Article pubs.acs.org/JPCA
Electronic Spectra of Cycl[3.3.2]azine and Related Compounds: Solvent Effect on Vibronic Couplings Yasuhiro Shigemitsu,*,†,‡ Motoyuki Uejima,§ Tohru Sato,§ Kazuyoshi Tanaka,§ and Yoshinori Tominaga∥ †
Industrial Technology Center of Nagasaki, 2-1303-8, Ikeda, Omura, Nagasaki 856-0026, Japan Graduate School of Engineering, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japan § Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Faculty of Environmental Studies, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japan ‡
S Supporting Information *
ABSTRACT: Quantitative ab initio calculations are presented for the ultraviolet−visible peaks of cycl[3.2.2]azine and its mono- and dibenzannulated polycyclic compounds at the multistate CASPT2 (MS-CASPT2) level of theory, with 11 nm deviation from the experimental S0 → S1 absorption. The electrophilic substitution reactions of cycl[3.2.2]azine, benzo[a]/[g]annulated cycl[3.2.2]azines, and 6dimethylamino[2.2.3]cyclazine-1-carboxylates with 3-cyano-4methylthiomaleimide gave the corresponding functionalized cycl[3.2.2]azine derivatives, which exhibited the absorption maxima around 510−630 nm. The first intense peaks were investigated by means of time-dependent density functional theory (TD-DFT). These peaks were systematically underevaluated by ∼50 nm, within the acceptable accuracies of TD-DFT. Furthermore, we calculated vibronic coupling constants of the electronic excited states of cycl[3.2.2]azine and simulated absorption spectra both in vacuo and in ethanol. The solvent effect is found to enhance oscillator strengths and vibronic couplings. This is because the solvent effect gives rise to changes in the electron density difference on the phenyl ring, and in turn, the intensified overlap between the electron density difference and the potential derivative in the phenyl ring leads to enhanced vibronic couplings in ethanol.
1. INTRODUCTION Since the pioneering synthesis of cycl[3.2.2]azine was first achieved,1−4 a series of studies in the cyclazine field have been actively pursued.5−7 These studies broadly cover the synthesis8−11 of cyclazines as well as their physicochemical12 and magnetic properties,13−15 and computational analysis.16−18 These fused cyclic systems are of interest owing to their aromaticity in relation to their structures (planar or distorted), which has been extensively studied in comparison with the structures of isoelectronic annulene families.19−22 In particular, cycl[3.2.2]azine, a prototypical tricycle of the family, has been intensively studied as an isoelectronic analogue to [10]annulene because cycl[3.2.2]azine has a simple structure with C2v spatial symmetry and 10π-peripheral conjugation.23 1H NMR shows that cycl[3.2.2]azine is diatropic,24 whereas cycl[3.3.3]azine (a 12π electronic system) is paratropic,25,26 indicating that the π-conjugative peripheral structure comprises a completely conjugated, sp2-hybridized carbon skeleton in planarity. These experimental findings have been qualitatively explained by a series of resonance energy analyses based on graph theory.27−29 From a chemical reactivity viewpoint, the synthetic mechanism of cyclazine derivatives also has been investigated,30,31 to determine whether the peripheral π-electron © 2012 American Chemical Society
system serves as a stable (and thus inactive) moiety or as a nucleophilic moiety with nature of pyrrole character. Actually, we have already reported a series of reactions between 4methythiomaleimides and electron-rich cyclazine derivatives that yield novel merocyanine dyes.32 In this study, we report on the novel electrophilic substitution reaction of cycl[3.2.2]azine and its benzannulated derivatives with 3-cyano-4-methylthiomaleimides. The structures and properties of cyclazines and related compounds have been studied experimentally and theoretically, including the vibronic transitions of acenaphthene,33 the electronic spectra of fluorene and its analogues,34 the strained structure of acepentalene,35 the aromaticity and geometrical distortions of heteroacepentalenes,36 and the electrochemistry of indolo[3,2,1-jk]carbazoles.37 To our knowledge, however, the electronic structures of cyclazines have not been fully elucidated using modern ab initio quantum chemical calculations. That is, no systematic and quantitative theoretical analysis has been reported so far for the ultraviolet−visible (UV−vis) spectra of these compounds, despite the existence of Received: May 27, 2012 Revised: August 12, 2012 Published: August 30, 2012 9100
dx.doi.org/10.1021/jp305148x | J. Phys. Chem. A 2012, 116, 9100−9109
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methylthiomaleimide gave the corresponding 4-(6dimethylamino[2.2.3]cyclazin-4-yl)-2,5-dioxo-1H-pyrrole-3-carbonitrile derivatives 4a−4d. The synthetic details for 3a−3c and 4a−4d are described in the Supporting Information.
