4-Hydroxybenzaldehyde and 2-acetylpyridine were purchased from Sigma-Aldrich (St. Louis - MO, USA) and were used without further purification. Sodium hydroxide was obtained from Alfa-Aesar (Haverhill - MA, USA). Following the Kröhnke reaction (Figure 1), TpyOH was obtained in moderate yield.

Synthetic route for 4-aryl substituted TpyOH

All measurements were performed at room temperature (20 ± 1°C) in HPLC quality ethanol. The stationary absorption spectra were measured using a Shimadzu UV-2401 PC double channel spectrophotometer (Kyoto, Japan) in a 1 cm quartz cell. The steady-state fluorescence emission and excitation spectra were recorded using a PTI Quantamaster™ 4000 spectrofluorometer (New Jersey, USA).

All calculations were performed using the Gaussian09 quantum chemical program package (12). The molecular structures of TpyOH in the electronic ground state were fully optimized with density functional theory (DFT) method using the parameter-free PBE0 hybrid functional and the 6-31+G(d,p) basis sets. To include the solvent effects, we used the Polarized Continuum Model (PCM) with the integral equation formalism (IEFPCM) developed by Tomasi et al (13, 14).

To simulate the absorption and emission spectra, vertical excitation energies in the singlet and triplet manifold were calculated using the PCM/TD-PBE0/6-31+G(d,p) level of theory. This approach has proven to predict accurate descriptions of excited states for several organic (15) and metal-organic complexes (16, 17, 18). Geometry optimizations for the S0 and S1 states were performed without symmetry constraints and were confirmed to be stationary points through vibrational frequency analysis.

TpyOH was synthesized under mild conditions following the Kröhnke reaction with a yield similar to the previously reported (11). After purification, TpyOH was crystallized in an orthorhombic crystal system with P2₁2₁2₁ symmetry. Full information of the structure was deposited at the CSD Database under the deposition number 1454127 (Table 1). The crystal-packing diagram (Figure 2) shows that the asymmetric unit of TpyOH contains one crystallographically independent molecule as part of four molecules per unit cell with similar bond distance and dihedral angles. The geometry of TpyOH corresponds to a nearly coplanar structure, where the central pyridinyl ring (Py2) makes a dihedral angle of about 5.03° with the phenolic ring (PhOH), and 6.05 and 12.2° with the pyridinyl substituent rings Py1 and Py3, respectively.

The differences in the dihedral angles observed on the terpyridyl moiety can be attributed to the intermolecular hydrogen bond O25-H26××××N18 formed between two adjacent molecules. As shown in Figure 1, the molecules are column-like stapled along the a-axe of the unit cell with a packing dominated mainly by intermolecular hydrogen bonds (1.85(3) Å), between the pyridinyl nitrogen of Py3 and the –OH over the phenol-group at the neighbour molecule and by π-p interactions (3.52(3) Å) between molecules of the same staple. Although the crystal structure of the target compound was published earlier (19) and the crystallographic data are quite similar, the cell volume here reported is a little bit smaller (about 68 Å), showing the possibility of polymorph formation for this compound (20).

X-Ray molecular structure of TpyOH. f1, f2, and f3 are the dihedral angles defined by atoms C₁₂–C₁₁–C₁₉–C₂₄, C₁–C₂–C₇–C₁₂, and C₁₈–C₁₃–C₉–C₁₀, respectively (left). And Cell packing of the four molecules in the unit cell along the a-axes, showing hydrogen bonding as red lines. The molecules are column-like stapled along the a-axe (right)

The Hirshfeld surface is a useful tool for describing the surface characteristics of molecules. The molecular Hirshfeld surface of TpyOH was generated by the CrystalExplorer 17.5 program (21), using a standard high surface resolution with the 3D dnorm surface (Figure 3) mapped over a fixed color scale of -0.66 (red) to 1.2 Å (blue). The Hirshfeld surface is shown to be transparent to permit visualization of the molecular moiety, and the 3D dnorm surface was used to identify close intermolecular interactions. The values of dnorm are negative or positive either the intermolecular contacts are shorter or respectively longer than the Van der Waals radii. The dnorm values were pictured onto the Hirshfeld surface using a red-blue-white color scheme, where red regions represent a closer contact with a negative dnorm value, a blue region represents longer contacts with a positive dnorm value, and a white region represents exactly the distances by van der Waals radii with a dnorm value of zero.

3D dnorm Hirshfeld-surface of the TPyOH molecule. Red spots indicate short distances between neighbour molecules. The two shortest distances between molecules are formed by hydrogen-bonds between OH-groups and nitrogen-atoms of neighbour molecules

The absorption and emission spectra of TpyOH in ethanol solution are shown in Figure 5. As can be observed, the absorption spectrum shows an intense band centred at λ = 286 nm and a weak long-wavelength absorption in the 330–410 nm range. The low-energy band is assigned to the intraligand charge transfer transition (ICT), while the high-energy band is a common feature of p–p* transitions of the Tpy moiety (22, 23, 24, 25). From the comparison between the UV-visible spectrum of TpyOH and the corresponding terpyridine systems bearing electron-donating or electron-withdrawing groups, is clear that the intensity of the ICT band is mainly governed by the electron donating strength of the substituent. Specifically, terpyridines carrying an -NR₂ electron donor substituent show a strong absorption band around 360-399 nm.

