Experimental and Theoretical Study of Model Ladder

Feb 16, 2008 - 237 BratislaVa, SloVakia, Leibniz Institute for Solid State and Materials Research, Dresden, Helmholtzstrasse. 20, D-010 69 Dresden, Ge...
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J. Phys. Chem. C 2008, 112, 3949-3958

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Experimental and Theoretical Study of Model Ladder Fluoranthenopyracylene with Two-Dimensional π-Conjugation upon Charging: Structure and Optical Properties Vladimı´r Lukesˇ,*,† Katarı´na Matuszna´ ,† Peter Rapta,†,‡ Roland Sˇ olc,† Lothar Dunsch,‡ Ade´ lia Justina Aguiar Aquino,£ and Hans Lischka£ Institute of Physical Chemistry and Chemical Physics, SloVak UniVersity of Technology, Radlinske´ ho 9, SK-81 237 BratislaVa, SloVakia, Leibniz Institute for Solid State and Materials Research, Dresden, Helmholtzstrasse 20, D-010 69 Dresden, Germany, and Institute for Theoretical Chemistry, UniVersity of Vienna, Wa¨hringerstrasse 17, A-1090 Wien, Austria ReceiVed: October 12, 2007; In Final Form: December 10, 2007

The spectroelectrochemical and theoretical study of thermally and chemically stable model ladder-type molecules with two-dimensional π-conjugation consisting of two repeating fluoranthenopyracylene units is reported. The model compound exhibits a reversible first electron transfer for both reduction and oxidation. Similar UV/vis/NIR spectra were observed for both the cation and the anion applying in situ ESR-UV/vis/ NIR spectroelectrochemistry. A single ESR line spectrum (line width: ∆Hp-p ) 0.25 mT) was detected for the radical cation, while its radical anion showed a distinct hyperfine pattern pointing to differences in the spin distribution of the positively and negatively monocharged fluoranthenopyracylene molecule. Density functional theory and Hartree-Fock time-dependent calculations were performed for the electronic ground (neutral and its positively/negatively charged forms) and lowest excited states to evaluate the optimal geometries. Two twisted conformations of the central ladder part are stabilized due to the presence of the mutual sterical repulsion of the lateral phenylene rings. Significant bond length changes upon the electric charging are indicated in the central part of the molecular skeleton. The anion shows larger changes in geometry in the central part upon optical excitation. Theoretical data for the absorption and fluorescence spectra are compared with the experimental data to demonstrate the value of the applied theoretical approach.

Introduction Conjugated aromatic molecules have received considerable attention due to their electrical, electrochemical, and optical properties.1,2 In oligomers and polymers with a single-stranded structure, like polyacetylene, poly-p-phenylene, polypyrrole, or polythiophene, the characteristic optoelectronic properties are strongly dependent on the geometry of the molecular backbone, where the mutual twisting of the structural subunits represents the major effect.3 A specific conformation can be decisively formed by the substitution pattern on the conjugated polymer backbone (configuration, tacticity), by the physical phase of the sample (solution, film, liquid crystalline phase), by the temperature, and also by the supramolecular arrangement. This often undesired dependence of the electronic properties on the steric conformation of molecules can be eliminated in double-stranded structures like the conjugated ladder-type structures. The characteristic electronic properties of the flattened and sterically restrained π-electron systems are then largely independent of the above-described influences and are only determined by the particular π-topology of the system. The synthesis of conjugated ladder-type systems includes preparation of soluble structures in which side groups enhancing solubility (e.g., alkyl chains) are introduced into the ladder-type molecule.4 The repetitive polyaddition reactions, mostly of Diels-Alder type, are very often used as formation principles of the ladder structures.5 * To whom correspondence should be addressed. E-mail: vladimir.lukes@ stuba.sk. † Slovak University of Technology. ‡ Leibniz Institute for Solid State and Materials Research. £ University of Vienna.

