Exciterad tillstånds(anti)aromaticitet och tung grupp 14 kemi: Grundvetenskapliga studier med potential för tillämpningar

Tidsperiod: 2016-01-01 till 2019-12-31

Projektledare: Henrik Ottosson

Finansiär: Vetenskapsrådet

Bidragstyp: Projektbidrag

Budget: 3 600 000 SEK

This research proposal is centered on two areas; the first devoted to investigations of excited state aromaticity and antiaromaticity effects (shortly, excited state (anti)aromaticity) or ES(A)A) and the second to the deepened understanding of conjugation topologies that are mixtures of pi- and sigma-conjugations. Both project areas feature organic molecules with intricate electronic structures, and in our research we will use a mix of experimental and theoretical/computational tools. The proposal is a continuation of a previous VR grant running for the period 2012-2015, yet, with a larger portion of ES(A)A projects than in the previous grant, as these projects are the most groundbreaking ones. It is expected that development of this area will have profound effects on the whole of chemistry, as outlined below.Aromaticity is one of the absolutely central concepts in chemistry, found in the literature nearly as often as the concepts of ionicity and acidity, and more frequently used than chirality. Yet, today the aromaticity concept is applied only to describe chemical properties and processes in the electronic ground state (S0). In this state, aromaticity is given by the well-known Hueckel´s rule. However, in 1972 Colin Baird provided the theoretical foundation of excited state (anti)aromaticity [J. Am. Chem. Soc.,1972, 94, 4941]. Based on qualitative perturbation molecular orbital theory he showed the pi-electron counts for aromaticity and antiaromaticity are reversed in the lowest pipi* excited triplet state (T1) as compared to the S0 state where aromaticity and antiaromaticity according to Hueckel´s rule require 4n+2 and 4n electrons, respectively. Yet, the ES(A)A concept is forgotten and essentially completely unexploited, except in studies of pericyclic photoreactions. My aim is to change this once and for all. Excited state (anti)aromaticity as given by Baird´s rule should be the next truly important and useful aromaticity form. Recently, my group showed that the ES(A)A concept can be used for retrospective rationalization of a very wide range of earlier experimental observations [Chem. Rev. 2014, 114, 5379]. My aim is now to investigate how the ES(A)A concept can be applied broadly, thereby showing its width and potential to the chemical community. The proposed research stretches from fundamental to applied, with potential applications in areas ranging from solar energy harvesting to efficient photochemical synthesis of medium-sized bioactive heterocycles. In the second research area described we will use the particular properties of molecular segments composed of heavier Group 14 elements (Si, Ge and Sn) in combination with pi-bonded segments. When aligned properly we have shown that one can obtain strong neutral (cross-)hyperconjugative effects [see e.g., Angew. Chem. Int. Ed. 2013, 52, 983, and Chem. Sci. 2014, 5, 360]. We now plan to explore such effects in structurally more intricate molecules as well as in oligomers of the previously synthesized compounds. The expectation is that our studies will provide a deeper understanding of how saturated molecular segments in otherwise pi-conjugated molecules, oligomers and polymers can influence electronic structural properties (conductance as well as optical properties). The proposed research is fundamental in character, yet, the new knowledge that will be derived should be clearly applicable in molecular and organic electronics. In this context we also plan to explore the single-molecule charge transport characteristics of a few of the generated (cross-)hyperconjugated compounds.The proposed research relies on a number of local, national and international collaborations as described in section 6 in the project description. It will continuously involve three PhD students and one postdoctoral fellow during 2016, all in the group of the applicant.