF-Al

z

2w s-out p-out


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Page 16 of 30

1.00 0.90

C p-in/p-out: +14.7%

C Z/E

0.80

Z/E

0.18

C

C

Z/E

Z/E

0.40 s-in/p-out: +13.4%

0.36 0.32 0.07

0.16

p-in/s-out: +13.3%

s-in/s-out: +12.4%

0.06 0.05

Figure 4: a) Illustration of polarization-resolved SHG experiment, for different combinations of the incoming (in) and outgoing (out) light. b) Polarization-dependent measurements of the 3-IF-Al/Si(111) interface. the incoming and outgoing light (see Fig. 4 a)). The p-polarized beam exhibits an electric field vector component of 45◦ with respect to the surface normal thus probing elements almost perpendicular to the interface, while the electric field vector of the s-polarized beam is oriented solely parallel to the surface. Figure 4 b) shows exemplarily the results for the 3-IF-Al/Si(111) interface (for the results of the other systems see ESI, Sec. S5). For a better comparison the signal levels are referenced to the level of the p-in/p-out combination. From these measurements we can derive the following observations: (1) For all polarization combinations different SHG signal amplitudes are observed for the open and closed form. (2) The signal amplitude of the closed state is above the one of the open form for all polarizations. (3) The signal is the highest for the p-in/p-out combination and more than one order of magnitude lower for the s-in/s-out beams. Based on the findings (2) and (3) we can conclude that the molecules are oriented mainly perpendicular with respect to the silicon surface. This is in agreement with calculations, i.e., the z-component of the hyperpolarizability vector is much larger than the x- and y-components (see Table 1), at least for C and E. For the 2-IF-Al/Si(111) the situation is similar while it changes significantly for the 2IF-Sp-Al/Si(111) (see ESI, Fig. S7). Due to the CH2 -spacer unit the fulgimide core is no longer oriented perpendicular to the surface. Thus the signal amplitude relations in the polarization-resolved measurements change completely. By far the highest SHG signal amplitude is observed in the p-in/s-out combination directly followed by the s-in/s-out com-

16


Page 17 of 30

bination. However, we find the highest NLO contrast of the here investigated systems of 32± 1 % for the polarization combination p-in/p-out. It is difficult to explain the special behaviour of 2-IF-Sp-Al with the help of calculated hyperpolarizabilities, because these are not very different from those of 2-IF-Al according to Tab.1: For the free molecular models, the CH2 -space has no big effect on NLO properties. It should be kept in mind, however, that the molecular z-axis as chosen in Fig.3(c) is somewhat arbitrary (more arbitrary than for 2-IF-Al and 3-IF-Al) by not reflecting properly the expected reorientation of the fulgimide core with respect to the surface. The reorientation may also cause repulsive intermolecular interactions which have been neglected here and which may hinder the flexibility of the 2-IF-Sp-Al switch in a SAM, more than for 2-IF-Al and 3-IF-Al. These effects can make the NLO response of 2-IF-Sp-Al special when compared to the other two switches.

(b)

(a) SHG Intensity (normalized)

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


2-IF-Al

1.00

CPSS

530nm

2-IF-Al

1.00

E/ZPSS

s = 4.2±0.5.10-20 cm2

0.95

0.95

0.90

0.90

0.85

E/ZPSS

365nm

CPSS

s = 3.4±0.6.10-19 cm2

0.80 0

250 500 Time [s]

0

0.85 0.80

250 500 Time [s]

Figure 5: Changes in the SHG signal amplitude as a function of illumination time for the 2-IF-Al/Si(111). a) Decrease in the SHG intensity due to illumination at λ = 530 nm inducting the ring-opening reaction. b) SHG intensity increase as a function of illumination time at λ = 365 nm induced by the ring-closure reaction. The solid lines in a) and b) represent exponential fits yielding the decay times used to calculate the respective effective cross section (σ) for the reversible photoinduced reaction (see experimental section).