Quantum chemical studies of π-conjugation in organic structures represent a considerable contribution in the design of optoelectronic materials. Many theoretical studies were done by semiempirical ab initio (with inclusion of correlation energies) approaches as well as density functional theory (DFT) calculations6-10 on equilibrium geometries in the electronic ground state. The calculation of the electronically excited states of these molecules is still a very challenging task, especially when vertical excitations and computing geometry relaxation effects in excited states are to be considered. Some of the most popular methods are both the Hartree-Fock (HF) and/or the DFT approach and their time-dependent (TD) extension for excited states,11-14 which were successfully used for studies of the optical properties of large organic oligomers15 for better understanding of excitation/de-excitation phenomena of π-molecular optoswitch systems.16 The ladder-type oligomers based on fluoranthenopyracylene fragments consist of a molecular skeleton with mutually connected naphthyl and phenylene aromatic rings.17 Due to the presence of the alkyl groups, these molecules are soluble in o-dichlorobenzene and form solid thin films by evaporation. First electrochemical experiments performed on these materials also exhibit a variety of optical properties upon reduction and oxidation, which are of high interest for organic electronic devices.18 Considering the state-of-the-art methods in the investigation of this type of molecules, the aim of this work is the theoretical and next spectroelectrochemical characterization of a recently prepared model fluoranthenopyracylene molecule with respect to its two-dimensional π-conjugation containing two repeating

10.1021/jp709948a CCC: $40.75 © 2008 American Chemical Society Published on Web 02/16/2008

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Figure 1. Numbering scheme of atoms, bonds, and fragments (a) and side and front view of DFT-optimized A (b) and B (c) conformations for the neutral form of the electronic ground state of the fluoranthenopyracylene unit.

units. The backbone of this system is formed by a set of naphthyl segments where eight lateral phenylene rings are located on both sides of the ladder (see Figure 1a). Calculations of the optimal geometries of neutral and charged forms in the electronic ground state and the lowest electronic excited state were realized at the density functional theory (DFT) and Hartree-Fock (HF) levels of theory. The analyses of electronic linear absorption and fluorescence quantities have been performed using the timedependent (TD)-DFT method. The evaluated theoretical values are used for the interpretation of experimental absorption and fluorescence measurements obtained in solution. Here, the influence of the electrochemical reduction/oxidation on the molecular structure of the charged states is to be theoretically interpreted. 2. Methodology 2.1. Materials and Instrumentation. The fluoranthenopyracylene sample was synthesized by Schlu¨ter et al. and is described elsewhere.5 The cyclic voltammograms were measured

in a three electrode system using a platinum wire as the working and counter electrodes and a silver wire as pseudoreference electrode. The potentials were corrected versus the ferrocene/ ferrocenium internal standard, which was added after every measurement series. ESR and optical measurements were carried out in o-dichlorobenzene (Aldrich) with 0.1 mol L-1 tetrabutylammonium tetrafluoroborate (TBABF4, Fluka) as the supporting electrolyte. TBABF4 was dried under reduced pressure at 340 K for 24 h and stored in a glovebox prior to use. Orthodichlorobenzene (o-DCB) was doubly distilled under argon over calcium hydride before use. The electron spin resonance (ESR) spectra were recorded by the EMX X-band ESR spectrometer (Bruker, Germany), and the UV/vis/NIR spectra in spectroelectrochemical experiments were recorded by the UV/vis/NIR spectrometer system TIDAS (J&M, Aalen, Germany). A PG 284 potentiostat (HEKA, Germany) was used for the potential control. In situ ESR and UV/vis/NIR spectroelectrochemical experiments were performed in an ESR flat cell filled with a 0.5 mM solution of the fluoranthenopyracylene. As a working

