Toward N-peri-Annulated Planar Blatter Radical through aza-Pschorr and Photocyclization

Preparation of the elusive N-peri-annulated planar Blatter radicals was attempted using aza-Pschorr and photocyclization methods. In both methods, substrates containing N–Me and N–Ac groups yielded a zwitterionic heterocycle lacking the N-substituent as the main product, while in one of them a carbazole derivative representing a new heterocyclic system was also obtained. The formation of the zwitterion and the carbazole suggests the formation of the desired planar Blatter radical, which undergoes facile fragmentation through homolysis of the N–R bond. This mechanism is supported by DFT computational results, which also suggest that N-Ar derivatives should be sufficiently stable for isolation. Electronic structures of three planar Blatter radicals annulated with the O, S, and N–Ph groups are compared.


■ INTRODUCTION
Benzo[e] [1,2,4]triazin-4-yls, 1,2 derivatives of the prototypical Blatter radical 3 (Blatter, Figure 1), are of increasing interest as building blocks and components of modern functional materials, 2 such as sensors, 4 metal−organic frameworks, 5 spin filters, 6,7 organic batteries, 8,9 and liquid crystals. 10,11For this reason, there is a concerted effort to develop chemistry of this exceptionally stable, π-delocalized, and redox active paramagnetic system.In this context, we have demonstrated planar Blatter radical analogues, 12 in which positions C (8) and C(ortho) in Blatter are connected with an oxygen (1O) or sulfur (1S) atom (Figure 1).This structural change resulted in a better spin delocalization and a red shift of the electronic absorption spectra 12 and enabled synthesis of new paramagnetic liquid crystals. 13,14Therefore, it is of interest to develop a synthetic route to radicals 1N, the nitrogen analogues of 1O and 1S.In contrast to chalcogens in 1O and 1S, the nitrogen atom in 1N is trivalent and the substituent R connected to the N atom can be used to tune the electronic system (Figure 1).
A recent attempt 15 at synthesis of the 4-methyl derivative of the parent radical 1N-a through a double cyclocondensation of a diamide resulted in the 4-methyl derivative of zwitterion 2 apparently through a loss of a hydrogen atom from the N(7) position (Figure 1).Since hydrogen transfer from a spincontaining position is an easy and common process, it could be speculated that substituting the N(7) position with a group, such as methyl (1N-b) or acetyl (1N-c), could lead to a stable and isolable radical.
Herein we report our efforts at obtaining radicals 1N-a and 1N-b by using two recently developed methods for the synthesis of planar Blatter radicals and appropriate substrates.Results of the reactions, including formation of a new heterocyclic ring system, were used to formulate reaction mechanisms supported with extensive DFT calculations.Stability of the expected radicals 1N as well as comparison of the electronic structures of planar Blatter radicals 1 are assessed using DFT computational methods.
Synthetic Strategy.There are four methods developed so far to access planar Blatter radicals with annulating oxygen 12,16−18 (E = O) and sulfur 12,18 (E = S) atoms (Figure 2), which could, in principle, give access to planar nitrogenannulated radicals 1N.All methods involve a (het)aryl substituent connected at the C(8) position of the benzo[e]- [1,2,4]triazine ring (I, Figure 2) and containing an appropriate functional group X in the adjacent position allowing for the formation of radical II.Only in the case of Method C, 17 photocyclization also occurs for nonfunctionalized (X = H) precursors.
Analysis of the four methods demonstrated that radicals 1N could, in principle, be obtained through the aza-Pschorr method 16 (Method B) using amines 3 (Figure 3) or through photocyclization 17 (Method C) of derivatives 4 or 5.The choice of the methods was dictated by the anticipated easier access to derivatives 3−5 than to halogen-substituted derivatives required for Methods A and D.
■ RESULTS AND DISCUSSION Synthesis of Precursors.Nitro derivative 4a was envisioned as the key precursor to derivatives 3 and 4. Initial attempts were focused on aromatic nucleophilic substitution reactions of the readily available 12 8-fluoro-3-phenylbenzo[e]- [1,2,4]triazine (6-F) under standard conditions: 12,17 DMSO solvent and the presence of NaH (Scheme 1).Despite prolonged reaction times (up to 72 h) and temperatures up to 120 °C, the expected product was not formed, and the unreacted starting 6-F was recovered.Other combinations of aprotic solvents and bases were equally unsuccessful.An attempt to N-arylate 8-amino-3-phenylbenzo[e] [1,2,4]triazine (7) with 2-fluoronitrobenzene in DMSO in the presence of NaH was also unsuccessful.The lack of success in these two processes is presumably due to the low nucleophilicity of both amines.In contrast, the reaction of 6-F with ammonia readily produced the amine 7 in a high yield (Scheme 1).
