High two-photon absorption efficiency promoted by intermolecular interactions
Two-photon absorption (TPA) is a nonlinear optical process in which two photons of equal or different frequencies are simultaneously absorbed by a material. TPA has proven to be of pivotal importance in numerous applications in many apparently disparate fields, such as optical limiting, 3D microfabrication, up-converted lasing, photodynamic therapy, data storage, and biomedical imaging.1 Because of this broad applicability, much effort is being dedicated to the design and synthesis of new molecular materials with optimized TPA efficiency.
In recent years, the structure-property relations governing TPA have been studied extensively, both experimentally and theoretically. Many molecular design strategies have been proposed and tested, establishing guidelines for the development of materials with high TPA efficiency, usually quantified in terms of TPA cross section (σTPA). The complexity of molecules being synthesized has increased from simple dipolar, to quadrupolar and octupolar dyes, to dendrimers.2 We term this approach intramolecular, since it aims to optimize TPA performance by modifying intramolecular properties, such as the nature of chemical group substituents, bonding structures, and symmetry.
Despite the remarkable results that can be obtained by optimizing intramolecular properties, there are arguably limits to this approach. High values of σTPA can be attained only at the cost of a significantly increased molecular complexity. One of the possible strategies to overcome this limit is to exploit excitonic coupling and intermolecular interactions. Instead of designing and synthesizing complicated linear and multidimensional molecular structures, we can start from monomers with relatively simple structures that are capable of self-associating into more complex supramolecular assemblies (aggregates). Our interest in this approach arises from the possibility of synthesizing comparatively simple TPA-efficient molecular units based on theoretical predictions. Their organization into supramolecular structures can be exploited to cooperatively enhance the TPA response through specific intermolecular interactions.3, 4
To test the validity of this approach, we performed experiments to investigate how molecular TPA efficiency is modified upon aggregation, comparing σTPA of molecules in both monomer (free molecules) and aggregated forms. We focused in particular on water-soluble porphyrins, since their self-aggregation can be conveniently controlled by screening charge repulsion by modifying the ionic strength and pH. In particular, the water-soluble diacid molecule 5, 10, 15, 20-tetrakis (4-sulfonatophenyl) porphyrin (H4TPPS2−) enables us to generate aggregates with different structures (either rod-like or fractal) by controlling conditions such as pH, concentration, ionic strength, and type of solvent5. It allows us to explore in detail how intermolecular interactions affect the efficiency of TPA response.
The σTPA of both monomers and aggregates of H4TPPS2− in solution was determined by the open aperture Z-scan technique, which detects changes in the transmittance caused by TPA.6 Figure 1(a) shows examples of Z-scan curves for the H4TPPS2− monomer (black) and rod-like aggregated (red) form under the same experimental conditions. The plot clearly indicates that the nonlinear absorption properties of the system are strongly enhanced upon aggregation by a factor of about 30.7 The same characterization performed on aggregates with fractal geometries confirmed this aggregation-derived enhancement.1 The data also indicate a slight enhancement of σTPA for the rod-like aggregates. The two values of σTPA, however, fall within the same confidence interval if the experimental error is considered, and, thus, a correlation between aggregate geometry and TPA efficiency cannot be categorically stated.
We also characterized the TPA properties of rod-like aggregates embedded in solid samples. We found that in electrostatically self-assembled multilayers (ESAM), intermolecular interactions could be exploited to promote self-aggregation of monomers into aggregates. This could be carried out at a higher level to force an alignment of the aggregates.8 For these samples, Z-scan measures showed a further enhancement by a factor of 1.7 with respect to isotropic samples. Figure 1(b) summarizes all the results obtained for the monomeric and aggregated species, highlighting the enhancement recorded for the different intermolecular approaches.
In conclusion, we showed that our intermolecular approach presents significant advantages. Firstly, we can obtain high TPA efficiency starting from simple molecular units without resorting to complicated molecular synthesis. Furthermore, structures with different geometries and, as a consequence, different efficiencies can be obtained by starting from those same molecular units by simply modulating the experimental conditions or introducing suitable additives. Secondly, the same intermolecular interactions promoting self-assembly can also be modulated to prepare materials in the solid state with a preferential orientational distribution of the chromophores, thus giving rise to an increase in the cross section through suitable alignment of the active units. The latter point makes the intermolecular approach particularly suited for the preparation of solid samples with improved TPA performances, confirming its scientific significance and technical potential.
Our next step will be to study self-assembling structures in which final aggregate geometries can be directed—for example, by starting from monomeric building blocks that connect together in a defined manner through the orientation of chemical groups. The idea is to identify methodologies for preparing materials with much larger nonlinear responses, capable of fully exploiting the potential of our intermolecular approach.
The author wishes to acknowledge Professor C. Ferrante, Professor R. Bozio, Professor L. M. Scolaro and Dr. M. Castriciano for their crucial contribution to the results reported in this paper.
Elisabetta Collini obtained her BSc and PhD degrees in chemistry at the University of Padova, where she is a research associate. Her interests focus on the characterization of nonlinear properties and excited state dynamics of multichromophoric systems and energy transfer processes. She was recently awarded with an ERC Starting Grant.