Ladder Fluoranthenopyracylene with 2D π-Conjugation electrode, a laminated Pt-µ-mesh (1024 meshes/cm2, active surface 0.1 cm2) was used. A platinum wire served as a counter electrode and a silver wire as a quasi-reference electrode, which was calibrated against the Fc/Fc+ redox couple. The cell was filled and tightly closed in a glovebox while experiments were performed in the optical ESR cavity (ER4104OR, Bruker, Germany) outside of the box. The spectrometer was triggered by a HEKA potentiostat PG 285 (scan rate 3 mV s-1), and the triggering was performed using the software package PotPulse 8.53 (HEKA Electronic, Germany). The UV/vis/NIR spectra were measured on the millisecond time scale at selected potentials during the cyclovoltammetric scan. The absorption spectra of the neutral fluoranthenopyracylene in o-DCB were measured on a Perkin-Elmer Lambda spectrometer. The emission spectrum of the same solution was recorded with a double-grating monochromator and photomultiplier tube in 10 mm pathway glass cell (Quartz SUPRASILHellma Optics), which gives transmission values of more than 80% over a spectral range between 200 and 250 nm for an empty cell. Excitation of the sample was done at 550 nm. 2.2. Calculations. All calculations were done using the Gaussian 0319 (TD-DFT/B3LYP level) and the Turbomole20 (time-dependent Hartree-Fock level, TD-HF) program package. The geometries of the electronic ground states for the neutral and charged forms of the studied molecule were optimized at the DFT (using the G03 B3LYP functional21) level. The optimal geometry of the electronic lowest singlet excited state was studied at the simpler time-dependent Hartree-Fock (TD-HF) level due to computational restrictions of the used program packages. On the basis of the optimized geometries, the electronic absorption and the luminescence spectra were calculated at the TD-DFT/B3LYP level. Vertical excitations were computed for the ground-state geometry, and the fluorescence transition was obtained as the vertical de-excitation at the optimized TD-HF geometry of the excited state. The polarized split-valence SV(P)22 basis set has been used. Partial charges were obtained using the natural bond orbital (NBO) partitioning analysis23 coded in the Gaussian 03 program package. Due to the extreme computational requirements, our theoretical model was restricted to the single molecule without taking any solvent or temperature effects into account as a standard simplification used in quantum chemical studies. Unlike the experimentally studied structure, the linear alkyl substituents located in the para positions of lateral phenylene rings were omitted due to computational limitations (see Figure 1a). Geometry optimizations were set to D2 or Ci symmetry. 3. Results and Discussion 3.1. Study of the Neutral Molecule. Because of the symmetry of the studied molecule, it is possible to follow the properties of only one-half of its structure. The ladder skeleton can be simplified as a mutually permuting set of naphthylene and/or phenylene chromophores. Structural details of the restricted fragments F and Ph as well as the bond length numbering scheme are given in Figure 1a. Two optimal conformations (A and B; see Figure 1b and c) with identical bond lengths (differences smaller than 0.0001 Å) but different distortions of the ladder part were identified from the B3LYP/ SV(P) optimization. The conformation A has a D2 symmetry (EDFT ) -3765.423706 hartree), and the ladder part is clockwise helically twisted. In the case of conformation B with a Ci symmetry (EDFT ) -3765.423601 hartree), the central part is distorted from the plane consisting of fragments F-I and F-I′. The orientation of these lateral phenylene rings and ladder

