Article pubs.acs.org/JPCA

Molecular Structures and Absorption Spectra Assignment of Corrole NH Tautomers Wichard Beenken,*,† Martin Presselt,‡ Thien H. Ngo,§ Wim Dehaen,∥ Wouter Maes,*,⊥ and Mikalai Kruk# †

Institute of Physics, Ilmenau University of Technology, P.O. Box 100565, 98684 Ilmenau, Germany Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany § International Center for Young Scientists (ICYS)/International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ∥ Molecular Design and Synthesis, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ⊥ Design & Synthesis of Organic Semiconductors (DSOS), Institute for Materials Research (IMO), Hasselt University, Universitaire Campus, Agoralaan 1 - Building D, B-3590 Diepenbeek, Belgium # Physics Department, Belarusian State Technological University, Sverdlova str. 13a, Minsk 220050, Belarus ‡

S Supporting Information *

ABSTRACT: The individual absorption spectra of the two NH tautomers of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole are assigned on the basis of the Gouterman four-orbital model and a quantum chemical TDDFT study. The assignment indicates that the red-shifted T1 tautomer is the one with protonated pyrrole nitrogen atoms N(21), N(22) and N(23), whereas the blue-shifted T2 tautomer has pyrrole nitrogen atoms N(21), N(22) and N(24) protonated. A wave-like nonplanar distortion of the macrocycle in the ground state is found for both NH tautomers, with the wave axis going through the pyrroles containing N(22) and N(24). The 7C plane determined by the least-squares distances to the carbon atoms C1, C4, C5, C6, C9, C16, and C19 is suggested as a mean corrole macrocycle plane for the analysis of out-of-plane distortions. The magnitude of these distortions is distinctly different for the two NH tautomers, leading to substantial perturbations of their acid−base properties, which are rationalized by the interplay of the degree of out-of-plane distortion of the macrocycle as a whole and the tendency of the pyrrole nitrogen atoms toward pyramidalization, with the former leading to a basicity increase whereas the latter enhances the acidity.

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INTRODUCTION Corroles constitute an important family of tetrapyrrolic macrocyclic compounds closely related to porphyrins, the key structural difference being the direct Ca−Ca linkage between two neighboring pyrrole rings instead of a regular methine bridge.1−22 The absence of one meso-carbon atom (Cm) changes the bond alternation pattern in the contracted porphyrinoid macrocycle. As a result, free base (Fb) corroles consist of three pyrrolic (−NH−) and one pyrrolenic (−N=) ring (rather than two of each, as is the case for Fb porphyrins). Therefore, corroles serve as trianionic ligands, binding trivalent metal ions. Noticeable distortions from planarity have been observed for Fb corroles, even in the absence of overcrowding peripheral substitution, due to the decreased core size and steric hindrance preventing the three inner pyrrolic NH protons to be in the same macrocyclic plane. These structural features result in a lower symmetry of the Fb corrole macrocycle compared to that of porphyrins (Cs vs D2h), and different corrole NH tautomers can be identified, i.e., structurally distinct isomers depending on the location of the three pyrrolic hydrogen atoms © 2014 American Chemical Society

within the macrocyclic core (Scheme 1). As a function of the peripheral substitution pattern, a different number (either two or four) of NH tautomers can be expected. Four NH tautomers, denoted as T1, T1′, T2, and T2′, are structurally distinguishable if all three meso-positions are occupied with different groups (R1 ≠ R2 ≠ R3, as in ABC-type corroles), whereas for AB2- and A3-type corroles (R1 = R3 and R1 = R2 = R3, respectively), only two NH tautomers are possible (T1 ≡ T1′ and T2 ≡ T2′). The coexistence of two corrole NH tautomers in (liquid) solution at room temperature has been discussed by several research groups,23−26 but only recently an experimental study has disclosed the individual absorption and fluorescence profiles of the two NH tautomers of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole (1) (Scheme 1) in both liquid and solid solutions.27−29 The long-wavelength bands in Received: November 9, 2013 Revised: January 13, 2014 Published: January 16, 2014 862