On the contrary, for the case of NO₂ substitution, the spectrum shows only one absorption band at higher energy, as expected for a p–p* transition centred on the ligand (22, 23, 24, 25). Furthermore, the emission spectrum of TpyOH shows a broad emission band centred at λ = 384 nm, with a very large Stokes shift of almost 100 nm, indicating that TpyOH undergoes with difficulty self-absorption processes. This suggests that TpyOH could be a good candidate for applications such as chemo-sensors. This emission band corresponds to energy relaxation from a nearly coplanar S1 state formed after the photo-absorption process (as it is shown in the following Figure 5).

Absorption (blue line) and emission (red line) spectra of TpyOH in ethanol. The fluorescence spectrum was taken with an excitation wavelength of 285 nm. The vertical sticks correspond to the vertical absorption and emission transitions calculated at the PCM/TD-PBE0/6-31+G(d,p) level of theory

DFT geometry optimization of TpyOH in the ground state was performed taking into account three different isomers, generally named as transoid, cisoid, and a mixture between them (from now on labelled as TpyOH 1, TpyOH 2, and TpyOH 3), which differ in the spatial orientation of the terpyridyl nitrogen atoms. Optimized ground state structures of these Tpy isomers in ethanol are depicted in Figure 6, while selected structural parameters are included in Table 2. For the TpyOH 1 isomer, the phenol substituent forms a dihedral angle of approximately 34° relative to central terpyridine ring, while the three terpyridine aromatic rings are nearly planar, with f2 and f3 of about -1.8° and -1.9°, respectively. For the TpyOH 2 isomer, the substituted phenol ring and the terminal terpyridine rings are twisted for about 38° and -139°, respectively, while for the TpyOH 3 corresponding dihedral angles (see Figure 2 for angles designations) are f1 = 36°, f2 = -144° and f3 = -3.7°.

Optimized molecular structures of TpyOH 1, TpyOH 2 and TpyOH 3 conformers in ethanol, calculated at the PCM/TD-PBE0/6-31+G(d,p) level of theory

TD-DFT computational studies were performed to elucidate the electronic structures of the S0 state of TpyOH 1. The calculated excitation energies with their contribution, oscillator strengths, and coefficients are reviewed in Table 3. The best agreement with the experimental data was obtained using the PCM/TD-PBE0/6-31+G(d,p) level of theory. Specifically, the calculated S0 --> S1 ICT transition is 3.98 eV (311 nm), while the S0 --> S2 transition is 4.15 eV (299 nm). This table also shows that the lowest-energy singlet --> singlet transition at 311 nm has its main contribution from the highest occupied molecular orbital HOMO (DFT orbital 85) to the lowest unoccupied molecular orbital LUMO (DFT orbital 86) and is assigned as an ICT transition.

Excitation energies (eV/nm), largest excitation coefficients, electronic transition configurations and oscillator strengths for TpyOH 1 in ethanol computed at the PCM/TD-PBE0/6-31+G(d,p) level of theory.

The frontier molecular orbitals and its composition are shown in Figure 7 and Table 4, respectively. From Table 4, on the one hand, it can be observed that the HOMO is composed of 34.0% p (Tpy) and 66.0% p (PhOH), whereas the LUMO orbital is localized on the terpyridine moiety with a 99.0% π* (Tpy). On the other hand, the next high-energy transitions at 299 nm (f = 0.4651, Table 3) and 289 nm (f = 0.3646, Table 3) dominate the energy absorption band. Table 3 also shows that the S2 state has its main contribution from the H --> L+1 transitions (96%), while the S3 state has contributions from the H --> L ICT transitions (29%) mixed with the H-1 --> L+1 (66%) transition. Both states have a p–p* character due to the p–p* transitions of the terpyridine and the phenol ring.

Frontier molecular orbitals of TpyOH 1 in ethanol calculated at the PCM/TD-PBE0/6-31+G(d,p) level of theory (isosurface value: 0.02)

Optimization of the S1 Franck−Condon state of TpyOH 1 in both gas phase and ethanol solution resulted in a more coplanar structure (Figure 8). Specifically, the dihedral angles f1, f2 and f3 decrease 15.1, 1.19, and 1.41° in the gas phase, and 12.5, 1.68, and 1.21° in ethanol, respectively. According to these results, we can conclude that the resonance interaction is more efficient in the first excited state than in the ground state. Table 5 lists the calculated emission energy, electronic transition configuration, and frontier molecular orbital compositions (%) in the S1 excited state of TpyOH 1. As can be observed, the calculated S1 --> S0 transition at 355 nm in ethanol has its main contribution from the HOMO and LUMO orbitals (90% contribution) and oscillator strength of 0.2383. This result shows the ICT character of the emission band.

In the S1 excited state, the HOMO is composed of 35.0% p (Tpy) and 65.0% p (PhOH), whereas the LUMO orbital is mainly localized on the terpyridine moiety with a 94.0% π* (Tpy). The frontier molecular orbitals in the S1 excited state of TPyOH 1 in ethanol are depicted in Figure 9.

Excitation energy (eV/nm), largest excitation coefficients, electronic transition configuration, oscillator strength f, and frontier molecular orbital compositions (%) in the S1 excited state for TpyOH 1 calculated at the PCM/TD-PBE0/6-31+G(d,p) level of theory

Frontier molecular orbitals in the S1 excited state of TPyOH 1 in ethanol calculated at the PCM/TD-PBE0/6-31+G(d,p) level of theory (isosurface value: 0.02).