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Article

8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design

by
Anton Georgiev
1,2 and
Liudmil Antonov
3,*
1
Department of Organic Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria
2
Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
3
Institute of Electronics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Micromachines 2020, 11(10), 901; https://doi.org/10.3390/mi11100901
Submission received: 8 September 2020 / Revised: 23 September 2020 / Accepted: 29 September 2020 / Published: 29 September 2020
(This article belongs to the Special Issue Recent Advances of Molecular Machines and Molecular Robots)
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Abstract

:
Theoretical design of conjugated proton cranes, based on 7-hydroxyquinoline as a tautomeric sub-unit, has been attempted by using ground and excited state density functional theory (DFT) calculations in various environments. The proton crane action request existence of a single enol tautomer in ground state, which under excitation goes to the excited keto tautomer through a series of consecutive excited-state intramolecular proton transfer (ESIPT) steps with the participation of the crane sub-unit. A series of substituted pyridines was used as crane sub-units and the corresponding donor-acceptor interactions were evaluated. The results suggest that the introduction of strong electron donor substituents in the pyridine ring creates optimal conditions for 8-(pyridin-2-yl)quinolin-7-ols to act as proton cranes.

1. Introduction

Excited state proton transfer reactions involve proton transfer through a ground state pre-existing hydrogen bond (either intra- or intermolecular), giving rise to another tautomer in the excited state [1,2,3,4,5]. Due to dramatic structural change, the excited tautomer possesses photophysical properties, different from those of the ground state specie. The ESIPT exhibiting molecules have become a field of active research [6] in the last decades, due to their applications as light-emitting materials and laser dyes [7,8,9], photo stabilizers [10] and photo switches [11,12].
Proton cranes are switching systems where a proton is transferred over a long distance within the same molecule. In general they contain a tautomeric sub-unit with clearly defined proton donor and proton acceptor parts, which exchange, under external stimuli, the tautomeric (or pseudo tautomeric) proton using a crane sub-unit as a proton delivery system. The overall process is sketched in Scheme 1, showing the sub-units linked by an axle. The proton is located in the tautomeric sub-unit in the single, solely existing, stable standby state. Under suitable irradiation, proton transfer occurs from the tautomeric to the crane sub-unit, leading to the unstable transfer state. Depending on the relative proton acceptor ability of the two competitive sites in the deprotonated tautomeric sub-unit, the protonated crane sub-unit might or might not rotate around the axle, leading, after release of the proton, to the end or to the standby state, correspondingly. The obtained end state can relax back to the standby either thermally or through applying another stimulus (another excitation wavelength for instance). The process of switching from the standby to the end state and back can be monitored easily by optical spectroscopy due to the different absorption spectra of the tautomers [2]. Although the rotation around the axle is not unidirectional, the proton cranes can be considered as simple prototypes of molecular motors [13] and, therefore deserve detailed investigation. Some of them, being named “small-molecule robotic arms”, have been used to deliver “cargo” from one side of the molecule to another [14].
Based on the structural design, the proton cranes can be classified depending on: (a) single- or double-bond character of the axle; (b) extent of conjugation between the tautomeric and crane sub-units; (c) heteroatoms being used as proton donor and proton acceptor sites. Several classes of compound could be mentioned as typical examples. Historically, the term “proton crane” was used for the first time by Varma et al. [15,16,17,18], who designed a single bond axle system, 8-(morpholinomethyl)quinolin-7-ol (1 in Scheme 2), where the proton is exchanged between O and N in the 7-hydroxyquinoline upon irradiation [19]. Proton crane 1 and its mimics [12,20,21] are based on an excited state proton transfer pathway upon irradiation as the only possibility to reach the energetically unfavorable end state. Another intriguing concept is the design of systems, where a moderate energy gap between tautomers of the tautomeric sub-unit provides proton exchange in a ground state [12,22,23,24].
In the rotary switches, where the axle is a C=N bond, the proton is transferred between oxygen atoms belonging either to β-dicarbonyl groups (2 in Scheme 2 as an example) [25,26,27,28] or 4-hydroxycoumarin [29,30,31], or proton transfer proceeds between oxygen and nitrogen in the switches designed by Aprahamian et al. [32,33,34,35]. The rotary switches operate mainly in ground state through acid/base catalyzed tautomerism [32].
The strongly conjugated proton cranes were theoretically introduced by Sobolewski [36] in the case of 5-(7-hydroxyquinolin-8-yl)-2H-1,4-oxazin-2-one (3 in Scheme 2). The theoretical design was later expanded by using a variety of tautomeric (7-hydorxyquinolines [37,38,39,40,41,42], 3-OH and 3-NH2 pyridines [43]) and crane (carbaldehydes [37,39], carboxamides [40,42], pyridines [38,43] and pyrimidines [41]) sub-units. The experimentally studied systems are limited to a few examples of 7-hydroxyquinoline (4) [39], 3-hydroxypyridine [44,45,46,47] and amide [48] tautomeric sub-units. The common feature of these systems is the excited state proton transfer either from E1K* or from K2E* (Scheme 3) to K1K* or K2K* respectively, which results to a conical intersection in the transition state between K1K* and K2K* (tautomeric and crane sub-units being perpendicular). This populates the ground state K1K and K2K, leading then to restoration of the existing ground state equilibrium over time.
The theoretical design of such proton transfer-based systems faces many challenges related to correct prediction of the ground state tautomerism, solvent effects and excited state dynamics upon irradiation. Recently, we have been able to accumulate knowledge about using density functional theory (DFT) calculations for correct prediction of the ground and excited state tautomerism of dyes and heterocycles [49,50,51,52,53], which gives a good opportunity to attempt the design of proton cranes. 8-(Pyridin-2-yl)quinolin-7-ol, where 7-hydroxy quinoline (4) serves as a tautomeric sub-unit, has been selected as a platform to investigate the effect of the substituents in the pyridine crane sub-unit (Scheme 4). The use of 4 as a tautomeric sub-unit gives the advantage of monitoring the switching easily since the end keto tautomer absorption is red shifted in respect of the enol standby state.
The aim of the study is to investigate the effect of the structural modifications and the solvent of the ground and excited state energy landscape by using DFT calculations and then to define structural conditions needed for a potentially working conjugated proton crane. Although the exited state energy landscape of 5 and 7 has been studied theoretically in the gas phase before [38], the current investigation gives new in-depth information about the possible performance of both ground and excited states and considers the solvent and substituents effects together.