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Scheme 1. NH Tautomerization in Meso-Substituted Fb Corroles (Left) with Atom Numbering According to the IUPAC Nomenclature, and the Molecular Structure of the Studied meso-Pyrimidinylcorrole 1 (Right)

the absorption spectra and the bands in the fluorescence spectra (in a wide temperature range) could be unambiguously assigned to the two individual corrole NH tautomers. It was found that the fluorescence from the long-wavelength T1 tautomer dominates the total emission spectrum at room temperature, in contrast to the low temperature case, where the short wavelength T2 tautomer dominates the emission profile.28 This phenomenon of temperature-controlled “switching” between the fluorescence emissions from the two corrole NH tautomers was observed for the first time and was explained by a reduced T2 → T1 tautomerization rate at low temperature. Energy level diagrams explaining the excitation energy deactivation channels at different temperatures have been proposed. H/D substitution of the core pyrroles led to a substantial decrease in the NH tautomerization rate, resulting in an increase of the contribution of the T2 tautomer to the total fluorescence spectrum at the expense of the T1 tautomer. The observed temperature dependence of the fluorescence spectra in the temperature range 277−332 K was used to evaluate the spectral profiles for the individual NH tautomers. The pronounced increase of the overall fluorescence quantum yield upon going from 332 to 277 K was explained in terms of the difference in fluorescence quantum yields of the individual NH tautomers, which were found to be 0.045 and 0.155 for the T1 and T2 tautomer, respectively. The red-shifted NH tautomer T1 has been tentatively assigned to the molecular structure with protonated pyrrole nitrogen atoms N(21), N(22), and N(23), and the blue-shifted tautomer T2 to the isomer with protonated pyrrole nitrogen atoms N(21), N(22), and N(24) (atom numbering according to IUPAC nomenclature; Scheme 1) by comparison of their spectral features with the absorption spectra of N-aryl-substituted corrole derivatives.30,31 The two NH tautomers were found to have a different basicity as well, which was expected to originate from a difference in their molecular conformations.27 Although the presence of two corrole NH tautomers for 5,10,15-triarylcorroles in solution at room temperature was experimentally proven,27,28 two important questions remained. The first question to be addressed is the detailed interpretation of the ground state absorption spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole32−37 (1) and the direct

assignment of the spectral features to the two distinct NH tautomers T1 and T2. The second thing that remained unanswered so far is the direct attribution of the two optically identified NH tautomers to specific molecular structures, with defined proton positions in the macrocyclic core (as shown in Scheme 1), which was done only indirectly so far (see above), and elucidation of the structural differences between the two corrole NH tautomers. Thus, the present paper is addressing these items by giving a detailed interpretation of the absorption spectra of corrole 1 in the visible range within the framework of the Gouterman four-orbital model, in the mean time providing evidence for the above-mentioned corrole NH tautomer assignment on the basis of quantum-chemical DFT studies.

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EXPERIMENTAL AND THEORETICAL METHODS 10-(4,6-Dichloropyrimidin-5-yl)-5,15-dimesitylcorrole (1) (Scheme 1) was prepared in accordance with the previously published synthetic procedure.32,34 Ground state absorption spectra at 293 K were measured with a UV−vis Varian Carry 100 spectrophotometer. The measurements at 77 K were done with a cryogenic quartz holder. Corrole concentrations in solution were measured spectrophotometrically by means of preliminarily determined extinction coefficients.27 No aggregation was noticed for a corrole concentration range up to 6.0.10−5 M, wherein all the experiments have been carried out. To attribute the two spectroscopically identified tautomers T1 and T2 to distinct chemical structures, we performed quantum-chemical calculations by means of density functional theory (DFT) for ground state geometry optimization and its time-dependent version (TD-DFT) for absorption spectra determination. The geometry optimizations were performed using the program Turbomole38,39 applying the GGA (generalized gradient approximation) BP86 exchange−correlation functional, the def2-TZVP triple-ζ basis set,40,41 and the MARI-J approximation.42 This combination has been shown to give excellent geometries, electron density distributions, and vibrational spectroscopic properties in many cases at very reasonable computational cost.43−49 A continuous polarizable solvent environment with ε = 10 was simulated in the Turbomole calculations as well, whereas in the case of the Gaussian calculations, dichloromethane was directly specified in 863

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the solvent option. To predict the UV−vis absorption spectra, TD-DFT calculations were performed by using not only the BP86 but also the B3LYP hybrid functional (see ref 50 and references therein) and the range-separated functionals CAMB3LYP and LC-ωPBE (both as implemented in Gaussian 0951) involving preceding reoptimization of the geometry applying the respective functional. For plotting all absorption data presented and visualization of the molecular orbital contributions to electronic transitions, the program Mathematica (Version 8) was used.52

Table 1. Band Analysis of the Absorption Spectrum of 10(4,6-Dichloropyrimidin-5-yl)-5,15-dimesitylcorrole (1) at Room Temperature by Means of Second Derivative Minima, and the Band Assignment to the Transitions of the Two Tautomers T1 and T2 in the Visible Range by Means of the Gouterman Modela experimental peak position

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RESULTS AND DISCUSSION Corrole NH Tautomer Absorption Spectra in the Framework of the Four-Orbital Model. The ground state absorption spectra for 10-(4,6-dichloropyrimidin-5-yl)-5,15dimesitylcorrole (1) in 2-Me-THF measured at 293 and 77 K are shown in Figure 1. Besides the general bandwidth

