Light Matters: from upconverted emission to charge generation

Light Matters: from upconverted emission to charge generation

Dr Ermelinda Maçôas outlines her work at IST using optical spectroscopy and microscopy to develop materials for biomedical and optoelectronic applications.

The way that plants harvest sunlight and convert it into energy and generate electrons that are transferred in the right direction on ultrafast timescales with minimal losses has always fascinated scientists. Understanding how plants do it can help scientists and engineers to design more efficient materials and devices to convert light into electricity or chemical energy. Advances in powerful ultrafast pulsed laser and in spectroscopy and microscopy provide ever more sensitive tools to interrogate and control fundamental light-driven processes. At IST, we use advanced optical spectroscopy and microscopy tools to understand how molecules and soft materials respond to light, and devise more clever ways to optimise and take advantage of that response in biomedical and optoelectronic applications.

We have home-built set-ups for measuring the emission decay in a time scale ranging from microseconds (1 µs = 10-6 s) to femtoseconds (1 fs = 10-15 s), a home-built nonlinear fluorimeter and a laser scanning confocal and multiphoton fluorescence microscope. We can make hyperspectral and lifetime imaging by recording the spectrum and the lifetime at each pixel. These tools are used to interrogate the relationship between the structure and the optical properties of optically responsive systems, being it molecules, polymers, hybrid nanoparticles or carbon nanomaterials.

The optical response can be conversion of the excitation light into light of a different wavelength (photoluminescence emission)1 or a different polarisation (from unpolarised light to circularly polarized light)2 (see Fig. 1) the transfer of the excitation light into a secondary system to trigger a chemical or physical process elsewhere (e.g. isomerisation, cyclisation or cycloreversion, singlet oxygen generation)3, or the generation of charges to produce an electrical response (see Fig. 2).4

Nonlinear dyes for bioimaging

A lot of our work is dedicated to materials that can emit upconverted light upon nonlinear excitation in the near-infrared region (NIR) of the electromagnetic spectrum. Nonlinear excitation means that we have more than one photon interacting with the system simultaneously, and this results in a response that depends nonlinearly on the intensity of the incident light. The most common process requires the simultaneous interaction with two-photons in the NIR. The emission that follows is typically in the visible region of the electromagnetic spectrum, meaning that the emitted photons are more energetic than those used in the excitation process.

Hence, the excitation light is upconverted. We have conducted extensive work on the design of new compounds with improved nonlinear brightness that are specially adapted for multiphoton imaging. The nonlinear brightness is the figure of merit for applications based on nonlinear emission and it is here defined as the product of the two-photon absorption (TPA) cross-section and emission quantum efficiency. Multiphoton imaging has well-known advantages in fluorescence based biomedical imaging and biosensing over conventional linear excitation methods, which include low background noise and low phototoxicity due to NIR excitation, deep penetration in scattering biological tissues, and reduced photobleaching due to increased localisation of the excitation light.

One of the most successful outcomes of our work is the development of a new nonlinear fluorophore with excellent performance with respect to state-of-the-art commercial labels.1 This dye can penetrate all the cell compartments, including the nucleus, and due to its sensitivity towards the presence of DNA, it can produce a multicolour image of the cytoplasm and the nucleus using fluorescence lifetime imaging microscopy. We are currently working on a related approach to improve the photostability and enable selective staining of specific cell organelles.

While fluorescent labels based on small molecules will be increasingly important in the context of emerging optical nanoscopic methods, there is a practical limit on the nonlinear brightness of small molecules.

To overcome such limitations, researchers have turned into nanomaterials. By far, the most efficient nonlinear emitters are semiconductor quantum dots and lanthanide nanocrystals that have TPA cross-sections orders of magnitude higher than known organic molecules. None of these systems are ideal due to severe cytotoxicity issues that must be circumvented by surface passivation.

Graphene quantum dots (GQDs) are now emerging as promising nonlinear fluorophores due to their exceptionally high nonlinear brightness combined with water solubility, biocompatibility, photostability and versatile surface chemistry. However, the lack of rational synthetic approaches for obtaining GQDs with predictable nonlinear optical properties is precluding their application in imaging and sensing.

The first report showing that the nonlinear excitation of GQDs can be valuable for 3D-imaging in biological and biomedical applications was published five years ago, and only a few reports exist on the application of nonlinear emission of GQDs for sensing. Thus, the challenge we now face is that of finding clear design guidelines for GQDs with optimised nonlinear emission.

