New method could enable faster and more efficient modeling of two-photon absorption in large molecules

Absorption spectroscopy, a vital analytical chemistry tool, can identify the presence of specific substances in samples by gauging light absorption across various wavelengths. This technique divulges crucial data about electronic structure, quantum states, sample concentration, and more, offering insights into a substance’s behavior and potential applications, including its interaction with other molecules.

Molecules with a propensity to simultaneously absorb two low-energy photons have diverse applications, from high-resolution microscopy to data storage and medicinal treatments. Yet, directly studying this phenomenon through experimentation is challenging. Computer simulations are invaluable but computationally intensive, especially for large molecules.

To address this issue, physicist Tárcius Nascimento Ramos and his team proposed an alternative computational approach, presented in The Journal of Chemical Physics. They revisited a semi-empirical method, INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic Parameterization), often overlooked due to its approximative nature. This method significantly reduces computation time, making extensive simulations feasible.

The INDO/S method approximates complex calculations by using tabulated values derived from experimental spectroscopic data. It offers efficiency for studying large molecular compounds, even those with more than 200 atoms, which are flexible and change their electronic properties when they alter shape.

The study applied this method to bridge the gap in characterizing one- and two-photon absorption spectra for large molecules in solution. Surprisingly, the semi-empirical method proved highly suitable for predicting these absorption spectra, hinting at new possibilities for molecular engineering.

Understanding one- and two-photon absorption is essential. These processes depend on molecules achieving compatible excited states with the photon’s energy. While one-photon absorption has specific selection rules, two-photon absorption permits more varied excited states. This distinction, along with its non-linear optical nature, makes two-photon absorption ideal for high-resolution applications in microscopy and data storage.

This research advances the field, particularly in computer modeling of two-photon absorption by organic molecules in solution, providing new avenues for innovative compound development.

Source: FAPESP

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