Switching efficiencies. An essential parameter for a photoswitchable interface is the efficiency of the photoinduced reactions. To gain insights into the switching efficiencies, 17



Page 18 of 30

we quantified the photostimulated ring-opening/closure reaction pathways of the fulgimidefunctionalized SAMs. Thereby, we calculated effective cross sections (σ) by using the change in the SHG signal as a function of illumination time as a measure for the switching process, i.e., we correlated the obtained signal changes with the amount of switched molecules (see experimental section). Figure 5 shows exemplarily the results for the 2-IF-Al/Si(111) interface. For the ring-opening reaction we derived a cross section of σC→E/Z = 4.2±0.5 · 10−20 cm2 . For the ring-closure reaction the cross section is σE/Z→C = 3.4±0.6 · 10−19 cm2 . The corresponding data for the 3-IF-Al/Si(111) and 2-IF-Sp-Al/Si(111) systems can be found in the ESI (Sec S6). The cross section values for all measured systems are summarized in Table 2 together with the literature values of the 3-IF-Am/Si(111) interface. 5 For the three systems studied here, the cross sections for the ring-closure reaction are approximately one order of magnitude higher compared to the values for the ring-opening process. The cross sections for the reversible reactions in 3-IF-Al/Si(111) and 2-IF-Sp-Al/Si(111) are similar, while for 2-IF-Al/Si(111) the cross sections are much lower. Thus, the introduction of the -CH2 -group in the 2-indolylfulgimide SAM provokes a cross section increase of nearly one order of magnitude compared to the same system without the spacer unit. In comparison, the 3-IF-Am/Si(111) possesses a cross section for the ring-opening reaction which is one order of magnitude higher (σC→E/Z = 1.2±0.3 · 10−18 cm2 ) and the σE/Z→C for the ringclosure process is in the same range (2.3±0.3 · 10−18 cm2 ). Such a high cross section is also observed in 2-IF-Sp-Al/Si(111) for the photoinduced ring-closure pathway. These high cross Table 2: Effective cross sections (in cm2 ) for the ring-opening/closure reactions in the fulgimide containing SAMs on Si(111). *The values for the 3-IF-Am/Si(111) have been adapted from Ref. 5 CP SS to E/ZP SS E/ZP SS to CP SS

3-IF-Al 1.3±1.2 · 10−19 7.1±1.3 · 10−19

2-IF-Al 4.2±0.5 · 10−20 3.4±0.6 · 10−19

2-IF-Sp-Al 1.2±0.5 · 10−19 2.0±0.7 · 10−18

3-IF-Am* 1.2±0.3 · 10−18 2.3±0.3 · 10−18

sections unambiguously indicate an effective electronic decoupling of the fulgimide from the substrate and an electronic as well as steric decoupling within the molecular layer. 6,45–48 18


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Note that the cross sections for fulgimides in solution are also in the range of 10

−18

cm2 . 29

In contrast, switches adsorbed directly on a metal or semimetal substrate, e.g., azobenzenes or spiropyrans on Au or Bi, exhibit cross sections, which are orders of magnitude lower. 49–55 However, utilizing an alkyne linker with a phenyl ring connecting the fulgimide switch led to a decrease in the switching efficiency compared to fulgimide immobilized via the amide linker system. Decoupling the switch from the phenyl-alkyne linker system with the -CH2 -spacer unit (2-IF-Sp-Al) results in switching efficiencies as measured for the fulgimide-amide-system (3-IF-Am). Implication and prospect: Merging together the obtained results we can conclude that changing the linker system from the amide (Am) to the alkyne (Al) linker led to an increase of the NLO contrast. The 3-IF-Al possessed the highest value. But the change in the linker system induced a thermal instability, i.e., both the 3-IF-Al and the 2-IF-Al undergo a thermally-driven ring-opening reaction. In addition, the cross section for the reversible photoinduced ring-open/closure reactions in 3-IF-Al and the 2-IF-Al are much smaller than in 3-IF-Am. The introduction of the -CH2 -spacer group, which electronically decouples the fulgimide switch from the phenyl-alkyne linker system caused a recovery of the thermal stability in 2-IF-Sp-Al. Moreover, the cross sections for the reversible photoisomerization increase and exhibit similar values as obtained for the 3-IF-Am. Thus, the utilization of an alkyne linker functionalized with an fulgimide via a Sonogashira reaction is only reasonable for systems in which the fulgimide is decoupled from the phenyl-alkyne linker system using for instance a –CH2 -group. Based on our results we may propose that a 3-indolyfulgimide with a spacer group using an alkyne linker on Si(111) would provide the best performance, viz. thermal stability, high cross sections, and a high NLO contrast. With the goal to “tailor” molecules with high NLO contrasts, we performed predictive, quantum chemical calculations of the type described above, by theoretically modifying the 3-IF-Al species which so far gave the highest NLO contrast between C and E/Z forms experimentally. In a first so far hypothetical species which we call “3-IF-Al-A”, we removed

