Article pubs.acs.org/JPCA
Conformational Effects on the Lowest Excited States of BenzoylPyrrolopyridazine: Insights from PCM Time-Dependent DFT Dan Maftei,† Gheorghita Zbancioc,‡ Ionel Humelnicu,† and Ionel Mangalagiu*,‡ †
Department of Physical and Theoretical Chemistry and ‡Department of Organic Chemistry, Al. I. Cuza University, 11 Bd. Carol I, 700506 Iasi, Romania S Supporting Information *
ABSTRACT: Time-dependent density functional theory (TD-DFT) computations and steady-state electronic spectroscopy measurements are performed on two recently synthesized pyrrolopyridazines to account for the detrimental effect of benzoyl substitution on the blue fluorescence emission. In the case of the highly fluorescent ester derivative, planar in ground state, we show that TD-DFT using the PBE0 and B3LYP hybrid functionals in the state-specific solvation approach provides an accurate description of absorption and emission properties. In benzoyl-pyrrolopyridazine, the (pretwisted) orientations of the benzoyl group and the solvent polarity are both found to modulate the nature of the lowest excited states. The first excited state has nπ* character at ground-state geometry of the main conformer (carbonyl group facing the diazine ring) in nonpolar solvents and become nearly degenerate with a ππ* state in polar solvents. The latter, lower than the nπ* state at the ground state geometry of a minor conformer, relaxes into a twisted intramolecular charge transfer. Experimental absorption and excitation spectra are consistent with the conformational-dependent picture of the lowest excited state (as derived from TD-DFT). A rather qualitative agreement in predicting the fluorescence emission wavelength is achieved in computations employing the CAM-B3LYP and BH&HLYP functionals, whereas global hybrids with low or moderate amounts of exact exchange exhibit the expected TD-DFT failure with up to 1 eV underestimated transition energies.
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yet modest molar extinction coefficients for the first absorption band located in the near-UV region and little variation of maximum absorption wavelength with the number or the nature of the substituents. Because both absorption and emission wavelengths show little differences from one derivative to another, the electronic spectra for several of PPs were satisfactory modeled by time-dependent density functional theory (TD-DFT) calculations performed on the unsubstituted PP heterocycle.4 Large Stokes shifts were also predicted to originate in a bond-equalization process within the pyridazine ring in the first excited state of PP. This suggests that absorption and fluorescence spectra, while showing small variations from one derivative to another, arise essentially from the same electronic transitions taking place within the heterocycle and leave the substituents with the key role in tuning the electronic transitions by electron-withdrawing or electron-donating effects. In a systematic approach to establish how functionalization of PP is influencing the fluorescence emission, Zbancioc et al.9 show that the best results in terms of quantum efficiency are obtained in PPs bearing two or three ester groups at the pyrrole ring. From a large series of compounds, while a certain
INTRODUCTION Photophysics of indolizines and azaindolizines has received increasing interest during the last years driven by a wide range of potential applications, from electroluminescent materials1 to macrocyclic fluorescent sensors.2,3 In particular, the interest in 5-azaindolizine derivatives arises from their highly efficient blue fluorescence emission,4−6 which makes them attractive materials in optoelectronics for blue organic light-emitting diodes. Indolizine is an aromatic 10 π-electron N-fused heterocycle isomeric with indole, containing a bridgehead nitrogen atom shared by an electron-excessive pyrrole and an electrondeficient six-membered ring.7 In addition to the uneven πelectron distribution between the two fused rings, the planar geometry of the indolizine is an important feature that makes room for electron delocalization within the entire heterocycle skeleton. The planarity is preserved in the presence of an additional nitrogen atom in the six-membered ring, as in isoelectronic azaindolizines, or upon substitution with different groups. In the case of 5-azaindolizine, referred to in the following as PP, few experimental1,8,9 and combined computational and experimental insights4,10 exist, aiming at a rational description of the relative effects induced by the substituents on the absorption and fluorescence properties. In the former studies, the PPs were shown to exhibit efficient fluorescence emission © 2013 American Chemical Society
Received: November 19, 2012 Revised: March 20, 2013 Published: March 25, 2013 3165
dx.doi.org/10.1021/jp311396m | J. Phys. Chem. A 2013, 117, 3165−3175
The Journal of Physical Chemistry A
Article
