Next Article in Journal
Efficient Catalytic Hydrogenation of Lignin-Derived Phenolics Under Mild Conditions
Previous Article in Journal
DFT Investigation of the Mechanism of Methoxycarbonylation of Styrene by Palladium Chloride
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Chemistry
Volume 6
Issue 6
10.3390/chemistry6060097
chemistry-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

8-(Pyridin-2-yl)quinolin-7-ol and Beyond: Theoretical Design of Tautomeric Molecular Switches with Pyridine as a Proton Crane Unit

by
Lidia Zaharieva
1,
Daniela Nedeltcheva-Antonova
1,2 and
Liudmil Antonov
1,*
1
Institute of Electronics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
2
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Chemistry 2024, 6(6), 1608-1621; https://doi.org/10.3390/chemistry6060097
Submission received: 31 October 2024 / Revised: 3 December 2024 / Accepted: 4 December 2024 / Published: 6 December 2024
(This article belongs to the Section Physical Chemistry and Chemical Physics)
Browse Figures
Versions Notes

Abstract

:
Long-range proton transfer in several conjugated proton cranes, originating from 7-hydroxy quinoline as a proton transfer platform, has been investigated theoretically by means of DFT and TD-DFT methodology. Major emphasis was given to their applicability to provide clean switching upon irradiation. The border conditions require the existence of a single enol tautomer in the ground state, which under excitation through a series of consecutive exited and ground state intramolecular proton transfer steps is transferred to the keto tautomer. It was shown that the most suitable candidates are based on using iso-quinoline, pyrimidine and 4-nitropyridine as proton crane units. Their suitability is a function of aromaticity changes, the basicity of the nitrogen atom from the proton crane unit and the structural effects originating from their conjugation with 7-hydroxy quinoline.

1. Introduction

Proton cranes are monomolecular switching systems, where a proton is transferred like a cargo over a large distance within the same molecule under light irradiation. Some typical, experimentally existing, examples based on the 7-hydroxy quinoline (7OHQ) platform (HMMQ, HQC, 7HQ-SB and HQBT) are shown in Scheme 1. Since the overall process is based on long-range proton transfer (PT), their switching mechanism differs substantially from the switching based on synchronous or consecutive double short-range PT, as it is in HQCA and HQB, where the end-station proton acceptor (the nitrogen atom in the 7OHQ moiety) is populated by a proton, but with not by the same proton, which leaves the proton donor OH group on the other side of the molecule. In fact, proton cranes use PT as an elementary process, which makes them a less exploited alterative to the traditional inner motion monomolecular photoswitches mainly based on E/Z isomerization [1,2,3,4,5,6].
Chemistry 06 00097 sch001
Scheme 1. Tautomeric forms of 7-hydroxy quinoline, long-range PT-based proton cranes (in chronological order HMMQ [7,8,9], HQC [10], 7HQ-SB [11,12] and HQBT [13]) and double short-range PT switching systems (HQCA [14,15] and HQB [16]).
Scheme 1. Tautomeric forms of 7-hydroxy quinoline, long-range PT-based proton cranes (in chronological order HMMQ [7,8,9], HQC [10], 7HQ-SB [11,12] and HQBT [13]) and double short-range PT switching systems (HQCA [14,15] and HQB [16]).
Chemistry 06 00097 sch001
The overall mechanism of action of the proton cranes is based on excited-state intramolecular PT [17,18,19,20,21,22] (ESIPT) as the first step, which leads to the transfer of the proton from the tautomeric backbone (7-hydroxy quinoline is the most popular example, although other cases are also listed in the literature [23]) to the proton crane unit which is expected to transport the proton further. The character of this intermediate state determines the way in which the proton cargo can reach its final destination—the quninolyl nitrogen atom. If the initial ESIPT process leads to the formation of a bipolar, zwitterionic, structure with a protonated proton crane unit and a deprotonated tautomeric backbone, the proton is transferred by intramolecular rotation and intramolecular neutralization (another ESIPT) to the end-station nitrogen atom entirely in the excited state. These proton cranes are called unconjugated and their mechanism of action has been described in detail by a number of studies [7,8,9,24]. If the initial ESIPT process leads to a classical tautomeric rearrangement, the obtained intermediate excited-state tautomer KE* (Scheme 2) returns back to the ground state via conical intersection (where PTP and PCU are perpendicular each other), simultaneously populating KE and KK states, which potentially relax, respectively, to the initial E and final K forms. This excited state behavior is a result of the change in the nature of the axle, connecting PTP and CPU, from a single bond in E to a double bond in KE. The overall switching mechanism has been described in detail theoretically [25] and proven experimentally [13]. In both conjugated and nonconjugated proton cranes, the obtained end K form returns back thermally to E in the ground state.
Chemistry 06 00097 sch002
Scheme 2. Tautomeric states in a conjugated proton crane with a rigid proton crane unit (PCU), containing nitrogen as a proton acceptor/donor site and 7-hydroxy quinoline as a proton transfer platform (PTP). The proton donor and proton acceptor atoms are given in red and blue, resp. The twisting angle α is indicated in magenta. In systems with flexible PCUs, like 7HQ-SB, the efficiency of switching is low due to side processes, as discussed in detail in [26].
Scheme 2. Tautomeric states in a conjugated proton crane with a rigid proton crane unit (PCU), containing nitrogen as a proton acceptor/donor site and 7-hydroxy quinoline as a proton transfer platform (PTP). The proton donor and proton acceptor atoms are given in red and blue, resp. The twisting angle α is indicated in magenta. In systems with flexible PCUs, like 7HQ-SB, the efficiency of switching is low due to side processes, as discussed in detail in [26].
Chemistry 06 00097 sch002
Very recently, we have described the conjugated proton crane HQBT (8-(benzo[d]thiazol-2-yl)quinolin-7-ol, Scheme 1), where clean forward switching (i.e., from pure E to pure K) occurs in acetonitrile upon irradiation with light at 365 nm [13]. Both forward and backward processes can be monitored by conventional spectroscopy. The experimental investigations allowed us to shed light on the factors underlying the overall action of the proton cranes—the relative stabilities of the tautomers in the ground state, the basicity of the proton acceptor in the PCU, the strength of the stabilizing intramolecular hydrogen bonding, the possibilities for steric hindrance, etc. Each of these factors can facilitate or hinder the switching process and has to be taken into account in the molecular design.
In the current investigation, we consider, by using DFT and TD-DFT methodology, the ground-state tautomerism and the excited-state PT in a series of proton cranes based on 7-hydroxy quinoline as a PTP and pyridine as a PCU (HQPy, Scheme 3), with respect to their potential suitability to achieve clean switching. The effects of the extended aromaticity (HQ1iQ, HQ3iQ and HQ2Q) and changed basicity (HQPm, HQPy′ and HQPy″) are considered as well. In addition, we will take as examples for comparison BPOH [27,28,29,30] and HQBT [13], where experimental data are available, and DMAPP, where the PTP structurally approximates the tautomeric backbone in 7-hydroxy quinoline.
Chemistry 06 00097 sch003
Scheme 3. Structures under discussion: HQPy (8-(pyridin-2-yl)quinolin-7-ol), HQ1iQ (8-(isoquinolin-1-yl)quinolin-7-ol), HQ3iQ (8-(isoquinolin-3-yl)quinolin-7-ol), HQ2Q ([2,8′-biquinolin]-7′-ol), HQPm (8-(pyrimidin-2-yl)quinolin-7-ol), HQPy′ (8-(4-nitropyridin-2-yl)quinolin-7-ol), HQPy″ (8-(4-(dimethylamino)pyridin-2-yl)quinolin-7-ol), BPOH ([2,2′-bipyridin]-3-ol) and DMAPP (3-(dimethylamino)-2-(pyridin-2-yl)phenol).
Scheme 3. Structures under discussion: HQPy (8-(pyridin-2-yl)quinolin-7-ol), HQ1iQ (8-(isoquinolin-1-yl)quinolin-7-ol), HQ3iQ (8-(isoquinolin-3-yl)quinolin-7-ol), HQ2Q ([2,8′-biquinolin]-7′-ol), HQPm (8-(pyrimidin-2-yl)quinolin-7-ol), HQPy′ (8-(4-nitropyridin-2-yl)quinolin-7-ol), HQPy″ (8-(4-(dimethylamino)pyridin-2-yl)quinolin-7-ol), BPOH ([2,2′-bipyridin]-3-ol) and DMAPP (3-(dimethylamino)-2-(pyridin-2-yl)phenol).
Chemistry 06 00097 sch003

