Profiling Energetics and Spectroscopic Signatures in Prototropic

Article pubs.acs.org/JPCA

Profiling Energetics and Spectroscopic Signatures in Prototropic Tautomers of Asymmetric Phthalocyanine Analogues Victor N. Nemykin* and Jared R. Sabin Department of Chemistry and Biochemistry, 1039 University Drive, University of Minnesota, Duluth, Minnesota 55812, United States S Supporting Information *

ABSTRACT: Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used to explain discrepancies in UV−vis and MCD spectra of the metal-free tribenzo[b,g,l]thiopheno[3,4-q]porphyrazine (1), substituted tribenzo[b,g,l]porphyrazine (2), and 2,3-bis(methylcarboxyl)phthalocyanine (3). On the basis of gas-phase and polarized continuum solvation model (PCM) DFT and TDDFT calculations, it was suggested that both NH tautomers contribute to the spectroscopic signature of 1, whereas the Q-band region of 2 and 3 is dominated by a single NH tautomer. For all tested compounds, it was found that the combination of the BP86 exchange−correlation functional, 6-31G(d) basis set, and TDDFTPCM approach provides the best accuracy in energies of the Qx- and Qy-bands of the individual NH tautomers as well as correctly describes their relative energy differences, which are important in understanding of experimental spectroscopy of the target systems.

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INTRODUCTION Traditionally, aromatic tetrapyrrolic macrocycles (i.e., porphyrins, phthalocyanines, and their analogues) were studied because of their industrial importance as dyes, pigments, and catalysts as well as their ability to mimic biologically relevant electron- and atom-transfer reactions.1 During the last several decades, however, in addition to the traditional areas of applications, these chemically and thermally robust systems with welldefined redox, photophysical, and coordination properties were intensely investigated as promising photosensitizers for photodynamic therapy of cancer (PDT), charge carriers for copiers and printers, redox-driven fluorescence markers and materials for molecular electronics, light-harvesting components of dyes sensitizing solar cells, and materials for gas sensors.2 Because of their key role in PDT application as well as importance as precursors for main-group, transition-metal, and lanthanide complexes, metal-free phthalocyanines and their analogues were intensely investigated over the past several decades.3 Specifically, tautomerization of NH protons (i.e., proton-transfer reaction between two inner NH protons and four inner nitrogen atoms) in metal-free porphyrins (H2Ps) has been the subject of great interest of several research groups using experimental and theoretical approaches.4 Following the results of these studies, it is now commonly accepted that the trans-NH tautomer of the metal-free porphyrins is more stable compared to the elusive cis-tautomer, which is considered as the reaction intermediate in the NH tautomerization process (Figure 1).4 In contrast, the NH tautomerization reaction in symmetric metal-free phthalocyanines and their analogues (i.e., tetraazaporphyrins and naphthalocyanines) has been rarely © 2012 American Chemical Society

investigated using experimental methods predominantly because the smaller inner cavity size, higher rigidity of the macrocycle, and higher acidity of inner NH protons result in a significantly lower activation barrier for the reaction.5 One of the interesting problems of NH tautomerism arises for lowsymmetry metal-free phthalocyanines and their analogues.6 Indeed, in these compounds (i.e., AAAB and ABAB) two possible trans-NH tautomers should have different energies, whereas in the case of AABB type only one trans-NH tautomer should be present (Figure 2). Early investigations indicate that in the case of simple benzanulated low-symmetry tetraazaporphyrins, two NH protons are predominantly connected to the pyrrolic rings with smaller aromatic substituents.7 Later, Kobayashi and co-workers confirmed the presence of both possible trans-NH tautomers in the series of low-symmetry tetraazaporphyrins using UV−vis and MCD methods accompanied by semiempirical ZINDO/S calculations.8 These findings were further re-examined by Jiang, Bai, and co-workers using the DFT approach.9 Although the semiempirical ZINDO/S approach provides an excellent agreement between experimentally observed and calculated vertical excitation energies for all possible NH tautomers studied by Kobayashi and co-workers8 as well as recently by Wöhrle and coworkers,10 their predicted relative energies were significantly overestimated for AAAB and ABBB low-symmetry systems. On the other hand, although DFT calculations by Jiang, Bai, and Received: May 7, 2012 Revised: June 12, 2012 Published: June 12, 2012 7364

dx.doi.org/10.1021/jp304386x | J. Phys. Chem. A 2012, 116, 7364−7371

The Journal of Physical Chemistry A

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Figure 1. Simplified energy profile of DFT-predicted (ref 9) NH tautomerization process in the metal-free tetraazaporphyrin.