subsequent fitting of spectra recorded in chloroform and acetonitrile. Steady-state fluorescence excitation and emission spectra were measured using a Horiba Fluoromax-4 spectrofluorometer and automatically corrected for instrumental effects. Given the very weak fluorescence emission, measurements were performed in the case of compound 2 at concentrations roughly 5−10 times higher than the corresponding solutions of 1. Normalized fluorescence excitation and emission spectra are provided in the Supporting Information. Computational Methods. All electronic structure computations were performed using the Gaussian 0911 suite of programs. Unless specified, equilibrium geometries and vertical transition energies were computed at the DFT and TD-DFT levels using the parameter-free PBE012 hybrid density functional (PBE1PBE in Gaussian) and the 6-31+G(d) basis set. This theory level is reported to provide reliable geometries and good estimates of transition energies at affordable computational costs.13−15 An “ultrafine” integration grid and tight convergence thresholds were used thoroughly in both selfconsistent field (r.m.s density matrix < 10−8 a.u.) and geometry optimization (r.m.s. force < 10−5 a.u.) cycles. Ground-state geometries were optimized starting from different initial orientations of the two functional groups with respect to the planar PP to account for conformational preferences arising from hindered rotations about the exocyclic C5−C8 and C7−C10 bonds. Namely, each of the two torsional coordinates, dihedral angles φ1 (∠N7a−C7−C10-O11) and φ2 (∠C4a−C5−C8-O9), has been initially set at roughly 20 and 160° for 1, whereas in 2 a slightly larger deviation from planarity has been adopted at C7 in the starting geometries to avoid the creation of too-small interatomic distances. Molecular symmetry was ignored in all equilibrium geometry calculations. Vibrational frequencies were computed following each optimization run to ensure that stationary points located represent minima of the ground-state (GS) potential energy surface (PES). Subsequent geometry optimization was carried out in cyclohexane, chloroform, and acetonitrile for each of the stationary points located in the gas-phase computations. Bulk solvent effects were included via the polarizable continuum model (PCM) in the integral equation formalism (IEFPCM),16 as implemented by default in Gaussian 09. Vertical excitation energies and oscillator strengths were computed for each conformer both in gas-phase and in PCM solvent at the corresponding geometries. Two different approaches have been adopted in the computation of electronic spectra in the condensed-phase, including a calculation of the first up to six singlet excitations in linear-response solvation, followed by a state-specific computation of the excited state of interest. The two methods of including implicit solvent effects on electronic transitions basically consist of (i) direct computation of vertical transition energies, without computing the excited-state electron density, which is the straightforward conventional linear-response (LR-PCM) procedure, and (ii) an iterative mutual optimization of the electron density in a specific excited state with solvent degrees of freedom17,18 in the state-specific (SS-PCM) approach, respectively. The two approaches might yield completely different estimates for electronic transitions involving a large change in electron density or in the case of strong solvent−solute interactions.19,20 Excited-state geometry of compound 1 was optimized in a preliminary step starting from the GS geometry. Next, rigid
influence of the nature and the position rather than the number of substituents is found, the group in position 7 of the pyrrole ring (Scheme 1) was considered to play a crucial role in Scheme 1. Chemical Structures of Dimethyl 5,7-Pyrrolo[1,2b]pyridazine-dicarboxylate (1) and Methyl 7-(4Chlorobenzoyl)-pyrrolo[1,2-b]pyridazine-carboxylate (2)a
a
IUPAC and custom numbering scheme adopted for selected atoms is shown for 1.
fluorescence emission of PP. When an ester or amide group from position 7 of a highly fluorescent derivative 1 is replaced with a benzoyl moiety as in 2 in an attempt to further increase π-electron conjugation in either ground or excited state, the resulting compound is nearly nonfluorescent. Such undesirable effect lacks a satisfactory description in simple terms of electron conjugation and suggests the existence of nonradiative funnels dominating the excited-state decay. If a design strategy of highly fluorescent compounds is concerned, then a rational description of the effect of the substituents on the excited state of PP needs thorough investigation. Investigated in this paper is the detrimental effect of the benzoyl group on the fluorescence emission of PPs as compared with an ester analogue. In this respect, TD-DFT calculations including solvent effects and steady-state absorption and fluorescence measurements are performed on two previously synthesized disubstituted PPs including the highly fluorescent ester derivative 1 and the nearly nonfluorescent benzoyl-substituted PP 2. Electronic spectra computations are also performed on the unsubstituted PP for comparison. New insights into the photophysics of pyrrolopyridazines allow us to revise some of the conclusions formulated in the previous work and also to derive a picture of the effects of conformation and solvent polarity on the first excited states of benzoyl-substituted PPs.