2. Theoretical Methodology

Quantum chemical calculations were performed using the Gaussian 16 C.01 program suite [31]. All structures (in both ground and excited state) 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 implicit solvation was described using the Polarizable Continuum Model [32] (the integral equation formalism variant, IEFPCM, as implemented in Gaussian 16). The transition states were estimated using the STQN method [33] and again verified by performing frequency calculations in the corresponding environment.
The M06-2X [34,35] functional with the TZVP [36] basis set was used for the structure optimizations in the ground state. The use of M06-2X provides very good predictability in the ground state [11,13,37,38,39] of tautomeric composition in tautomeric compounds and proton cranes in solution, as well as the E/Z isomerization ratio in some rotary switches.
The TD-DFT method [40,41,42] was used for the singlet excited-state optimizations. For consistency with the ground-state calculations, again the M06-2X functional with the TZVP basis set was used for the optimizations. Previously, we have shown [13] that in the case of HQBT, the results for the excited-state PESs, obtained by M06-2X and CAM-B3LYP, are essentially the same. In addition, the DFT results for both ground and excited state were validated [13] by using domain-based local pair natural orbital coupled cluster theory with single, double and perturbative triple excitations (DLPNO-CCSD(T) [43]) and domain-based local pair natural orbital-similarity transformed equation of motion coupled cluster theory with singles and doubles (DLPNO-STEOM-CCSD [44]), respectively.
The Harmonic Oscillator Model of Aromaticity [45,46,47,48] (HOMA) indices, based on the M06-2X optimized ground-state geometries, were calculated by using the Multiwfn software package [49,50].
Bearing in mind that M06-2X systematically underestimates the absorption band positions [51], the UV-Vis spectral data were predicted by using the B3LYP [52] functional (TZVP basis set) with the M06-2X optimized ground-state geometries. The spectra were simulated according to the methodology described previously by us [53] by using a single Gauss-shaped band half-band width ( Δ ν 1 / 2 ) of 3000 cm−1.

