Interpretation and prediction of optical properties: novel fluorescent dyes as a test case

The rapid development of modern quantum mechanical theories and computational resources facilitates extended characterization of molecular systems of increasing size and complexity, including chromophores of biochemical or technological interest. Efficient and accurate computations of molecular structure and properties in the ground and excited electronic states are routinely performed using density functional theory (DFT) and its time-dependent (TD-DFT) counterpart. However, the direct comparison with experiment requires simulation of electronic absorption or emission spectra, for which inclusion of vibrational effects leads to more realistic line shapes while at the same time allowing for more reliable interpretation and prediction of optical properties and providing additional information that is not available from experimental low-resolution UV-vis spectra. Computational support can help identify the most interesting chromophores among a large number of potential candidates for designing new materials or sensors, as well as unraveling effects contributing to the overall spectroscopic phenomena. In this perspective, recently developed viologen derivatives (1,1 & PRIME;-disubstituted-4,4 & PRIME;-bipyridyl cation salts, viol) are selected as test cases to illustrate the advantages of spectroscopic theoretical methodologies, which are still not widely used in "chemical" interpretation. Although these molecules are characterized by improved stability as well as the dual function of chromism and luminescence, their detailed spectroscopic characterization is hampered due to the availability of only low-resolution experimental spectra. DFT-based absorption and emission spectra are exploited in the analysis of optical properties, allowing detailed investigation of vibrational effects and gaining more insights on the structure-spectra relationship, which can be extended to develop further viologen dyes with improved optical properties.