differences, which may or may not originate from the NH tautomerism in compound 1. The second reason for choosing of 1−3 is because these compounds represent low-symmetry heterocycle-containing tribenzo[b,g,l]thiopheno[3,4-q]Scheme 1. NH Tautomers of Compounds 1−3

Figure 2. Nonequivalent NH tautomeric forms of AAAB and ABAB types of low-symmetry phthalocyanine analogues.

co-workers resulted in more reasonable energetics for individual NH tautomers, their vertical excitation energies were not calculated.9 As shown earlier, TDDFT and TDDFT-PCM methods provide very reasonable vertical excitation energies for porphyrins, phthalocyanines, and their analogues, but these approaches have not been used for the prediction of both energetics and vertical excitation energies of the individual NH tautomers.11 Thus, in this paper, for the first time, we profile both the spectroscopic signatures and energetics of the individual NH tautomers in low-symmetry phthalocyanine analogues (Scheme 1) using a combined TDDFT and TDDFT-PCM approaches. We choose the reaction sequence shown in Scheme 1 for the two major reasons. First, published earlier by one of us,12 experimental data on compounds 1−3 (Scheme 1) suggest that the spectroscopic UV−vis and MCD spectra of compound 1 are quite different from those obsereved for compounds 2 and 3. Specifically, in UV−vis and MCD spectra of 1, additional intense closely spaced bands were observed between classic Qx and Qy bands, which were not observed for compounds 2 and 3. At that time, however, available theoretical methods did not allow us to explain these 7365



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porphyrazine (1), substituted tribenzo[b,g,l]porphyrazine core (2), and phthalocyanine (3), which allow us to test simultaneously the performance of TDDFT and TDDFTPCM for three independent classes of porphyrinoids.

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COMPUTATIONAL DETAILS All calculations were conducted using the Gaussian 09 software package running under either a Windows or UNIX OS.13 The molecular geometries were obtained either in a gas phase or in dichloromethane via optimization with the hybrid (∼20% of Hartree−Fock exchange) Becke three-parameters exchange functional14 and the Lee−Yang−Parr nonlocal correlation functional 15 (B3LYP) or GGA (0% of Hartree−Fock exchange) Becke’s exchange functional16 and the Perdew nonlocal correlation functional17 (BP86). The 6-31G(d) basis set18 for all atoms was used in all calculations because reported earlier11 as well as our test calculations suggested only minimal changes in calculated energies and vertical excitations in phthalocyanines and their analogues when larger (i.e., 6311G(d) and 6-311+G(d)) basis sets were employed. In all cases, solvation effects were taken into consideration using polarized continuum model19 (PCM; dichloromethane was used as a solvent). For all optimized structures, frequency calculations were carried out to ensure that optimized geometries represented local minima. For the sake of simplicity and symmetry considerations, the ester groups in compound 2 were eliminated, whereas those in compound 3 were replaced with cyano groups. Such simplification resulted in C2v, Cs, and C2v symmetries for compounds 1−3, respectively. TDDFT and TDDFT-PCM calculations were conducted using BP86 and B3LYP exchange−correlation functionals coupled with the 631G(d) basis set for all atoms. The first 50 excited states were calculated to ensure that all excitations in the Q-band region are covered. Single point calculations were performed at the same level of theory as TDDFT and TDDFT-PCM computations. When necessary, the percent contributions of atomic orbitals to molecular orbitals were calculated using the VMOdes program.20

Figure 3. Vis and MCD spectra of compound 1 (A), 2 (B), and 3 (C) in the Q-band region (in M−1·cm−1 and deg·M−1·cm−1·T−1 units) for UV−vis and MCD spectra, respectively.

728 and 646 nm and are associated with the strong negative and positive Faraday B-terms in the MCD spectrum observed at 724 and 644 nm, respectively (Figure 3a). The observation of two closely spaced intense bands at 693 and 674 nm, which are associated with the strong Faraday pseudo A-term in the MCD spectrum centered at 688 nm is indicative of the presence of the minor NH tautomer form in solution. On the other hand, only two intense Qx and Qy bands were observed in the UV−vis and MCD spectra of the compounds 2 and 3 (Figure 3b,c), suggesting large energy differences between major and minor forms of NH tautomers in these molecules. To gain insight into the spectroscopic signatures and Boltzmann distribution for the minor and major forms of NH tautomers in compounds 1−3, gas-phase TDDFT and solution TDDFT-PCM calculations were performed for all compounds using four optimized geometries and two exchange−correlation functionals (GGA BP86, 0% Hartree− Fock exchange and hybrid B3LYP, ∼20% Hartree−Fock exchange) coupled with a 6-31G(d) basis set. As shown previously, the nature of the exchange−correlation functional plays an important role in accurate prediction of the energies and spectroscopic signatures of phthalocyanine analogues, whereas the use of the larger than 6-31G(d) basis set provides only minor improvements on the calculated properties of these systems.11 Because all DFT and DFT-PCM calculations considered in this work provide similar trends for all compounds, we will only discuss in details results obtained using DFT-PCM B3LYP//BP86 approach as a typical example.