3. Results and Discussion

The available information for the switching mechanism of HQBT [13] allows us to define the most important elements of the ground-state potential energy surface (PES) as screening conditions whether a compound is potentially suitable or not to be a proton crane. First, the clean switching requires a single tautomer in the ground state, not a mixture of forms. In this particular case, the most stable E (for the abbreviations, see Scheme 2) is considered an off-state. The on-state, K, has to be substantially higher in energy. Second, since the population of the ground state upon irradiation occurs through a conical intersection between S1 and S0, with PCU and PTF being perpendicular in the first singlet excited state, the resulting intermediate states KE and KK should be less stable compared to E and K, respectively. This allows, along with the relatively low PT transfer barriers (TS PT(O-N) between E and KE, and TS PT(N-N) between KK and K), for a smooth transfer of the relaxed molecules to the end (E and K) forms. The third requirement is not related to the PES, but is also very important: the spectra of the tautomers have to allow for easy monitoring of the switching process. This will be discussed in detail later.
The relative energies of the tautomers of the studied molecules along with important structural information are collected in Table S1. The ground-state PESs in toluene and acetonitrile are visualized in Figure 1 and Figure S1. The data are obtained for toluene and acetonitrile since either the appearance of additional, competitive channels for long-range proton transfer by forming solvent wires transferring the proton via the intermolecular solvent-assisted mechanism [54,55] or the hindering of the intramolecular PT channel by breaking the intramolecular hydrogen bonding (or both together) are the major disadvantages of the proton cranes in proton donor and proton acceptor solvents. Of course, the proton can be delivered fully intermolecularly by using free bases (like ammonia [56], formates [57] or some heterocycles [58]) or the process can be triggered by ion addition [59,60,61], but these systems are outside the scope of the concept for proton cranes.
The curves in Figure 1 and Figure S1 can be considered from the viewpoint of the available information about HQBT and the reliability of the theoretical DFT methodology proven for this compound. As seen in almost all of studied compounds, the relative stabilization of KK with respect to K is the most critical factor. In HQBT, KK and K are approximately equally stable in toluene, which corresponds to switching of 65% to K, while the higher polarity of the latter gains substantial stabilization in acetonitrile, leading to a complete (>95%) shift to it. The exact values are given in Table S1.
As shown in Table S1 and seen from Figure 1, considering the assumptions for successful switching defined above, it seems that HQPy and HQ2Q are unsuitable for proton cranes in toluene due to the unsuitable relative stability of KK and K. The same can be concluded for HQPy″ and DMAPP. The relatively small difference between the dipole moments of K and KK (Table S1) does not allow to achieve additional noticeable stabilization of the former in acetonitrile (even the changes in HQPy″ are in the opposite direction; see Figure S1). In HQ1iQ, due to the substantial steric hindrance between PTP and PCU, the PT N-N barrier is large (especially in acetonitrile), being comparable to the twisting transition state, which could make the direct relaxation to E more favorable after KK and KE are populated. The situation with BPOH is doubtful, because no experimental evidence for obtaining KK and K is found [27,29,30,62] and it can be concluded that the transfer from KK directly to E is more efficient. The transition from KK to K in this case occurs through a five-membered ring and, therefore, is energy-demanding. The remaining compounds fulfill the basic requirements for proton cranes, and the twisting barriers provide a variety of faster backward relaxation rates compared to HQBT. The predicted absorption spectra of HQ3iQ, HQPm and HQPy′ are shown in Figure S2. As seen, HQ3iQ has a spectral pattern very similar to HQBT—although the spectra of E and K are overlapped, the switching event can be monitored by the appearance of the red-shifted band of K. The monitoring could be even easier in HQPm and in HQPy′, where the overlapping is low and the appearance of K can be conveniently detected. However, the slightly decreased intensity of the spectra of the E tautomer in the visible range (and hence the low efficiency of the irradiation) also has to be taken into consideration. Although the simulated spectra of KK are also shown in the figure for completeness, the relative stabilities of KK and K and the almost barrierless PT between them (see Table S1) suggest that there are no conditions in which one can expect a measurable amount of KK to be detected in these three compounds.
Once it has been predicted that HQ3iQ, HQPm and HQPy′ are suitable for switching, based on long-range PT, it is important to look for the possible reasons for their suitability and the unsuitability of the others. As seen, there are two critical points to be explained—the relative stability of the pair KK/K, which is the key to clean switching to K, and the height of the twisting barrier (TS(KE-KK)), which determines the backward relaxation rate from K to E and, hence, the efficiency of the accumulation of K over the time.
The switching cycle from E to K and back in the studied compounds potentially includes four different tautomers, whose stability is affected, among the other structural and electronic factors [63], by the aromaticity of the involved rings [64,65,66,67,68,69,70,71,72,73]. Very recently, London and co-authors [74] have shown that the change in the aromaticity of the PTP can strongly affect the tautomerism in molecular switches with salicylideneaniline PCUs. The calculated HOMA (Harmonic Oscillator Model of Aromaticity [45,46,47,48]) indices, shown in Figure 2, allow us to make some interesting comparisons. Considering 7OHQ as an example for heterocyclic tautomerism, it is seen that the transfer of the proton of O to N leads to a reduction in the aromatic character of the aromatic rings, more substantially in the quinoid A ring (Figure 2), which is the “price” for K being substantially less stable (Table S1). In all compounds, where 7OHQ plays a role of PTP, the enol tautomer has a benzenoid structure which determines its stability. As seen, in all compounds from this series, the aromatic character of the A-ring is reduced, compared to 7OHQ, while the B-ring’s aromaticity increases (more substantially in HQBT). The changes in the aromaticity of K can indicate its relative stability with respect to E. As seen, the relative energy of K becomes substantially less compared to 7OHQ. One of the reasons, but not the sole reason, is the improved aromaticity of the B-ring and approximately the same in the A-ring. In the case of BPOH and DMAPP, the change from E to K leads to a substantial reduction in the aromaticity of the A-ring, which affects the relative stabilities more strongly. The well-defined charge transfer character of K, compared to E, also plays a role in the overall stabilization. HQBT and HQPy′, in which the charge transfer is well defined, have the most stable and almost planar K forms. Of course, planarity is also a function of the strength of the intramolecular hydrogen bonding, which will be discussed below. As seen from Table S1, the length of the axle could also indicate the extent of the charge transfer—it is shorter in K for all studied compounds, but the difference is most pronounced for HQBT, HQPm and HQPy′.
