Modulating the Photoisomerization Mechanism of ... - ACS Publications

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

C: Physical Processes in Nanomaterials and Nanostructures

Modulating the Photoisomerization Mechanism of Semiconductor-Bound Azobenzene Functionalized Compounds Luis G.C. Rego, and Graziele Bortolini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11057 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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 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 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.

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 22 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

Modulating the Photoisomerization Mechanism of Semiconductor-Bound Azobenzene Functionalized Compounds Luis G. C. Rego∗ and Graziele Bortolini Department of Physics, Universidade Federal de Santa Catarina, SC, 88040-900, Brazil E-mail: [email protected]

∗

To whom correspondence should be addressed

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 The competing interplay of charge transfer and structural relaxation in the photoisomerization mechanism of azobenzene-functionalized semiconductor complexes is revealed by nonadiabatic excited-state molecular dynamics simulations. It is shown that fundamentally different structural dynamics occur in azo-compounds due to electronic charge transfer. If charge transfer occurs first the photoinduced isomerization mechanism is quenched and the intramolecular relaxation occurs mainly through vibration excitations. To demonstrate the effect, azobenzene/π-bridge/TiO2 semiconductor structures were engineered to have very different interfacial electron transfer (IET) rates, albeit having similar molecular structures. This was accomplished with π-bridges constituted of biphenyl and 2,6,2’,6’-tetramethyl-biphenyl moieties, which exhibit very different electric conductances due to the twist angle of the comprising aromatic rings. It is demonstrated that ultrafast IET quenches the trans→cis photoisomerization in azo-compounds bound to semiconductor surfaces, due to the dissociation of the electron-hole pair excitation. The present study aims at enlightening the role of the electronic dynamics in the photoisomerization mechanism of azo-compounds coupled to the environment, highlighting the importance of molecular tailoring for the design of photo-responsive materials.

1

Introduction

Photochromic, photoelastic and photoresistive molecules are constitutive building blocks for functionalized materials that can be engineered at the molecular level to operate in the microscopic and macroscopic domains. 1–4 Amongst the most versatile photo-responsive molecular structures, azobenzene-based compounds have raised a great deal of attention due to their relevance for fundamental science 5–10 and technological applications. 2,11,12 Despite the wide applicability of azobenzene and its reliability as a photochromic compound, the excited state dynamics of azobenzene remains a subject of debate, as indicated by recent

2


Page 2 of 22

Page 3 of 22 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


studies. 11,13–16 To study such systems, static model calculations have been important for providing understanding of the underlying energetics and to calculate rates of processes. However, as the complexity of such structures increase, the relaxation processes in which electronic and structural dynamics become strongly coupled require an inherently dynamical description. Herein we demonstrate by means of nonadiabatic excited-state molecular dynamics simulations the occurrence of such an effect, an electronic-structural race condition, described schematically in Figure 1, that is the suppression of photoinduced isomerization in azobenzene-based dye-semiconductor complexes caused by ultrafast electron transfer.

Figure 1: Scheme of the excited-state relaxation dynamics in azobenzene dye-semiconductor complexes, with two competing relaxation mechanisms: the interfacial electron transfer from the azo-compound to the TiO2 and electronic relaxation in the TiO2 conduction band (blue arrows), which quenches trans→cis photoisomerization and the intramolecular relaxation (purple arrows) that activates photoisomerization in the azo-dye. The vertical red arrow describes the photoexcitation of the system. It was recently reported 16 that the structural dynamics of the photoexcited push-pull azo-compound para-Methyl Red (compound 1 in Figure 2) differs fundamentally whether it is isolated or forming a charge transfer (CT) complex with the TiO2 semiconductor (CT-1 in Figure 2). The p-methyl-red belongs to a class of azobenzene compounds designated pushpull azobenzene systems, in which electron donating and withdrawing groups are substituted at the 4 and 4’ (para) positions of the two azo rings. The substitution of electron donating 3