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RESULTS AND DISCUSSION Electronic Structures and Relative Energies of NH Tautomers of Compounds 1−3. As suggested earlier for the simple benzoanulated metal-free tetraazaporphyrins,8 the presence of the minor NH tautomeric form could be detected using UV−vis and MCD spectroscopy in the low-energy Qband region. Specifically, the presence of relatively intense band(s) in UV−vis spectra located between the traditional Qx and Qy bands of the major NH tautomer, as well as observation of the related to these additional band(s) Faraday pseudo Aterm in MCD spectra were associated with the presence of the minor NH tautomer.8 The absence of these spectroscopic signatures was associated with a large energy difference between major and minor forms of NH tautomers. In addition, the minor NH tautomers in the low-symmetry phthalocyanines were detected using variable-temperature UV−vis and fluorescence approaches.21 In agreement with the hypothesis proposed by Kobayashi and co-workers,8 the UV−vis and MCD spectra of compounds 1−3 (Figure 3) suggest the presence of the minor NH tautomer in compound 1 and a large energy difference between the major and minor forms of NH tautomers in compounds 2 and 3.22,23 Indeed, the Qx and Qy bands of the major NH tautomer of macrocycle 1 are located at 7366



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Molecular orbital compositions, energies, and surfaces for the key orbitals in the NH tautomers of compounds 1−3 are presented in Figures 4−9 and Supporting Information Table 1.

Figure 7. DFT-PCM (B3LYP//BP86) molecular orbitals composition in NH tautomers of compound 2.

Figure 4. DFT-PCM (B3LYP//BP86) predicted MO energies and key surfaces for 1H2X and 1H2Y tautomers of compound 1.

Figure 5. DFT-PCM (B3LYP//BP86) molecular orbitals composition in NH tautomers of compound 1. Figure 8. DFT-PCM (B3LYP//BP86) predicted MO energies and key surfaces for 3H2X and 3H2Y tautomers of compound 3.

Figure 6. DFT-PCM (B3LYP//BP86) predicted MO energies and key surfaces for 2H2X and 2H2Y tautomers of compound 2.

Because the first two excited states (which are responsible for observation of Qx and Qy bands) in compounds 1−3 resemble almost pure HOMO (a1u type, π MO) → LUMO (b2g type, π* MO) and HOMO (a1u type, π MO) → LUMO+1 (b3g type, π* MO) transitions in the parent metal-free phthalocyanine,3a,22 we will focus on these MOs energies and compositions in compounds 1−3. In agreement with the previous DFT calculations on phthalocyanine-type macrocycles,3a,11,22 the HOMO in NH tautomers 1H2X and 1H2Y is an a2 symmetry π-orbital, which resembles the transition-metal as well as metalfree phthalocyanine a1u MO. The energies of the HOMOs are

Figure 9. DFT-PCM (B3LYP//BP86) predicted molecular orbitals composition in NH tautomers of compound 3.

almost the same for 1H2X and 1H2Y tautomers most likely because they are delocalized over the entire macrocycle. The HOMOs in 1H2X and 1H2Y tautomers are well separated in energy from the other occupied MOs. In both tautomers, the LUMO and LUMO+1 MOs have b2 and a2 symmetries and resemble b3g and b2g MOs of metal-free phthalocyanine. The 7367



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Table 1. Energetics and Boltzman Distribution Analysis for Individual Tautomers of Compounds 1−3 Predicted at DFT and DFT-PCM Levels methoda DCM

gas

DCM

gas

DCM

gas

B3LYP//B3LYP B3LYP//BP86 BP86//B3LYP BP86//BP86 B3LYP//B3LYP BP86//BP86 B3LYP//B3LYP B3LYP//BP86 BP86//B3LYP BP86//BP86 B3LYP//B3LYP BP86//BP86 B3LYP//B3LYP B3LYP//BP86 BP86//B3LYP BP86//BP86 B3LYP//B3LYP BP86//BP86