2. Theoretical Simulations

Quantum-chemical calculations were performed using the Gaussian 09 D.01 program suite [54]. If not specified otherwise, the M06-2X [55,56] functional with the TZVP [57] basis set were used for the calculations in the ground and excited states, respectively. The M06-2X functional was selected for three reasons. First, it has shown that there is good empirical relations between the relative stability of the tautomers, defined as ΔE, and the experimentally determined ΔGo values of tautomeric azo and azomethyne dyes in cyclohexane and acetonitrile [50,58,59], and of heterocyclic tautomers [49,53]. Second, the use of M06-2X provides very good predictability of the E/Z isomerization ratio in some rotary switches [51,60]. Third, the use of this function has given acceptable results in describing the solvent assisted proton transfer in 4 [61,62].
The implicit solvation was described using the polarizable continuum model (the integral equation formalism variant, IEFPCM, as implemented in Gaussian 09) [63]. All ground state structures were optimized without restrictions, using tight optimization criteria and an ultrafine grid in the computation of two-electron integrals and their derivatives. The true minima were verified by performing frequency calculations in the corresponding environment. The transition states were estimated using the Berny algorithm [64] and again verified by performing frequency calculations in the corresponding environment.
The time dependent DFT (TD-DFT) method [65,66,67] was used for singlet excited state optimizations, again without restrictions, applying tight optimization criteria and an ultrafine grid in the computation of two-electron integrals and their derivatives. The excited state calculations are limited to S1, because, as shown in Table S1, the S2 state is forbidden, and the proton crane action is linked to the S1 landscape according to previous results [38]. In the case of 5 the excited energy landscape was confirmed by using CAM-B3LYP [68]. The absorption spectra of the tautomers were simulated by using B3LYP [69] density functional taking into account its good predictability [67].
The natural bond orbitals (NBO) [70] analysis was performed as implemented in Gaussian 09) [63].