Gouterman model

assignment

nm

cm−1

cm−1

T1

635 599 585 569 553 548 537 529 518 511 494 480

15740 16695 17085 17570 18085 18250 18620 18910

15740 16695 17040 17570 17995 18250 18340 18870 19295 19550 20170 20850

S1 (0−0)

19575 20250

T2 S1 (0−0)

S1 (0−1) S2 (0−0) S1 (0−1) S2 (0−0) S1 (0−2)? S2 (0−1) S1 (0−2)? S2 (0−1) S2 (0−2) S2 (0−2)?

a

Bands expected by the model but not found in the analysis of the experimental spectrum are given in italic. Questionable assignments are marked by “?”.

The Gouterman four-orbital model suggests that for each tetrapyrrolic macrocycle there are four electronic transitions, two in the visible region forming the Q-bands (S0 → S1 and S0 → S2 transitions) and two in the UV range forming the Soret Bband (S0 → S3 and S0 → S4 transitions). Each of the two transitions S1 and S2 possesses an equidistant vibrational progression with vibrational spacing value Δν. From analysis of the fluorescence spectra the Δν value was found to be 1300 ± 25 cm−1 for both NH tautomers.28 We have found that a very weak second overtone (0−2) band is also detectable in the fluorescence spectra of corroles. Therefore, it is reasonable to suggest that the two overtones (0−1) and (0−2) contribute to the absorption spectra of each of the NH tautomers as well. Because the vibrational progressions of the S1 and S2 transitions are equal, the energies of all vibronic bands in the absorption spectrum are accessible, provided that the respective pure electronic (0−0) transitions are known. Because at 293 K both the T1 and the T2 tautomer coexist in comparable concentrations, whereas at 77 K the T2 tautomer dominates the absorption spectrum, the long-wavelength band at 635 nm (15 740 cm−1), which distinctly appears in the room temperature absorbance but is barely observable at 77 K, can be identified as the S1(0−0) transition of the T1 tautomer. Adding the energy spacing of Δν = 1300 cm−1, its first vibrational overtone S1(0−1) should peak at 17 040 cm−1. This value fits well to the weak band found in the experimental spectrum at 585 nm (17 085 cm−1). Adding 1300 cm−1 once more, the second vibrational overtone S1(0−2) could be found at 18 340 cm−1 and might be assigned to a very weak band at 537 nm (18 620 cm−1). Thus, the second overtones seem to contribute to the corrole absorption indeed. The band at 599 nm (16 695 cm−1), which is prominent in both spectra, has consequently to be assigned to the S1(0−0) transition of the T2 tautomer, which first vibrational overtone S1(0−1) is then to be expected at 17 995 cm−1 and might match to the experimental band found at 553 nm (18 085 cm−1). This assignment of the S1(0−

Figure 1. Visible range ground state absorption spectra of Fb 10-(4,6dichloropyrimidin-5-yl)-5,15-dimesitylcorrole (1) in 2-Me-THF in equimolar concentrations (9.6 × 10−6 M) at 293 K (solid line) and 77 K (dashed line). Numbers refer to band maxima in the spectrum taken at 293 K, as revealed with the second derivative method (see text for details).

narrowing and small blue shift upon lowering the temperature, a specific change in the shape of the spectrum is apparent as well. The latter has been explained by a change of the relative concentrations of the two NH tautomers T1 and T2, and the consequently altered contributions of their individual spectra to the measured total absorption spectrum.27 One can see that, as shown before,28 the short wavelength T2 tautomer dominates the spectrum at 77 K, whereas at room temperature both T1 and T2 tautomers may be present in comparable concentrations. The absorption band maxima in the visible range of the spectrum recorded at 293 K, obtained by searching for minima in the second derivatives of the absorption spectrum,53,54 are reported in Table 1. The peak positions correspond well to those derived by means of the same procedure from the spectrum obtained at 77 K, with the exception of the peak at 635 nm belonging to the T1 tautomer. In the following paragraphs, the observed bands are assigned to the electronic transitions and their vibrational overtones of the two tautomers on the basis of the Gouterman four-orbital model.55,56 864