As the nucleophilic substitution failed to yield 4a, attention was turned to metal-catalyzed processes.Initial experiments with an Ullmann-type reaction of 8-bromo-3-phenylbenzo[e]- [1,2,4]triazine (6-Br) with 2-nitroaniline in the presence of CuI (20 mol %), 1,10-phenanthroline (5 mol %), and Cs 2 CO 3 in dry DMF again did not yield the desired product 4a, even with elongated reaction times (up to 4 days) and temperatures up to 110 °C.Finally, Pd-catalyzed C−N cross coupling of 6-Br and 2-nitroaniline conducted in toluene in the presence of Pd 2 (dba) 3 , DavePhos and Cs 2 CO 3 gave the desired product 4a as red crystals in yields up to 82% (Scheme 2).
The obtained nitroaniline 4a was subsequently converted to the N-methyl (4b) and N-acetyl (4c) derivatives using the standard conditions.Thus, a reaction of 4a with excess MeI in DMA in the presence of NaH gave N-Me derivative 4b in 89% yield, while 4c was obtained in 76% yield by reaction of 4a with excess acetic anhydride in the presence of ZnCl 2 at 80 °C.The reaction flasks were protected from light since the products appeared to be photosensitive.For comparison purposes, compounds 5a and 5b were obtained in an analogous way starting from 6-Br and aniline (Scheme 3).Aniline 3b, required for the aza-Pschorr reaction, was obtained by catalytic reduction of the corresponding nitro analogue 4b (Scheme 2).In the case of reduction of 4c, slow cyclization of the resulting amine 3c to the benzimidazole 8 was observed and pure amine 3c free of 8 could not be isolated.The process was acid-catalyzed, as demonstrated by formation of 8 in a nearly quantitative yield by treatment of the   The Journal of Organic Chemistry reaction mixture in EtOH with a drop of HCl.The starting 8bromo derivative 6-Br was obtained in a way analogous to that of the preparation 12 of the 8-fluoro analogue 6-F and will be described elsewhere.
Cyclization.Investigation of the formation of 1N started with the aza-Pschorr cyclization 16 of amine 3b.Thus, treatment of 3b with t-BuONO in PhCl under N 2 at 70 °C resulted in the formation of two major products: a yellow and a more polar dark green, according to TLC analysis of the crude reaction mixture (Scheme 4).Chromatographic separation and extensive spectroscopic analysis revealed that the less polar yellow solid product is 11-methyl-3-phenyl-11H-[1,2,4]triazino[6,5-a]carbazole (9) isolated in 10% yield, while the green solid was identified as zwitterion 2 obtained in 23% yield.The former represents a new heterocyclic system, while the isolation of 2 suggests that the desired radical 1N-b was formed as a transient, unstable species under the reaction conditions (vide inf ra).Therefore, the milder photocyclization method 17 was tested for the preparation of 1N.
Irradiation of dilute (∼1 mM) solutions of the N-Me derivative 4b in CH 2 Cl 2 with a 300 W halogen lamp resulted in full consumption of the starting material after 2 h, and zwitterion 2 was obtained in 40% yield as the only isolable product (Scheme 5).The N-Ac derivative 4c did not photocyclize in CH 2 Cl 2 solutions; however, changing the solvent to EtOH led to the formation of zwitterion 2 isolated in 10% yield after ca. 5 days of irradiation, along with recovered unreacted triazine 4c.The N-unsubstituted derivative 4a was stable under these conditions: it did not show any photocyclization products after 24 h and was quantitatively recovered.
For comparison purposes, N-Me derivative 5b lacking the nitro group was also irradiated under the conditions described above for 4b.No photochemical products were observed after 4 days, and the starting material was recovered in full.
Molecular and Crystal Structures.The molecular structure of carbazole derivative 9 was confirmed with the single-crystal X-ray diffraction analysis of a yellow needleshaped monoclinic crystal belonging to the P2 1 /n space group.The crystals of 9 were grown from AcOEt by a slow evaporation method.Results are shown in Figure 4, and full data are provided in the Supporting Information.
