なぜかピレン系のエキシマー発光には興味がある。
Metal Ion Enhanced Charge Transfer in a Terpyridine-bis-Pyrene System
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Introduction
In the last decades, in order to gain more insight into electron transfer processes, extensive studies have been carried out on the optical behavior of chromophoric molecular systems consisting of electron donor and acceptor groups connected via different bridges .
Because of its long fluorescence lifetime (up to 450 ns) [3], high fluorescence quantum yield and its ability to act as a donor as well as acceptor , pyrene has often been chosen as an ideal charge transfer partner.
In some cases, the pyrene moiety was attached to chelating “polypyridyl” systems, which are known to coordinate d7 to d10 metal-ions, and through this specific metal ion complexation the excited state properties can be tuned.
As conjugated spacers between the pyrene and the metal centers, phenylene or ethynylene-phenylene have been reported. Among the innocent metal ions able to coordinate bipyridine units, Zn(II) has received attention because of its ability to enhance the emission of the ligands and tune some of the properties.
For example, in the poly- and oligo-phenylene vinylene (OPV) derivatives covalently linked to a bipyridine system reported by Wasielewski et al. an enhancement of the electronic delocalization on the polymer backbone and a red shift of the emission was observed upon addition of particular ions [e.g. Zn(II)].
The emissive excited state was suggested to have charge transfer character. More recently, systems have been used in order to show the occurrence of emissive charge-transfer states in their zinc (II) complexes, such as bipyridine linked to a donor group or to a pyrene unit via an oligo-phenylene bridge , conjugated pyrene-thiophene-terpyridine , or OPV terminated by a terpyridine unit.
Also in free ligands such as bipyridine bound to pyrene and terpyridine linked in its 4′-position to a dimethylaniline group charge transfer emission was observed.
The special interest in the terpyridine derivative systems lies in e.g. the strong blue emission of their zinc complexes.
Furthermore, the zinc can be used to assemble different units and build up long rod-like linear structures which can be considered coordination polymers. Due to the dynamic nature of the systems and to the high emission quantum yield in the blue region, zinc bis(phenylterpyridine) derivatives have attracted the attention in the material-science field.
Two recent reviews on polypyridyl systems in conjunction with aromatic units and with respect to molecular wires are available.
In order to design systems where the emission can be tuned and to have a full understanding of the free ligand and its behavior once coordinated to zinc, we have investigated the properties of a pyrene substituted terpyridine, its zinc complex and the bis-protonated form of the ligand.
The synthesis, electrochemical behavior and the photophysics of these species composed of a terpyridine (Tpy) ligand which is linked to a phenyl substituted with two pyrene units in meta positions (Figure 1) are described.
Figure 5.Emission spectraof TpyPhPyrene2 in THF (—), in valeronitrile (---), in propionitrile (⋯) and in acetonitrile (–·–) at room temperature (inset: spectrum at 77K in butyronitrile matrix) (λex= 310 nm).
The emission spectrum of the free ligand in THF (Figure 5) shows a broad but somewhat structured emission band centered at 400 nm that is attributed to the local excited (LE) state of the pyrene. However, in nitrile solvents the spectra show a dual emission with a second band arising around 470 to 480 nm (with increasing polarity).
The appearance of the low energy emission band for the TpyPhPyrene2 ligand in highly polar solvents implies the existence of a low lying excited state polar in nature and stabilized by the solvent that is due to a charge separation between the electron donor (pyrene) and the electron acceptor (terpyridine) moieties of the molecule.
Upon closer examination of Figure 5, it can be seen that the so called local excited state emission also changes shape with polarity.
Whereas the spectra in THF and valeronitrile have a typical shape that can be attributed to “substituted pyrene”, the spectra in propionitrile and acetonitrile clearly have a different shape.
This is in contrast with a π–π* transition localized on the pyrene (Ham effect: increase of 0-0 transition with polarity). In fact, these spectral features are very similar to tolyl-terpyridine (TpyTol) emission in polar solvent.
It thus appears that several close lying excited states are present and a reordering of states occurs upon solvent polarity change. A comparison of the emission spectrum of TpyPhPyrene2 with that of
Figure 6. Comparison the emission spectra of TpyTol in ethanol (—) and TpyPhPyrene2 (---) in acetonitrile (λex= 310 nm).
Clearly, a simple picture with only one LE state is not fully correct and a manifold of two or more states must be considered as fluorescent levels. Of course, it is not possible to fully rule out the possibility that the 425 nm band is just an enhancement of the vibronic structure of the pyrene (or 1,3-dipyrenyl-benzene) that gains intensity vs the other vibration levels due to the change in solvent.
The excited state lifetimes were determined in a number of solvents (see table 2). The LE state displays a lifetime between 17 and 20 ns in most non-polar solvents. In polar media, a bi-exponential decay of the emission can be observed due to the formation of a lower lying 1ILCT state (Figure 5) that has a shorter lifetime (approximately 6 ns, Table 2) and a lower energy maximum with increasing the polarity of the solvent.