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0) transitions is further supported by the fluorescence spectra measurements as recently reported.28,29 The remaining prominent bands at 569 nm (17 570 cm−1) and 529 nm (18 910 cm−1), which increase in absorbance going down from room temperature to 77 K, are consequently the S2(0−0) transition of the T2 tautomer and its first vibrational overtone S2(0−1), expected by the model at 18 870 cm−1. Interestingly, in contrast to other cyclic tetrapyrroles such as porphyrins, the second overtone S2(0−2) of the T2 tautomer, which has been calculated to be at 20170 cm−1, is clearly visible in the absorption spectrum at 494 nm (20 250 cm−1). This means that, certainly for the S2 transition of the T2 tautomer and most probably also for the S1 transition of the T1 tautomer (see above), the vibronic coupling is about double that found in porphyrins. Finally, the S2(0−0) transition of the T1 tautomer has to be assigned to the weak band found at 548 nm (18 250 cm−1), with its first vibrational overtone S2(0−1), calculated at 19 550 cm−1, being apparent at 511 nm (19 575 cm−1) in the room temperature spectrum. Both these bands only slightly increase at 77 K, compared to those bands already assigned to the T2 tautomer, supporting the assignment of these bands to the T1 tautomer. Due to the weakness of the fundamental S2(0−0) transition of the T1 tautomer, the corresponding second overtone S2(0− 2), to be expected at 20 850 cm−1 (480 nm), is only vaguely perceptible. Notably, the second overtone of the S1 transition of the T2 tautomer, expected at 19 295 cm−1 (518 nm) but missed in the band analysis, is almost hidden by the S2(0−1) transition of the T1 tautomer. Nevertheless, this hidden band may be the reason that in this spectral region the absorption at 77 K is still higher than at room temperature. Therefore, the statement above that the vibronic coupling in 10-(4,6-dichloropyrimidin5-yl)-5,15-dimesitylcorrole (1) is about the double of that found in porphyrins seems to hold for all electronic transitions in the visible range. The difference between the S1(0−0) transitions of the two NH tautomers T2 and T1 is about 955 ± 10 cm−1 in the absorption spectrum, matching the value of 965 ± 10 cm−1 measured for the fluorescence spectrum. Furthermore, it coincides with the splitting found in the Soret region as well. Between the S2(0−0) transitions of T1 and T2, which are in reverse energy order compared to the S1(0−0) transitions, we obtained a difference of 680 ± 10 cm−1. This means that the main difference between the spectra of the two NH tautomers in the visible range is a diminishing of the splitting between the electronic transitions S2 and S1 from 2510 ± 10 cm−1 in the case of the T1 tautomer to 875 ± 10 cm−1 in the case of the T2 tautomer, i.e., of about 1635 cm−1. This may indicate that the conformation of the corrole macrocycle may be heavily affected by the NH tautomerization. We will return to an analysis of this issue below, when reporting the details of the quantumchemically calculated geometries of the two corrole NH tautomers. Finally, we want to point out the interesting feature that the experimental value of the S2(0−0)−S1(0−0) splitting for the T2 tautomer fits well to those observed for several monoprotonated porphyrins, which are in the range 850−1050 cm−1, whereas the value of the S2(0−0)−S1(0−0) splitting for the T1 tautomer seems to be closer to those found in Fb porphyrins, e.g., 2940 cm−1 for the Fb tetramesitylporphyrin.57 Thus, the main characteristic features of the pyrimidinylcorrole NH tautomers absorption spectra are the following: the long-wavelength band (635 nm) in the absorption spectrum belongs to the 0−0 transition of the T1 tautomer (with

protonated pyrrole nitrogen atoms N(21), N(22), and N(23), by definition), and the T2 tautomer (with protonated pyrrole nitrogen atoms N(21), N(22), and N(24)) has a 0−0 transition shifted to the short wavelength region for up to 1000 cm−1. The energy gap between the first and second electronic transition, ΔE(S2−S1), is about 3 times larger for the T1 tautomer than for the T2 tautomer. Both features lead to an increase in energy of the electronic transitions in the visible range in the order S1(T1) < S1(T2) < S2(T2) < S2(T1). Corrole NH Tautomers Geometry Optimization and out-of-Plane Macrocycle Distortions. The ground state molecular geometries of the two pyrimidinylcorrole NH tautomers were obtained by DFT-based geometry optimization with the BP86 exchange−correlation functional. The reoptimization of molecular geometry done with other functionals (B3LYP, CAM-B3LYP, LC-ωPBE) has revealed essentially the same results and for these minor quantitative differences they are not discussed below (for detailed information as xyz-files, see the Supporting Information). The corrole macrocycle has been found to be essentially nonplanar for both NH tautomers (Figure 2). The corrole macrocycle core is known to be smaller

Figure 2. Side views of the optimized structures of the T1 (top) and T2 (bottom) NH tautomers of the studied pyrimidinylcorrole (mirror images of the structures as denoted in Scheme 1). The macrocycles are oriented in such a way that the plane of pyrrole A (closest to the viewer) is perpendicular to the picture plane. The yellow line represents the hydrogen bond between pyrroles C and D.