ΔE (cm−1)b 330 270 330 270 400 340

ΔE (kJ/mol)b

Compound 1 (330)

(260) (410) (340) Compound 2 −1620 (−1440) −1490 −1430 −1390 (−1935) −1680 (−1680) −1470 (−1460) Compound 3 610 (600) 620 610 620 (590) 580 (550) 560 (530)

3.95 3.23 3.95 3.23 18.54 18.30 −19.38 −17.82 −17.11 −16.63 −20.10 −17.59 7.30 7.42 7.30 7.42 6.94 6.70

(3.95)

(3.11) (4.91) (4.07) (−17.23)

(−19.56) (−20.10) (−17.46) (7.18)

(7.06) (6.58) (6.34)

ratioc 17:83 21:79 17:83 21:79 13:87 16:84 100:0 100:0 100:0 100:0 100:0 100:0 5:95 5:95 5:95 5:95 6:94 6:94

(17:83)

(22:78) (12:88) (17:83) (100:0)

(100:0) (100:0) (100:0)

(5:95)

(6:94) (7:93) (7:93)

a Geometry Optimization // Single Point. bE(H2X) −E(H2Y); Zero-Point energy given in parentheses cBoltzmann distribution ratio: H2X: H2Y; Zero-Point distribution given in parentheses

LUMO to LUMO+1 energy gap is significantly larger for the 1H2X tautomer compared to that calculated for 1H2Y compound. Interestingly, DFT-PCM calculations predict that the a′ symmetry HOMOs in 2H2X and 2H2Y tautomers are localized on the sulfur-containing fragment of the molecule and do not contribute into the intense Qx and Qy transitions (see below). Because of the presence of NH protons at the same fragment in 2H2X, this MO is significantly stabilized compared to the 2H2Y tautomer. The HOMO−1 in 2H2X and 2H2Y tautomers has π-character and is delocalized over entire macrocycle and resembles the a1u symmetry HOMO in metal-free phthalocyanine, a transition from which contributes the most intensity to the Q-band region of compound 2. Unlike the energies of HOMOs, the energies of HOMO−1s in 2H2X and 2H2Y tautomers are almost identical, similar to the case for compound 1. Again, HOMO and HOMO−1 are well separated in energy from the rest of the occupied MOs in 2H2X and 2H2Y tautomers. In both tautomers, the LUMO and LUMO+1 MOs have a″ and a′ symmetries and resemble b3g and b2g MOs of metal-free phthalocyanine. Unlike in tautomers 1H2X and 1H2Y, the LUMO - LUMO+1 energy gap is very close in 2H2X and 2H2Y compounds. Finally, the HOMO in NH tautomers 3H2X and 3H2Y is an a2 symmetry π-orbital and resembles that of the transition-metal and the metal-free phthalocyanine a1u MO. The energies of HOMOs are almost the same for 3H2X and 3H2Y tautomers most likely because they are delocalized over entire macrocycle. The HOMOs in 3H2X and 3H2Y tautomers are well separated in energy from the other occupied MOs. In both tautomers, the LUMO and LUMO+1 MOs have π*-characted and a2 and b1 symmetries and resemble b2g and b3g MOs of metal-free phthalocyanine. The energy order of these MOs is opposite to that calculated for 1H2X and 1H2Y tautomers and reflects the strong electron-withdrawing effect of the peripheral cyanogroups in 3H2X and 3H2Y compared to the slightly electron-donating effect of the thiophene moiety in

1H2X and 1H2Y. Similarly, the LUMO to LUMO+1 energy gap is significantly larger for the 3H2Y tautomer compared to that calculated for the 3H2X compound, which is opposite to the trend observed for 1H2X and 1H2Y. Gas-phase DFT and solution DFT-PCM energies and Boltzman distribution analysis for tautomers 1−3 are presented in Table 1. It is important to note that the relative energies and trends in tautomeric pairs are very close for all combinations of the exchange−correlation functional and media (i.e., gas phase and solution) used in this work. In excellent agreement with experimental observation, the DFT and DFT-PCM predicted energy differences for 1H2X and 1H2Y tautomers are quite small (Table 1) with 1H2Y tautomer being more stable. A small energy difference suggests the presence of both tautomers in solution at room temperature (see discussion below). On the other hand, DFT and DFT-PCM calculated energy differences in 2H2X and 2H2Y as well as 3H2X and 3H2Y pairs are significantly larger (Table 1), which is indicative of the dominance of only one tautomer in solution at room temperature. Similarly to earlier predictions,8,9 the most stable tautomer of compound 2 (2H2X) has two NH protons predominantly connected to the pyrrolic rings with smaller number of aromatic substituents. Boltzmann distribution analysis of all tautomers was conducted using eq 1: n1/n2 = exp( −ΔE /kT )