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3951 skeleton for both structures is nearly perpendicular (dihedral angles are between 82-83°), causing a negligible π-conjugation between these molecular fragments. The opposite mutual arrangement of the neighboring lateral rings is not parallel. The smallest distance of 2.92 Å is found for the carbon atoms connecting the phenylene rings to the skeleton (see bonds a in Figure 1a), while the largest distance of 4.48 Å is calculated for the most remote carbon atoms (see bonds d in Figure 1a). This deformation is caused by the inter-ring van der Waals interaction, especially the repulsion electrostatic forces dominating over the attractive dispersion forces24,25 in the restricted distances for aromatic systems.26,27 The presence of this repulsion has also a strong secondary effect on the structure of the ladder part of the molecule. The planarity of naphthyl fragments (F-II and F-II′) containing lateral phenylene units is perturbed, and the ladder part is twisted (see also the side and frontal views on the optimized structure in Figure 1b). It is worth noting that the similar fluoranthenopyracylene oligomers formed from a set of naphthyl and phenylene segments (see also molecule A in Scheme 1 of ref 18) have a planar ladder skeleton due to the absence of a mutual repulsion of the nearest lateral rings.28 The selected C-C-optimized B3LYP bond lengths (for A and B structures) of the central ladder skeleton for the studied systems in the neutral electronic ground state are visualized in Figure 2 using circle symbols. The largest distances between the C-C bonds are found for the connections of the individual chromophoric fragments (see bonds. 5 and 13) and bond 9 in fragment F-II in Figure 1. The shortest bond lengths are existent in the central part of naphthyl ring of fragment F-III (bond 1) and in fragments F-I and F-II (bonds 19 and 15 and 11 and 7, respectively). Both the structure of the ladder part and the related size of the π-conjugation primarily affect the photophysical properties of the molecule. Experimental absorption spectra of the neutral form measured in o-dichlorobenzene can be characterized by two dominant bands (see Figure 3). Intensive absorption bands are detected in the spectral range of 2.28 to 2.60 eV. In fact, it is a single band which exhibits three vibronic peaks, where the 0 f 0 vibronic peak of the lowest electronic transition is the most intense. The equidistant energy difference (0.17 eV) between the vibronic peaks indicates a harmonic character of the vibrational motion. The second and very broad band starts at 3.0 eV and is localized at the beginning of the ultraviolet scale. Its vibrational structure is unresolved. The calculated excitation energies and the oscillator strengths are also given in Figure 3 (see right axis). The theoretical value of the lowest excitation energy obtained for the isolated molecule is shifted by 0.06 eV (see TD-DFT energy) from the experimental one. Although the differences between the next TDDFT and the experimental excitation energies are larger than 0.10 eV, the agreement with the experimental observation is acceptable with respect to the applied theoretical approach. The mutual comparison of theoretical oscillator strengths shows that the following energetically nearest excited states (second/third) can also contribute to the fluorescence phenomena due to their relatively large intensities. The excitation energies obtained for the optimal HF geometries show larger differences (0.27 eV; see dotted lines in Figure 3) in comparison with the experimental observations. In this context, it is useful to examine the HOMO and LUMO orbitals of the molecule because it is the HOMOLUMO excitation that plays a dominant role in this energetically lowest optical transition according to the frontier molecular orbital theory. Its contribution to this effect is 97.5%. These

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Figure 2. Selected optimized C-C bond lengths of the neutral and charged (cation/anion) forms of the electronic ground state and the neutral electronic excited state of the fluoranthenopyracylene. See Figure 1 for bond numbering.

Figure 3. Experimental and theoretical absorption and fluorescence spectra (solid lines for absorption and dotted lines for fluorescence) of the fluoranthenopyracylene. The left axis stands for the experimental measurements and the right axis for the theoretical TD-DFT results. The excitation energies based on the optimal DFT geometries are depicted by solid bars, and the excitation energies calculated for the HF geometries are indicated by dotted bars. The fluorescence energy (Efl) is calculated on the optimal TD-HF geometry.

selected orbitals are visualized in Figure 4, demonstrating that the MOs are dominantly localized in the central part of the ladder. However, the shape of the LUMO orbital shows an interring bonding character. Its lobes are spread along the ladder part and connect fragment F-III with its neighboring fragments. The electronic excitation of the neutral molecule leads to the formation of a quinoid-type structure. Similar to the charged cases, bonds 2, 4, 5, and 10 are mostly shortened, while bonds 1, 3, 6, 7, 9, and 11 are elongated (see Figure 2). Especially, bonds 2 and 3 reveal a large sensitivity upon optical excitation. The calculations at the TD-HF theoretical level show identical changes for both the A and B conformations at distances up to 0.05 Å. The C-C bond lengths between the ladder and lateral phenylene rings as well as the bond lengths within the lateral phenylenes are not effected by the electronic excitation. In both

cases, the values given by the HF approximation are 1.504, 1.393, 1.387, and 1.389 Å. The positions of the experimental fluorescence maxima reveal three different values at 2.23, 2.09, and 1.92 eV (see Figure 3). The small Stokes shift in solution (0.05 eV ) 427 cm-1) indicates minimal geometrical changes between the electronic ground and the excited states. In comparison with the theoretical excitation energy calculated from the electronic ground-state HF geometry, the theoretical fluorescence energy gained from the electronic lowest-excited-state HF geometry is in better agreement (EFlu ) 2.19 eV) with the experimental value (2.22 eV). It can be described better by the estimation of the optimal excited-state geometry using the TD-HF approach than in the case of the electronic ground state. On the basis of the fluorescence energy and the oscillator strength, the radiative lifetime might be computed for spontaneous emission by using the Einstein transition probabilities according to the following formula (in atomic units)29