than that of porphyrins due to the direct Ca−Ca linkage, leading to closer N−N distances and enhanced steric repulsion of the three inner hydrogen atoms.58 As a result, an alternative up and down tilting of the pyrrole rings, which has been named with the term “macrocycle puckering”, occurs.59 The formation of “classical” alternate tilting, as observed in porphyrins, is prevented by the inner NH···N hydrogen bonding, providing some driving force back to macrocycle planarization. The interplay of these opposite trends may explain the calculated molecular conformations for the two NH tautomers (see Figure 2 and corresponding xyz-files in the Supporting Information). 865

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seven carbon atoms C1, C4, C5, C6, C9, C16, and C19 are practically in the same plane for both NH tautomers, whereas the two Ca atoms of pyrrole C (C11 and C14) and the two adjacent Cm carbons of the methine bridges (C10 and C15) deviate from this 7C plane significantly. The magnitude of these deviations differs noticeably between the two NH tautomers. Because the macrocycle mean plane should be almost invariant under NH tautomerization to enable structural analysis of both NH tautomers in the same frame, the 7C plane given by the least-squares distances to the carbon atoms C1, C4, C5, C6, C9, C16, and C19, seems to be a better choice than the 11C plane. This requirement is reasonable, also because the rate constant of NH tautomerization is fast,27,64 and consequently, the equilibrium mean structure should be defined in an invariant frame. Structural Implications for Differences in Basicity and Acidity of the NH Tautomers. Within the framework of the 7C mean macrocycle plane, the optimized structure of the T1 tautomer has larger out-of-plane (oop) displacements than the T2 tautomer. The relative displacement magnitudes range from 15 to 25%, being maximal for the Cb carbons. We should stress here that for the T2 tautomer with the smaller out-of-plane distortions a wave-type distortion is still clearly visible. The planes of the three pyrroles B, C, and D belong to the family of planes that parallel the line passing through the two Ca atoms of pyrrole B. As a result, the proton-free nitrogen atom N(23) of pyrrole C lies almost in the mean plane, with a pyrrole C tilt angle of 1.3°. On the contrary, pyrroles A, B, and D of the T1 tautomer are essentially not parallel but tilted down to form a shallow cone-like surface, and the proton-free nitrogen atom N(24) of pyrrole D is noticeably exposed out of the mean plane, with a tilt angle increased up to 5.6°. The differences in the position of the proton-free pyrrole ring for the T1 and T2 tautomers are in line with our earlier observations of differences in the protonation rate between the two NH tautomers.27 Differences in the degree of nonplanar distortions between the two NH tautomers have been suggested as the main reason for their different basicity. It is well-known that the formation of nonplanar conformers of porphyrins, where the pyrrole nitrogen atoms are exposed out of the mean plane, leads to a substantial increase in basicity.65,66 Thus, on the basis of the calculated geometry differences of the two NH tautomers, the higher basicity should be assigned to the T1 tautomer due to the larger tilt of pyrrole D, and the lower basicity to the T2 tautomer due to the lower tilt of pyrrole C. One more feature characteristic to both corrole NH tautomers is the substantial deviation of the positions of the hydrogen atoms from the respective pyrrole planes (Table 2). These displacements, which correspond to the pyramidalization of the pyrrole nitrogen atoms, imply that they acquire some sp3 2 character. A raw estimation of this mixed hybridization spλ can

The general type of nonplanar distortion for both NH tautomers can be assigned as a “wave-like” distortion with the wave axis going through the pyrroles containing N(22) and N(24), denoted in Figure 3 as pyrroles B and D, respectively.

Figure 3. Representation of the wave-like distortion of the corrole macrocycle.

Pyrroles A and C, containing N(21) and N(23), respectively, have a substantially different orientation in relation to the wave axis. This distortion seems to originate from the trend toward coplanarization of pyrroles C and D due to the earlier mentioned NH···N hydrogen bonding. On the other hand, it may also increase the dihedral angle between pyrroles D and A. Therefore, the term wave distortion should be considered within the framework introduced by Smith et al.,60 implying the need to take into account the asymmetry distinguishing the structures of corroles and porphyrins in more than only one meso-carbon atom. Due to this asymmetry, there is some arbitrariness in the direction of the wave axis. However, analysis of the optimized molecular structures of both tautomers leads us to the conclusion that pyrrole B is the most symmetrical element. Thus the wave axis should pass pyrrole B through the N(22) nitrogen atom bisecting the Cb−Cb bond (Figure 3). Because all meso-aryl substituents in corrole 1 have bulky groups in the ortho-positions, the aryl rings are almost perpendicular to the macrocycle. Quantitative analysis of the macrocycle distortion depends on the choice of the reference plane. In our opinion, all previous approaches to define a corrole macrocycle mean plane have two substantial shortcomings, namely that they do not take into account the general asymmetric character of the macrocycle distortions and NH tautomerization. Due to the total asymmetry of the corrole structure it is impossible to define the macrocyclic plane in a simple way. Furthermore, not all atoms are equivalent with respect to their importance for the macrocycle plane definition. All approaches used to date deal with complete sets of elements of a given type: the 19C+4N plane consists of all carbon and nitrogen atoms of the macrocycle,60−62 the 19C plane consists of all carbon atoms of the macrocycle,24 the 11C plane consists of all Ca and Cm atoms of the macrocycle,61 and the 4N plane consists of the four pyrrole nitrogens.63 The only documented case of an asymmetrical set is the 3N plane of unsubstituted pyrroles, which has been used for analysis of out-of-plane deviations of the core hydrogen atoms in N-aryl-substituted corroles.30,31 From the two side views of the optimized structures of both NH tautomers shown in Figure 2, one can see that the pyrrole nitrogens and Cb carbons are substantially deviating from the 11C plane formed by all other macrocycle carbon atoms. Thus the 11C plane seems to be the best approximation for a complete-set macrocycle mean plane but does not really consider the macrocycle asymmetry. On the other hand, the

