The project is directed towards aromaticity effects in electronically excited states. We apply quantum chemical calculations using DFT and electron correlated wave function methods (primarily CASSCF and CASPT2, as well as coupled cluster methods). The calculations are closely connected to on-going experimental studies in the group. The topic concerns excited state aromaticity and it is based on Baird's rule which tells that species with 4n pi-electrons are aromatic and those with 4n+2 pi-electrons are antiaromatic in the lowest pipi* excited triplet and singlet states. A series of processes and properties can be examined and rationalized in terms of excited state aromaticity. In the next grant period we will use calculations (i) to examine singlet excited state (anti)aromaticity and homo(anti)aromaticity; (ii) to design new compounds with low or inverted singlet-triplet energy gaps; (iii) to investigate the role of excited state (anti)aromaticity and antiaromaticity on photochemical reactivity, in particular, drug photodegradation; (iv) to design organic dyes for singlet fission photovoltaics; (v) to screen potential triplet state quenchers for fluorophore bioimaging; (vi) to investigate the importance of aromaticity in porphyrinoids and other macrocycles; (vii) to investigate the importance and interplay of Baird and Clar structures on high-spin polycyclic aromatic hydrocarbons.