The situation with the relative stability of KK and K is very complicated because both compounds have quinoid structures that involve a combination of different rings—A–C in KK and A–B in K. Generally, in all KK forms, due to the tautomeric double-bond rearrangement, the axle has a double-bond character, which leads to a strong tendency for planarization. HQ1iQ, in which the steric hindrance between the C=O group from the PTP and the condensed aromatic ring from the PCU is too strong, is an exception. As seen from the summed natural charges in Figure 2, this leads to a much stronger charge transfer character compared to K. Considering the series from HQPy to HQPm, the aromaticity of the C-ring is lower in KK, whereas the opposite tendency is observed for B. However, both rings are aromatic, while in both tautomers the aromaticity of the A-ring is low, especially in KK. Perhaps the clear non-aromaticity of A in HQPy′ and HQPm can be attributed to the destabilization of KK with respect to K, which is much needed to fulfil the requirements for clean switching. In the series with quinoline and iso-quinoline PCUs (HQ2Q, HQ1iQ and HQ3iQ), the PTP follows the same pattern as above, while the aromaticity of the C-ring slightly decreases from KK in HQ3iQ. The latter is probably compensated by a slight increase in the D-ring. It is clear that how the dependence of the aromatic character of the condensed rings affects the relative stability of KK with respect to K does not seem straightforward—the non-aromatic character in the oxygen connected A-ring in KK could be an explanation for K being more stable, but HQ2Q, where KK is predicted to be more stable, shows exactly the same pattern.
Except for the aromaticity of the PTP and PCU, the relative stability of KK and K should be a function of the basicity of the competing nitrogen atoms, i.e., the ability of NPTP and NPCU to attract the proton in the KK and K tautomer, respectively. It is possible to find in the literature the pKa values for most of the PCUs used here as an estimate of the basicity of the corresponding nitrogen atom: 1.2 for benzothiazole [75], 5.23 for pyridine, 1.23 for pyrimidine, 1.61 for 4-nitropyridine, 9.17 for 4-aminopyridine, 4.90 for quinoline and 5.40 for iso-quinoline [76]. These data can be roughly interpreted as an indicator of how strongly the tautomeric proton in the K tautomer is attracted by NPCU, which can potentially lead to the formation of KK. Compared to HQPy, where the stability of the KK and K tautomers are approximately the same, by looking for a tendency related to the pKa values, the KK tautomer is less stable in HQPm and HQPy′, but more stable in HQ2Q. In the switches, where the iso-quinoline is used as a PCU, the KK form is more stable, although the pKa values of iso-quinoline are higher, compared to pyridine. It is impossible to estimate experimentally the basicity of the nitrogen atom in the K tautomer of 7OHQ, and therefore, as an approximation for the basicity of NPTP (in KK) and NPCU (in K), the natural charges could be used (Table S1). Here, the more negative charge of NPTP in KK could mean stronger attraction of the proton and the formation of K (i.e., K more stable). As seen from Table S1, this is the case in HQ1iQ, HQ3iQ and HQPy′, but not in HQPm. The values for HQBT and HQPy do not indicate that K and KK have approximately the same stability. Obviously, the reasons for KK or K being more stable or less stable are not simple to define, and it is most probable that the reasons are related to aromaticity, basicity and some steric factors together.
The recent experimental and theoretical study of the photochemical behavior of HQBT revealed the switching mechanism in the conjugated proton cranes [13,24]. It is illustrated in Figure 3, where the excited-state PESs of the studied compounds are shown. The only existing ground-state E tautomer, being excited upon irradiation, gives a relatively high energetic Frank–Condon state, which undergoes a very fast ESIPT into KE*. Then, the intramolecular hydrogen bonding is broken in a substantial part of the excited molecules and the PCU rotates, reaching the conical intersection (CI) region with a perpendicular position with respect to the PTP. The excited molecules return back to the ground state exactly in the twisting region (TS(KE-KK), see Figure 1), simultaneously populating KE (and E) and KK (and K under suitable conditions). The switching mechanism is based on two photochemical cycles: the fast, determined by the ESIPT and excited-state twisting, E→E*→KE*→CI→KE→E; and the relatively slow, determined by thermal backward relaxation, E→E*→KE*→CI→KK→K→E (the last step here is thermal). This combination allows the first cycle to play a role of a pumping process, which operates until all molecules return back to E from the excited state are transferred into K, providing clean switching from E to K. The efficiency is favored on one side by the as-fast-as-possible ESIPT, excited-state twisting to the CI and the relaxation of KE to E in the ground state, and on the other side by the fast transfer of KK into K and the as-slow-as-possible thermal backward process from K to E. Some of the factors, such as the relative stability of KK with respect to K and the ground-state IPT barriers (TS(PT O-N) and TS(PT N-N)) have already been discussed above.
Although the PES of HQBT looks flat, the CI region is reached in a few picoseconds following an ESIPT in the femtosecond time scale [13]. According to the experimental measurement, the rate constant of the overall switching process from E to K is 0.19 ± 0.01 s−1 in toluene (0.23 ± 0.07 s−1 in acetonitrile), while for the thermal backward process, these values are 0.0408 ± 0.002 s−1 and 0.055 ± 0.024 s−1, respectively. Returning back to Figure 3, it is clear that a Frank–Condon E state with higher energy and a ballistic ESIPT with simultaneous twisting around the axle could be very useful. HQBT is planar in all of its tautomers, while in the rest of the compounds, due to slighter or stronger (as in HQ1iQ) steric hindrance, the PCU in the E form is already twisted with respect to the PTP (see Table S1 and Figure 2). This makes excited-state twisting much easier. As seen from Figure 3, in most of the compounds, the Frank–Condon state is followed by a ballistic ESIPT with simultaneous twisting, which directly ends in the CI region (HQ1iQ, HQ2Q and HQPy′ as the most representative cases), which means that in these compounds, the first pumping photochemical cycle could be faster with respect to HQBT. Then, the accumulation of K depends on one side on the relative stability of KK and TS(PT N-N), which finally leads to the selection of HQ3iQ, HQPm and HQPy′ as suitable candidates. Compared to the ground-state twisting barrier which determines the rate of the backward process, according to the data shown in Figure 1 and collected in Table S1, ground-state relaxation to E should be substantially faster in HQ3iQ and HQPm (the energy barrier of 17–19 kcal/mol against 24–26 kcal/mol in HQBT in toluene), while in HQPy′ the rate should be approximately the same. This makes the latter compound a suitable switching system to be synthesized and studied experimentally.

