Molecular thin film device efficiencies are strongly dependent on the hierarchical organisation of organic semiconductor molecules. This is because the intermolecular interactions that govern fundamental optical properties such as emission and energy transfer are sensitive to the spatial and orientational arrangement of molecules. While we can explain optical properties arising in amorphous systems and nanoaggregates at small length scales (< 10 nm), and liquid crystals at large length scales (>1000 nm) as shown in Fig 1, a significant challenge remains in its description at mesoscopic length scales (at 10s and 100s of nm). One unique way to describe optical properties at varied length scales has been by the use of chiral side chains grafted to the semiconducting molecular backbone that enables the use of chiroptical spectroscopy techniques (i.e., using circularly polarised light) to study light matter interactions. 

In our research we use molecular chirality as a marker to study light-mater interactions to predict optical properties at mesoscopic length scales that can transform technologies. Some examples from our research are described below.

Faraday rotation in lead halide perovskites

Faraday Rotation is a type of magneto-optical phenomenon that rotates the polarization of light in the presence of magnetic field that is parallel to the propagation direction of light. Faraday rotators are used at the source of broadband and other communication technologies, blocking reflected light that would otherwise destabilise lasers and amplifiers. They are also used in optical switches and fibre-optic sensors. The global optical switches market alone is worth more than $US4.5 billion and is growing. We demonstrated, for the first time, Faraday rotation in lead bromide perovskites. We showed that they can rival commercial TGG standards for certain wavelengths in the visible spectrum. We carried out temperature dependent studies and found that degree of optical rotation obtained in methylammonium lead bromide perovksites is less dependent on temperature compared to TGG.

Collaborators: Udo Bach (Monash Uni), Anita Ho-Baillie (Uni Sydney) and Makhsud Saidaminov (Uni Victoria, Canada)

Excited state chiroptical phenomena in singlet fission dimers

Singlet fission (SF), the conversion of one singlet exciton into two triplets, offers a promising pathway to overcome thermalization losses and surpass the detailed balance limit. Insights into the SF process have been developed in the last few years led by investigations of the excited state dynamics in designer molecular systems where SF-based chromophores are covalently linked by a bridging molecule. However, little is known about chromophore-bridge interactions that is crucial towards the rational design of efficient SF-based chromophores. We study a designer pentacene dimer with chiral binaphthyl unit acting as the bridge. With the help of ground and excited state circular dichroism (CD), we sensitively probe SF chromophore-bridge interactions and demonstrate that a significant contribution from the bridge to the high-energy singlet transitions. We propose that the wavefunction overlap between the pentacene units and the axially chiral (binaphthyl) bridge aids in the significant suppression of the triplet-triplet recombination and emphasise the role of the frontier molecular orbitals of the bridge on the decay dynamics of the TT state. Our work underlines the promising role of chirality and chiroptical techniques to sensitively investigate chromophore-bridge interactions and the impact on multiexciton dynamics.

Collaborators: Lüis Campos (Columbia Uni), Akshay Rao (Cambridge Uni), Dane McCamey and Murad Taybjee (UNSW) and Stavros Anthanasopoulos (Madrid Uni).

Supramolecular chirality

Supramolecular gels are an important class of soft materials composed of hierarchically ordered three-dimensional networks of individual gelator molecules useful for a wide ranging of applications in catalysis, sensing and drug delivery. Insights into the nature of molecular packing across different length scales are crucial to achieve control over the final assembled gel structure. Using a combination of chiroptical techniques – CD, FDCD and MCD – we comprehensively demonstrate stepwise hierarchical self-assembly of perylene imides to gels. This study offers insight into gelation mechanism, which will be relevant to the formation of fibrillar networks and other large supramolecular structures in general.

Collaborators: Pall Thordarson (UNSW), Markus Müllner and Asaph Widmer-Cooper (Sydney Uni).

Chiroptical phenomena in semiconductors

Studying light-matter interactions in organic semiconductor molecules and polymers in challenging due to the subtle interplay between inter-molecular interactions and the inherent disorder that exists in aggregates of these molecules. To this end, we have advanced the scope and applicability of two chiroptical (i.e. using circularly polarised light) spectroscopy methods – fluorescence detected circular dichroism and circularly differential scattering – on a range of different molecules and novel nanomaterials and developed new protocols for experimental analysis. Through this work, we have accurately quantified the impact of intrinsic disorder in molecular aggregates on their use in optoelectronic device, for e.g. optical switches.

Origin of Chiroptical Phenomena

Together with Stefan Meskers (TU Eindhoven), we have work towards developing models to understand the origin behind the chiroptical phenomena in different materials. In the past, we have used a classical coupled oscillator model to explore the possibility of a general theory that can unify two century-old standalone theories on exciton coupling at a molecular level and liquid crystals at a mescoscopic length scale. Bringing together elements from spectroscopy and  theory, we showed that the crossover in optical properties at different length scales can be explained in a phenomenological way using an anisotropic dielectric tensor. This  contribution was featured in J. Phys. Chem. A (116, 1121) with a front cover.

Selected publications

1. A. Sharma, S. Athanasopoulos, E. Kumarasamy, C. Phansa, A. Asadpoordarvish, R.P. Sabatini, R. Pandya, K.R. Parenti, S.N. Sanders, D.R. McCamey, L.M. Campos, A. Rao, M.J.Y. Tayebjee, G. Lakhwani Pentacene–Bridge Interactions in an Axially Chiral Binaphthyl Pentacene Dimer J. Phys. Chem. A 125, 7226 (2021)
2. R.P. Sabatini, C. Liao, S. Bernardi, W. Mao, M.S. Rahme, A. Widmer-Cooper, U. Bach, S. Huang, A.W.Y. Ho-Baillie, G. Lakhwani Solution‐Processed Faraday Rotators Using Single Crystal Lead Halide Perovskites Adv. Sci. 7, 1902950 (2020)
3. A. Sharma, J.P. Wojciechowski, Y. Liu, T. Pelras, C.M. Wallace, M. Müllner, A. Widmer-Cooper, P. Thordarson, G. Lakhwani The Role of Fiber Agglomeration of Perylene-Based Fiber Networks Cell Reports Physical Science, 1, 100148 (2020)
4. G. Long, R. Sabatini, M.I. Saidaminov, G. Lakhwani, A. Rasmita, X. Liu, E.H. Sargent, W. Gao Chiral-perovskite optoelectronics Nat. Rev. Mater. 5, 423 (2020)
5. A. Sharma, S.Athanasopoulos, P.C. Tapping, R.P. Sabatini, O.F.McRae, M. Müllner, T.W. Kee, G. Lakhwani, Emission Decay Pathways Sensitive to Circular Polarization of Excitation, J. Phys. Chem. C, 112, 23910 (2018)
6. G. Lakhwani, S.C.J. Meskers, Insights from Chiral Polyfluorene on the Unification of Molecular Exciton and Cholesteric Liquid Crystal Theories for Chiroptical Phenomena J. Phys. Chem. A, 4, 1121 (2012)
7. G. Lakhwani, S.C.J. Meskers Circular Selective Reflection of Light Proves Cholesteric Ordering in Thin Layers of Chiral Fluorene Polymers J. Phys. Chem. Lett., 2, 1497 (2011)
8. R. Abbel, J. Gilot, G. Lakhwani, E.W. Meijer, A.P.H.J. Schenning, S.C.J. Meskers Polymer solar cells sensitive to the circular polarization of light Adv. Mat., 22, E131 (2010)