Article pubs.acs.org/IC
Long-Lived Charge Transfer Excited States in HBC-Polypyridyl Complex Hybrids Anastasia B. S. Elliott,† Raphael Horvath,‡ Xue-Zhong Sun,‡ Michael G. Gardiner,§ Klaus Müllen,∥ Nigel T. Lucas,*,† Michael W. George,*,‡,⊥ and Keith C. Gordon*,† †
MacDiarmid Institute for Advanced Materials and Nanotechnology and Department of Chemistry, University of Otago, Dunedin, New Zealand ‡ Department of Chemistry, University of Nottingham, Nottingham, United Kingdom § School of Physical Sciences (Chemistry), University of Tasmania, Private Bag 75, Hobart 7001, Australia ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany ⊥ Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Talking East Road, Ningbo 315100, China S Supporting Information *
ABSTRACT: The synthesis of two bipyridine-hexa-perihexabenzocoronene (bpy-HBC) ligands functionalized with either tBu or C12H25 and their Re(I) tricarbonyl chloride complexes are reported and their electronic properties investigated using spectroscopic and computational methods. The metal complexes show unusual properties, and we observed the formation of a long-lived excited state using time-resolved infrared spectroscopy. Depending on the solvent, this appears to be of the form Rebpy•HBC•+ or a bpy-centered π,π* state. TD-DFT calculations support the donor−acceptor charge transfer character of these systems, in which HBC is the donor and bpy is the acceptor. The ground state optical properties are dominated by the HBC chromophore with additional distinct transitions of the complexes, one associated with MLCT 450 nm (ε > 17 000 L mol−1 cm−1) and another with a HBC/metal to bpy charge transfer, termed the MLLCT band (373 nm, ε = 66 000 L mol−1 cm−1). These assignments are also supported by resonance Raman spectroscopy.
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INTRODUCTION Hexa-peri-hexabenzocoronenes (HBCs) have attracted considerable attention because of the combination of their electronic properties and their propensity to self-organize into discotic liquid crystals.1−5 This makes them an attractive framework upon which to build molecular devices in which order and specific electronic properties are required. The ability to selforganize in HBCs is strongly correlated to substitution on the periphery of the structure with linear alkyl (and other) units favoring liquid crystalline phases and bulky groups such as tBu inhibiting such phase formation.4 However, the electronic properties of the HBC core are resilient to such substituents, with the optical properties showing strong π,π* transitions at ca. 365 nm and emission at 486 nm, both with distinctive vibronic structures.6 The use of strongly electroactive substituents and metal-based units can cause some perturbation of the HBC core. In a study of HBC substituted by diketopyrrolopyrrole,7 good liquid crystal forming properties were maintained while the compound displayed new optical properties with strong transitions up to 600 nm. However, these optical properties are a linear combination of the HBC and the diketopyrrolopyrrole unit. Dössel et al.8 investigated HBC linked to perylene diimide units and observed Förster © XXXX American Chemical Society
resonance energy transfer between the subunits, but again the excited states appear to be solely based on HBC or perylene diimide. Several studies of HBC linked to transition metal units have been published. If the HBC core is σ-bonded to a Pt unit then there are dramatic changes to the photophysics of the HBC complex, with strong and sharp phosphorescence exhibited at 578 nm.6 Importantly, this is a characteristic HBC-based phosphorescence and the nature of the excited states is not altered by the presence of the Pt moiety.6 The use of modified HBCs to construct Pt(II) acetylide complexes has also been reported,9−11 and such substitution has resulted in remarkable changes in the relative dynamics of HBC photophysics affecting the relative populations of the singlet and triplet manifolds. However, the excited states remain HBC based, and there is little evidence for perturbation of the nature of the excited states formed. In this paper we investigate two compounds based on HBC that have been singly substituted with a 2,2′-bipyridine (bpy) moiety as well as their corresponding Re(I) complexes (see Received: November 10, 2015
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DOI: 10.1021/acs.inorgchem.5b02602 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Article
RESULTS AND DISCUSSION Synthesis and Solid-State Structure. The 2,2′-bipyridine-functionalized bpy-HBC(R)5 ligands (3) were synthesized in excellent yield and subsequently coordinated to [Re(CO)3Cl] as shown in Scheme 1. Diffusion of methanol into a chloroform/dichloromethane solution of 4a at 4 °C afforded single crystals suitable for an Xray diffraction structure determination (Figure 1). The
Scheme 1). The compounds differ in the substitution of the remaining positions of the HBC core, which are occupied by Scheme 1. Synthesis and Structures of Compounds 3 and 4
Figure 1. (a) ORTEP diagram and atom-numbering scheme for an Xray crystal structure of 4a·3CHCl3 (chloroform molecules omitted for clarity). Ellipsoids are shown at the 50% probability level. Selected bond lengths (Angstroms): Re1−Cl1 2.4495(16), Re1−N1 2.168(4), Re1−N2 2.193(5), Re1−C101 1.906(7), Re1−C102 1.939(6), Re1− C103 1.949(5), C101−O101 1.160(8), C102−O102 1.132(7), C103− O103 1.123(7). (b) Perspective view of a centrosymmetric offset dimer of 4a in the solid state (CHCl3 solvent omitted).