The asymmetric unit contains one molecule of 9 (Figure 4).The heterocyclic core of the molecule is nearly planar, while the C(3)-phenyl group is twisted relative to the core plane by 5.9°.The dimensions of the benzo[e][1,2,4]triazine fragment in 9 are similar to those found in 3-phenylbenzo[e][1,2,4]triazine (10). 19Thus, the most differences in the interatomic distances are within 2σ with the average of 0.005 Å for all values.The largest changes in bond lengths in parent 10 upon fusing with indole are observed for the C(7)−C(8) bond, the common edge with the indole fragment (+0.036Å), and the C(4a)−C(8a) bond (+0.019Å).Without these two distances, the average difference between the two structures is less than 1σ (0.002 Å).Similar results are obtained for the indole fragment in which the C(2)−C(3) bond in the parent structure 20 (the common edge connecting indole and 10) and N(1)−C(2) are expanded in 9 by +0.052 Å and +0.019 Å, respectively, while the C(3a)−C(7a) is contracted by −0.029 Å.
The N(11)−Me bond length in 9 (1.459(2)Å) is close to that reported for N-Me carbazole (1.456(4) Å), 21 but unlike in the latter, the methyl group in 9 assumes a nearly ideal eclipsed conformation relative to the carbazole system.
Molecules of 9 form slipped stacks with the interplanar distance of 3.387 Å and the slippage angle of 23.8°.The stacks are arranged along the [010] direction in an alternating synand anticlinic pattern with the tilt angle between the stacks of 56°(Figure 4).The crystal packing of 9 is governed by short CH  The Journal of Organic Chemistry required for the N(1) attack (ΔG ‡ 298 = 1.50 kcal mol −1 ) is lower than that for the C( 7) attack (ΔG ‡ 298 = 2.85 kcal mol −1 , Scheme 6).This 1.35 kcal mol −1 difference in the activation energies corresponds to an approximate 7:1 ratio (at 70 °C) of the two products, which is consistent with the observed yield of carbazole 9 (10%).The calculated activation energies for the cyclization of 11 are significantly lower than those for the oxygen analogue: ΔG ‡ 298 = 3.29 kcal mol −1 for the N(1) attack and ΔG ‡ 298 = 10.46 kcal mol −1 for the C(7) attack.Due to a large difference between the two energies (ΔΔG ‡ 298 = 7.17 kcal mol −1 ), radical 1O is the only observed product of the aza-Pschorr cyclization. 16The higher rate of formation of radical 1N-b than adduct 12 is associated with a significantly larger exergonic effect (by about 21 kcal mol −1 ) of the former cyclization process.
For a better understanding of the nature of radicals 1N, their thermodynamic stability against fragmentation and formation of zwitterion 2 was assessed with the DFT method.Results shown in Figure 5 indicate that all fragmentation processes are endergonic, with the highest energy change ΔG 298 = 53.53kcal mol −1 for the parent radical 1N-a.Since hydrogen transfer processes are generally easy (e.g., to O 2 ), it can be assumed that this process dominates in the formation of 2 instead of the direct homolysis of the N−H bond.
The free energy change ΔG 298 , calculated to be below 24 kcal mol −1 , indicates that homolysis of the N−Me and N−Ac bonds in radicals 1N and formation of 2 are feasible at ambient temperature, which is consistent with experimental observations.In contrast, the calculated free energy change in the homolysis of the N−Ph bond in 1N-d is significantly higher.The calculated ΔG 298 = 34.79kcal mol −1 suggests that this process is ineffective at temperatures below 100 °C and indicates that N-Ar derivatives 1N could be stable to isolation.
The energetics of the photocyclization process of three model precursors, 4b′, 4c′, and 5b′, in which the C(3) phenyl group was replaced with an H atom, was investigated at the TD-B3LYP/6-31G(d,p) level of theory in CH 2 Cl 2 dielectric medium.Analysis of geometries optimized at the S 1 state revealed a particularly short nonbonding distance of 2.743 Å between the NO 2 group ipso carbon atom and the N(1) nitrogen atom in 4b′.This is consistent with intramolecular donor−acceptor interactions between the two sites.The ipso carbon becomes more electrophilic upon photoinduced shift of electron density from the NMe group toward the NO 2 group.Such a reorganization of electron density upon photoexcitation is less efficient for the NAc derivative 4c′ and impossible for the 5b′ analogue lacking the NO 2 group.Consequently, the N(1)•••C distance is longer in 4c′ with relaxed S 1 geometry, although still well inside the van der Waals separation (3.175 Å), while in 5b′ the distance is 3.742 Å.This demonstrates the particular role of the NO 2 group in efficient photocyclization processes.