and withdrawing groups at opposite ends of the parent molecule breaks the symmetry of the aromatic system. There are several time-resolved femtosecond studies of related push-pull azobenzene compounds in solution, 17–20 which describe the similarities of the trans-to-cis photoisomerization with the parent azobenzene. They describe that the initial relaxation (internal conversion) takes a few hundreds of femtoseconds and the photoinduced isomerization is detected in less than a picosecond. They report furthermore a strong influence of the solvent on the isomerization dynamics for such substituted azobenzenes. It was argued, 16 in regard to CT-1, that the photoisomerization of the CT azo-compound is quenched due to the ultrafast electron transfer (ET) between the azobenzene moiety and the TiO2 semiconductor. Instead of the photoinduced trans→cis isomerization usually observed for isolated molecular azobenzene, the structural relaxation of the cationic CT-1 compound occurs mainly by vibronic excitations of the -N=N- bond. In addition, previous studies have examined the suppression of the trans→cis photoisomerization in densely packed self-assembled monolayers (SAM) of azo-compounds due to intermolecular steric hindrance. 5,6 Gahl et al. 7 reported, furthermore, that intermolecular excitonic coupling among azobenzene chromophores can also quench trans→cis photoisomerization in densely packed SAMs. Contrary reports, 8 nonetheless, about collective photoisomerization in SAMs fill the debate. The role of the molecule-surface coupling has also been investigated. For instance, photoisomerization of azobenzene molecules suspended from Au(111) surfaces by tert-butyl (C4 H9 ) ligands was investigated for various strengths of the molecule-surface coupling, 9,10 revealing that photoisomerization occurs only when the molecule is lifted off the surface. However, the actual underlying mechanism remains unclear. The present study aims at shedding light on issues concerning electronic coupling of molecular photo-switches with the environment.

4


Page 4 of 22

Page 5 of 22 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

Model System

To investigate the interplay of charge transfer and photoisomerization dynamics in azobenzene charge transfer complexes we designed three azo-dyes, shown in Figure 2. They are comprised of a (dimethylamino)azobenzene moiety, a molecular spacer and a carboxyl end group that functions as the anchor of the dye. For the sake of comparison, system 1 corresponds to the original para-Methyl Red without the spacer. System 2 contains a biphenyl π-conjugated bridge as spacer and System 3 has a 2,6,2’,6’-tetramethyl-biphenyl bridge. Venkataraman et al. 21 used an STM (scanning tunneling microscope) to measure the electric conductance of various molecular bridges between gold electrodes, they observed that the electric conductance of the bridge decreases with the twist angle of the comprising aromatic rings, which decreases the π-conjugation in the bridge. In particular, the electric conductance measured along the biphenyl bridge was 1.1×10−3 G0 whereas for the 2,6,2’,6’-tetramethyl-biphenyl bridge it was 14.5 times smaller (G0 = 2e2 /h is the quantum of electric conductance, e is the charge of the electron and h is Planck’s constant). Therefore, the aforementioned bridges were chosen as spacers in model systems CT-2 and CT-3, respectively, due to their disparate electric conductances albeit similar molecular structures. Jia et al. 1 used methylene spacers likewise to isolate a molecular photo-switch from graphene nano-contacts. Geometry optimization was performed on azo-systems 1, 2 and 3 by density-functional theory (DFT) and molecular mechanics (MM) methods to obtain the torsional angles ϕ1 and ϕ2 between the aromatic rings of the bridge, as shown in Figure 2. DFT calculations using the B3LYP exchange-correlation functional and the 6-31G(d) basis set yielded ϕ1 = ϕ2 = 35° for system 2 and ϕ1 = 36.7° and ϕ2 = 90.4° for system 3. Geometry optimization performed with the MM method yielded ϕ1 = 32.3° and ϕ2 = 32.7° for system 2 and ϕ1 = 33.5° and ϕ2 = 90.1° for system 3. These results are in good agreement with calculations performed elsewhere. 21 Details concerning the MM force field are provided in the Supporting Information. Subsequently, we created charge transfer model systems {CT-1, CT-2, CT-3} shown in Figure 2. A TiO2 anatase cluster of area 20.46 Å × 15.12 Å and thickness ∼ 13 5