τ)

c3 2(EFlu)2f

(1)

where c is the velocity of light, EFlu is the transition energy, and f is the oscillator strength. The computed value of 3.85 ns shows a reasonable agreement with the experimentally observed lifetime (3.70 ns). We would like to note that this experimental value represent its least value since the method of its direct measurements using single photon counting does not automatically include the subtraction of the quantum yield. In order to estimate the origin of the vibronic structure detected in the fluorescence and absorption spectra, an additional calculation of vibrational frequencies was performed in the ONIOM approach30 for conformation A. In this treatment, the outer parts of the molecule were described using the AM1 Hamiltonian, while for the description of the central molecular segment, the B3LYP functional was used. In the case of large molecules, this strategy represents a reasonable compromise between the reliability of the obtained data and computational contingency. It can be seen in Figure 2 that the ONIOM optimal

Ladder Fluoranthenopyracylene with 2D π-Conjugation

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Figure 4. Plots of the HOMO and LUMO (calculated at the B3LYP/SV(P) level) for the neutral form of the conformation A in the electronic ground state of the fluoranthenopyracylene.

C-C bond lengths for the electronic ground state do not reveal any significant differences from the trends indicated by the DFT method. The calculated vibrational spectrum starts (see Figure 5a) from a frequency of 400 cm-1, corresponding to ring torsions, ring deformations, and in-plane bending of the whole aromatic system with small intensities. The first intense peaks are located around 700 cm-1 (see Figure 5b). Many of the modes include in- and out-of-plane CH and CC bending torsional vibrations and CC ring stretches (see peaks in the region between 800 and 1600 cm-1). Although every vibrational mode is not presented here, we obtained a wide variety of ring bending bands in the central part of the molecules which match the vibrations of the substituted aromatic rings in the experimental region of 1000-1200 cm-1 (see also Figure 5b). The theoretical aromatic CH-stretch vibrations are located at about 3200 cm-1. The reasonable correspondence of the theoretical modes of the largest intensities (1358 and 1399 cm-1) with the experimental value of 0.17 eV (1371 cm-1) let us assume a negligible energy potential change of the electronic ground-state geometry upon vertical excitation. 3.2. Study of Electrically Charged Forms. The redox reactions and optical properties of the model molecule were experimentally studied by cyclic voltammetry and in situ ESRUV/vis/NIR spectroelectrochemistry. The sample exhibits both reversible reduction and oxidation for the first electron transfer (Figure 6), and consequently, the one-electron transfer should result in an initial formation of stable ion radicals. The evidence for the formation of stable ion radicals formed during the oxidation and reduction is given by ESR spectroscopy. Figure 6 shows the ESR spectra measured in the cathodic reduction (Figure 6a) and the anodic oxidation (Figure 6b) of the

fluoranthenopyracylene, taken at the maximum of the first voltammetric reduction and oxidation peak, respectively. An intense single ESR line (line width: ∆Hp-p ) 0.25 mT) in the region of the first oxidation peak was observed, in contrast to its radical anion with a hyperfine pattern, confirming differences in the spin distribution in positively and negatively charged fluoranthenopyracylenes (see Figure 7). The single-line ESR signal observed for the radical cation points to an extended delocalization of the unpaired electron of this compound. The spectrum of the corresponding anion radical is more complex and is substantially broader, with a hint of hyperfine splitting as compared to the cation. The changes in the ESR intensity during the reduction and oxidation of the fluoranthenopyracylene correlates well with the changes in the intensity of the absorption bands simultaneously measured in the visible region at 1.52.2 eV. Figure 8a shows the representative UV/vis absorption spectra of ion radicals of the molecule (see the solid line for the anion and the dashed line for the cation in the region of 1.5-2.2 eV), taken at the maximum of the first voltammetric reduction and oxidation peak, respectively. The difference UV/ vis/NIR spectra (the initial solution of the investigated sample was taken as a reference in this case) observed at selected electrode potentials during the in situ oxidation of the model compound are shown in Figure 8b. The negative absorbance illustrates a decrease of the absorption peaks (2.28, 2.45, and 3.38 eV) of the initial neutral compound under oxidation. The positive absorbance shows new absorption bands of the cation radical (1.71 and 1.89 eV), which increase by the increasing oxidation potential. No additional peaks indicating the formation of follow-up reaction products of the cation were observed both in the ESR and the UV/vis/NIR spectra, confirming its high

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Figure 5. Part 1 of 2.