19



Figure 6: Structures and “hyperpolarizability vectors” β = (βx , βy , βz ) (arrows) of the three isomers of two modified systems 3-IF-Al-A (a) and 3-IF-Al-B (b) of the 3-IF-Al fulgimide. For 3-IF-Al-A we removed the phenyl ring from the linker, while for 3-IF-Al-B we replaced the peripheral phenyl by a naphtyl unit. The structures were optimized at the B3LYP/631G∗ level of theory. The coordinate system was chosen as mentioned in Fig. 3. the phenyl ring at the alkyl linker of 3-IF-Al (see Fig. 6a). In a second hypothetical molecule, “3-IF-Al-B”, we replaced the terminal phenyl ring of the molecule by a naphtyl group (see Fig. 6b). Both modified molecules are shown in Fig. 6, in their optimized C, E and Z forms. Analyzing now the three NLO measures as introduced above theoretically, the following is found: For 3-IF-Al-A the quantities β0 , βHRS , βEFISHG (see Tab. S4 in the ESI) are very much comparable to those of 3-IF-Al. Thus removing the phenyl group in the linker unit seems to have only a minor influence on NLO properties and accordingly on NLO contrast. The reasons behind are the decoupling of the linker and the indolylfulgimid moiety, and also the fact that the linker units (with and without phenyl) are not much different for the closed and open forms. In contrast, replacing the terminal phenyl group of the indolylfulgimid moiety 20


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by naphtyl leading to 3-IF-Al-B, all three NLO measures β0 , βHRS , and βEFISHG increase substantially for the closed form C (by a factor of two to three). Since the magnitude of these quantities changes only much more moderately for the E/Z forms (see Table S4 in the ESI), also the NLO contrast is expected to increase considerably for 3-IF-Al-B compared to all other fulgimides studied so far. For 3-IF-Al-B, the NLO contrast changes more because of the extended peripheral π-system, which also changes orientation during the switching process. Hence, extending the peripheral π-system, could be a promising route towards high-contrast materials in the future. However, one must also be aware of the possibility that an extended π-system could increase the steric hindrance, which in fact could reduce the switching probability.

Conclusion In summary, we have studied the switching ability and the nonlinear optical (NLO) contrast between different switching states of three unequal fulgimide-functionalized self-assembled monolayers (SAMs) on Si(111) using second harmonic generation (SHG), density functional theory and wave function method calculations. To achieve high NLO contrasts controlled variations of the linker system and the fulgimides have been conducted. For all fulgimidefunctionalized interfaces a photoinduced reversible ring-opening/closure reaction has been demonstrated via the NLO response. Thereby, the closed form of the fulgimide exhibited a higher SHG signal amplitude compared to the open form, due to the extended π-electron system of the closed state. The NLO contrast measured in terms of the SHG signal change between the two photostationary states of open and closed form is for all systems above 20%, for one particular case a NLO contrast of 32% has been detected. In addition, by chemical modifications high cross sections for the light induced reversible switching process in the order of 10−18 cm2 have been observed, which are similar to the values known for fulgimides in solution. Thus, fulgimide-functionalized SAMs formation on Si(111) has facilitated a very

21



efficient reversible photoisomerization. Based on our results further chemical modifications of both the linker system and the molecular switch may even increase the NLO contrast between the open and closed fulgimide. The theoretical prediction of a system with a switchable, extended peripheral π-system suggests that in particular the switching unit is a promising target for improving NLO contrasts. Various measures for the NLO signal and contrast have been tested, most of them based on hyperpolarizabilties but also simple absorption spectra seem to be useful, approximate NLO indicators. In general, the generation of twodimensional NLO switching materials using functionalized photoswitchable silicon interfaces may pave the way for applications in photonic and optoelectronic devices.

Electronic supplementary information Additional information about the fulgimides and the on-surface synthesis, spectroscopic data of the fulgimides, SHG experimental results and the quantum chemical calculations.

Acknowledgments Funding by the German Research Foundation (DFG) through the collaborative research centers SFB 658 ”Elementary processes in molecular switches at surfaces” (projects B6 and C2) and SFB 1249 ”N-Heteropolycylcles as functional materials” (project B06) are gratefully acknowledged. M.H. acknowledges financial support from the Heidelberg Graduate School of Fundamental Physics. P.S. and C.R. acknowledge support by the Cluster of Excellence 304 “Unifying Concepts in Catalysis”, coordinated by the Technical University of Berlin.