Figure 2: Chemical structure of azo-compounds: 1) para-Methyl Red molecule (parent system), 2) parent molecule with a biphenyl spacer, 3) parent molecule with a 2,6,2’,6’tetramethyl-biphenyl spacer. ϕ1 and ϕ2 designate torsional angles between aromatic rings. Bottom of the Figure: charge transfer model systems {CT-1, CT-2, CT-3} formed by azodyes 1, 2 and 3 adsorbed on the (101) surface of the TiO2 anatase. Å, which corresponds to 4 atomic planes, is used to simulate the semiconductor substrate, with periodic boundary conditions applied along the (101) plane. The nuclei comprising the molecular azo-compounds and 30 nuclei of the TiO2 cluster nearby the anchoring site are allowed to evolve by molecular mechanics; such atoms are shown in bold representation in 6


Page 6 of 22

Page 7 of 22 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


Figure 2. The remaining nuclei of the TiO2 cluster are kept constrained during the classical and excited-state nonadiabatic dynamics. However, for simulating the charge transfer effect, the electronic degrees of freedom of all atoms of the CT model systems, free and constrained, are treated as quantum mechanical. The azo-compounds are bound to the (101) surface of the TiO2 anatase through the carboxyl group in a bidentate bridge geometry.

3 3.1

Theory Nonadiabatic Quantum-Classical Dynamics Method

In the following we summarize the theoretical method, which has been described in detail elsewhere. 15,16,22 The time-dependent Schrödinger equation (TDSE) is solved for the electronic degrees of freedom

iℏ

∂ ˆ el (Rt ) |Ψ(r; Rt , t)⟩ , |Ψ(r; Rt , t)⟩ = H ∂t

(1)

where r designates the electronic coordinates, Rt ≡ R(t) are the time-dependent nuclear ˆ el (Rt ) is the time-dependent extended Hückel hamiltonian operator. The coordinates and H nuclei are described by the molecular mechanics method, with the following force field (FF)

MM VGS ({R}) =

∑

Kb (R − R0 )2 +

bonds

+

∑ i,j̸=i

[( 4εij

σij Rij

)12

∑

Kθ (θ − θ0 )2 +

angles

(

−

σij Rij

∑ ∑ torsions

)6 ] +

∑

Cn (cos ϕ)n

n

qj qi , 4πϵ 0 Rij i,j̸=i

(2)

where R is the atomic position and Rij = |Ri − Rj | is the distance between atoms i and j, which have fixed partial charges qi and qj . The dynamics of the nuclei is given by the

7



Page 8 of 22

classical equations of motion

·

R = P/M ,

(3)

P = −∇R ⟨Ψ(r; t)|V (r, R)|Ψ(r; t)⟩r ,

(4)

·

where the forces acting upon the nuclei, positioned at R, are calculated via the electron-nuclei coupling potential V (r, R) averaged over the electron-hole wavefunction. The single-particle electron-hole excitation is written as the spinor state Ψ = {Ψel , Ψhl }, whose components are the wavepackets that describe the excited electron, Ψel (t), and the hole, Ψhl (t). The method assumes the following approximation [ ] MM ⟨Ψ(r; R, t)|V (r, R)|Ψ(r; R, t)⟩r ≈ VGS (R) + VEH Ψel (R, t), Ψhl (R, t) ,

(5)

by means of which the excited-state interatomic potential is separated as the contribution MM from the ground state force field potential, VGS , plus a correction that is due to the electron[ el hl ] hole excitation, VEH Ψ , Ψ . The second term is responsible for the coupling between

quantum and classical degrees of freedom in the excited state. It is calculated on-the-fly as [ ] [ ] VEH Ψel (R, t), Ψhl (R, t) = T r ρEH (R, t)H(Rt )

(6)

where ρEH = |Ψel ⟩⟨Ψel | − |Ψhl ⟩⟨Ψhl | is the electron-hole density matrix. The force produced on atom N by the e-h excitation is, therefore, given by [ ] FN = −∇RN VEH Ψel (R, t), Ψhl (R, t) .

(7)

The total energy of the excited state is comprised of the classical energy of the nuclei (kinetic plus potential), as given by the molecular mechanics formalism, and the quantum [ ] energy of the excited electron-hole pair EQM = T r ρEH H .