Relaxed scans of the potential energy surface (PES) in the S 1 state for all three model precursors revealed that the barrier to cyclization at the N(1) position is the lowest for the N-Me derivative 4b′ with ΔE SCF = 2.72 kcal mol −1 (Figure 6).The   The Journal of Organic Chemistry moderate intensity bands in the visible range extending to the near IR region (Figure 7).The lowest energy absorption maximum was found at 763 nm (log ε = 3.19), and the optical band gap was determined from the onset of absorption as 1.50 eV.The spectrum of 2 is qualitatively similar to that of the recently reported 4-methyl derivative with a maximum of the lowest energy band at 725 nm in the same solvent. 15ccording to the TD-DFT results, the lowest excitation is calculated at 673 nm (f = 0.076), involving almost exclusively the HOMO → LUMO transition (97%).Both FMOs are approximately evenly distributed over the heterocyclic core, and the difference in their energies is 2.33 eV (Figure 8).Electrochemical analysis of zwitterion 2 revealed two quasireversible reduction processes with half-wave potentials E°1 /2 at −1.41 and −2.01 V and a poorly reversible oxidation process with E°1 /2 = 0.38 V vs the Fc/Fc + couple (Figure 7).
Comparison of Planar Blatter Radicals.Finally, the effect of the annulating heteroatom on the electronic properties of the planar Blatter radical was assessed by using DFT methods.For comparison purposes, radical 1N-d with the N(7)−Ph group was used as the most likely to be isolable stable species.
Data listed in Table 1 demonstrate that annulation and consequently planarization of the Blatter radical result in significant bathochromic and hyperchromic shifts of the low energy absorption band from 480 nm (f = 0.028) in Blatter to 666 nm (f = 0.063) in 1N-d.In Blatter and 1O the S 0 → S 2 excitation is over an order of magnitude more probable than the S 0 → S 1 process and hence more relevant to experimental observations.The main component of this excitation is the β-HOMO → β-LUMO transition in all four radicals.The density distributions of these two β-FMOs are similar in all three planar Blatter radicals and shown for 1N-d in Figure 9.
The bathochromic shift calculated in the series is related to the progressively increasing energy of β-HOMO from −6.48 eV in prototypical Blatter to −5.56 eV in 1N-d.At the same time, the β-LUMO is destabilized in the latter by over 0.11 eV relative to that in Blatter.In contrast, the chalcogens exert weaker effects on the β-HOMO and stabilize the β-LUMO up to 0.23 eV for 1S.Analysis of the α electron manifold indicates that the annulating nitrogen atom significantly destabilizes the SOMO (α-HOMO) by 0.32 eV, while the chalcogens have a marginal impact.
The annulating heteroatom also affects the spin density distribution in the radicals.Calculations of the inverse of the Radical Delocalization Value 22 (RDV −1 ) demonstrate that planarization of the Blatter radical indeed enhances spin delocalization and the N−Ph annulating group is the most effective in changing the RDV −1 value from 3.931 in Blatter to 4.746 in 1N-d (Table 1).It needs to be stressed that the N−Ph group is orthogonal to the heterocycle plane, and hence, the two π systems do not interact (Figure 9).Consequently only 1% of spin is transferred to N( 7      The Journal of Organic Chemistry fragment and stabilization of the zwitterionic resonance forms, as shown in Figure 10.This is consistent with high spin concentration on the peri-annulating heterocycle: it is 6.9% for N−Ph in 1N-d, while it is about 3.3% for O and S in 1O and 1S, respectively.In agreement with the proposed mechanism, spin density in the N-Me derivative 1N-b is higher, 7.2%, while that in the N-Ac 1N-c is significantly lower (1%).