3. Results and Discussion

The structure of 4 suggests possibility for enol (E) and the keto (K) tautomer exchange. However, the large distance between the proton donor (O) and proton acceptor (N) sites makes the direct intramolecular proton transfer (PT) not possible, and thus, solvent-assisted PT is the only possible mechanism for conversion between the tautomers in this molecule [71,72]. Absorption and emission are observed from the E tautomer only in non-protic solvents at room temperature [73], whereas in aqueous solution, fluorescence was observed from the K form only [74]. Dual emission was observed in alcohols [75,76,77]. In the ground state, the E tautomer is the only existing form in non-protic and dry protic solvents, while both tautomers are stable in comparable amounts in the presence of water. Solvent-assisted excited-state proton transfer was proposed, based on the experimental data, in protic solvents [72,76,78], where the process is facilitated by the increased excited state acidity/basicity of the enol/imino groups [79]. It should be noted that concentration effect also plays a substantial role [73,80] in the PT through intermolecular interactions of species. There is a large number of papers where the solvent assisted PT mechanism has been proven by a variety of experimental measurements [81,82,83,84] and theoretical simulations [61,62,85,86].
The energy landscape of 4 is quite simple and represents an idealistic case for potential PT-based switching. As seen in Figure 1a and Figure 2a, Table 1, and according to the experimental results discussed above, the compound is presented as a single E tautomer (E1K) in the ground state, which upon excitation (E*) goes through ESPT to the more stable K*, relaxing then to the unstable K (K2E) and finally reaching again the E state through a thermal relaxation. NBO analysis of the tautomeric states, shows donor-acceptor interaction between OH group and quinoline nitrogen with a charge transfer from the enol to the quinoline nitrogen (Table S3, Scheme S1). Sufficient charge transfer in the opposite direction is observed in the keto tautomer. The substantially larger dipole moment of the keto tautomer (Table 2) leads to increased stability in ground state with increasing polarity of the solvent (Figure 1a), but not in an extent to be eventually observed in solution. A similar, but weaker, effect is observed in the excited state. The simulated absorption spectra (Figure S2a) clearly show the advantage of the PT-based switching. The enol tautomer absorbing around 300 nm under suitable condition can give rise of a strongly red shifted keto form.
Another option to make the proton transfer in 4 intramolecular is to attach a crane sub-unit which performs series of consecutive intramolecular PT steps through intramolecular hydrogen bonding (for example, compounds 1 and 3 in Scheme 2) [18,19,36]. In the case of the conjugated proton cranes the overall mechanism includes four tautomers and the corresponding transition states (TS1-3) between them as sketched in Scheme 3. Correspondingly, the energy landscape in ground and excited state is governed by the energy and relative stability of these structures. The existence of intramolecular hydrogen bonding suggests limited specific solute-solvent interactions (except in TS2) and the solvent can be considered as a continuum solvation, affecting the relative stability through dipole moments of the corresponding structures. Therefore, solvents, covering a wide range of dielectric constants, were selected in the current study. Toluene (ε = 2.37) and acetonitrile (ε = 35.6) are aprotic solvents, while formamide (ε = 108.9) is a strongly polar solvent with dualistic proton acceptor and proton donor nature [87,88], selected to provide the upper border of the experimentally affordable dielectric constant. It should be noted that in all studied compounds the major, environmentally defined changes in the energy landscape are observed from a vacuum to toluene and then to acetonitrile. The differences between acetonitrile and formamide are negligible both as shapes and values.
The attachment of the unsubstituted pyridine crane sub-unit (compound 5) gives interesting information for comparison. Compared with 7-hydroxyquinoline 4, the atomic charge of the quinoline nitrogen in 5E1K increases. (Tables S3 and S4) following the increased resonance π-delocalization and charge transfer between the aromatic sub-units. Moreover, the structure is almost planar (Table 2), supporting the effective π-delocalization. Compared to the unsubstituted pyridine, the atomic charge of the nitrogen atom of the crane sub-unit rises in 5, indicating increased basicity. However, the basicity of the pyridine nitrogen atom remains insufficient to attract the tautomeric proton from the oxygen (no K1K form). Further NBO analysis of 5E1K displays almost negligible donor–acceptor interaction by the quinoline and pyridine sub-units (Scheme S1). According to Table 1 and, as shown in Figure 1b, the enol E1K form remains the most stable (single) tautomer. Although some additional stabilization of the end state K2E is achieved depending on the environment, the values of the relative energies do not suppose its existence in solution before the excitation. Since the K1K form is not presented, the ground state PT occurs directly to K2K. As seen, the respectively large barrier (perpendicular TS2, Figure 