8


Page 9 of 22 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


We work solely with localized atomic orbitals of the Slater-type, ⟨r|α(t)⟩ = fa (r−RA (t)), ∑ whereby the electron-hole wavepackets are written as |Ψ(t)⟩ = α cα (t)|α(t)⟩. The numerical approach for solving the time-dependent Schrödinger equation is to divide the total evolution ∏ operator into short segments – time slices δt – then apply the scheme U(t) ≈ j U (j) (δτ ), where the wavefunction propagation within the time-slice δt is obtained by the simple recursive series |Ψ(t + δt)⟩ ≈ |Ψ(t)⟩ +

) K ( ∑ −iδt k=1

ℏk

H(j) |Ψk−1 ⟩ ,

(8)

with H(j) ≡ H(jδt), Ψ0 ≡ Ψ(t) and Ψk = −iδt/(ℏk)H(j) Ψk−1 . Within the present diabatic representation we obtain a compact expression for the force produced by the electron-hole excitation that is very efficient for numerical computation

FAB = −2

∑∑

⟨α|∇β⟩ {χ ◦ ρR + H′ ρR }βα ,

(9)

α∈A β∈B

where (A ◦ B)βα = Aβα Bβα designates the Hadamard product and ρR ≡ ρAO . We also R make explicit use of the extended Hückel hamiltonian, written as Hαβ = χαβ Sαβ , with χαβ ≡ Kαβ (hα + hβ )/2, and Sαβ = ⟨α|β⟩ designates the overlap matrix. The current method has been applied to study the trans→cis and cis→trans photoisomerization in azobenzene and stilbene molecules in gas phase, as well as vibronic internal conversion of highly excited aromatic compounds, wherein results have been compared with first-principle quantum chemistry calculations. 15 As Supporting Information, Figure S9 presents a comparison between the self-consistent Molecular Mechanics/Extended Hückel Ehrenfest method and the complete active space self-consistent field (CASSCF) method, namely between energies of molecular orbitals calculated on-the-fly during the trans→cis S2 (π-π ∗ ) photoisomerization of azobenzene with CASSCF(14/12)/6-31G∗ total energy calculations performed by Oliboni et al. for the geometries obtained from the semiempirical NA-MD simulations. The comparison shows a strong qualitative agreement, particularly concerning the position of the conical intersection, and a good accord among the energy lev9



els calculated with both methods. Furthermore, in regard to present results, a detailed study about the photoisomerization of the para-Methyl-Red in gas phase has been performed, 16 where results were compared to time-resolved femtosecond spectroscopy studies carried out for similar push-pull azo-compounds.

3.2

System Preparation

The charge transfer model systems {CT-1, CT-2, CT-3} consist of the molecular azocompounds {1, 2, 3} bound to the (101) surface of TiO2 anatase by the carboxyl group, in a bidentate bridge geometry. All the nuclei comprising the molecular azo-compounds and 30 nuclei nearby the anchoring site at the TiO2 cluster are allowed to evolve by molecular mechanics. Such atoms are shown in bold representation in Figure 2. The remaining nuclei of the TiO2 cluster are constrained during classical and excited-state nonadiabatic dynamics. For simulating the interfacial electron transfer, the electronic degrees of freedom of all the atoms of the CT model systems, free and constrained, are treated as quantum mechanical. Before carrying out excited state dynamics simulations, the model systems were thermalized. We first performed molecular mechanics simulations in the ground state keeping the system in contact with a Berendsen thermostat at 300K until the molecular structure of the CT model system was stabilized. Then, additional molecular mechanics simulations were performed with the system in contact with a Nosé-Hoover thermostat at 300K. Sample structures were collected at intervals of 10 ps to serve as initial conditions for the excited state dynamics simulations. During the thermalization process, molecular mechanics simulations were performed with a time step δt = 0.1 fs, for CT-1 and CT-2 systems, and δt = 0.07 fs for the bigger CT-3 system. A cut-off radius for electrostatic interactions of 50 Å was used. After thermalization, the photoexcited system was prepared by setting the electron in the π ∗ -orbital of the azo-compound and the hole in the n-orbital of the azo-compound, including the carboxyl group. The coupled quantum/classical method for dynamics of electrons in molecules (DynEMol) was then used for nonadiabatic self-consistent quantum-classical 10