■ CONCLUSIONS
The N-peri-annulated planar Blatter radicals 1N still remain elusive.Two investigated approaches to this class of radicals using several precursors yielded the zwitterion 2 as the main product, which suggests that the desired radicals were formed as the transient species before undergoing fragmentation.This observation augmented with DFT calculations indicates the low thermal stability of N-alkyl and N-acyl derivatives 1N against homolytic bond cleavage.Further support for this conclusion is provided by the isolation of small quantities of carbazole derivative 9. Product 9 is apparently formed by the Pschorr cyclization process of the transient phenyl radical intermediate, which can cyclize either at the C(7) position of the benzo[e][1,2,4]triazine giving 9 or at the N(1) position.The latter leads to radical 1N that undergoes fragmentation with the loss of the N(7) substituent and the formation of the isolated zwitterion 2.
In accordance with the proposed mechanism, DFT calculations indicate that the loss of the Me and Ac substituent from the N(7) position is endergonic with ΔG 298 less than 24 kcal mol −1 and hence feasible at ambient temperature.A much higher ΔG 298 is calculated for the N-Ph derivative (1N-d), which suggests the stability of N-aryl radicals 1N under ambient conditions.The ΔG 298 calculated for N-H radical 1Na is the highest among the four radicals.The lack of observation of 1N-a is presumably related to the oxidative instability of this species under Pschorr reaction conditions and the presence of atmospheric oxygen.
Analysis of the electronic structure of the three planar radicals indicates that the peri-annulating nitrogen atom has the strongest effect among the three heteroatoms on the energy of β-FMOs, and hence on the position of the low energy absorption bands and also on spin delocalization.Despite the orthogonal orientation of the N-aryl group, it can affect the radical's π system through the inductive effect, hence controlling the electronic properties of the system.For these reasons, N-aryl radicals 1N constitute attractive synthetic targets for materials with tunable properties.

■ COMPUTATIONAL DETAILS
Quantum-mechanical calculations were carried out using the Gaussian 09 suite of programs. 23Geometry optimizations were undertaken at the (U)B3LYP/6-311G(d,p) level of theory in a vacuum.Vibrational frequencies were used to characterize the nature of the stationary points and to obtain thermodynamic parameters.Transition states for the formation of 1N-b and 9 in the Pschorr reaction were located using the UB3LYP/6-311G(d,p) method and the QST3 algorithm.The three input geometries were obtained from relaxed PES scans.The final SCF energies for mechanistic considerations were obtained at the (U)B3LYP/6-311++G(d,p)//(U)B3LYP/6-311G(d,p) level of theory in the PhCl dielectric medium requested with the SCRF(Solvent = C6H5Cl) keyword (PCM model). 24lectronic excitation energies in the CH 2 Cl 2 dielectric medium were obtained for derivatives 1 and 2 at the (U)B3LYP/6-311++G(d,p)//(U)B3LYP/6-311G(d,p) level of theory using the time-dependent 25 DFT method supplied in the Gaussian 09 package.Solvation models in calculations were implemented by the PCM model 24 using the SCRF-(Solvent=CH2Cl2) keyword.
A relaxed scan of the potential energy surface (PES) in the S 1 state for model compounds was conducted at the TD-B3LYP/6-31G(d,p) level of theory in the CH 2 Cl 2 dielectric medium using the "TD=(singlets, root=1, NStates=14)" keyword and 0.04 Å step from the S 1 state equilibrium geometry.
