Light Harvesting and Chromogenic Nanomaterials for Energy Saving and Conversion

We aim to develop advanced (nano)materials following novel and emerging concepts and approaches, whose optical properties (i.e. absorption and emission) can be fine-tuned by different external stimuli.

Even though this work represents nowadays state-of-the-art and frontier research, our long-term vision is to bring these new materials into functional hybrid devices and marketable prototypes. These systems are of key relevance in important applications such as ophthalmic (which represents the largest field for photochromic dyes), smart windows, helmets, sport-glasses or automotive industry, among many others. Other long-term envisaged applications for these systems are temperature sensors, rewritable devices, logic gates, energy storage, optical memories and aesthetic purposes.

For this reason, we mostly use the combination of commercially available polymers, phase change materials and molecular dyes (such as spirooxazines, spiropyrans, porphyrins and polycyclic aromatic hydrocarbons) within micro/nanostructures to enhance their performance. This guarantees ready-to-use ending products, suitable to be scaled up following well-established manufacturing processes.

Several collaborators have strongly contributed to the development of this work, with special emphasis on Dr. Jordi Hernando (Uni. Autònoma de Barcelona, Spain) and Prof. Loredana Latterini (Univ. Perugia, Italy).

I. Photochromism

Photochromic materials and thin films exhibit reversible colour changes upon exposure to UV or visible radiation so its use is projected to address the tuneable optical transparency of glass and plastic substrates.

Our main contribution in this research area has been the “notion” of liquid-like photochromic solid materials. This simple but disruptive approach, which was achieved through the encapsulation of photochromic materials in core-shell micro and nanocapsules, allows for the obtaining of different solid materials with liquid like behaviour and opens novel research and application areas in the field of photochromism, as described next:

  • Fast responsive (direct and reverse) photochromism

The oil core guarantees the fast isomerization of the photochromic dyes even after the embedment of the capsules in a polymeric matrices. In particular, both available thermal back isomerization processes, i.e. fading (in the case of direct photochromism) and darkening (in reverse photochromism), become much faster if compared to the rate of the same photochrome in rigid polymeric matrices.

 

 

  • Switchable photochromism

The interconversion between direct and reverse photochromism is reversibly fine-tuned with the co-encapsulation of dyes and acidic phase change materials (PCM). By controlling the state (solid/liquid) of the encapsulated PCM with temperature and irradiating at a suitable wavelength, it is possible to induce the coloration or fading of the material at will (see image). Moreover, the same film, kept above or below the melting point of the PCM reversibly changes its color (thermochromism).

Selected publications:
 Dyes and Pigments 2017, 145, 359-364
Angewandte Chemie International Edition 2016, 128 (48), 15268-15272
Advanced Optical Materials 2013, 1 (9), 631-636
WO2013132123A1
WO2017105666A1
WO2017191346A1

In addition, our early advances in photochromism have prompted us to pursue market-oriented research in this area set through a dedicated spin-off (Futurechromes SL).

 

III. Thermochromism

Reversible (or irreversible) colour changes can also be induced at will by using temperature as external stimulus and the following nanomaterials:

  • encapsulation of pH and/or hydrogen-bonding sensitive dyes in combination with acidic colour developers,
  • use of smart spin crossover materials,
  • aggregation/disaggregation of fluorescent dyes upon encapsulation in polymeric nanoparticles with different glass transition temperatures (thermofluorochromism).

Using these materials, we work on the following research areas:

  • Positive and negative thermochromism

  • Near-infrared thermochromism

Selected publications:
 Journal of Materials Chemistry C 2016, 4 (25), 5879-5889
Inorganic chemistry 2015, 54 (14), 6776-6781
Advanced Functional Materials 2015, 25 (26), 4129-4134
Scientific reports 2013, 3, 1708
Angewandte Chemie International Edition 2008, 120 (10), 1883-1886

 

  •  High-temperature threshold fluorescent sensor

 

 III. Fluorescence

We very recently reported tuneable absorbing and fluorescent nanoparticles based on semiconductive polymers. The optical properties can be finely tuned by reducing the length and/or the conjugation length of the polymers accurately emulsified with ultrasounds. Interestingly, the nanoparticles maintain the tuneable absorbing and fluorescent properties both, in solution and solid films, opening a broad range of applications for these systems, from white light emission to biosensors. In the past, we also made efforts to integrate fluorescent molecules on hybrid surfaces and devices as pH sensors and actuators.

  • Tunable fluorescence

Selected publications:
 Particle & Particle Systems Characterization, 2018, DOI: 10.1002/ppsc.201700322.
Chemical Communication 2011, 47, 6864-6866
Nanoscale 2010, 2 (9), 1781-1788
Small 2008, 4 (12), 2131-2135
Chemistry-A European Journal 2008, 14 (31), 9754-9763
Langmuir 2008, 24 (7), 2963-2966

IV. Upconversion

Upconversion (UC) is the anti-Stokes shift of lower energy exciting radiation into more energetic emission. This phenomenon, and the materials providing it, find application in near infrared (NIR) induced luminescence (bioimaging) or to convert the NIR portion of the solar radiation into visible light, which could be used to enhance the efficiency of photovoltaic cells or activate photocatalytic and photochemical process.

Our strategy is based on aggregation induced upconversion in paraffin which allowed:

  • triplet-triplet annihilation-based UC in solid materials,
  • switchable UC-phosphorescence emissions.

Selected publications:
Chemistry of Materials 2016, 28 (3), 738-745