Page 10 of 22

Page 11 of 22 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


simulations with a time step δt = 0.02 fs for both electrons (QM) and nuclei (MM). Azobenzene has one weak electronic resonance in the visible at 440 nm which is attributed to an n-π ∗ excitation and a stronger π-π ∗ transition in the UV around 310 nm. For push-pull azo-compounds, the substitution of electron donating and withdrawing groups at opposite ends of the parent molecule breaks the symmetry of the aromatic system and shifts its absorption spectra to the visible. Differently from the parent azobenzene, in the case of dimethylaminoazobenzenes (DA) 19,23,24 and particularly the para-Methyl-Red, 25–27 the S1 (π, π ∗ ) becomes the lowest optical excitation, followed closely by the S2 (n, π ∗ ). For such systems these transitions nearly coincide and overlap with a charge transfer (CT) transition absent in azobenzene. 23,24 It is observed that such mixture of states becomes more pronounced due to structural distortions promoted by thermal motion. In Supporting Information, a detailed comparison shows the agreement between electronic structure calculations performed with ab-initio, at the DFT/B3LYP/6-31G(d) level of theory, and the parametrized Extended Hückel method.

4

Results and Discussion

The photoinduced electronic-structural coupled relaxation dynamics of the CT systems is hereafter studied with a self-consistent quantum-classical method that incorporates nonadibatic excited-state electronic quantum dynamics into molecular mechanics. 15,16,22 Details concerning the system preparation are provided as Supporting Information. Previously, the CT systems were thermalized in the ground state at 300 K. After thermalization, the photoexcited system was prepared by setting an electron in the π ∗ -orbital and a hole in the n-orbital of the molecular azo-compounds, which includes the carboxyl anchor group. Figure 3 presents the main features of the photoinduced electronic-structural relaxation dynamics: a) the survival probability of the electron in the dye, Pdye , b) the NN bond length and c) the CNNC dihedral angle as a function of time, for model system CT-1. The

11



Figure 3: The main features of the photoinduced electronic-structural relaxation in model system CT-1 as a function of time, for 4 independent simulation runs:w: a) electron survival probability (Pdye ) in the para-Methyl-Red, showing ultrafast electron transfer; b) NN bond length (in angstrom, Å), revealing a strong excitation of the NN bond stretching mode; c) CNNC dihedral angle (in radian), which does not evince signs of isomerization. simulations show that photoexcitation of the CT-1 complex triggers an ultrafast electron transfer from the para-Methyl Red dye to the TiO2 cluster that oxidizes the azo-compound with a time constant τ ≈ 5 fs. 12


Page 12 of 22

Page 13 of 22 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


In a recent publication, 16 a molecular mechanism for the trans→cis excited-state isomerization of azo-compounds has been proposed whereby the adiabatic force F(n, π ∗ ), produced by the (n,π ∗ ) electron-hole pair excitation, gives rise to a torque on the NN bridge of the azo-group that drives azo-compounds toward the conical intersection, possibly inducing the trans→cis isomerization. Such isomerization mechanism relies on the presence of the electron-hole excitation in the azobenzene moiety. However, the hole excitation alone in the n-orbital of the cationic azo-compound produces a force F(n) along the NN bond, which strongly excites the NN stretching mode, thereby suppressing the isomerization process. Such photoinduced structural relaxation is evinced in Figures 3-b) and c) for the CT-1 complex, for 4 independent simulation runs. The following results aim at providing support to the proposed mechanism by means of the engineered charge transfer model systems CT-2 and CT-3, which have very different interfacial electron transfer (IET) rates, albeit having similar molecular structures. Behavior similar to CT-1 is observed for model system CT-2, whose dye incorporates the biphenyl π-conjugated bridge between the azobenzene moiety and the TiO2 cluster. Figure 4 presents two dynamics simulations obtained from independent initial conditions (left and right sides of the panel). Despite inclusion of the spacer, most of the photoexcited electron is transferred within 100 fs, which is faster than the characteristic time of the photoinduced trans→cis isomerization. The twist angle between phenyl rings (ϕ1 and ϕ2 ) of approximately 35° is not enough to disrupt the π-conjugation through the bridge. Thus, after the interfacial electron transfer the stretching mode of the NN bond is strongly excited due to the fast oxidation of the dye, for both simulations shown in Figure 4. The simulations indicate that the NN bond excitation effect is robust enough to be detected, for instance, by femtosecond stimulated Raman 28 or pump-degenerate four-wave mixing 29 spectroscopic methods. Morever, no evidence of isomerization is observed in the CNNC dihedral angle that dwells steadily around ϕCN N C = π, corroborating the relaxation mechanism of the CT-1 complex.