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Ladder Fluoranthenopyracylene with 2D π-Conjugation

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Figure 5. Part 2 of 2. Calculated infrared spectra of A conformation (a) and the pictorial representation of the selected most intensive vibrational modes (b) for the electronic ground state in B3LYP/AM1 ONIOM treatment (ip refers to in-plane, oop to out-of-plane, defm to deformation, tor to torsion, and str to stretching). The visualization of the most intensive normal modes (A conformation) of the fluoranthenopyracylene calculated.

stability at room temperature. We observed a significant sensitivity of the fluoranthenopyracylene anion radical to the moisture. Depending on the solvent purity and sample storage, additional ESR spectra were observed (see small sharp ESR lines in the left part of the ESR spectrum marked with asterisks

in Figure 7). It indicates lower stability of the anion as compared to that of the cation. The visible pattern and the vibrational fine structure of the absorption spectra of both the anion and cation are very similar, in contrast to the ESR spectra (see Figure 8a). In order to

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Figure 6. Cyclic voltammetry of the fluoranthenopyracylene (0.5 mM solution in o-DCB/TBABF4 electrolyte; scan rate 0.2 V s-1; Pt wire working electrode; the potentials are referred to the ferrocene internal standard).

Figure 7. ESR spectra of the (a) anion radical and (b) cation radical of the fluoranthenopyracylene (0.5 mM solution in o-DCB/TBABF4 electrolyte; Pt-mesh working electrode) measured in the region of the first voltammetric cathodic and anodic peak, respectively (*-followup product).

interpret the experimental results of the in situ spectroelectrochemistry, geometry optimization of the charged structures was performed in the first step. The calculations started from the optimized neutral ground-state conformations (A and B). The obtained optimal charged A and B conformations show practically identical bond lengths. These results are illustrated in

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Figure 8. (a) UV/vis spectra of the corresponding cation radical (solid line, solid arrow) and anion radical (dashed line, dashed arrow) of the fluoranthenopyracylene (the peak maxima of the neutral compound are marked with doted arrows) in o-DCB/TBABF4 electrolyte, measured in the region of the first cathodic and anodic peak, respectively. (b) Difference UV/vis/NIR spectra observed at certain potentials during in situ oxidation of the fluoranthenopyracylene.

Figure 2. A mutual comparison of the optimal geometries for the neutral and radical cation/anion indicates a defect generation due to the positive or negative charging at the naphthyl moieties, which extends toward the neighboring C-C bonds on the next aromatic fragment in the ladder skeleton (e.g., see bonds 2, 3, 5, 6, and 9). The amplitude of the structural modifications decreases progressively from the central part of the molecules. Here, the negative charging has larger effects on the structure. On the contrary, bonds 6 and 12 in fragment F-II exhibit a small shortening for the positive charging and an evident prolongation after negative charging. Although the lateral phenylene rings do not reveal any geometrical changes after oxidation and reduction, the largest distance between the remote carbon atoms on the phenylene rings decreases from 4.49 to 4.41 Å. The calculated bonds a, b, c, and d are constant for neutral and charged forms with the values of 1.501, 1.404, 1.397, and 1.399 Å, respectively. Because of the fact that the lateral phenylene rings do not affect the π-conjugation due to their quasi-perpendicular orientation and do not change in their geometry upon charging, only small total partial charges occur at these fragments. This is illustrated in Table 1, where the total sums of NBO partial charges for individual molecular fragments have been collected (see also Figure 1a). For cationic forms, the positive partial charges are concentrated in the central part of the ladder (fragment F-III). The negative charge at the anion is less cumulated in the middle part (fragments F-III and F-II). The

Ladder Fluoranthenopyracylene with 2D π-Conjugation

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3957 TABLE 1: Sums of NBO Relative Atomic Charges Included in Selected Fragments of Neutral and Charged Forms Calculated with the SV(P) Basis Set; See Figure 1a for the Notation F-I (F-I′)

F-II (F-II′)