4. Conclusions

The applicability of a series of compounds, originating from 7OHQ, as conjugated proton cranes was investigated by using DFT and TD-DFT methodology in toluene and acetonitrile. HQBT, a very efficient proton crane, developed by us, was used as a model compound. The screening was provided on the basis of the number of requirements, the most important of which were as follows: the efficient population of the end K form (achieved by the suitable relative stability of the KK and K tautomers, and the ground-state twisting barrier) and the fast excited-state intramolecular proton transfer. The reasons for KK or K being more stable or less stable are complex and are most probably related to aromaticity, basicity and some steric factors together. In all studied compounds, the excited-state proton transfer and rotation are expected to be faster compared to HQBT. According to the obtained results, several of the most suitable candidates were selected, namely HQ3iQ, HQPm and HQPy′, with the last one as the favorite.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/chemistry6060097/s1: Table S1: Relative energies, natural charges and structural parameters, related to the switching, in toluene. The corresponding values for acetonitrile are given in brackets; Figure S1: Ground state PESs of the studied compounds in acetonitrile presented as relative energies (left column) and as relative Gibbs free energies (right column). The values are given in Table S1; Figure S2: Simulated absorption spectra of (from top to down) HQ3iQ, HQPm and HQPy’ in toluene.

Author Contributions

Conceptualization, L.A.; methodology, L.A. and D.N.-A.; investigation, L.Z., L.A. and D.N.-A.; resources, L.A.; writing—original draft preparation, L.A.; writing—review and editing, L.A. and D.N.-A.; visualization, L.A.; supervision, L.A.; project administration, L.A.; funding acquisition, L.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Bulgarian National Science Fund, National Research Program VIHREN grant number KP-06-DV-9/2019.

Data Availability Statement

The data are available upon request.

