Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed-shell singlet state (normally ground state, S-0) and lowest * triplet state (T-1 or T-0), as given by Huckel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T-1 and S-0 states. This effect is caused by a reversal in the dipole moment when going from S-0 to T-1 as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S-0 reduces (enhances) aromaticity in T-1, allowing for rationalizations of the triplet state energies (E-T) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted E-T. As a means to increase the -electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6-dicyanopentafulvenes can be a route to on-demand formation of a fulvenium dication with a triplet ground state (T-0). The five-membered ring of this species is markedly Baird-aromatic, although less than the cyclopentadienyl cation known to have a Baird-aromatic T-0 state.

This thesis describes work that could help development of new photochemical reactions and light-absorbing materials. Focus is on the chemical concept "aromaticity" which is a proven conceptual tool in developing thermal chemical reactions. It is here shown that aromaticity is also valuable for photochemistry. The influence of aromaticity is discussed in terms of structure, reactivity and properties. With regard to structure, it is found that photoexcited molecules change their structure to attain aromatic stabilization (planarize, allow through-space conjugation) or avoid antiaromatic destabilization (pucker). As for reactivity, it is found that stabilization/destabilization of reactants decrease/increase photoreactivity, in accordance with the Bell-Evans-Polanyi relationship. Two photoreactions based on excited state antiaromatic destabilization of the substrates are reported. Finally, with respect to properties, it is shown that excited state energies can be tuned by considering aromatic effects of both the electronic ground state and the electronically excited states. The fundamental research presented in this thesis forms a foundation for the development of new photochemical reactions and design of compounds for new organic electronic materials.

2-Aryltellurophenols substituted in the aryltelluro or phenolic part of the molecule were prepared by lithiation of the corresponding O-THP-protected 2-bromophenol, followed by reaction with a suitable diaryl ditelluride and deprotection. In a two-phase system containing N-acetylcysteine as a co-antioxidant in the aqueous phase, all compounds quenched lipid peroxyl radicals more efficiently than α-tocopherol with 3 to 5-fold longer inhibition times. Compounds carrying electron donating para-substituents in the phenolic or aryltelluro part of the molecule showed the best results. The mechanism for quenching of peroxyl radicals was discussed in the light of calculated OH bond dissociation energies, deuterium labeling experiments and studies of thiol-consumption in the aqueous phase. 

Cyclic conjugation that occurs through-space and leads to aromatic properties is called homoaromaticity. Here we formulate the homoaromaticity concept for the triplet excited state (T₁) based on Baird's 4n rule and validate it through extensive quantum-chemical calculations on a range of different species (neutral, cationic and anionic). By comparison to well-known ground state homoaromatic molecules we reveal that five of the investigated compounds show strong T₁ homoaromaticity, four show weak homoaromaticity and two are non-aromatic. Two of the compounds have previously been identified as excited state intermediates in photochemical reactions and our calculations indicate that they are also homoaromatic in the first singlet excited state. Homoaromaticity should therefore have broad implications in photochemistry. We further demonstrate this by computational design of a photomechanical “lever” that is powered by relief of homoantiaromatic destabilization in the first singlet excited state.

Compounds that can be labeled as "aromatic chameleons" are pi-conjugated compounds that are able to adjust their pi-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenbzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T-1) of the compounds. Decreases in T-1 energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or transbent connectivities. The T-1 energies decreased down to those of the parent (isolated) 4n pi-electron units. Simultaneously, we observe an increased influence of triplet State aromaticity of the central 4n ring as given by Baird's rule and evidenced by geometric, magnetic, and electron density based aromaticity indices (HOMA, NICS-XY, ACID, and FLU). Because of an influence of,triplet state aromaticity in the central 4n pi-electron units,, the most stabilized, compounds, retain the triplet excitation in Baird pi-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar pi-sextet character. Interestingly, the T-1 energies go down as the total number of aromatic cycles within a molecule in the T-1 state increases.

The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first * triplet and singlet excited states (T-1 and S-1). Baird's rule says that the -electron counts for aromaticity and antiaromaticity in the T-1 and S-1 states are opposite to Huckel's rule in the ground state (S-0). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4n-electron heterocycles with two or more heteroatoms represent limitations.

For the concept of aromaticity, energetic quantification is crucial. However, this has been elusive for excited-state (Baird) aromaticity. Here we report our serendipitous discovery of two nonplanar thiophene-fused chiral [4n]annulenes ^Th4COT _Saddle and ^Th6CDH _Screw, which by computational analysis turned out to be a pair of molecules suitable for energetic quantification of Baird aromaticity. Their enantiomers were separable chromatographically but racemized thermally, enabling investigation of the ring inversion kinetics. In contrast to ^Th6CDH _Screw, which inverts through a nonplanar transition state, the inversion of ^Th4COT _Saddle, progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird’s theory, the planar conformation of ^Th4COT _Saddle is stabilized in the photoexcited state, thereby enabling lower activation enthalpy than that in the ground state. The lowering of the activation enthalpy, i.e., the energetic impact of excited-state aromaticity, was quantified experimentally to be as high as 21–22 kcal mol^–1.

Baird's rule tells that the electron counts for aromaticity and antiaromaticity in the first pi pi* triplet and singlet excited states (T-1 and S-1) are opposite to those in the ground state (S-0). Our hypothesis is that a silacyclobutene (SCB) ring fused with a [4n]annulene will remain closed in the T-1 state so as to retain T-1 aromaticity of the annulene while it will ring-open when fused to a [4n + 2]annulene in order to alleviate T-1 antiaromaticity. This feature should allow the SCB ring to function as an indicator for triplet state aromaticity. Quantum chemical calculations of energy and (anti)aromaticity changes along the reaction paths in the T-1 state support our hypothesis. The SCB ring should indicate T-1 aromaticity of [4n]annulenes by being photoinert except when fused to cyclobutadiene, where it ring-opens due to ring-strain relief.

Summary

The concept of excited-state aromaticity is receiving much attention in that completely reversed aromaticity in the excited state (so-called aromaticity reversal) provides crucial insight into photostability, photoreactivity, and its application to the photosynthetic mechanism and photoactive materials. Despite this significance, experimental elucidation of excited-state aromaticity is still unsolved, particularly for the excited singlet state. Here, as an unconventional approach, time-resolved IR (TRIR) spectroscopy on aromatic and anti-aromatic hexaphyrin congeners shed light on excited-singlet-state aromaticity. The contrasting spectral features between the Fourier transform IR and TRIR spectra reveal the aromaticity-driven structural changes, corroborating aromaticity reversal in the excited singlet states. Our paradigm for excited-state aromaticity, the correlation of IR spectral features with aromaticity reversal, provides another fundamental key to understanding the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited singlet state of π-conjugated molecular systems.