C12H25 or tBu groups. The degree of aggregation of HBC derivatives is strongly dependent on the substitution of the HBC disk. Flexible, long alkyl chains imbue the HBC molecule with increased solubility and allow the moieties to stack effectively. This is in contrast to the decrease in self-association when the HBC moiety is functionalized with bulky tBu groups, affording some control over the aggregation. We find evidence for transitions involving HBC and the appendage. While any changes in the UV−vis absorption and emission spectra are subtle, using time-resolved infrared (TRIR) spectroscopy we were able to identify very unusual excited states that remain hidden using more conventional techniques. At early times after photoexcitation we observe evidence for a charge-separated state, which converts to a bpycentered excited state. Such a phenomenon has been previously observed in an HBC-fullerene hybrid,12 but this is the first example in which a metal is also incorporated into the system. Interestingly, the order of these states appears to be very sensitive to the nature of the solvent.
asymmetric unit contains one rhenium complex along with a cluster of three chloroform molecules. The crystal structure of 4a reveals two important structural characteristics. First, the angle between the HBC plane and the bipyridine plane is 11.4(3)°; this implies significant π-conjugation between the two moieties. Second, the system shows stacking in the crystalline phase with a distance between the HBC cores of 3.41 Å (Figure 1b); however, the shortest interatomic contact between the HBC cores is 3.31 Å (C1···C41), shorter than the interplanar separation of graphite (3.35 Å; Figures S1 and S2, Supporting Information). UV−vis Spectroscopy. The electronic absorption spectra of the ligands and their complexes are shown in Figure 2. The spectra of the ligands are typical for HBC systems.6,11 The bands at 350, 365, 395, and 405 nm are assigned as β′-, β-, P-, B
DOI: 10.1021/acs.inorgchem.5b02602 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
nature of the transitions parametrized through electron transition density (ETD), in which charge movement from differing parts of the molecule (the bpy, HBC, and [Re(CO)3Cl] moieties) are calculated. The calculations are based on the model ligand bpy-HBC(Me)5 and thus do not differentiate between the two ligand types used in this study. A good correlation between the calculated and the experimental data is found with the transition energies and relative intensities being consistent with experimental results. The calculations indicate that a number of transitions involve charge transfer between the HBC and the bpy moieties. For the metal complexes, the lowest energy transitions are predicted to be somewhat more complicated than a simple MLCT transition. The ETD analysis suggests that the HBC and [Re(CO)3Cl] moieties both act as donor groups, with the metal center being the primary donor. Furthermore, the transition associated with the 373 nm band also shows significant charge transfer character, with the HBC being the primary donor (with donation of electron density from both metal and HBC to bpy; Table 1, bold). Resonance Raman Spectroscopy. The bandwidth of the electronic transitions makes a definitive analysis of the types of transitions present difficult. A more incisive way of examining the nature of the electronic transitions is to utilize resonance Raman spectroscopy in which enhancement of modes associated with the resonant chromophore are observed.16−18 The Raman spectra of the ligands are shown in Figure S3 (see SI) and provide enhancement patterns that are characteristic for excitations of particular electronic bands. Previously, the resonance Raman spectra of unsubstituted HBC have been investigated and have shown major bands at ca. 1300 and 1600 cm−1 with several weaker bands in the 1200−1400 cm−1 region.19 The off-resonance spectra (λex = 1064 nm) of the ligands studied here appear considerably more complex. Most notably, the 1600 cm−1 band appears to be split, which is consistent with symmetry breaking due to substitution. The resonance Raman spectra of Re(CO)3Cl{bpy-HBC(tBu)5} (4a; Figure 3) are discussed as representative for both complexes. The off-resonance spectra are included for
Figure 2. Electronic absorption spectra of both ligands and their corresponding complexes in CH2Cl2 at concentrations of 1 × 10−6 mol L−1.
and α-bands.13 The effect of complexation is 2-fold; the HBC bands are blue shifted (by 550 cm−1) and new bands (λ ≈ 450 nm) are observed. As is shown using resonance Raman (see below), the 450 nm transition contains significant metal to ligand charge transfer (MLCT) character. The MLCT transition for Re(bpy)(CO)3Cl lies at 387 nm;14 thus, the transitions observed in these complexes are red shifted by over 3000 cm−1. In terms of inductive electron-withdrawing effects, this is similar to effects of using a NO2 substituent on the bpy,15 and it implies that the HBC and bpy interact strongly in the complex with the HBC extending the bpy π* MO conjugation. One other, more subtle, spectral feature is observed. In the complexes there is band at 373 nm that lies between the β and the P band. This is assigned as a metal and ligand to ligand charge transfer (MLLCT) band with reference to DFT calculations and resonance Raman data (see below). The electronic spectra of the ligands and complexes were modeled using time-dependent (TD) DFT calculations. The results are presented in Table 1 along with the analysis of the
Table 1. Experimental Electronic Absorption Data Acquired in CH2Cl2 Correlated to Calculated Electronic Transitions calculateda
experiment R
t
Bu, λ, nm (ε, mM−1 cm−1)
bpy-HBC(R)5 (3) α′ 443 (7.6) α 403 (35) P 395 (61) β 364 (165) β′ 348 (90) β″ 332 (48) IL