References (1) Bouas-Laurent, H.; D¨ urr, H. Organic Photochromism. Pure Appl. Chem. 2001, 73, 639–665. 22


Page 22 of 30

Page 23 of 30 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


(2) Browne, W. R.; Feringa, B. L. Molecular Switches; Wiley-VCH Verlag, New York, 2011. (3) Coe, B. J. Molecular materials prossessing switachable quadratic nonlinear optical properties. Chem. Eur. J. 1999, 5, 2464–2471. (4) Delaire, J. A.; Nakatani, K. Linear and nonlinear optical prperties of photochromic molecules and materials. Chem. Rev. 2000, 100, 1817–1845. (5) Schulze, M.; Utecht, M.; Hebert, A.; R¨ uck-Braun, K.; Saalfrank, P.; Tegeder, P. Reversible Photoswitching of the Interfacial Nonlinear Optical Response. J. Phys. Chem. Lett. 2015, 6, 505–509. (6) Schulze, M.; Utecht, M.; Moldt, T.; Przyrembel, D.; Gahl, C.; Weinelt, M.; Saalfrank, P.; Tegeder, P. Nonlinear optical response of photochromic azobenzenefunctionalized self-assembled monolayers. Phys. Chem. Chem. Phys. 2015, 17, 18079– 18086. (7) Parthenopoulos, D. A.; Rentzepis, P. M. Three-dimensional optical storage memory. Science 1989, 245, 843–845. (8) Marder, S. R.; Kippelen, B.; Jen, A. K.-Y.; Peyghambarian, N. Design and synthesis of chromophores and polymers for electro-optic and photorefractive applications. Nature 1997, 388, 845–851. (9) Dalton, L. R.; Steier, W. H.; Robinson, B. H.; Zhang, C.; Ren, A.; Garner, S.; Chen, A.; Londergan, T.; Irwin, L.; Carlson, B.; et al., From molecules to opto-chips: organic electro-optic materials. J. Mater. Chem. 1999, 9, 1905–1920. (10) Samyn, C.; Verbiest, T.; Persoons, A. Second-order non-linear optical polymers. Macromol. Rapid Commun. 2000, 21, 1–15. (11) Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. A novel non-destructible readout molecular memory. Opt. Commun. 2003, 223, 61–66. 23



(12) Walker, E.; Rentzepis, P. M. Two-photon technology-a new dimension. Nature Photonics 2008, 2, 406–408. (13) Gust, D.; Andréasson, J.; Pischel, U.; Moore, T. A.; Moore, A. L. Data and signal processing using photochromic molecules. Chem. Commun. 2012, 48, 1947–1957. (14) Champagne, B.; Plaquet, A.; Pozzo, J.-L.; Rodriguez, V.; Castet, F. Nonlinear optical molecular switches as selective cation sensor. J. Am. Chem. Soc. 2012, 134, 8101–8103. (15) 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. (16) Parola, S.; Julián-López, B.; Carlos, L. D.; Sanchez, C. Optical Properties of Hybrid Organic-Inorganic Materials and their Applications. Adv. Funct. Mater. 2016, 26, 6506–6544. (17) 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. (18) Boubekeur-Lecaque, L.; Coe, B. J.; Clays, K.; Foerier, S.; Verbiest, T.; Asselberghs, I. Redox-switching of nonlinear optical behavior in Langmuir-Blodgett thin films containing a ruthenium(II) ammine complex. J. Am. Chem. Soc. 2008, 130, 3286–3287. (19) Priimagi, A.; Ogawa, K.; Virkki, M.; Mamiya, J.; Kauranen, M.; Shishido, A. Highcontrast photoswitching of nonlinear optical response in crosslinked ferroelectric liquidcrystalline polymers. Adv. Mater. 2012, 24, 6410–6415. (20) Xing, X.-S.; Sa, R.-J.; Li, P.-X.; Zhang, N.-N.; Zhou, Z.-Y.; Liu, B.-W.; Liu, J.; Wang, M.-S.; Guo, G.-C. Second-order nonlinear optical switching with a record-high 24