Table 2. Angle δ between the N−H Bond and the Pyrrole Ring Plane for the T1 and T2 Tautomers of Corrole 1 δ, deg

866

pyrrole/N

T1

T2

A/N(21) B/N(22) C/N(23) D/N(24)

2 25 2

17 15 9

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be done with the equtaion 1 + λ2 cos θ = 0, where θ is the mean angle for the C−N−C and the two H−N−C bonds,67 considering that all three angles (Ca−N−C′a, Ca−N−H and C′ a −N−H) are different. For example, the maximum pyramidalization was found for pyrrole B in the T1 tautomer with a value λ2 = 2.19. Notably, the pyramidalization patterns are clearly different for the two NH tautomers. In the case of the T1 tautomer, as mentioned above, pyrrole B has the maximum value, which results in an angle between the N−H bond and the pyrrole ring plane of δ = 25°. In this case the pyrrole plane is defined by the triangle spanned by the N and the two Cb atoms. The δ values for pyrroles A and C do not exceed 2°. In the T2 tautomer, the maximum δ value is lower, but the values for all three pyrroles are of the same order of magnitude: 17° for A, 15° for B, and 9° for D, respectively. The significant out-of-plane displacement of the pyrrole protons has been noticed for the first time by Gross et al.,63 and the degree of displacement has been related to the acidity; i.e., the higher the pyramidalization, the higher the acidity of given corrole macrocycle. The observed differences in pyramidalization of the pyrrole nitrogens for the T1 and T2 tautomers of corrole 1 account for the experimentally observed acidity difference reported recently.29 It seems that the lower pyramidalization for the three pyrrole nitrogens of the T2 tautomer is more important than the higher pyramidalization for one pyrrole nitrogen of the T1 tautomer; i.e., the T2 tautomer is the more acidic one. This finding means that any of the three pyrrolic protons can dissociate under basic conditions (after proton removal the two remaining protons arrange to adopt a stable trans-configuration, which is characteristic for tetrapyrrolic macrocycles having two core hydrogen atoms23). Increased pyramidalization of three pyrrole nitrogens increases the deprotonation rate in the T2 tautomer compared to the T1 tautomer where only pyrrole N−H has an enhanced dissociation rate. TD-DFT Calculations of the Absorption Spectra of the Corrole NH Tautomers. To verify the assignment of the spectral features to the two NH tautomers, time-dependent density functional theory (TD-DFT) calculations of the ground state absorption spectra of the two optimized molecular structures were carried out (Figure 4). The performances of the classical GGA and hybrid density functionals (BP86, B3LYP) and those including long-range corrections (CAMB3LYP, LC-ωPBE) have been compared for uncertainties in non-multireference methods for tetrapyrrolic compounds. Fortunately, all calculations with the different functionals uniformly reproduce the order of the 0−0 transitions of the T1 and T2 tautomers: the longest wavelength band in the absorption spectrum belongs to the 0−0 transition of the T1 tautomer and the next electronic transition is the 0−0 transition of the T2 tautomer. This means for the crucial criterion the assignment of the spectroscopically defined tautomers T1 and T2 to the respective chemical structures (Scheme 1) is correct. However, the calculated energy differences between these transitions obtained by BP86 or LC-ωPBE (185 and 170 cm−1, respectively) are too small compared to the experimentally measured value of 955 ± 10 cm−1. Calculations with B3LYP increase this difference up to 500 cm−1, and finally calculations with the CAM-B3LYP functional provided a satisfactory accuracy, providing an energy difference of 730 cm−1. Furthermore, the correct order of the S0 → S2 transitions of the T1 and T2 tautomers is reproduced by both B3LYP and CAM-B3LYP functional-based calculations. Thus, the results of