1b), due to its large dipole moment (~8D in vacuum), is substantially affected by the solvent environment. The competitive basicity of the nitrogen atoms (Table S2) makes the K2K and K2E forms almost equally stable with the proton being practically delocalized (negligible TS3). The comparable stability of K2K and K2E does not allow the pure end state of switching, as defined in Scheme 1, to be achieved. This conclusion is also confirmed by the NBO analysis of the aforementioned tautomeric states (Scheme S1), showing that the protonated pyridine sub-crane of K2K acts as very strong donor in respect of the deprotonated tautomeric sub-unit. Most probably, the charge transfer character of K2K is responsible for its stabilization in respect of K2E. As mentioned above, the ground state-relative energies of the tautomeric forms indicate that E1K is the solely presented form, which can be excited. The excited state energy landscape (Figure 2b) approximates the shape suggested by Sobolewski et al. [36,38] in structurally similar conjugated proton cranes. As an additional verification, the excited energy landscapes obtained by the currently used M06-2X density functional and the CAM-B3LYP one, showing essentially the same changes, are compared in Figure S1. In a gas phase a very fast ESIPT proton transfer from the excited enol leads to a perpendicular minimum and populates both E1K and K2K in the ground state through a conical intersection. The landscape in toluene is essentially the same. In polar solvents, the ESIPT from E1K* should yield K1K* and eventually K2K* through a very low barrier. The strong red shift of the absorption of K2K and K2E, comparing to E1K, is the overall effect expected in the optical spectra (Figure S2b).
However, the relative stability gradient from K1K* to K2E* does not seem sufficient to achieve the needed ground state population of the latter. This fact and the equal stability of K2K and K2E make compound 5, at least according to these simulations, unsuitable for a proton crane. One of the options for improvement is to tune the basicity of the pyridine nitrogen atom through substitution in positions 4′ and 5′ in order to avoid steric hindrance with the tautomeric sub-unit.
The substitution of pyridine crane sub-unit at 4′- and 5′- positions by different electron withdrawing (EW) and electron donating (ED) groups influences the basicity of pyridine nitrogen and is insignificant for the tautomeric sub-unit (Scheme 4) [89]. The 4′-substituent is considered as para in respect to the pyridine nitrogen and affects the π-π delocalization depending on its donor-acceptor properties. Compared to the unsubstituted 5, the donor character of NH2 in 6 (+M and +I effects) leads to strong involvement of the nonbonding electron pairs of amine nitrogen to the aromatic π-delocalization. This increases the π-electron density of the ring and the basicity of pyridine nitrogen, which favors the proton attraction form the tautomeric sub-unit. As seen from Figure 1c and Figure 2c, the introduction of an amino (or dimethylamino, Figure S3a) in para position in respect of the pyridine nitrogen changes the energy landscape substantially. The increased basicity of the N1′ leads to stabilization of the K1K form in polar solvents to the extent that it could be observed in acetonitrile/formamide (<1% according to Table 1). The red shift of the K1K absorption in respect of E1K (Figure S2c) provide conditions for selective excitation even if both are presented simultaneously in solution. The increased basicity effect leads to stabilization of K2K in respect of K2E. Upon excitation of E1K in solution the vertically excited state relaxes to K1K* and then should follow the decreasing energy trend to K2E*. The situation is more favourable in toluene, where the TS2* and TS3* are fairly low, promising a fast switching process. According to Figure S2c the process can be monitored by optical spectroscopy due to the good band separation.
The meta (5′) positioned amino group reduces the pyridine nitrogen basicity and changes the relative stability of K2K (Figure 1d, Figure S3b for 9) in comparison with 5, making the K2E form slightly more stable. The excited E1K relaxes in solution either directly (in toluene) or through a series of consecutive PT steps (in acetonitrile and formamide) to K2E*. The replacement of the amino with dimethylamino group in position 4′ or 5′ (8 and 9) does not change the ground state landscape in comparison with 6 and 7. The weak σ-donating ED CH3 group in 10 by its +I effect leads (Figure S3c) to the same ground state relative stability of the tautomeric forms as in 5.
As expected, the introduction of electron acceptor group in the crane decreases basicity of the pyridine nitrogen atom (due to the –M effect in 11–12, Scheme S1; and –I effect in 13) and increases the conjugation between the sub-units, leading to a larger barrier of rotation around the axle. As a result the K2K forms in 11–13 become less stable in comparison with the K2E ones (Figure 1e,f and Figure S3d). As shown in Figure 2e,f, the excitation of the enol tautomer in 11 and 12 leads to a perpendicular TS2* state, which should populate upon relaxation both ground state E1K and K2K, resulting in low efficiency of switching to K2E. In both compounds K2E* forms are substantially less stable in respect of TS2*, reducing the possibility for direct relaxation to the ground end state.