Acknowledgments

The authors thank the Bulgarian National Science Fund, National Research Program VIHREN by the project T-Motors (contracted as KP-06-DV-9/2019) for the financial support for this investigation. The theoretical calculations were carried out using the infrastructure purchased under the National Roadmap for RI, financially coordinated by the MES of the Republic of Bulgaria (grant No D01-325/01.12.2023).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Molecular Switches, 2nd ed.; Feringa, B.L.; Browne, W.R. (Eds.) Wiley-VCH: Weinheim, Germany, 2011; ISBN 978-3-527-63440-8. [Google Scholar]
  2. Molecular Photoswitches; Pianowski, Z.L. (Ed.) Wiley-VCH: Weinheim, Germany, 2022; Volume 1, ISBN 978-3-527-35103-9. [Google Scholar]
  3. Kathan, M.; Hecht, S. Photoswitchable Molecules as Key Ingredients to Drive Systems Away from the Global Thermodynamic Minimum. Chem. Soc. Rev. 2017, 46, 5536–5550. [Google Scholar] [CrossRef] [PubMed]
  4. Crespi, S.; Simeth, N.A.; König, B. Heteroaryl Azo Dyes as Molecular Photoswitches. Nat. Rev. Chem. 2019, 3, 133–146. [Google Scholar] [CrossRef]
  5. Lameijer, L.N.; Budzak, S.; Simeth, N.A.; Hansen, M.J.; Feringa, B.L.; Jacquemin, D.; Szymanski, W. General Principles for the Design of Visible-Light-Responsive Photoswitches: Tetra-Ortho-Chloro-Azobenzenes. Angew. Chem. Int. Ed. 2020, 59, 21663–21670. [Google Scholar] [CrossRef] [PubMed]
  6. Feringa, B.L. The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture). Angew. Chem. Int. Ed. 2017, 56, 11060–11078. [Google Scholar] [CrossRef] [PubMed]
  7. de Bekker, E.J.A.; Geerlings, J.D.; Varma, C.A.G.O. Mechanism of a Photoinduced Solvent-Assisted Transfer of a Proton to a Specified Remote Target. J. Phys. Chem. A 2000, 104, 5916–5927. [Google Scholar] [CrossRef]
  8. de Bekker, E.J.A.; Pugzlys, A.; Varma, C.A.G.O. Elementary Processes in Photoinduced Proton Transfers in 2-Hydroxy-1-(N-Morpholinomethyl)Naphthalene and 7-Hydroxy-8-(N-Morpholinomethyl)Quinoline in Liquid Solutions. J. Phys. Chem. A 2001, 105, 399–409. [Google Scholar] [CrossRef]
  9. van der Loop, T.H.; Ruesink, F.; Amirjalayer, S.; Sanders, H.J.; Buma, W.J.; Woutersen, S. Unraveling the Mechanism of a Reversible Photoactivated Molecular Proton Crane. J. Phys. Chem. B 2014, 118, 12965–12971. [Google Scholar] [CrossRef]
  10. Vetokhina, V.; Nowacki, J.; Pietrzak, M.; Rode, M.F.; Sobolewski, A.L.; Waluk, J.; Herbich, J. 7-Hydroxyquinoline-8-Carbaldehydes. 1. Ground- and Excited-State Long-Range Prototropic Tautomerization. J. Phys. Chem. A 2013, 117, 9127–9146. [Google Scholar] [CrossRef]
  11. Georgiev, A.; Yordanov, D.; Ivanova, N.; Deneva, V.; Vassilev, N.; Kamounah, F.S.; Pittelkow, M.; Crochet, A.; Fromm, K.M.; Antonov, L. 7-OH Quinoline Schiff Bases: Are They the Long Awaited Tautomeric Bistable Switches? Dye. Pigment. 2021, 195, 109739. [Google Scholar] [CrossRef]
  12. Nakashima, K.; Petek, A.; Hori, Y.; Georgiev, A.; Hirashima, S.; Matsushima, Y.; Yordanov, D.; Miura, T.; Antonov, L. Acylhydrazone Subunits as a Proton Cargo Delivery System in 7-Hydroxyquinoline. Chem. Eur. J. 2021, 27, 11559–11566. [Google Scholar] [CrossRef]
  13. Rehhagen, C.; Argüello Cordero, M.A.; Kamounah, F.S.; Deneva, V.; Angelov, I.; Krupp, M.; Svenningsen, S.W.; Pittelkow, M.; Lochbrunner, S.; Antonov, L. Reversible Switching Based on Truly Intramolecular Long-Range Proton Transfer─Turning the Theoretical Concept into Experimental Reality. J. Am. Chem. Soc. 2024, 146, 2043–2053. [Google Scholar] [CrossRef] [PubMed]
  14. Tang, K.-C.; Chen, C.-L.; Chuang, H.-H.; Chen, J.-L.; Chen, Y.-J.; Lin, Y.-C.; Shen, J.-Y.; Hu, W.-P.; Chou, P.-T. A Genuine Intramolecular Proton Relay System Undergoing Excited-State Double Proton Transfer Reaction. J. Phys. Chem. Lett. 2011, 2, 3063–3068. [Google Scholar] [CrossRef]
  15. Liu, Y.; Yang, Y.; Jia, X.; Ma, Q.; He, Y.; Zhai, H.; Zhang, Y.; Liu, Y. Theoretical Study of the Excited State Intramolecular Double Proton Transfer and Spectral Behaviors of 7-Hydroxyquinoline-8-Carboxylic Acid. J. Mol. Liq. 2020, 302, 112552. [Google Scholar] [CrossRef]
  16. Kumar, G.; Paul, K.; Luxami, V. Deciphering the Excited State Intramolecular Charge-Coupled Double Proton Transfer in an Asymmetric Quinoline–Benzimidazole System. New J. Chem. 2020, 44, 12866–12874. [Google Scholar] [CrossRef]
  17. Chou, P.-T. The Host/Guest Type of Excited-State Proton Transfer; a General Review. J. Chin. Chem. Soc. 2001, 48, 651–682. [Google Scholar] [CrossRef]
  18. Lim, S.-J.; Seo, J.; Park, S.Y. Photochromic Switching of Excited-State Intramolecular Proton-Transfer (ESIPT) Fluorescence: A Unique Route to High-Contrast Memory Switching and Nondestructive Readout. J. Am. Chem. Soc. 2006, 128, 14542–14547. [Google Scholar] [CrossRef]
  19. Demchenko, A.P.; Tang, K.-C.; Chou, P.-T. Excited-State Proton Coupled Charge Transfer Modulated by Molecular Structure and Media Polarization. Chem. Soc. Rev. 2013, 42, 1379–1408. [Google Scholar] [CrossRef]
  20. Sedgwick, A.C.; Wu, L.; Han, H.-H.; Bull, S.D.; He, X.-P.; James, T.D.; Sessler, J.L.; Tang, B.Z.; Tian, H.; Yoon, J. Excited-State Intramolecular Proton-Transfer (ESIPT) Based Fluorescence Sensors and Imaging Agents. Chem. Soc. Rev. 2018, 47, 8842–8880. [Google Scholar] [CrossRef]
  21. Jankowska, J.; Sobolewski, A.L. Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview. Molecules 2021, 26, 5140. [Google Scholar] [CrossRef]
  22. Joshi, H.C.; Antonov, L. Excited-State Intramolecular Proton Transfer: A Short Introductory Review. Molecules 2021, 26, 1475. [Google Scholar] [CrossRef]
  23. Böhnke, H.; Bahrenburg, J.; Ma, X.; Röttger, K.; Näther, C.; Rode, M.F.; Sobolewski, A.L.; Temps, F. Ultrafast Dynamics of the ESIPT Photoswitch N-(3-Pyridinyl)-2-Pyridinecarboxamide. Phys. Chem. Chem. Phys. 2018, 20, 2646–2655. [Google Scholar] [CrossRef] [PubMed]
  24. Slavova, S.; Antonov, L. Azaindolizine Proton Cranes Attached to 7-Hydroxyquinoline and 3-Hydroxypyridine: A Comparative Theoretical Study. Phys. Chem. Chem. Phys. 2024, 26, 7177–7189. [Google Scholar] [CrossRef] [PubMed]
  25. Sobolewski, A.L. Reversible Molecular Switch Driven by Excited-State Hydrogen Transfer. Phys. Chem. Chem. Phys. 2008, 10, 1243–1247. [Google Scholar] [CrossRef] [PubMed]
  26. Rode, M.F.; Nedeltcheva-Antonova, D.; Antonov, L. Long-Range Proton Transfer in 7-Hydroxy-Quinoline-Based Azomethine Dyes: A Hidden Reason for the Low Efficiency. Molecules 2022, 27, 8225. [Google Scholar] [CrossRef] [PubMed]
  27. Bulska, H.; Grabowska, A.; Grabowski, Z.R. Single and Double Proton Transfer in Excited Hydroxy Derivatives of Bipyridyl. J. Lumin. 1986, 35, 189–197. [Google Scholar] [CrossRef]