F-III

Ph

Ph′

total charge

0.034 0.187 -0.133

-0.087 0.086 -0.224

0.056 0.249 -0.207

0.006 0.025 -0.009

0.006 0.026 -0.010

0 1 -1

mined from voltammetric and spectroscopic data are in good agreement with the calculated data. The small energy difference ∆E(cat) between the HOMO and SOMO (see alpha orbital in Figure 10) in the monocation (0.32 eV) compared with that between the SOMO and LUMO in the monoanion (∆E(an) ) 0.90 eV) also points to a different charge delocalization in positively and negatively charged structures. 4. Conclusions

Figure 9. Plots of spin densities for (a) cation and (b) anion radicals for A conformations in the electronic ground state (calculated at the B3LYP/SV(P) level) of the fluoranthenopyracylene.

Figure 10. Schematic representation of the energy of the SOMO, HOMO, and LUMO of the fluoranthenopyracylene calculated at the B3LYP/SV(P) level of theory for the neutral and charged forms of the electronic ground state. Grey lines stand for the experimentally determined values for the neutral molecule18 and the dashed line indicates the Fermi energy level in the neutral species. The scale corresponds to the neutral molecular orbitals.

distribution of the unpaired radical electron can be also described using the spin densities as a difference between the alpha and beta electrons. These distributions are visualized in Figure 9 and support the concept of the different electron localization within the anion and cation. In the case of the positively charged structure, the unpaired electron is equally situated on the bonds which are perpendicularly oriented to the ladder (see bond 4 in fragment F-III) and the carbon atoms connected with the lateral phenylene rings. The unpaired electron in the anion is mostly localized in fragment F-III along the ladder part (see bonds 2 and 5) and bonds 8 and 10 in fragment F-II. The relative values of the highest double-occupied (HOMO), single-occupied (SOMO), and lowest-occupied (LUMO) energies of the molecular orbitals for the neutral and charged states are shown in Figure 10. Experimental HOMO and LUMO values for the neutral form (see gray-colored symbols) deter-

We presented a theoretical and experimental study of a thermally and chemically stable model ladder-type molecule with two-dimensional π-conjugation consisting of two repeating fluoranthenopyracylene units. Theoretical geometries were obtained using the DFT method for the electronic neutral ground state and for the positively as well as negatively charged forms. This fluoranthenopyracylene structure has no planar ladder skeleton due to the repulsion by its lateral rings. Geometrical changes on the central part of the ladder are found in the molecule by one-electron oxidation and/or reduction. Relatively large changes are indicated for the cationic system, whereby the lateral phenylene rings do not reveal any geometrical changes. The calculated TD-DFT electronic spectra and the HOMO-LUMO gap values are in a good agreement with experimental data of the molecule in solution. Despite the fact that the lateral phenylene rings do not affect the π-conjugation due to their perpendicular orientation, the repulsive forces occurring between the mutually oriented rings are responsible for the helical twisting of the ladder part. The optimal geometry of the lowest excited state was studied at the TD-HF level, and structural changes were discussed with respect to the electronic ground-state geometries. The obtained theoretical results give physical and chemical insight into the mechanisms controlling the molecular optical response of a molecule after electrochemical charging and its optical excitation since this information is indeed valuable for the understanding of properties that might lead to the future application of this system or its larger oligomer counterparts in structures like photoelectronic devices. From the experimental point of view, the investigated system exhibits a reverse fast electron transfer for both reduction and oxidation, whereby such redox phenomena in π-linked complexes were previously utilized also for electromechanical systems reported by Kim et al. 31 Acknowledgment. The authors would like to thank to Dr. Niels Schulte and Prof. Arnulf-Dieter Schlu¨ter (ETH Zu¨rich) for preparing the compounds under study. This work was supported by the Austrian Science Foundation, Special Research Program F16 (ADLIS) and No. P18233-NO2 by the Slovak Scientific Grant Agency (Projects No. 1/3036/06, 1/2021/05, 1/3566/06, and 1/3579/06) and by the Slovak Ministry of Education (Project MVTP 2007). Financial support by the Alexander von Humboldt Foundation (Projekt 3 Fokoop DEU/ 1063827) is also duly acknowledged. K. Matuszna´ and R. Sˇ olc thank to the John von Neumann-Institut for the use of the supercomputer in FZ Ju¨lich.

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