13



Figure 4: Main features of the photoinduced electronic-structural relaxation in model system CT-2 as a function of time, for two independent simulation runs: a) survival probability (Pdye ) of the electron in the azo-dye; b) NN bond length (in angstrom, Å), revealing a strong excitation of the NN bond stretching mode; c) CNNC dihedral angle (in radian), which does not evince signs of isomerization. The relaxation behavior changes qualitatively for model system CT-3, whose simulations results are presented in Figure 5 for three independent initial conditions. In this case we observe that the orthogonal orientation between the aromatic rings of the 2,6,2’,6’-tetramethylbiphenyl spacer effectively breaks the π-conjugation along the bridge, hindering the electron transfer from the azobenzene moiety to the TiO2 . Therefore, the electron survival probability, Pdye , remains above 0.5 during the entire relaxation dynamics. These results corroborate the electric conductance measurements that were performed for the spacer molecules in metallic nano-junctions. 21 Furthermore, concerning the aforementoined excited-state isomerization mechanism, due to the presence of the photoexcited (n,π ∗ ) electron-hole pair in the azobenzene moiety, the structural dynamics of the molecular dye changes qualitatively. As evidenced by the data in Figure 5, while the molecule is in the excited state there is a steady elongation of the NN bond, albeit without much change in the stretching mode 14


Page 14 of 22

Page 15 of 22 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


Figure 5: The main features of the photoinduced electronic-structural relaxation dynamics is model system CT-3 as a function of time, for three independent simulation runs, showing: a) the survival probability (Pdye ) of the electron in the azo-dye, b) the NN bond length (in angstrom, Å) and c) the CNNC dihedral angle (in radian). The inset shows two structures, which correspond to the initial and final frames of simulation, as indicated by the arrows. amplitude. As the azobenzene reaches the conical intersection, the NN bond length returns to its ground state value of ∼ 1.3 Å. This indicates the rupture of the -N=N- double bond during the excited-state dynamics and its restoration as the system decays to the ground

15



state S0 , regardless of the occurrence of isomerization. Notice that for isolated azobenzene 11 and push-pull azobenzene molecules, 17–20 in gas phase or solution, the trans→cis photoisomerization rate is around 20-30%. The simulations also show a much wider variation of the CNNC dihedral angle, resulting eventually in the trans→cis photoisomerization for two of the simulation cases. In fact the graphs of NN bond length and CNNC dihedral angle reveal the coupling between these degrees of freedom. For the sake of completeness we also show, at the corner of Figure 5, the structure of model system CT-3 at the beginning and at the end of a simulation, as indicated by the arrows. The present simulation results of {Pdye , NN bond, CNNC dihedral} evidence the overall qualitative difference between quantum nonadiabatic dynamics of systems CT-1,2 versus system CT-3. The goal here is not to determine photoisomerization yields, but rather to enlighten the effect of competing electron transfer and structural relaxation dynamics in azobenzene compounds, which has never been studied before by direct nonadiabatic dynamics.

5

Conclusions

The paper described by means of nonadiabatic excited-state molecular dynamics simulations the competing interplay of charge transfer and structural relaxation in the photoisomerization mechanism of azobenzene-functionalized compounds. It demonstrates that ultrafast electron transfer out of the azobenzene moiety quenches the trans→cis photoisomerization in azocompounds. Therefore, for the ensuing cationic azo-compound, the structural relaxation occurs mainly through vibronic excitations of the NN stretching mode. To corroborate the effect azobenzene/π-bridge/TiO2 semiconductor structures were engineered to have very different interfacial electron transfer (IET) rates, despite having similar molecular structures. This was accomplished with π-bridges constituted of biphenyl and 2,6,2’,6’-tetramethylbiphenyl moieties, which exhibit very different electric conductances due to the twist angle

16


Page 16 of 22

Page 17 of 22 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


between the comprising aromatic rings. The effect is ascribed to the dissociation of the (n,π ∗ ) electron-hole pair excitation, which is responsible for the adiabatic force F(n, π ∗ ) that gives rise to a torque on the NN bridge that drives the system toward the conical intersection responible for the trans→cis isomerization. 16 Due to the fast oxidation, the remaining hole excitation in the n-orbital of the cationic azo-compound produces a force F(n) that simply excites the NN stretching mode, thus suppressing the isomerization process. The above results evidence the pivotal role of the n-π ∗ excited state in the photoinduced isomerization of azo-compounds. The present study aims at enlightening the role of the electronic dynamics in the photoisomerization mechanism of azo-compounds bound to the environment, as well as highlighting the importance of molecular tailoring for the design of photo-responsive materials.