Figure 4. Ground-state absorption spectra of the T1 (red) and T2 (blue) corrole NH tautomers calculated with B3LYP (upper panel) and CAM-B3LYP (bottom panel) functionals. Note that the spectra contain the pure electronic transitions only, without vibronic overtones.

the quantum-chemical calculations with B3LYP and CAMB3LYP functionals support our previous tentative NH tautomer assignment28 and match to the results of the absorption spectrum analysis with the Gouterman four-orbital model. These two functionals, which are expected to provide the most adequate description of the corrole NH tautomers, will hence mainly be used in the following discussion on the nature of the transitions. The best match of the calculated absorption peaks with the experimental maxima measured for the lowest transitions has been found using the BP86 functional. Though better in absolute values for these transitions than calculations using other functionals, the BP86 calculations are not able to reproduce the energetic relations between the first and the second electronic transitions for both NH tautomers and contain additional absorption peaks between the Q and the Soret bands, which have almost charge transfer character. This problem, which does not occur in the calculations using CAMB3LYP or LC-ωPBE functionals, obviously results from the expected shortcoming of non-range-separated functionals in describing charge transfer states. Unfortunately, using the longrange corrected LC-ωPBE functional results in transition energies that are far above the experimental values and fails also to predict a correct order of the first and second electronic transitions for both NH tautomers. Only in the case of the B3LYP and CAM-B3LYP functionals do the calculated absorption spectra, which are moderately blue-shifted compared to the experimental one (compare Figures 1 and 4), reproduce the energetic relations between the electronic transitions, as identified from the experimental spectra, in a sufficient manner. This means that for the correct energetic gap between the S0 → S1 and S0 → S2 transitions, the exchange interaction, better taken into account by hybrid functionals like B3LYP, is crucial. Using the B3LYP and CAM-B3LYP functionals, we have determined the Kohn−Sham molecular orbital energies for the 867

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T1 and T2 tautomers (Table 3). These data do not show dramatic changes in the orbital energies between the two NH Table 3. Calculated Kohn−Sham Orbital Energies for the T1 and T2 Tautomers (eV) B3LYP

CAM-B3LYP

MO

T1

T2

T1

T2

LUMO+1 LUMO HOMO HOMO-1

−2.273 −2.595 −5.244 −5.484

−2.201 −2.606 −5.240 −5.515

−1.280 −1.716 −6.267 −6.514

−1.139 −1.760 −6.319 −6.519

tautomers. In accordance with previously published results of quantum-chemical calculations, the LUMO and LUMO+1 molecular orbitals are not degenerate as in the case of porphyrins.26,62 Our results demonstrate that the energy difference between the frontier occupied orbitals ΔE(HOMO − HOMO−1) is even smaller than that between the unoccupied ones ΔE(LUMO+1 − LUMO). It should be noticed that the ΔE(HOMO − HOMO−1) remains practically unchanged in going from the T1 to the T2 tautomer, whereas the ΔE(LUMO+1 − LUMO) increases slightly. The latter seems to be the only pronounced effect of NH tautomerization on the molecular orbital level. The orbital origin for each of the four molecular orbitals remains unchanged for both NH tautomers (Figure 5). The HOMO can be treated as a full analogue of the porphyrin a2u orbital (high electron density on the meso-positions and pyrrole nitrogens), whereas the HOMO−1 looks like a porphyrin a1u orbital (high electron density on the Cb and nodes at the pyrrole nitrogens). The T2 tautomer has distinct nodes at all the pyrrole nitrogens, but the T1 tautomer has asymmetry in the electron density distribution, resulting in a clear node on pyrrole B only. The electronic spectra calculations with the CAM-B3LYP functional demonstrate that the electronic transitions involve few strong contributions, i.e., not more than two configurations with squared coefficients larger than 0.05. None of them involves unoccupied orbitals energetically higher than LUMO +1 (Figure 5). The vertical arrows show the electron−hole configurations contributing to each electronic transition. The widths of these arrows scale with the coefficients in the configuration interaction (CI) expansion, thus enabling estimation of the particular molecular orbital weights in the electronic transitions. The contributions are depicted with squared coefficients larger than 0.05. On this level, the electronic transitions in the UV−vis range can be satisfactory explained with the Gouterman four-orbital model for both corrole NH tautomers. In the spectra calculated with the B3LYP functional additional low intensity bands appear, which are absent in the CAM-B3LYP calculated ones (Figure 4). Because these “additional” transitions were not detected in the experiments and due to their charge transfer character, they are likely to be artifacts resulting from the known limitations of non-rangeseparated functionals. The only significant difference in the nature of the transitions obtained by the B3LYP and CAMB3LYP functionals is observed for the lower (from two) Soret transitions, calculated for the T1 tautomer at 406.9 and 378.5 nm by using B3LYP and CAM-B3LYP, respectively. This transition complies with the Gouterman four-orbital model when the long-range corrected CAM-B3LYP functional is used and shows no notable charge transfer character. In the case of

Figure 5. Frontier molecular orbitals of the T1 (left) and T2 (right) corrole NH tautomers, as calculated using the CAM-B3LYP functional and the TZVP basis set, and the transition diagram. The wavelengths of the transitions are denoted at the top of each of the grayish transition panels. The thickness of the vertical arrows relates to the coefficients in the CI expansion, visualizing the weight of the particular molecular orbitals to the electronic transitions.