  28. Lipkowski, J.; Grabowska, A.; Waluk, J.; Calestani, G.; Hess, B.A. Ground State Structures of Molecules ?Prepared? For Phototautomerization: 2,2?-Bipyridyl-3,3?-Diol and 2,2?-Bipyridyl-3-Ol. J. Crystallogr. Spectrosc. Res. 1992, 22, 563–572. [Google Scholar] [CrossRef]
  29. Kaczmarek, Ł.; Balicki, R.; Lipkowski, J.; Borowicz, P.; Grabowska, A. Structure and Photophysics of Deazabipyridyls. Excited Internally Hydrogen-Bonded Systems with One Proton Transfer Reaction Site. J. Chem. Soc. Perkin Trans. 2 1994, 2, 1603–1610. [Google Scholar] [CrossRef]
  30. Borowicz, P.; Grabowska, A.; Leś, A.; Kaczmarek, Ł.; Zagrodzki, B. New Phototautomerizing Systems: Non-Symmetric Derivatives of [2,2′-Bipyridyl]-3,3′-Diol. Chem. Phys. Lett. 1998, 291, 351–359. [Google Scholar] [CrossRef]
  31. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Petersson, G.A.; Nakatsuji, H.; et al. Gaussian 16 Rev. C.01; Gaussian Inc.: Wallingford, CT, USA, 2016. [Google Scholar]
  32. Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999–3094. [Google Scholar] [CrossRef]
  33. Peng, C.; Ayala, P.Y.; Schlegel, H.B.; Frisch, M.J. Using Redundant Internal Coordinates to Optimize Equilibrium Geometries and Transition States. J. Comput. Chem. 1996, 17, 49–56. [Google Scholar] [CrossRef]
  34. Zhao, Y.; Truhlar, D.G. Density Functionals with Broad Applicability in Chemistry. Acc. Chem. Res. 2008, 41, 157–167. [Google Scholar] [CrossRef]
  35. Zhao, Y.; Truhlar, D.G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215–241. [Google Scholar] [CrossRef]
  36. Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. [Google Scholar] [CrossRef] [PubMed]
  37. Kawauchi, S.; Antonov, L. Description of the Tautomerism in Some Azonaphthols. J. Phys. Org. Chem. 2013, 26, 643–652. [Google Scholar] [CrossRef]
  38. Rayne, S.; Forest, K. A Comparative Examination of Density Functional Performance against the ISOL24/11 Isomerization Energy Benchmark. Comput. Theor. Chem. 2016, 1090, 147–152. [Google Scholar] [CrossRef]
  39. Antonov, L. Tautomerism in Azo and Azomethyne Dyes: When and If Theory Meets Experiment. Molecules 2019, 24, 2252. [Google Scholar] [CrossRef]
  40. Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic Excitations within the Adiabatic Approximation of Time Dependent Density Functional Theory. Chem. Phys. Lett. 1996, 256, 454–464. [Google Scholar] [CrossRef]
  41. Improta, R. UV-Visible Absorption and Emission Energies in Condensed Phase by PCM/TD-DFT Methods. In Computational Strategies for Spectroscopy; Barone, V., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2011; pp. 37–75. ISBN 978-1-118-00872-0. [Google Scholar]
  42. Liu, J.; Liang, W. Analytical Approach for the Excited-State Hessian in Time-Dependent Density Functional Theory: Formalism, Implementation, and Performance. J. Chem. Phys. 2011, 135, 184111. [Google Scholar] [CrossRef]
  43. Guo, Y.; Riplinger, C.; Becker, U.; Liakos, D.G.; Minenkov, Y.; Cavallo, L.; Neese, F. Communication: An Improved Linear Scaling Perturbative Triples Correction for the Domain Based Local Pair-Natural Orbital Based Singles and Doubles Coupled Cluster Method [DLPNO-CCSD(T)]. J. Chem. Phys. 2018, 148, 011101. [Google Scholar] [CrossRef]
  44. Berraud-Pache, R.; Neese, F.; Bistoni, G.; Izsák, R. Unveiling the Photophysical Properties of Boron-Dipyrromethene Dyes Using a New Accurate Excited State Coupled Cluster Method. J. Chem. Theory Comput. 2020, 16, 564–575. [Google Scholar] [CrossRef]
  45. Krygowski, T.M.; Cyrański, M.K. Structural Aspects of Aromaticity. Chem. Rev. 2001, 101, 1385–1420. [Google Scholar] [CrossRef] [PubMed]
  46. Krygowski, T.M.; Szatylowicz, H. Aromaticity: What Does It Mean? ChemTexts 2015, 1, 12. [Google Scholar] [CrossRef] [PubMed]
  47. Zhu, Q.; Chen, S.; Chen, D.; Lin, L.; Xiao, K.; Zhao, L.; Solà, M.; Zhu, J. The Application of Aromaticity and Antiaromaticity to Reaction Mechanisms. Fundam. Res. 2023, 3, 926–938. [Google Scholar] [CrossRef] [PubMed]
  48. Yan, J.; Slanina, T.; Bergman, J.; Ottosson, H. Photochemistry Driven by Excited-State Aromaticity Gain or Antiaromaticity Relief. Chem. A Eur. J. 2023, 29, e202203748. [Google Scholar] [CrossRef] [PubMed]
  49. Lu, T. A Comprehensive Electron Wavefunction Analysis Toolbox for Chemists, Multiwfn. J. Chem. Phys. 2024, 161, 082503. [Google Scholar] [CrossRef]
  50. Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580–592. [Google Scholar] [CrossRef]
  51. Antonov, L.; Kawauchi, S.; Okuno, Y. Prediction of the Color of Dyes by Using Time-Dependent Density Functional Theory. Bulg. Chem. Commun. 2014, 46, 228–237. [Google Scholar]
  52. Becke, A.D. Density-functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648–5652. [Google Scholar] [CrossRef]
  53. Nedeltcheva-Antonova, D.; Antonov, L. Ground-State Tautomerism and Excited-State Proton Transfer in 7-Hydroxy-4-Methyl-8-((Phenylimino)Methyl)-2H-Chromen-2-One as a Potential Proton Crane. Physchem 2024, 4, 91–105. [Google Scholar] [CrossRef]
  54. Marx, D. Proton Transfer 200 Years after von Grotthuss: Insights from Ab Initio Simulations. ChemPhysChem 2007, 8, 209–210. [Google Scholar] [CrossRef]
  55. Ekimova, M.; Hoffmann, F.; Bekçioğlu-Neff, G.; Rafferty, A.; Kornilov, O.; Nibbering, E.T.J.; Sebastiani, D. Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism. J. Am. Chem. Soc. 2019, 141, 14581–14592. [Google Scholar] [CrossRef] [PubMed]
  56. Meuwly, M.; Bach, A.; Leutwyler, S. Grotthus-Type and Diffusive Proton Transfer in 7-Hydroxyquinoline·(NH3)n Clusters. J. Am. Chem. Soc. 2001, 123, 11446–11453. [Google Scholar] [CrossRef] [PubMed]
  57. Codescu, M.-A.; Weiß, M.; Brehm, M.; Kornilov, O.; Sebastiani, D.; Nibbering, E.T.J. Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate. J. Phys. Chem. A 2021, 125, 1845–1859. [Google Scholar] [CrossRef] [PubMed]
  58. Codescu, M.-A.; Kunze, T.; Weiß, M.; Brehm, M.; Kornilov, O.; Sebastiani, D.; Nibbering, E.T.J. Ultrafast Proton Transfer Pathways Mediated by Amphoteric Imidazole. J. Phys. Chem. Lett. 2023, 14, 4775–4785. [Google Scholar] [CrossRef]
  59. Shekhovtsov, N.A.; Bushuev, M.B. Enol or Keto? Interplay between Solvents and Substituents as a Factor Controlling ESIPT. J. Mol. Liq. 2022, 361, 119611. [Google Scholar] [CrossRef]
  60. Shekhovtsov, N.A.; Nikolaenkova, E.B.; Ryadun, A.A.; Samsonenko, D.G.; Tikhonov, A.Y.; Bushuev, M.B. ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-Yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence. Molecules 2023, 28, 1793. [Google Scholar] [CrossRef]