4. Conclusions

The ground and excited state energy landscape of conjugated proton cranes based on substituted 8-(pyridin-2-yl)quinolin-7-ols was investigated by means of DFT and TD-DFT calculations in various environments. The results indicate that the appropriate condition for switching requests different tautomeric states of the tautomeric sub-unit to be achieved in ground and in excited states. According to the simulations, such conditions were suggested when the electron donating amine substituent is introduced in the pyridine crane sub-unit. The effect of the solvent also plays a substantial role through stabilization based on the relative polarity of the tautomers.

Supplementary Materials

The following are available online at https://www.mdpi.com/2072-666X/11/10/901/s1, Figure S1: Comparison between the excited (S1) state energy landscapes (in relative energies in kcal/mol units) of of 5 obtained by CAM-B3LYP/TZVP (a) and M06-2X/TZVP (b, the same as in Figure 2b) in vacuum (green), toluene (red), acetonitrile (blue) and formamide (violet). The filled circles represent optimized structures.; Figure S2: Predicted absorption spectra (B3LYP/TZVP//M06-2X/TZVP) in toluene (red) and in acetonitrile (blue) of the different tautomers of 4–7 (a–d) and of 11–12 (e–f): E1K—solid line, K2E—dashes, K2K—dots, K1K (only in 6 in acetonitrile) – black dots. The spectra in formamide are practically identical to those in acetonitrile.; Figure S3: Ground state energy landscape (change of the relative energies in kcal/mol units) of 8–10 (a–c) and 13 (d) in vacuum (green), toluene (red), acetonitrile (blue) and formamide (violet). The filled circles represent optimized structures.;, Table S1: Frank-Condon states (vertical transitions) of the E1K form of 4–7 and 11–12.; Table S2: Important structural parameters of the tautomers of 4–7 and 11–12 in ground and excited S1 (in brackets) state.; Table S3: Natural charges in vacuum in selected atoms in 4 and 5 in ground state and excited state (in brackets).; Table S4: Milliken atomic charges in vacuum in selected atoms in ground state.; Scheme S1: Ground state NBO charges of the different tautomers of 4–7 and of 11–12. The donor (in blue)–acceptor (in red) interactions in the molecular backbones are presented by summing the natural charges of the different parts in the molecules.

Author Contributions

Conceptualization, L.A.; methodology, L.A.; validation, A.G. and L.A.; investigation, L.A. and A.G.; resources, A.G.; writing—original draft preparation, review and editing, L.A. and A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Bulgarian National Science Fund, project DN09/10/2016 MolRobot.