B3LYP, the strongest contribution to this Soret band is a transition from the HOMO−1 to the LUMO+2. Because the LUMO+2 is located at the meso-pyrimidinyl group, whereas all occupied orbitals involved are located at the tetrapyrrolic macrocycle, this contribution results in a certain charge transfer character for B3LYP calculations. For the T2 tautomer, the electronic transitions computed with B3LYP are similar to those of the T1 tautomer, except that all dominating contributions are in full accordance with the Gouterman fourorbital model; i.e., there is no sign of charge transfer character for the lowest Soret band. The higher Soret transition possesses a HOMO−1 to LUMO+1 excitation as the dominant contribution for both functionals and both NH tautomers, which complies perfectly with the Gouterman four-orbital concept. Notably, the Gouterman model is based on the configuration interaction between the four frontier orbitals as an essential element, which causes the splitting between Q and Soret bands. This may explain why a correct consideration of the exchange interaction, as provided by the hybrid functional 868

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edge BELSPO for the frame of the IAP P7/05 network, facilitating their collaboration. M. Presselt gratefully acknowledges financial support by the Carl-Zeiss-Foundation. M. Presselt and W. Beenken are also grateful to H. Schwanbeck from the Ilmenau University Computer Center for technical assistance. T. H. Ngo further acknowledges the International Center for Young Scientists (ICYS).

B3LYP and its derivative CAM-B3LYP, is so crucial for the correct order of electronic transitions in the absorption spectra of both NH tautomers. Nevertheless, all TD-DFT calculations together confirm the original assignment of the chemical structures of the T1 and T2 tautomers.

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CONCLUSIONS A combined experimental and theoretical study was carried out to reveal the individual absorption spectra of the NH tautomers of AB2-meso-pyrimidinylcorrole 1. The assignment of the experimentally observed absorption bands in the ground state absorption spectrum to the individual NH tautomers, based on the Gouterman four-orbital model, was confirmed by determining optimized structures using DFT and the corresponding spectra by TD-DFT. The red-shifted T1 tautomer is the one with protonated pyrrole nitrogens N(21), N(22), and N(23), whereas the blue-shifted T2 tautomer has pyrrole nitrogens N(21), N(22), and N(24) protonated. The general type of nonplanar distortion for both NH tautomers can be assigned as wave-like, with the wave axis going through the pyrroles containing N(22) and N(24). The total magnitude of the out-of-plane distortion is noticeably higher for the T1 tautomer. The results of the geometry optimization demonstrate a distinct difference between the out-of-plane distortions for the two NH tautomers, particularly for the pyrrole ring tilting. The acid−base properties of the corrole NH tautomers can be rationalized by the interplay of the degree of out-ofplane distortion of the macrocycle as a whole and the tendency of the pyrrole nitrogens toward pyramidalization. The possibility of preferential solvation of one of the NH tautomers brings additional complexity by particular core stabilization (deprotonated, free base, or protonated) and leads to pronounced solvent dependence of the ground state absorption spectra of corroles. The presented results shed light on the long-lasting problem of detailed interpretation of the ground state absorption spectra of meso-triarylcorroles.

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ASSOCIATED CONTENT

S Supporting Information *

Frontier molecular orbitals and electronic transitions as calculated using the B3LYP functional (Figure S1), comparison of TD-DFT calculated absorption spectra using different functionals (Figure S2), and xyz-data of both corrole NH tautomers. This information is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*W. Beenken: tel, 03677693258; e-mail, [email protected]. *W. Maes: tel, (+32) 11268312; fax, (+32) 11268299; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been carried out with financial support from FP7 project DphotoD-PEOPLE-IRSES-GA-2009-247260. W. Maes, T. H. Ngo, and W. Dehaen thank the FWO (Fund for Scientific Research − Flanders), the KU Leuven, Hasselt University, and the Ministerie voor Wetenschapsbeleid for continuing financial support. The Belgian coauthors acknowl869

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dx.doi.org/10.1021/jp411033h | J. Phys. Chem. A 2014, 118, 862−871