Demonstration of Baird's rule complementarity in the singlet state with implications for excited-state intramolecular proton transfer

Abstract

Greater aromaticity in the ground state leads to greater antiaromaticity in the excited state (and vice versa) which helps rationalize previously unexplained behavior of ESIPT fluorophores.

Excited-state proton transfer relieves antiaromaticity in molecules

“…See T 1 state results in SI Appendix, Table S13. A recent experimental paper reported similar findings for other derivatives of HBO (42). Together these evidence (21,23,24,42) suggest that the π-conjugation patterns of ESIPT compounds may be designed to tune the magnitude of their Stokes' shifts.…”

Detailed Visualization of Aromaticity Using Isotropic Magnetic Shielding

“…Such contour plots can be used to quantitatively assess differences in aromatic stability for both ground and excited state molecules and promise to become a valuable tool to rationalize aromatic behavior. [10] Computational Details…”

“…[9] The more feature-rich IMS contour plots presented here furnish a complete picture of the local and global aromaticities of each molecule which supports the more commonly accepted view that the 6-membered ring of benzene is more aromatic than its naphthalene counterparts. [10] Finally, while undoubtedly useful in certain applications, one-dimensional aromatic ring chemical shielding (ARCS) [11] and NICS-XY-scan [12] plots furnish different and much more limited information that misses the link between shielding and bonding. Indeed, we show that IMS maps provide one of the most insightful and succinct means of visualizing the aromaticity of molecules, even in comparison to other 2D and 3D approaches to visualize aromatic character (vide infra).…”

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.…”

“…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.…”