Page 24 of 30

Page 25 of 30 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


contrast for a photochromic and thermochromic bistable crystal. Chem. Sci. 2017, 8, 7751–7757. (21) Heller, H. G.; Koh, K.; Elliot, C.; Whittall, J. Fulgides and Fulgimides for Practical Applications. Mol. Cryst. Liq. Cryst. 1994, 246, 79–86. (22) Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. Synthesis and photochemistry of photochromic fluorescing indol-2-ylfulgimides. J. Mater. Chem. 2000, 10, 2477–2482. (23) Dirk, C. W.; Twieg, R. J.; Wagniére, G. The Contribution of π Electrons to Second Harmonic Generation in Organic Molecules. J. Am. Chem. Soc. 1986, 108, 5387–5395. (24) Shen, Y. R. Surface studies by optical second harmonic generation: An overview. J. Vac. Sci. Technol. B 1985, 3, 1464–1466. (25) Guyot-Sionnest, P.; Chen, W.; Shen, Y. R. General considerations on optical secondharmonic generation from surfaces and interfaces. Phys. Rev. B 1986, 33, 8254–8263. (26) Shen, Y. R. Surface properties probed by second -harmonic and sum-frequency generation. Nature 1989, 337, 519–525. (27) Shen, Y. R. Surface probed by nonlinear optics. Surf. Sci. 1993, 299/300, 551–562. (28) Takeuchi, N.; Kanai, Y.; Selloni, A. Surface Reaction of Alkynes and Alkenes with H-Si(111): A Density Functional Theory Study. J. Am. Chem. Soc. 2004, 126, 15890– 15896. (29) Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. Synthesis of novel photochromic fluorescing 2-indolylfulgimides. Tetrahedron Lett. 1999, 40, 8067–8069. (30) Liang, Y.; Dvornikov, A.; Rentzepis, P. M. Solvent and ring substitution effect on the photochromic behavior of fluorescent 2-indolylfulgide derivatives. J. Photochem. Photobiol. A: Chem. 1999, 125, 79–84. 25



(31) Weber, C. Real-time studies of coupled molecular switches in photoresponsive materials. Ph.D. thesis, Humbolth Universität zu Berlin, 2015. (32) Schulze, M.; Hänsel, M.; Tegeder, P. Hot Excitons Increase the Donor/Acceptor Charge Transfer Yield. J. Phys. Chem. C 2014, 118, 28527–28534. (33) Hänsel, M.; Belova, V.; Hinderhofer, A.; Schreiber, F.; Broch, K.; Tegeder, P. Ultrafast Excited State Dynamics in Diindenoperylene Films. J. Phys. Chem. C 2017, 121, 17900–17906. (34) Utecht, M. M. Zur Optimierung und dem Auslesen molekularer Schalter: Quantenmechanische Untersuchungen an vier Beispielen. Ph.D. thesis, Universität Potsdam, 2015. (35) Plaquet, A.; Guillaume, M.; Champagne, B.; Castet, F.; Ducasse, L.; Pozzo, J.-L.; Rodriguez, V. In silico optimization of merocyanine-spiropyran compounds as secondorder nonlinear optical molecular switches. Phys. Chem. Chem. Phys. 2008, 10, 6223– 6232. (36) Suponitsky, K. Y.; Tafur, S.; Masunov, A. E. Applicability of hybrid density functional theory methods to calculation of molecular hyperpolarizability. J. Chem. Phys. 2008, 129, 044109. (37) Tonnelé, C.; Pielak, K.; Deviers, J.; Muccioli, L.; Champagne, B.; Castet, F. Nonlinear optical responses of self-assembled monolayers functionalized with indolinooxazolidine photoswitches. Phys. Chem. Chem. Phys. 2018, 20, 21590–21597. (38) Song, P.; Gao, A.-H.; Zhou, P.-W.; Chu, T.-S. Theoretical study on photoisomerization effect with a reversible nonlinear optical switch for dithiazolylarylene. J. Phys. Chem. A 2012, 116, 5392–5397.