Supporting Information Available Includes: (1) classical force field parameterization; (2) parameterization of the quantum method; (3) comparison with quantum-chemistry methods; (4) additional data for electrostructural relaxation dynamics in complex CT-3.

This material is available free of charge

via the Internet at http://pubs.acs.org/.

Acknowledgement The authors acknowledge insightful discussions with Dr. A. Migliore. This study was financed by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Brasil (CAPES) Finance Code 001, by the Brazilian National Counsel of Technological and Scientific Development (CNPq) and the National Institute for Organic Electronics (INEO).

17



References (1) Jia, C. et al. Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity. Science 2016, 352, 1443–1445. (2) Browne, W. R.; Feringa, B. L. Making molecular machines work. Nat. Nanotechnol. 2006, 1, 25–35. (3) Gust, D.; Moore, T. A.; Moore, A. L. Mimicking Photosynthetic Solar Energy Transduction. Accounts Chem. Res. 2001, 34, 40–48. (4) Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; van Gondelle, R. Lessons from nature about solar light harvesting. Nat. Chem. 2011, 3, 763–774. (5) Titov, E.; Granucci, G.; Götze, J. P.; Persico, M.; Saalfrank, P. Dynamics of Azobenzene dimer photoisomerization: electronic and steric effects. J. Phys. Chem. Lett. 2016, 7, 3591–3596. (6) Cantatore, V.; Granucci, G.; Rousseau, G.; Padula, G.; Persico, M. Photoisomerization of self-assembled monolayers of Azobiphenyls: simulations highlight the role of packing and defects. J. Phys. Chem. Lett. 2016, 7, 4027–4031. (7) Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E. R.; Freyer, W.; Carley, R.; Reuter, K.; Weinelt, M. Structure and Excitonic Coupling in Self-Assembled Monolayers of Azobenzene-Functionalized Alkanethiols. J. Am. Chem. Soc. 2010, 132, 1831–1838. (8) Elbing, M.; Baszczyk, A.; von Hanisch, C.; Mayor, M.; Ferri, V.; Grave, C.; Rampi, M. A.; Pace, G.; Samori, P.; Shaporenko, A.; Zharnikov, M. Single Component Self-Assembled Monolayers of Aromatic Azo-Biphenyl: Influence of the Packing Tightness on the SAM Structure and Light-Induced Molecular Movements. Adv. Funct. Mater. 18, 2972–2983.

18


Page 18 of 22

Page 19 of 22 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


(9) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fréchet, J. M. J.; Trauner, D.; Louie, S. G.; Crommie, M. F. Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface. Phys. Rev. Lett. 2007, 99, 038301. (10) Wolf, M.; Tegeder, P. Reversible molecular switching at a metal surface: A case study of tetra-tert-butyl-azobenzene on Au(111). Surf. Sci. 2009, 603, 1506 – 1517. (11) Bandara, H. M. D.; Burdette, S. C. Photoisomerization in different classes of Azobenzene. Chem. Soc. Rev. 2012, 41, 1809–1825. (12) Marturano, V.; Ambrogi, V.; Bandeira, N. A. G.; Tylkowski, B.; Giamberini, M.; Cerruti, P. Modeling of Azobenzene-based compounds. Phys. Sci. Rev. 2017, 2, 20170138. (13) Bockmann, M.; Marx, D.; Peter, C.; Site, L. D.; Kremer, K.; Doltsinis, N. L. Multiscale modelling of mesoscopic phenomena triggered by quantum events: Light-driven azomaterials and beyond. Phys. Chem. Chem. Phys. 2011, 13, 7604–7621. (14) Tan, E. M.; Amirjalayer, S.; Smolarek, S.; Vdovin, A.; Zerbetto, F.; Buma, W. J. Fast photodynamics of Azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy. Nat. Commun. 2015, 6, 5860. (15) Oliboni, R. S.; Bortolini, G.; Torres, A.; Rego, L. G. C. A nonadiabatic excited state molecular mechanics/extended Hückel Ehrenfest method. J. Phys. Chem. C 2016, 120, 27688–27698. (16) Torres, A.; Prado, L. R.; Bortolini, G.; Rego, L. G. C. Charge Transfer Driven Structural Relaxation in a PushPull Azobenzene DyeSemiconductor Complex. J. Phys. Chem. Lett. 2018, 9, 5926–5933. (17) Poprawa-Smoluch, M.; Baggerman, J.; Zhang, H.; Maas, H. P. A.; De Cola, L.; Brouwer, A. M. Photoisomerization of disperse red 1 studied with transient absorp19