To meet the challenge, we are searching for the missing link between the structure and the nonlinear emission of GQDs enabling the controlled synthesis and engineering of such material. Our systematic approach combines traditional top-down and bottom-up synthetic procedures with step-by-step fully controlled synthesis of nanographene molecules. In this quest we have partnered with a group of very skilled organic chemists from the University of Granada, Spain.2 Our goal is to provide better tools for biomedical research and nanomedicine based on the nonlinear response of GQDs.

NIR antennae for functional materials

Optimised two-photon absorbers can also operate as nonlinear NIR-antennae in functional materials for data storage. The excess energy captured by the NIR antenna is transferred to an active unit positioned at a nearby distance and used to induce a physical transformation that signal the written bit. The spatial confinement and increased penetration depth provided by nonlinear excitation in the NIR is at the heart of the multilayer writing and addressing strategy for increasing the density of optical data storage media. This strategy competes with effort made towards decreasing the bit size by reducing the excitation wavelength (eg. Blue-ray disc).

Together with a team from the university of Wuhan we have studied the use of polymers based on the 1, 3, 5-triazine unit as NIR-antenna in multilayer data storage.3 The system with the highest nonlinear brightness was used as NIR antennas for multilayer data storage in a composite simulating an optical memory. The written bit had a good contrast using lower excitation power and three orders of magnitude lower exposure times when compared with previously studied composites using similar operating principles (see Fig. 2). We anticipate that the performance can be improved by covalently linking the active photochromic units to the NIR antennae.

Materials for optoelectronic applications

In addition to upconverted light and energy transfer upon optical excitation, we are also interested in photoinduced charge-generation (see Fig. 2). Together with a team from Instituto de Engenharia de Sistemas e Computadores, Investigação e Desenvolvimento (INESC-ID) at Lisbon we have studied the charge generation at organic heterojunctions having at least one of the components in the single crystal form.4 We have shown that charge-transfer interfaces between organic single crystals can have orders of magnitude higher photoconductive quantum efficiency when compared with the isolated components or even with the same components in a different form (e.g. film, polycrystalline).

At the interface between a highly ordered rubrene single-crystal and an amorphous fullerene film, a large photoresponse was observed with responsivity values two-order of magnitude higher than the corresponding bulk-heterojunctions, and higher than the isolated single-crystal. Most interestingly, this system showed an enhanced electrical response to light in the red part of the visible spectrum, presenting an exceptionally charge high photogeneration yield for lower excitation energies. The enhanced response in the red resulted from a very efficient harvesting of excitons generated in the electron acceptor layer, a phenomenon that was not previously well explored due to the low absorption cross-sections of the most common n-type organic semiconductors.

We have also studied non-fullerene molecules in crystalline charge-transfer interfaces with remarkable observations on the effect of molecular distortion in exciton diffusion. We showed that even for highly distorted molecules electronic transport is not significantly compromised if the molecular packing in the crystal allows for orbital overlap in multiple directions creating a vast network for exciton diffusion and charge migration.
This knowledge is now used by the INESC-ID team to develop miniaturised photonic devices based on organic single crystals.

The work outlined shows that at IST we embrace the challenge of research for a better society starting from the very basic light-matter interactions to the establishment of clear design guidelines for optically responsive materials with application in biomedical research and optoelectronics.

The work at IST has been supported by Fundação para a Ciência e a Tecnologia (RECI/CTM-POL/0342/2012, IF/00759/2013, PTDC/QUI-QFI/
29319/2017 and PTDC/NAN-MAT/29317/2017).

1 Nanoscale, 2018 10(26) 12505 and Chem. Commun., 2011, 47(26)7374
2 Chem. Sci. 2018, 9 3917 and Angew. Chemie Int. ed., 2018 ASAP, doi: 10.1002/anie.201808178
3 J Mater Chem C, 2015, 3, 10775 and J. Mater. Chem., 2012, 22(33)16781
4 J. Am. Chem Soc, 2015,137,7104 and J. Mater Chem C, 2014,23639 and Nat. Commun., 2013, 4, 1842

Dr Ermelinda Maçôas
CQFM and CQE at Instituto Superior Técnico
+351 218 419 206

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