Acknowledgments

The financial support from Bulgarian National Science Fund (project DN09/10/2016 MolRobot) is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Micromachines 11 00901 sch001 550
Scheme 1. Proton crane in standby, transfer and end states (from left to right). The proton donor parts are given in red, while the proton acceptors are marked in blue.
Scheme 1. Proton crane in standby, transfer and end states (from left to right). The proton donor parts are given in red, while the proton acceptors are marked in blue.
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Micromachines 11 00901 sch002 550
Scheme 2. Examples for proton cranes: single-bond axle non-conjugated (1) and conjugated (3), double-bond axle (2).
Scheme 2. Examples for proton cranes: single-bond axle non-conjugated (1) and conjugated (3), double-bond axle (2).
Micromachines 11 00901 sch002
Micromachines 11 00901 sch003 550
Scheme 3. Possible tautomers in 5 given as an example of the studied molecules.
Scheme 3. Possible tautomers in 5 given as an example of the studied molecules.
Micromachines 11 00901 sch003
Micromachines 11 00901 sch004 550
Scheme 4. Studied compounds presented as the most stable ground state form. The C1C6C2′N1′ dihedral angle is marked as α.
Scheme 4. Studied compounds presented as the most stable ground state form. The C1C6C2′N1′ dihedral angle is marked as α.
Micromachines 11 00901 sch004
Micromachines 11 00901 g001 550
Figure 1. Ground state energy landscape (relative energies in kcal/mol units) of 4–7 (a–d)) and 11–12 (e,f) in vacuum (green), toluene (red), acetonitrile (blue) and formamide (violet). The filled circles represent optimized structures.
Figure 1. Ground state energy landscape (relative energies in kcal/mol units) of 4–7 (a–d)) and 11–12 (e,f) in vacuum (green), toluene (red), acetonitrile (blue) and formamide (violet). The filled circles represent optimized structures.
Micromachines 11 00901 g001
Micromachines 11 00901 g002 550
Figure 2. Excited (S1) state energy landscape (relative energies in kcal/mol units) of 4–7 (a–d)) and 11–12 (e,f) in vacuum (green), toluene (red), acetonitrile (blue) and formamide (violet). The filled circles represent optimized structures. The corresponding vertical transitions are given in dashes.
Figure 2. Excited (S1) state energy landscape (relative energies in kcal/mol units) of 4–7 (a–d)) and 11–12 (e,f) in vacuum (green), toluene (red), acetonitrile (blue) and formamide (violet). The filled circles represent optimized structures. The corresponding vertical transitions are given in dashes.
Micromachines 11 00901 g002
Table
Table 1. Relative energies of the tautomers of 4–7 and 11–12 in ground and excited S1 (in brackets together with the corresponding oscillator strength) state.
Table 1. Relative energies of the tautomers of 4–7 and 11–12 in ground and excited S1 (in brackets together with the corresponding oscillator strength) state.
CompoundEnvironmentΔE, kcal/mol
E1KK1KK2KK2E
4vacuum0.0
(20/0.08)		--15
(0.0/0.08)