tion spectroscopy and quantum chemical calculations. J. Phys. Chem. A 2006, 110, 11926–11937. (18) Hoffman, D. P.; Mathies, R. A. Photoexcited structural dynamics of an Azobenzene analog 4-nitro-4[prime or minute]-dimethylamino-azobenzene from femtosecond stimulated Raman. Phys. Chem. Chem. Phys. 2012, 14, 6298–6306. (19) Schmidt, B.; Sobotta, C.; Malkmus, S.; Laimgruber, S.; Braun, M.; Zinth, W.; Gilch, P. Femtosecond fluorescence and absorption dynamics of an Azobenzene with a strong push-pull substitution. J. Phys. Chem. A 2004, 108, 4399–4404. (20) Nancoz, C.; Licari, G.; Beckwith, J. S.; Soederberg, M.; Dereka, B.; Rosspeintner, A.; Yushchenko, O.; Letrun, R.; Richert, S.; Lang, B.; Vauthey, E. Influence of the hydrogen-bond interactions on the excited-state dynamics of a push-pull Azobenzene dye: The case of Methyl Orange. Phys. Chem. Chem. Phys. 2018, 20, 7254–7264. (21) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Dependence of single-molecule junction conductance on molecular conformation. Nature 2018, 442, 904–907. (22) Torres, A.; Oliboni, R. S.; Rego, L. G. C. Vibronic and coherent effects on interfacial electron transfer dynamics. J. Phys. Chem. Lett. 2015, 6, 4927–4935. (23) Biswas, N.; Umapathy, S. Study of solvent effects on the molecular structure and the reorganization energies of 4-nitro-4-dimethylaminoazobenzene using resonance Raman intensities. J. Raman Spec. 2001, 32, 471–480. (24) Biswas, N.; Umapathy, S. Resonance Raman study of the solvent dynamics for ultrafast charge transfer transition in 4-nitro-4-dimethylamino-azobenzene. J. Chem. Phys. 2003, 118, 5526–5536.

20


Page 20 of 22

Page 21 of 22 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


(25) Zhang, L.; Cole, J. M. TiO2 -assisted photoisomerization of Azo dyes using selfassembled monolayers: Case study on para-Methyl red towards solar-cell applications. ACS Appl. Mater. Interfaces 2014, 6, 3742–3749. (26) Sowmiya, M.; Senthilkumar, K. Opto-electronic and interfacial charge transfer properties of Azobenzene dyes on anatase TiO2 (001) surface The effect of anchoring group. J. Photochem. Photobiol. A: Chemistry 2017, 346, 372 – 381. (27) Zhang, L.; Cole, J. M.; Waddell, P. G.; Low, K. S.; Liu, X. Relating electron donor and carboxylic acid anchoring substitution effects in Azo dyes to dye-sensitized solar cell performance. ACS Sustain. Chem. Eng. 2013, 1, 1440–1452. (28) Frontiera, R. R.; Dasgupta, J.; Mathies, R. A. Probing interfacial electron transfer in Coumarin 343 sensitized TiO2 nanoparticles with femtosecond stimulated Raman. J. Am. Chem. Soc. 2009, 131, 15630–15632. (29) Abraham, B.; Fan, H.; Galoppini, E.; Gundlach, L. Vibrational spectroscopy on photoexcited dye-sensitized films via pump-degenerate four-wave mixing. J. Phy. Chem. A 2018, 122, 2039–2045.

21



Graphical TOC Entry

22


Page 22 of 22

Modulating the Photoisomerization Mechanism of ... - ACS Publications

Recommend Documents