Excited-state proton transfer relieves antiaromaticity in molecules

Abstract

Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4n + 2] π-aromatic in the ground state, become [4n + 2] π-antiaromatic in the first 1ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o-Salicylic acid undergoes ESPT only in the “antiaromatic” S1 (1ππ*) state, but not in the “aromatic” S2 (1ππ*) state. Stokes’ shifts of structurally related compounds [e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates] vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.

Electron-driven proton transfer relieves excited-state antiaromaticity in photoexcited DNA base pairs

“…Whether or not EDPT happens is determined by changes in the p-electronic structure of the photoexcited base. These ndings have immediate implications for the design of photostable unnatural base pairs, as well as other light-driven proton transfer 43 and electron transfer processes.…”

Hydrogen bond design principles

“…Conversely, hydrogen bonds that decrease excited-state antiaromaticity in compounds are strengthen (Figure 13b), and in the extreme, relief of excited-state antiaromaticity can drive excited-state proton transfer reactions. 111 Although not properly recognized in a large body of supporting examples, [114][115][116][117][118] this relationship-between excited-state (anti) aromaticity and excited-state hydrogen bonds-explains why photoexcitation strengthens some hydrogen bonds but weakens others.…”

Impact of benzannulation on ESIPT in 2-(2′-hydroxyphenyl)-oxazoles: a unified perspective in terms of excited-state aromaticity and intramolecular charge transfer

“…6) was rationalized in terms of the Baird's rule. 13,14 Despite these compounds are structurally similar, they exhibit very different ESIPT emission with a maximum at 470 nm for 1H2NBO and at 30 Wu et al suggested that the relaxation of K* to the hot ground state relieves more antiaromaticity in 1H2NBO than in 2H3NBO, as the latter is considerably less antiaromatic in S 1 . 14 The enol form of 1H2NBO in S 1 exhibits two complete antiaromatic Clar's sextets that remains unchanged upon tautomerization, whereas in 2H3NBO the formation of the keto isomer implies the loss of one of those sextets (Fig.…”

“…NICS(1) zz values were computed using the PW91 functional with the IGLOIII basis set. For the estimation of NICS(1) zz in the S 1 state, NICS calculations were performed as open-shell triplet states employing the geometries optimized at the S 1 state as reported by Wu et al 14 Visualization and graphics rendering were carried out with GaussView 5.0.8 (ref. 41) and VMD 1.9.3.…”

“…11 The authors then expanded the investigation toward other naphthalene-fused 2-(2 0 -hydroxyaryl)benzazoles, nding that ESIPT emission of these dyes was blue-shied compared to the model compound HBO, but without proposing a possible explanation for this effect. 12 Almost another ten years later, this unconventional behaviour has been revisited by different authors 13,14 under the light of the Baird's rule. 15 This rule, according to which [4n + 2] p-aromatic annulenes become antiaromatic in the pp* S 1 or T 1 state, has been successfully applied to rationalize the ESIPT emission prole of different benzannulated HBO derivatives by connecting the relative stability of the tautomers to their aromatic (antiaromatic) character in the ground (excited) state.…”