  61. Shekhovtsov, N.A.; Nikolaenkova, E.B.; Vorobyeva, S.; Ryadun, A.A.; Bushuev, M.B. Coordination-Driven Molecular Switch on the Base of an ESIPT-Capable Pyrimidine Ligand: Synthesis, Fine-Tuning of Emission by the Halide Anion and Theoretical Studies. J. Photochem. Photobiol. A Chem. 2025, 459, 116091. [Google Scholar] [CrossRef]
  62. Kaczmarek, Ł.; Zagrodzki, B.; Kamieński, B.; Pietrzak, M.; Schilf, W.; Leś, A. Synthesis and NMR Study of New Derivatives of [2,2′-Bipyridyl]-3,3′-Diol and [2,2′-Bipyridyl]-3-Ol. J. Mol. Struct. 2000, 553, 61–72. [Google Scholar] [CrossRef]
  63. Taylor, P.J.; van der Zwan, G.; Antonov, L. Tautomerism: Introduction, History, and Recent Developments in Experimental and Theoretical Methods. In Tautomerism: Methods and Theories; Antonov, L., Ed.; Wiley: Weinheim, Germany, 2013; pp. 1–24. ISBN 978-3-527-65882-4. [Google Scholar]
  64. Gordon, P.F.; Gregory, P. Azo Dyes. In Organic Chemistry in Colour; Springer: Berlin/Heidelberg, Germany, 1987; ISBN 978-3-642-82959-8. [Google Scholar]
  65. Fabian, J.; Hartmann, H. Light Absorption of Organic Colorants: Theoretical Treatment and Empirical Rules; Springer: Berlin/Heidelberg, Germany, 1980; ISBN 978-3-642-67587-4. [Google Scholar]
  66. Antonov, L.; Lyčka, A. A Correction of the Neglected Tautomerism in the Spectra Prediction of O-Methoxyaniline Terminated Monoazonaphthols: DFT and 15N NMR Study. J. Mol. Liq. 2024, 406, 125134. [Google Scholar] [CrossRef]
  67. Raczyńska, E.D.; Kosińska, W.; Ośmiałowski, B.; Gawinecki, R. Tautomeric Equilibria in Relation to Pi-Electron Delocalization. Chem. Rev. 2005, 105, 3561–3612. [Google Scholar] [CrossRef]
  68. Elguero, J. Tautomerism: A Historical Perspective. In Tautomerism: Concepts and Applications in Science and Technology; Antonov, L., Ed.; Wiley-VCH: Weinheim, Germany, 2016; pp. 1–10. ISBN 978-3-527-69571-3. [Google Scholar]
  69. Gorelik, M.V. Current State of the Problem of Aromaticity. Russ. Chem. Rev. 1990, 59, 116–133. [Google Scholar] [CrossRef]
  70. Elguero, J.; Katritzky, A.R.; Denisko, O.V. Prototropic Tautomerism of Heterocycles: Heteroaromatic Tautomerism—General Overview and Methodology. In Advances in Heterocyclic Chemistry; Elsevier: Amsterdam, The Netherlands, 2000; Volume 76, pp. 1–84. ISBN 978-0-12-020776-3. [Google Scholar]
  71. Filarowski, A.; Kochel, A.; Cieslik, K.; Koll, A. Steric and Aromatic Impact on Intramolecular Hydrogen Bonds in o-hydroxyaryl Ketones and Ketimines. J. Phys. Org. Chem. 2005, 18, 986–993. [Google Scholar] [CrossRef]
  72. Filarowski, A.; Kochel, A.; Kluba, M.; Kamounah, F.S. Structural and Aromatic Aspects of Tautomeric Equilibrium in Hydroxy Aryl Schiff Bases. J. Phys. Org. Chem. 2008, 21, 939–944. [Google Scholar] [CrossRef]
  73. Martyniak, A.; Majerz, I.; Filarowski, A. Peculiarities of Quasi-Aromatic Hydrogen Bonding. RSC Adv. 2012, 2, 8135. [Google Scholar] [CrossRef]
  74. Kalapos, P.P.; Kunfi, A.; Bogner, M.M.; Holczbauer, T.; Kochman, M.A.; Durbeej, B.; London, G. Salicylideneaniline/Dithienylethene Hybrid Molecular Switches: Design, Synthesis, and Photochromism. J. Org. Chem. 2024, 89, 16–26. [Google Scholar] [CrossRef]
  75. Albert, A.; Goldacre, R.; Phillips, J. 455. The Strength of Heterocyclic Bases. J. Chem. Soc. 1948, 2240–2249. [Google Scholar] [CrossRef]
  76. Perrin, D.D. Dissociation Constants of Organic Bases in Aqueous Solution—Supplement 1972; Pure and Applied Chemistry Supplement; Butterworths: London, UK, 1972; ISBN 978-0-408-70408-3. [Google Scholar]
Chemistry 06 00097 g001
Figure 1. Ground-state PESs of the studied compounds in toluene presented as relative energies (left column) and as relative Gibbs free energies (right column). As seen, most of the intramolecular proton transfers can be considered barrierless if the Gibbs free energies are taken into account (see also the data in Table S1). The corresponding curves in acetonitrile are shown in Figure S1.
Figure 1. Ground-state PESs of the studied compounds in toluene presented as relative energies (left column) and as relative Gibbs free energies (right column). As seen, most of the intramolecular proton transfers can be considered barrierless if the Gibbs free energies are taken into account (see also the data in Table S1). The corresponding curves in acetonitrile are shown in Figure S1.
Chemistry 06 00097 g001
Chemistry 06 00097 g002aChemistry 06 00097 g002b
Figure 2. Tautomeric forms of the studied compounds with: HOMA aromaticity indices (in orange), the sum of natural charges of the atoms (including hydrogens) in the corresponding ring obtained by NBO analysis (in brown), and the twisting angle α (in magenta). All parameters are given for the toluene solution. The rings are indicated as A–C, shown as an example in the case of 7OHQ and HQBT.
Figure 2. Tautomeric forms of the studied compounds with: HOMA aromaticity indices (in orange), the sum of natural charges of the atoms (including hydrogens) in the corresponding ring obtained by NBO analysis (in brown), and the twisting angle α (in magenta). All parameters are given for the toluene solution. The rings are indicated as A–C, shown as an example in the case of 7OHQ and HQBT.
Chemistry 06 00097 g002aChemistry 06 00097 g002b
Chemistry 06 00097 g003
Figure 3. First singlet excited-state PESs of the studied compounds in toluene, presented as relative energies. The enol Frank–Condon states of E are given with “-“ on the left side.
Figure 3. First singlet excited-state PESs of the studied compounds in toluene, presented as relative energies. The enol Frank–Condon states of E are given with “-“ on the left side.
Chemistry 06 00097 g003
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zaharieva, L.; Nedeltcheva-Antonova, D.; Antonov, L. 8-(Pyridin-2-yl)quinolin-7-ol and Beyond: Theoretical Design of Tautomeric Molecular Switches with Pyridine as a Proton Crane Unit. Chemistry 2024, 6, 1608-1621. https://doi.org/10.3390/chemistry6060097

AMA Style

Zaharieva L, Nedeltcheva-Antonova D, Antonov L. 8-(Pyridin-2-yl)quinolin-7-ol and Beyond: Theoretical Design of Tautomeric Molecular Switches with Pyridine as a Proton Crane Unit. Chemistry. 2024; 6(6):1608-1621. https://doi.org/10.3390/chemistry6060097

Chicago/Turabian Style

Zaharieva, Lidia, Daniela Nedeltcheva-Antonova, and Liudmil Antonov. 2024. "8-(Pyridin-2-yl)quinolin-7-ol and Beyond: Theoretical Design of Tautomeric Molecular Switches with Pyridine as a Proton Crane Unit" Chemistry 6, no. 6: 1608-1621. https://doi.org/10.3390/chemistry6060097

APA Style

Zaharieva, L., Nedeltcheva-Antonova, D., & Antonov, L. (2024). 8-(Pyridin-2-yl)quinolin-7-ol and Beyond: Theoretical Design of Tautomeric Molecular Switches with Pyridine as a Proton Crane Unit. Chemistry, 6(6), 1608-1621. https://doi.org/10.3390/chemistry6060097

Article Metrics

Back to TopTop