(1)

where n1 and n2 are fractions of tautomer 1 and 2 in solution, ΔE is the energy difference between energies of tautomer 1 and 2, k is the Boltzmann constant, and T is temperature. Boltzmann distribution analysis indicates that the roomtemperature ratios of 1H2X and 1H2Y, 2H2X and 2H2Y, and 3H2X and 3H2Y should be close to 30/70%, 100/0%, and 5/ 95%, respectively. Such distribution suggests that only for compound 1 will both 1H2X and 1H2Y tautomers be observed in solution at comparable concentrations, whereas UV−vis and 7368



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Table 2. Experimentally Observed, TDDFT, and TDDFT-PCM Predicted Energies of Qx and Qy Bands in Compounds 1−3 methoda exp (UV−vis)b exp (MCD)b 1H2X

1H2Y

exp (UV−vis) exp (MCD) 2H2X

2H2Y

exp (UV−vis) exp (MCD) 3H2X

3H2Y

a

B3LYP//B3LYP B3LYP//BP86 BP86//B3LYP BP86//BP86 B3LYP//B3LYP B3LYP//BP86 BP86//B3LYP BP86//BP86



Qx, nm

Qy, nm

Compound 1 728 (693) 646 (674) 724 (695) 644 (681) 712 643 729 665 727 659 745 681 695 657 717 676 712 669 734 690 Compound 2 690 594 686 607 645 573 668 602 666 588 685 617 656 601 678 622 668 614 691 636 Compound 3 703 666 704 666 693 643 724 670 713 657 744 685 700 633 726 658 715 648 742 673

Qx, cm−1

Qy, cm−1

13740 (14440) 13810 (14390) 14040 13720 13760 13420 14390 13950 14050 13620

15480 (14840) 15530 (14680) 15550 15040 15170 14680 15220 14790 14950 14490

14490 14580 15500 14970 15020 14560 15240 14750 14970 14470

16840 16470 17450 16610 17000 16210 16640 16080 16290 15720

14220 14200 14430 13810 14026 13440 14290 13770 13990 13480

15020 15020 15550 14930 15220 14560 15800 15200 15430 14860

Geometry optimization//single point. bBands observed for minor tautomer of compound 1 are listed in parentheses.

MCD spectra of compounds 2 and 3 should be dominated by the 2H2X and 3H2Y tautomers, respectively. TDDFT-PCM Predicted Vertical Excitation Energies for Compounds 1−3. In agreement with the previous publications,11 TDDFT and TDDFT-PCM predicted vertical excitation energies in compounds 1−3 depend on the nature of employed exchange−correlation functional and “solvent” media (Table 2). Despite such expected dependence, the calculated trends for individual tautomers of compounds 1−3 remain the same and thus the only TDDFT-PCM B3LYP//BP86 calculations, which provide the best agreement between theory and experiment will be discussed in details below. TDDFT-PCM B3LYP//BP86 calculated vertical excitation energies of NH tautomers of compounds 1−3 in Q-band region along with the experimental UV−vis and MCD data are shown in Figures 10−12 and Supporting Information Figures 1−3. Similar to the bands for the metal-free phthalocyanine and its low-symmetry analogues, the Qx and Qy bands for all tautomers originate from the almost pure HOMO (compounds 1 and 3) or HOMO−1 (compound 2) to LUMO and LUMO +1 π−π* transitions (Supporting Information Table 3). Agreement between the TDDFT-PCM predicted energies for Qx and Qy transitions and experimental data for compounds 1− 3 is excellent (Table 1). Indeed, differences between the experimental and theoretical energies are in the ∼500 cm−1

Figure 10. Experimental vis (A) and MCD (B) as well as TDDFTPCM predicted (C) and (D) spectra of compound 1 in the Q-band region (M−1·cm−1 and deg·M−1·cm−1·T−1 units for UV−vis and MCD spectra, respectively). The cumulative spectral curve in part D is generated using Boltzmann distribution analysis. Red bars and lines correspond to 1H2X, and blue bars and lines, to 1H2Y tautomers. A bandwidth of 500 cm−1 was used in all cases.