■ EXPERIMENTAL SECTION General Methods.Reagents and solvents were commercially available.Reactions were protected from moisture with a N 2 atmosphere.Anhydrous Na 2 SO 4 was used for drying organic extracts, and all volatiles were removed under reduced pressure.Heat in reactions involving elevated temperatures was supplied using oil baths, and reported temperature refers to that of the bath.All reaction mixtures and column eluents were monitored by thin layer chromatography (TLC) using commercial silica gel TLC plates.The plates were observed under UV light at 254 and 365 nm.NMR spectra were obtained at 400 MHz ( 1 H) and 100 MHz ( 13 C) MHz in CDCl 3 .Chemical shifts were referenced to the solvent ( 1 H and 13 C: 7.26 and 77.16 ppm for CDCl 3 ) 26 using a Bruker Avance 400 spectrometer.Melting points were determined on a Melt-Temp II apparatus in capillaries, and they are uncorrected.High-resolution mass spectrometry (HRMS) measurements were performed by using a Varian 500 MS LS Ion Trap spectrometer.In all cases, little or no fragmentation was observed, and the M+ and MH+ peaks were the most intense signals.IR spectra were measured in KBr pellets with a Bruker Alpha ATR spectrometer.UV−vis spectra were recorded on a Jasco V770 spectrophotometer in spectroscopic-grade CH 2 Cl 2 at concentrations of (1−10) × 10 −5 M. Extinction coefficients were obtained by fitting the maximum absorbance at 308 nm against the concentration in agreement with Beer's law.Irradiations were conducted with a 300 W halogen lamp ("portable halogen work lamp" without the protecting front glass window) equipped with a T3 double-ended RSC base J118 light bulb.[1,2,4]triazin-8-yl)benzene-1,2-diamine (3b).To a solution of amine 3b (72.0 mg, 0.22 mmol) in dry PhCl (1 mL), under N 2 atm, t-BuONO (158 μL, 1.32 mmol, 6 equiv) was added dropwise with stirring.The resulting mixture was gradually heated to 70 °C over a period of 15 min and kept at 70 °C for 2.5 h, after which time TLC analysis showed complete consumption of substrate 3b.The reaction mixture was cooled, volatiles were evaporated, and the dark residue was chromatographed on passivated silica gel using AcOEt/pet.ether mixture (1:4) as eluent.The first yellow fraction contained 7.0 mg (10% yield) of carbazole 9, which was recrystallized from AcOEt/hexane.The second, more polar green fraction contained 15.0 mg (23% yield) of zwitterion 2, which was also recrystallized from AcOEt.For analytical data, see below.

The Journal of Organic Chemistry
Attempted Preparation of Radicals 1N-b and 1N-c by Photocyclization.A 1 mM solution (100 mL, 0.1 mmol) of N-methyl derivative 4b or 5b in CH 2 Cl 2 or N-acetyl 4c in EtOH was placed in a round-bottom flask fitted with a magnetic stirrer and a reflux condenser.The solution was stirred and irradiated with a 300 W halogen lamp, which was placed approximately 30 cm from the flask.The irradiation warmed up the reaction mixture to 30−35 °C.Progress of the reaction was monitored by TLC (AcOEt/hexane, 1:4), and the irradiation was stopped either after all of the starting material was consumed (2 h for 4b) or after an extended period of time (up to 5 days for 4c).The solvent was evaporated, the residue was adsorbed on a passivated silica gel (1% Et 3 N), and the products were separated by column chromatography, using passivated silica gel (1% Et 3 N) and with a AcOEt/hexane mixture (1:4) as the eluent.The isolated product was recrystallized from AcOEt.Both precursors gave the same zwitterion 2 with yields of 40% for precursor 4b (in CH 2 Cl 2 ) and 10% for 4c (in EtOH).In the latter case, the unreacted starting 4c was recovered in 60% yield, while, in the case of irradiation of 5b, the entire starting material was recovered.

Figure 4 .
Figure 4. Left: Molecular structure of 9. Atomic displacement ellipsoids are drawn at the 50% probability level.The red arrows point to bonds most affected by ring fusion of 10 with indole.Right: Partial crystal packing of 9.

N
Scheme 6. Proposed Mechanism for the Formation of Zwitterion 2 and Carbazole 9 a

Figure 6 .
Figure 6.Relative energy of PES relaxed scans along the N(1)•••C vector leading to 1N-b′ and 1N-c′ in the S 1 state of models 4b′, 4c′, and 5b′ obtained at the TD-B3LYP/6-31G(d,p) level of theory in the CH 2 Cl 2 dielectric medium.
)−Ph, relative to 5% transferred to the coplanar C(2)−Ph in 1N-d, through a spin polarization mechanism.The high effectiveness of the N− R group in spin delocalization is related to the more efficient injection of electron density into the benzo[e][1,2,4]triazine

d
Obtained with the UB3LYP/EPR-II method in the CH 2 Cl 2 medium.e The S 0 → S 2 excitation.f The S 0 → S 1 excitation.

Table 1 .
Comparison of DFT Derived Electronic Properties of the Blatter Radical and Its Planar Analogues a a Single-point calculations in the CH 2 Cl 2 dielectric medium for geometry obtained at the UB3LYP/6-311G(d,p) level of theory.b Lowest energy excitation with significant f involving mainly the β-HOMO → β-LUMO transition.c Obtained with the TD-UB3LYP/6-311++G(d,p) method.