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MATERIALS AND METHODS Experimental Section. Dimethyl 5,7-pyrollo[1,2-b] pyridazine-dicarboxylate (1) and methyl 7-(4-chlorobenzoyl)pyrollo[1,2-b] pyridazine-carboxylate (2) have been prepared using a microwave-assisted synthetic route previously described elsewhere.9 Electronic absorption spectra were recorded in a 600−250 nm wavelength range on dilute solutions (∼10−5 M) prepared in spectroscopic grade acetonitrile, chloroform, and cyclohexane at room temperature using a Shimadzu UV-1800 double-beam spectrophotometer using the corresponding solvent as reference. Recorded absorption spectra were deconvoluted using a multipeak fitting procedure. A Gaussian profile function was initially assigned to each observable peak in cyclohexane, and the corresponding parameters defining the position of maximum, the peak area, and it is full width at halfmaximum were allowed to vary simultaneously in an iterative nonlinear least-squares minimization. Values of these parameters, as refined in cyclohexane, were used as starting values in 3166
dx.doi.org/10.1021/jp311396m | J. Phys. Chem. A 2013, 117, 3165−3175
The Journal of Physical Chemistry A
Article
excited-state PES scans were performed independently on each of the two torsional coordinates to account for conformational effects. Lowest minima located on each PES scan were subjected to geometry optimizations followed by vibrational frequencies calculations. S0/S1 and S0/S2 vibronic couplings were computed from analytical/numerical vibrational frequencies and normal modes21,22 using the FCclasses code23 and including solvent (cyclohexane) and finite temperature effects24 to model the band shape at 300 K. In the case of compound 2, successfully converged geometry optimizations were initiated from GS of the two most stable conformers. Three additional excited states were constantly included in geometry optimization to check for the occurrence of root-flipping events. To overcome the well-known failure of commonly used functionals in dealing with long-range charge-transfer (CT) states,25 four additional exchange-correlation functionals were considered. At the (TD-)PBE0/6-31+G(d) ground (excited) state geometries, subsequent calculations of electronic excitation (emission) energies were performed using (i) the widely used B3LYP global hybrid, (ii) the long-range corrected CAM-B3LYP,26 (iii) the BH&HLYP hybrid, which has been recently recommended by Aittala et al.27 for computing the electronic spectra of pyridil-indolizines, and (iv) the full Hartree-Fock meta-hybrid functional M06-HF28 reported to outperform BH&HLYP for charge-transfer excitations.29 Results from configurational interaction singles (CIS) computations are included for comparison. Because the aforementioned caveat of conventional hybrids (including PBE0) affects the landscape of the excited-state PES,30,31 with a systematic bias toward twisted (often spurious) intramolecular CT minima, excited-state geometries of compound 2 were computed also using the CAM-B3LYP, BH&HLYP, and M06-HF functionals in both gas-phase and PCM solvents.
Figure 1. Ground-state equilibrium geometries of the two most stable conformers of 1 (top) and 2 (bottom).
ratio of the two mixed syn-anti and anti-syn conformations shows little variation with the environment. Conformational preferences may be considered as a result of an interplay between electronic interactions that favor a planar orientation of the functional groups and steric repulsion having an opposite effect. It turns that in 1 the former prevails at both C5 and C7, whereas steric repulsion is more effective in compound 2 at C7 yielding twisted orientations of the benzoyl group in both syn-syn (φ1 ≈ 24°) and syn-anti (φ1 ≈ 147°) most stable conformers. Overall, computed free-energy barriers to rotation for compound 1 are lower than those reported for nitroso-indolizines34 and lower at C7 (5.8 to 6.2 kcal/mol, depending of the rotamer downhill) than for the corresponding rotation at C5 (9.0 to 10.6 kcal/mol). This is not unexpected because the two ester groups receive, in planar orientation, different effects from the proximity of the diazine ring. At C5, a syn conformation may be favored by a weak intramolecular CH···O interaction between the electron-rich carbonyl oxygen and the hydrogen atom from the arylic C4. In contrast, the proximity of the nitrogen atom, N1, to the ester group of C7 induces a steric repulsion with either carbonyl or alkoxy oxygen, which may explain the little energy difference between the syn and anti orientations at C7 and also the lower interconversion barrier (as compared with C5). The same considerations hold for the GS conformational preferences of 2, although the large benzoyl group forbids a planar geometry at C7 in both syn and anti orientations and explains the lower barrier to rotation (3.7 to 4.1 kcal/mol). Moreover, in both of the most stable conformers of 2, the phenyl group is further twisted with respect to carbonyl at ∼25° and the exocyclic bond at C7 is ∼10° out of the pyrrole ring. Electronic Spectra and Excited-State Geometries. Ester-Substituted PP (1). Maximum absorption wavelengths and oscillator strength values for the first three singlet transitions of the most stable syn-syn conformer of 1 (PCM TD-PBE0/6-31+G(d) GS geometry) are listed in Table 1. Computed absorption spectra for the other three conformers (omitted) show little absolute differences of