extensive synthetic studies fueled by the compounds’ industrial and pharmaceutical applications. In the present study, time-dependent density functional theory (TD-DFT), complete active space self-consistent field (CASSCF), and second-order CAS perturbation theory (CASPT2) quantum chemical analyses were used to obtain detailed insight into the UV−vis spectra of cycl[3.2.2]azine and its extended π-peripheral systems (1a−1c, 2a,and 2b in Figure 1). In addition, newly synthesized cyclazine derivatives linked
3. COMPUTATIONAL DETAILS The optimized geometries of the compounds were obtained by means of DFT at the (B3LYP)/6-311G(d,p) level in vacuo. TD-DFT spectral calculations were carried out using the three exchange-correlation (XC) functionals B3LYP, PBE, and CAMB3LYP with the 6-31+G(d,p) basis set. The solvent effects for ethanol were considered with the conventional linear-response polarized continuum model (PCM).40 DFT and TD-DFT calculations were performed using Gaussian09 software.41 For more accurate predictions of the vertical excitation energies, CASSCF and CASPT2 single-point calculations for the DFT-optimized geometries were performed using MOLCAS 7.4 software.42,43 For 1a, eight state-averaged (SA)CASSCF(10e,12o) calculations were done and the eight singlestate (SS)-CASPT2(10e,12o) states were averaged within the multistate (MS)-CASPT2 scheme. For 1b, 1c, 2a, and 2b, four SA-CASSCF(10e,10o) and SA-CASSCF(10e,14o) calculations were done, and the four SS-CASPT2 states were averaged within the MS-CASPT2 scheme. The Atomic Natural Orbitals large sets (ANO-L: C,N,O[4s3p2d]/H[3s2p]) were employed throughout the present study. Furthermore, the detailed CASSCF and CASPT2 computational strategies are described in the Supporting Information for 1a. 4. OPTIMIZED GEOMETRIES AND ELECTRONIC SPECTRA OF CYCL[3.2.2]AZINE AND ITS BENZANNULATED POLYCYCLES The pioneering X-ray crystal structure analysis of 1,4dibromocycl[3.2.2]azine is problematic because of the large deviations between its chemically equivalent bonds.44 The 1:1 complex of cycl[3.2.2]azine with 1,3,5-trinitrobenzene has been successfully crystallized, and the geometry of cycl[3.2.2]azine has been well established.45 The electrochemical properties and geometries of dibenzo[a,d]cycl[3.2.2]azines were also investigated.46,47 Theoretically, the geometries of cycl[3.2.2]azine derivatives have been examined using a series of primitive molecular orbital calculations.16−18,48,49 For 1a, Hü ckel molecular orbital (MO)48 and Pariser−Parr−Pople (PPP)MO geometry optimizations49 have been reported. As shown in Table 1, the key peripheral bond lengths optimized by these methods were qualitatively good within 0.02 Å deviations from the experiments, and no significant bond alternations were
Figure 1. Cycl[3.2.2]azine, its mono- and dibenzannulated polycyclic compounds, and the functionalized polycycles linked with maleimide ring.
with a maleimide moiety (3a−3c and 4a−4d in Figure 1) were computationally investigated to elucidate their electronic structures and UV−vis spectra. The detailed spectral structure of 1a, which is not observable in solution, can be restored computationally by considering the vibronically assisted sideband of the allowed intense peaks.
