PHOtoresponsive Nonlinear Optical Monolayers – PHONOM

The project is organized in work packages implying three teams of the Institute of Molecular Sciences, which possess complementary skills for fulfilling the project objectives.

1) Elaboration of organic photochromes with high NLO contrast
This task is devoted to the synthesis and characterization of NLO photo-switchable compounds. The strength of the donor or acceptor terminal units are modulated to tune the absorption properties of the compounds, their SHG responses and relaxation rates. The best candidtaes are then be selected for surface functionalization.

2) Elaboration and optimization of functionalized surfaces
This task consists in the design of surfaces functionalized with NLO switches. Surfaces with different chromophore concentration are elaborated and fully characterized. Three main figures of merit are addressed: i) the reversibility of the switching process, i.e. the targeted systems should show neither writing- nor reading-induced fatigue after a large number of illumination steps, ii) the amplitude of the SHG response of the NLO active state, and iii) the SHG contrast between the NLO active and inactive states (readability of the ON/OFF states).

3) Spectroscopic characterization
The reproducible, relatively rapid and sensitive measurement of the optical properties of the SAMs in their different switching states are technical challenges. All the WRITE/ERASE and READ, optical processes will be performed in situ on a single multimodal setup that will be specifically designed.

4) Numerical simulations
A computational approach combineing molecular dynamic (MD) simulations and DFT calculations is used to provide a fundamental understanding of the functionalized surfaces. MD simulations give access to the morphology of the NLO-active layer at the atomistic level, and enable the exhaustive statistical sampling of the multiple geometrical conformations spanned by the chromophores at the surface. Then, DFT calculations performed on individual molecular fragments extracted from the MD trajectories provide a molecular interpretation on how dynamical geometry distortions, induced by thermal and steric effects, impact the SHG responses of both states of the SAM.

1) Synthesis of AZO and DASA compounds: the C2M group synthesized four series of DASA derivatives with different acceptor groups (Meldrum acid, barbituric acid, isoxazolone and indanedione). Various amines with different acidobasic properties were also introduced as donor groups and some of them were functionalized with a terminal alkyne to allow the click reaction and subsequent grafting onto the monolayer. A series of azobenzene derivatives with different donor and acceptor groups were also synthesized.

2) Characterization of the ONL properties of AZO and DASA compounds in solution: the GSM group performed Hyper Rayleigh scattering measurements to measure the second harmonic responses of AZO and DASA compounds. The ONL properties were also modeled by the THEO group. The GSM group also studied the kinetics of the photo-commutation process for the different DASA derivatives. A first paper on the photo-commutation and ONL responses of DASA in solution is in progress.

3) Substrate tests: the GSM and C2M groups have carried out systematic tests on a series of commercial glasses in order to identify the best substrate for the SAMs. These measurements validated the accuracy, sensitivity and speed of the R-SHG spectrometer measurements with fixed (non-time modulated) polarizations. Based on the results of the R-SHG analyses, an in-plane isotropic substrate with low signal intensity was selected so that the second harmonic responses of the chromophores could stand out.

4) Synthesis and characterization of functionalized monolayers: A first series of surfaces functionalized by AZO and DASA derivatives was synthesized by the C2M group. Some samples were characterized by PM-IRRAS in order to verify the grafting level of the molecules. In parallel, the second harmonic responses have recently been systematically characterized by the GSM group at a first qualitative level in order to control the grafting and the polar orientation of the chromophores.

5) Experimental developments: The GSM group has developed and tested a new experimental device dedicated to the measurement of the second harmonic reflectivity which integrates a precise control of the polarizations (linear, elliptical and circular left and right). This ellipsometer R-SHG progressively integrates several functionalities such as the displacement of the sample in its (x, y) plane with an integrated bright field microscope in order to visualize the sample on several hundreds of micrometers (spatial field).

1) Synthesis and characterization of functionalized monolayers: the synthesis of surfaces functionalized by AZO and DASA derivatives is going on. The purchase of a new CCD camera programmed for the end of this year with a co-financing of the Région Nouvelle Aquitaine and the ANR will allow a fine characterization of the ONL responses of the monolayers.

2) Modeling: after simulations of the optical properties of AZO and DASA compounds in solution, the next step is to simulate the structure of the monolayers using classical dynamics approaches. Then, DFT calculations performed on finite-size supramolecular aggregates extracted from molecular dynamics simulations will be performed in order to study the impact of intermolecular interactions on the ONL responses of monolayers.

3) Experimental developments: The next step will consist in integrating in-situ vis-RAS (reflectivity absorption with polarization) measurements in order to locally control the presence of chromophores by vis-RAS and their polar orientation rate given by R-SHG.

C. Naim, F. Castet, E. Matito
Impact of Van der Waals interactions on structural and nonlinear optical properties of azobenzene switches
Phys. Chem. Chem. Phys. 2021, 23, 21227-21239

The design of responsive materials allowing remote and reversible commutation of their electronic, magnetic, or optical properties is one of the greatest challenges for the development of optoelectronic and photonic devices. In this context, organic photochromic compounds have been extensively studied and integrated into a variety of applications such as logic gates or high-density optical memories. However, most of optical devices rely on linear absorption spectroscopy both for writing/erasing and reading information onto the material, which often results in destructive readout processes, since the state of the photochromic molecules is easily altered upon irradiation. Exploiting the nonlinear optical (NLO) response of the molecules for the reading step, in particular the second harmonic generation (SHG), instead confers non-destructive readout ability to the devices, as it allows using near-infrared excitation wavelengths that avoid triggering uncontrolled photoconversions or side reactions.
The PHONOM project aims at designing self-assembled monolayers (SAMs) based on photoresponsive NLO switches, targeting primarily systems for applications in optical communication or data storage. The project relies on a multidisciplinary approach encompassing numerical simulations, molecular synthesis, surface engineering, and spectroscopic characterizations. SAMs incorporating azobenzene (AZO) derivatives, as well as visible light-triggered Donor–Acceptor Stenhouse Adducts (DASAs) will first be elaborated. In a second step, the PHONOM project will address the challenge of multi-encoding by elaborating SAMs mixing these two types of photoswitches, which can be independently (orthogonally) controlled by irradiation with different wavelengths. In addition to surfaces made of homogeneously distributed AZO and DASAs molecules, patterned surfaces based on domains exclusively composed of one type of NLO switches will also be designed by microcontact printing, in order to assess the spatial selectivity in the spectral addressability of the devices.
The project is organized in three work packages implying three teams of the Institute of Molecular Sciences, which possess complementary skills for fulfilling the project objectives. As a major outcome of the project, the experimental proof of concept for orthogonally commutable NLO 2D materials will be provided, which may lead in the short term to the fabrication of new prototypes of photonic devices. Other important outputs of the project will consist in the development of new computational methods and experimental setups allowing the complete in situ characterization of functionalized surfaces.