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(39) Scheres, L.; Giesbers, M.; Zuilhof, H. Self-Assembly of Organic Monolayers onto Hydrogen-Terminated Silicon: 1-Alkynes Are Better Than 1-Alkenes. Langmuir 2010, 26, 10924–10929. (40) 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. (41) Ségerie, A.; Castet, F.; Kanoun, M. B.; Plaquet, A.; Liégeois, V.; Champagne, B. Nonlinear Optical Switching Behavior in the Solid State: A Theoretical Investigation on Anils. Chem. Mater. 2011, 23, 3993–4001. (42) Nelson, C. A.; Luo, J.; Jen, A.; Laghumavarapu, R. B.; Huffaker, D.; Zhu, X.-Y. Time, energy, and phase resolved second harmonic generation at semiconductor interfaces. J. Phys. Chem. C 2014, 118, 27981–27988. (43) Wu, X.; Park, H.; Zhu, X.-Y. Probing Transient Electric Fields in Photo-excited Organic Semiconductor Thin Films and Interfaces by Time-resolved Second Harmonic Generation. J. Phys. Chem. C 2014, 118, 10670–10676. (44) Nénon, S.; Champagne, B. SCC-DFTB calculation of the static first hyperpolarizability: From gas phase molecules to functionalized surfaces. J. Chem. Phys. 2013, 138, 204107. (45) Tegeder, P. Optically and thermally induced molecular switching processes at metal surfaces. J. Phys.: Condens. Matter 2012, 24, 394001. (46) Wagner, S.; Leyssner, F.; Kördel, C.; Zarwell, S.; Schmidt, R.; Weinelt, M.; R¨ uckBraun, K.; Wolf, M.; Tegeder, P. Reversible photoisomerization of an azobenzenefunctionalized self-assembled monolayer probed by sum-frequency generation vibrational spectroscopy. Phys. Chem. Chem. Phys. 2009, 11, 6242–6248.

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(47) Jung, U.; Sch¨ utt, C.; Filinova, O.; Kubitschke, J.; Herges, R.; Magnussen, O. Photoswitching of azobenzene-functionalized molecular platforms on Au. J. Phys. Chem. C 2012, 116, 25943–25948. (48) Moldt, T.; Przyrembel, D.; Schulze, M.; Bronsch, W.; Boie, L.; Brete, D.; Gahl, C.; Klajn, R.; Tegeder, P.; Weinelt, M. Differing Isomerization Kinetics of AzobenzeneFunctionalized Self-Assembled Monolayers in Ambient Air and in Vacuum. Langmuir 2016, 32, 10795–10801. (49) Hagen, S.; Kate, P.; Leyssner, F.; Nandi, D.; Wolf, M.; Tegeder, P. Excitation Mechanism in the Photoisomerization of a Surface-bound Azobenzene Derivative: Role of the Metallic Substrate. J. Chem. Phys. 2008, 129, 164102/1–164102/8. (50) Hagen, S.; Kate, P.; Peters, M. V.; Hecht, S.; Wolf, M.; Tegeder, P. Kinetic analysis of the photochemically and thermally induced isomerization of an azobenzene derivative on Au(111) probed by two-photon photoemission. Appl. Phys. A 2008, 93, 253–260. (51) Bronner, C.; Schulze, G.; Franke, K. J.; Pascual, J. I.; Tegeder, P. Switching ability of nitro-spiropyran on Au(111): electronic structure changes as a sensitive probe during a ring-opening reaction. J. Phys.: Condens. Matter 2011, 23, 484005. (52) Schulze, G.; Franke, K. J.; Pascual, J. I. Induction of a photostationary ring-opening– ring-closing state of spiropyran monolayers on the semimetallic Bi(110) Surface. Phys. Rev. Lett. 2012, 109, 026102. (53) Bronner, C.; Priewisch, B.; R¨ uck-Braun, K.; Tegeder, P. Photoisomerization of an Azobenzene on the Bi(111) Surface. J. Phys. Chem. C 2013, 117, 27031–27038. (54) Bronner, C.; Tegeder, P. Photo-induced and thermal reactions in thin films of an azobenzene derivative on Bi(111). New J. Phys. 2014, 16, 053004.

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(55) Nickel, F.; Bernien, M.; Kraffert, K.; Kr¨ uger, D.; Arruda, L. M.; Kipgen, L.; Kuch, W. Reversible Switching of Spiropyran Molecules in Direct Contact With a Bi(111) Single Crystal Surface. Adv. Funct. Mater. 2017, 27, 1702280.

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Nonlinear optical

w (Laser pulse) 2w(SHG)

contrast

w

2w

fulgimides hn1 hn2 Si(111)

Si(111)

TOC Graphic

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2-Dimensional Nonlinear Optical Switching Materials: Molecular

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