Eumelanin is the black biopolymer responsible for photoprotection in living organisms. Lack of knowledge of the atomic-level structure limits understanding of its function and exploitation of its potential in material science. To overcome these limitations, we present a systematic density functional theory study of the stability and optical properties of a library of 830 dimers of 5,6-dihydroxyindole (DHI), which are minimal models of eumelanin oligomers. Our aim is to understand the principles that govern the formation of DHI oligomers, relate the optical properties of the dimers with their stability, and establish their possible role in the photophysics of the biopolymer. From the structural point of view, we find a preference for oxidized over reduced and cyclic over linear structures, which speaks in favor of polycyclic graphite-like structures for the larger oligomers. We present an electrocyclization mechanism leading to the cyclic structures. We also find that besides the widely considered quinone and quinone methide oxidation patterns where two heteroatoms per DHI fragment are oxidized, dimers with one or three oxidized sites per fragment and an interfragment double bond are also stable and may be present in eumelanin. As far as the optical properties are concerned, some oxidized dimers combine relative stability with absorption energies as low as 1.3 eV. Such fragments may be present as substructures in the naturally found oligomers and might have a relevant contribution to the absorption spectrum of the biopolymer. In addition to these insights into the struc-tural and optical properties of the oligomers, we introduce a new classification scheme and a representative set of 53 dimers combining thermodynamic stability with chemical diversity.<br>
<p><a></a><a>Electronic conjugation through covalent bonds is generally considered as the basis for the electronic transition of organic luminescent materials</a>. Tetraphenylethylene (TPE), an efficient fluorophore with aggregation-induced emission (AIE) character, its blue photoluminescence in aggregate state is always ascribed to the through-bond conjugation (TBC) among the four phenyl rings and the central C=C bond. Herein, systematic <a>spectrometry studies and ab initio theoretical simulation</a> were conducted for TPE and its derivatives, and intramolecular through-space interaction (TSI) between two vicinal phenyl rings is proved as the origin of the blue emission. Furthermore, aided by the evaluation of excited-state decay dynamics, the non-luminescent nature of TPE in solution is revealed as the result of excited-state evolution towards conical intersections via isomerization and cyclization. In aggregate state, the excited-state TSI (ESTSI) is stabilized by the restriction of intramolecular motions, and strong blue emission from through-space conjugation is induced. The mechanistic model of ESTSI delineated in this work provides a new strategy to design luminescent materials beyond the traditional theory of TBC, and expands the quantum understanding of molecular behavior into the aggregate level.</p>
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