toluene0.0
(21/0.13)		--11
(0.0/0.15)
acetonitrile0.0
(22/0.22)		--6.7
(0.0/0.27)
5vacuum0.0
(18/0.26)		-
(-)		9.6
(-)		9.7
(6.2/0.05)
toluene0.0
(12/0.38)		-
(-)		7.8
(-)		7.9
(1.37/0.10)
acetonitrile0.0
(9.7/0.54)		-
(1.22/0.32)		5.6
(1.18/0.52)		5.4
(0.0/0.20)
6vacuum0.0
(12/0.23)		-
(-)		7.4
(8.3/0.23)		9.1
(0.0/0.05)
toluene0.0
(-)		-
(6.4/0.30)		5.5
(5.1/0.34)		7.6
(0.0/0.10)
acetonitrile0.0
(-)		2.2
(3.3/0.52)		3.2
(1.6/0.44)		5.6
(0.0/0.19)
7vacuum0.0
(14/0.30)		-
(-)		11
(-)		10
(0.86/0.04)
toluene0.0
(11/0.46)		-
(-)		9.6
(-)		8.7
(0.0/0.07)
acetonitrile0.0
(9.6/0.68)		-
(5.6/0.37)		7.1
(4.4/0.55)		6.2
(0.0/0.15)
11vacuum0.0
(-)		-
(-)		11
(-)		9.7
(24/0.14)
toluene0.0
(-)		-
(-)		9.7
(-)		7.4
(21/0.13)
acetonitrile0.0
(-)		-
(-)		7.0
(1.06/0.24)		4.1
(6.8/0.15)
12vacuum0.0
(-)		-
(-)		9.7
(-)		8.1
(14/0.06)
toluene0.0
(-)		-
(-)		8.3
(-)		6.1
(11/0.17)
acetonitrile0.0
(4.9/0.88)		-
(-)		6.4
(1.20/0.87)		3.4
(0.88/0.85)
Table
Table 2. Important structural parameters of the tautomers of 4–7 and 11–12 in ground and excited S1 (in brackets) state.
Table 2. Important structural parameters of the tautomers of 4–7 and 11–12 in ground and excited S1 (in brackets) state.
CompoundEnvironmentE1KK1KK2KK2E
Dipole moment, in D
4vacuum/
acetonitrile		1.2/1.8
(1.3/1.9)		 7.9/12.8
(9.0/13.3)
52.2/3.1
(2.3/3.3)		-/-
(-/6.6)		5.4/8.7
(-/8.2)		6.1/10.0
(7.0/10.6)
63.8/5.4
(4.0/-)		-/8.3
(-/8.7)		5.6/8.7
(5.3/8.1)		4.8/8.0
(5.2/8.4)
73.2/4.3
(3.3/5.3)		-/-
(-/8.8)		6.5/10.3
(9.0/10.2)		6.1/9.7
(6.5/10.7)
113.4/4.0
(-/-)		-/-
(-/-)		8.6/12.3
(-/11.7)		10.7/15.5
(11.5/15.5)
126.2/7.8
(-/9.4)		-/-
(-)		6.5/8.9
(-/9.1)		9.0/13.1
(10.4/14.7)
dihedral angle α (Scheme 4), in o
5vacuum22
(16)		-
(-)		180
(-)		180
(160)
toluene24
(15)		-
(-)		180
(-)		173
(157)
acetonitrile25
(15)		-
(42)		180
(163)		164
(155)
6vacuum19
(13)		-
(-)		179
(169)		176
(160)
toluene21
(-)		-
(39)		179
(167)		174
(157)
acetonitrile24
(-)		2
(20)		179
(166)		169
(154)
7vacuum23
(15)		-
(-)		180
(-)		176
(163)
toluene24
(15)		-
(-)		179
(-)		168
(161)
acetonitrile26
(15)		-
(35)		178
(169)		162
(160)
11vacuum22
(-)		-
(-)		180
(-)		173
(159)
toluene23
(-)		-
(-)		180
(-)		171
(160)
acetonitrile24
(-)		-
(-)		179
(157)		168
(165)
12vacuum22
(-)		-
(-)		180
(-)		179
(160)
toluene23
(-)		-
(-)		180
(-)		176
(159)
acetonitrile25
(17)		-
(-)		180
(180)		169
(160)

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Georgiev, A.; Antonov, L. 8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design. Micromachines 2020, 11, 901. https://doi.org/10.3390/mi11100901

AMA Style

Georgiev A, Antonov L. 8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design. Micromachines. 2020; 11(10):901. https://doi.org/10.3390/mi11100901

Chicago/Turabian Style

Georgiev, Anton, and Liudmil Antonov. 2020. "8-(Pyridin-2-yl)quinolin-7-ol as a Platform for Conjugated Proton Cranes: A DFT Structural Design" Micromachines 11, no. 10: 901. https://doi.org/10.3390/mi11100901

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