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

Hydroxyphenyl-azoles are among the most popular ESIPT (Excited State Intramolecular Proton Transfer) scaffolds and as such, they have been thoroughly studied. Nevertheless, some aspects regarding the interplay between the emissive properties of these fluorophores and the size of their π-conjugated framework remain controversial. Previous studies have demonstrated that benzannulation of 2′-hydroxyphenyl-oxazole at the phenol group of the molecule can lead to either red- or blue-shifted fluorescence emission, depending on the site where it occurs. In this report, benzannulation at the heterocyclic unit (the oxazole site) is analysed in order to get the whole picture. The extension of π-conjugation does not significantly affect the ESIPT emission wavelength, but it leads instead to higher energy barriers for proton transfer in the first excited singlet state, as a consequence of dramatic changes in the charge transfer character of excitation caused by successive benzannulation. Theoretical calculations revealed an interesting connection between intramolecular charge transfer and excited-state aromaticity in the S1 state. The theoretical approach presented herein allows the behaviour of hydroxyphenyl-oxazoles in the excited state to be rationalized and, more generally, a deeper understanding of the factors governing the ESIPT process to be obtained, a crucial point in the design of new and efficient fluorophores.


1 H and 13 C NMR spectra of HNO
. 1 H (top) and 13 C NMR (bottom) of HNO in CDCl3.

Non-covalent interactions (NCI) analysis
For the analysis of the OH-- interaction the RDG function has been used ( ) = 1 2(3 2 ) 1/3 |∇ ( )| ( ) 4/3 (1) where ρ(r) is the total electron density and the RDG (r) is the reduced density gradient of the exchange contribution. According to Atoms in Molecules (AIM) theory the nature of a weak interaction depends on both the 2 eigenvalue and the electron density:  (r) = Sign (2(r))(r) When 2>0 the interaction is bonding, and the opposite if 2<0. In Fig. S9-S11 the scatter plots of RDG function vs. Ω(r) function are shown for HPO, HBO and HNO in the S0 and S1 states. It is important to note that the left spike (marked with red circle) shifts to the left in S1 in the case of HPO and HBO, indicating a strengthening in the OH− interaction upon excitation, whereas remains almost unaltered in HNO.
S 0 S 1

QTAIM analysis
According to the QTAIM approach, a bonding interaction is related to the existence of a path of maximum electron density ρ(r) (the bond path) connecting the corresponding atomic basins. A bond critical point (BCP) is a point along this bond path at the interatomic surface where ρ(r) reaches a minimum value. In the present case, we analysed the topology of ρ(r), looking for the presence of a BCP between the hydrogen atom and the N acceptor, and then we calculated ρ(r), its Laplacian, ∇ 2 ρ(r) and the potential energy density V(r) at that BCP. According to the equation proposed by Espinosa et al. [i], the H-bond energy can be estimated from V(r) as follows: Topological analysis revealed the existence of strong hydrogen bonds in all compounds in both S0 and S1 states ( Table 5 in main text). (Note: A H-bond is defined as "strong" when ρ(r) ≥ 0.03) [ ii ]. Good correlations were found between the H-N distance and ρ(r), ∇ 2 ρ(r), total electron energy density H(r) and H-bond energy (Fig. S8). According to QTAIM analysis results, the H-bond in HPO results considerably strengthened by 2.05 kcal/mol in S1, whereas in HBO the interaction increases its intensity in 1.17 kcal/mol. Unlike these cases, photoexcitation weakens the intramolecular H-bond of HNO by 0.18 kcal/mol, in agreement with the observed in IR calculations and NCI analysis.  Figure S8. Correlation between H-N bond length and ρ(r), ∇ 2ρ(r), H(r) and hydrogen bond energy estimated via potential energy density V(r) BCP. Figure S9. Jablonski diagram (in eV) related to the ESIPT process of HPO, HBO and HNO, calculated at B3LYP/6-31+G(d)//PCM in acetonitrile. Figure S10. Structures of S1 transition state for HPO, HBO and HNO showing the displacement vectors over the imaginary frequency mode.

Relevant information about stationary points
Figure S11. Density difference plots (Δρ = ρS1 -ρS0, isovalue=0.0004) for the E* (left), TS* (middle) and K* (right) stationary points on the S1 PES of HNO. The blue/red zones indicate a decrease/increase of electron density upon excitation, respectively.
Figure S12. (Left) Density difference plots (Δρ = ρS1 -ρS0, isovalue=0.0004) for the enol form of 1H2NBO (top), 2H1NBO (middle) and 2H3NBO (bottom). The blue/red zones indicate a decrease/increase of electron density upon excitation, respectively. (Right) electrostatic potential maps and dipole moment vectors for the three compounds. It is apparent that the ICT character is larger for 2H3NBO than for its isomers.