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negative and positive Faraday B-terms observed experimentally (Figure 10). Boltzmann distribution analysis predicts that the 1H2Y tautomer is 270 cm−1 (3.23 kJ/mol) more stable compared to 1H2X. This is in disagreement with the experimental data, which suggest the opposite trend. Despite this discrepancy, however, DFT-PCM and TDDFT-PCM data suggest that both tautomers should be present in solution at room temperature in agreement with the experimental observations. The TDDFT-PCM predicted energies of Qx bands in 2H2X (668 nm) and 2H2Y (678 nm) tautomers are close to the those observed experimentally in the MCD spectrum (negative Faraday B-term, 686 nm), whereas those calculated for Qy bands in 2H2X (602 nm) and 2H2Y (622 nm) tautomers are also close to those observed experimentally (positive Faraday B-term, 607 nm, Figure 11). Because of the very large energy difference 1490 cm−1 (17.82 kJ/mol) between the 2H2X and 2H2Y tautomers, Boltzmann population analysis suggests that only the former tautomer will contribute to the overall intensity of the Q-band region, in an excellent agreement with the experimental data. Indeed, only two significantly intense Faraday B-terms were observed in the experimental MCD spectrum of 2, which suggests the presence of only a single tautomeric form in solution. Finally, the MCD spectrum of the low-symmetry phthalocyanine 3 is dominated by a negative Faraday B-term located at 704 nm and a positive Faraday B-term observed at 666 nm. These correspond to the Qx and Qy bands observed at 703 and 666 nm in UV−vis spectrum (Figure 12). Interestingly, the energies of the TDDFT-PCM predicted Qx band for 3H2X (724 nm) and 3H2Y (726 nm) tautomers are almost identical, whereas the calculated energy of the Qy band in the same tautomers (670 and 658 nm, respectively) is slightly different. Boltzmann population analysis predicts that 2H2Y tautomer will be the major species in solution at room temperature (96%), whereas the 2H2X tautomers will only provide a small contribution to the overall UV−vis and MCD spectra intensities. Again on both qualitative and quantitative levels this prediction is in agreement with the experimental data.


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CONCLUSIONS Overall, DFT-PCM and TDDFT-PCM could accurately predict the simultaneous presence of minor and major NH tautomers in metal-free tribenzo[b,g,l]thiopheno[3,4-q]porphyrazine (1), which was experimentally observed in its UV−vis and MCD spectra. DFT-PCM and TDDFT-PCM was also correct in the prediction of the dominance of only a single NH tautomer in the case of the low-symmetry metal-free substituted tribenzo[b,g,l]porphyrazine (2) and 2,3-dicyanophthalocyanine (3). It was found that a combination of a BP86 exchange−correlation functional and a 6-31G(d) basis set provides very accurate energies of the Qx- and Qy-bands of the individual NH tautomers in compounds 1−3.


range, which is much better than the typical errors expected for TDDFT calculations (∼1600 cm−1).11 TDDFT-PCM calculations predict that because of the larger splitting between the LUMO and LUMO+1, the Qx and Qy bands in the 1H2X tautomer should be observed at 729 and 665 nm, which correspond to the strong negative and strong positive MCD Faraday B-terms observed at 724 and 644 nm. On the other hand, the TDDFT-PCM predicted energies for the Qx and Qy bands for the 1H2Y tautomer are close to one other and should be between the energies of the corresponding bands for the 1H2X tautomer. On a quantitative level, the predicted energies of the Qx and Qy bands in the 1H2Y tautomer were at 717 and 676 nm, respectively, which is close to the 693 and 674 nm

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ASSOCIATED CONTENT

* Supporting Information S

Experimental UV−vis and MCD data as well as TDDFT-PCM predicted vertical excitation energies for compounds 1−3 in energy (cm−1) scale. Room temperature Boltzmann distribution analysis for compounds 1−3. DFT-PCM optimized coordinates, tabulated MO compositions, and TDDFT-PCM predicted expansion coefficients for compounds 1−3. Full ref citations for refs 2c, 7a, and 13. This material is available free of charge via the Internet at http://pubs.acs.org. 7370



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support from NSF CHE-1110455 and Minnesota Supercomputing Institute to V.N. as well as University of Minnesota Duluth Undergraduate Research Opportunity Grants to J.R.S. are greatly appreciated. We also acknowledge Dr. N. Kobayashi’s help with collecting initial UV−vis and MCD spectra of compounds 1−3.

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