Table 1. Computed and Experimental C−C and C−N Bond Lengths (in Angströms) of 1a
2. SYNTHESIS Cycl[3.2.2]azine 1a and its benzannulated derivatives 1b and 1c were synthesized through the cycloaddition of indolidines bearing electron-withdrawing groups, such as cyano or carbamoyl groups, with acetylenic compounds, as reported previously.38,39 The reaction of 1a with 3-cyano-4-methylthiomaleimide in refluxing acetic acid gave 4-([2.2.3]cyclazin-2-yl)1-methyl-2,5-dioxo-1H-pyrrole-3-carbonitrile 3a. The reactions of benzo[a][2.2.3]annulated cyclazine 1b and benzo[g][2.2.3]annulated cyclazine 1c with 3-cyano-4-methylthiomaleimide afforded the corresponding novel compounds 3b and 3c, respectively. The reaction of methyl 6-dimethylamino[2.2.3]cyclazine-1-carboxylate or -1,2-dicarboxylates with 3-cyano-4-
theoretical Hückel N1−C2 N1−C11 C2−C4 C2−C7 C5−C6 C7−C9 C9−C11 a
9101
1.39 1.38 1.40 1.41 1.40 1.38 1.41
a
PPPb
DFT (this work)
exptlc
1.40 1.37 1.40 1.43 1.40 1.38 1.42
1.377 1.360 1.398 1.434 1.410 1.401 1.429
1.377 1.353 1.390 1.430 1.406 1.392 1.425
Reference 48. bReference 49. cReference 45. dx.doi.org/10.1021/jp305148x | J. Phys. Chem. A 2012, 116, 9100−9109
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In the present study, eight state-averaged (SA)-CASSCF(10,12) and MS-CASPT2/ANO-L calculations were carried out to quantitatively evaluate the several low-lying allowed excited states with 1A1 and 1B2 symmetry, as shown in Table 2. The active spaces include the spatially extended LUMO + 8 (7b1, Rydberg orbital) to appropriately treat the valence−Rydberg mixing, which in some cases significantly affects the valence excitation energies as has been reported for guanine.51 The good agreement of the predicted excitation energies between the SS-CASPT2 and the MS-CASPT2 indicates minor interaction between the SS-CASPT2 wave functions. The level shift technique with careful choice of shift value (0.3) seems to work well with the fairly large reference weight of the main configuration (ω = ∼0.7). The first MS-CASPT2 excitation energy (11A1 → 21B2, 399 nm) agreed quantitatively with the corresponding first peak at 420 nm with modest oscillator strength (0.11) and is described mainly by HOMO → LUMO excitation (66% of the configuration weight). The next transition (11A1 → 21A1, 265 nm) can be assigned to the second peak at 290 nm with a relatively small oscillator strength (0.026). The third transition (11A1 → 21B2, 249 nm) has a relatively small oscillator strength (0.014). The fourth transition (11A1 → 31A1, 231 nm) corresponds to the third intense peak at 230 nm, with a fairly large oscillator strength (0.61). The 31B2 and 41B2 states that are root-flipped at the SS-CASPT2 level are correctly restored at the MS-CASPT2 level. Next, we move on to the SA-CASSCF and MS-CASPT2 computations for the benzannulated tetra- and pentacycles 1b, 1c, 2a, and 2b (Table 3). The first (longest) peaks exhibited red shifts as the peripheral π-conjugation extended. The three major peaks appear to overlap with the associated peaks derived from vibronic interactions. For 1b, the solvent effect only slightly affects the MS-CASPT2 λmax shift by 1 nm, so we can safely ignore this effect on the spectra of the compounds. Nevertheless, the solvent effect plays a critical role in vibronic interactions, as is explained in section 6. The bathochromic shifts with the extension of peripheral π-conjugation were qualitatively reproduced within 50 nm deviation from the experiment at the MS-CASPT2(10e, 10o)/ANO-L level, but the results substantially depended on the active space selections. Our MS-CASPT2 computations reached satisfactory convergence but did not attain absolute agreement with the experimental results, even when the maximum (10e, 14o) active spaces with ANO-L basis set were employed. Table 4 shows the TD-DFT results for the first intense peaks of the compounds. Modern TD-DFT methods have been confirmed to achieve excellent performance that is compatible
predicted. The more refined DFT-optimized structures in this study also reproduced quantitatively the key bond lengths, indicating that the central nitrogen atom plays a minor role electronically in the peripheral π-conjugation, with a planar C2v structure being the most stable. The electronic spectra of cyclazines 1a−1c, 2a, and 2b comprise three major absorption bands in the UV−vis region,38,39 as shown in Figure 2. For 1a, the first band is
Figure 2. Electronic spectra of 1a, 1b, 1c, 2a, and 2b. Reprinted with permission from ref 38. Copyright 1988 The Japan Institute of Heterocyclic Chemistry.
located at 420 nm, the second band at 290 nm, and the third at 230 nm. The intense π−π* absorption peaks are derived from the allowed excitations within the low-lying π-MOs with (a2,b1) spatial symmetry. The UV−vis absorption peaks of the compound have been primitively analyzed using the variable β PPP method.50 That study identified three intense peaks at 3.97 (4.29), 4.75 (4.52), and 5.29 (5.08) eV (calcd/exptl) but failed to assign the first intense π−π* peak at 3.05 eV, and just one allowed transition was predicted with sufficiently large oscillator strength of >0.1. These results contradict the experimental results, which exhibited at least three major peaks in the UV−vis region that were
