Abstract
A series of diimine ligands has been designed on the basis of 2-pyridyl-1H-phenanthro[9,10-d]imidazole (L1, L2). Coupling the basic motif of L1 with anthracene-containing fragments affords the bichromophore compounds L3–L5, of which L4 and L5 adopt a donor–acceptor architecture. The latter allows intramolecular charge transfer with intense absorption bands in the visible spectrum (lowest λabs 464 nm (ε = 1.2 × 104 M–1 cm–1) and 490 nm (ε = 5.2 × 104 M–1 cm–1) in CH2Cl2 for L4 and L5, respectively). L1–L5 show strong fluorescence in a fluid medium (Φem = 22–92%, λem 370–602 nm in CH2Cl2); discernible emission solvatochromism is observed for L4 and L5. In addition, the presence of pyridyl (L1–L5) and dimethylaminophenyl (L5) groups enables reversible alteration of their optical properties by means of protonation. Ligands L1–L5 were used to synthesize the corresponding [Re(CO)3X(diimine)] (X = Cl, 1–5; X = CN, 1-CN) complexes. 1 and 2 exhibit unusual dual emission of singlet and triplet parentage, which originate from independently populated 1ππ* and 3MLCT excited states. In contrast to the majority of the reported Re(I) carbonyl luminophores, complexes 3–5 display moderately intense ligand-based fluorescence from an anthracene-containing secondary chromophore and complete quenching of emission from the 3MLCT state presumably due to the triplet–triplet energy transfer (3MLCT → 3ILCT).
Highlights
Multichromophore compounds, i.e. species combining two or more photofunctional units, offer wide possibilities to manipulate the energy of electronic transitions on the molecular level and within the bulk materials
Depending on the properties of the constituting blocks and the interplay between them, such molecules can be utilized for a diversity of light-harvesting, light-emitting, and charge transport purposes.[1−4] The efficiency of the targeted photophysical processes is defined by the dynamics of the excited state, which can be chemically tuned by proper molecular design
The family of chelating ligands based on a 2-pyridyl-1H-phenanthro[9,10-d]imidazole core was prepared according to the reaction sequence summarized in Scheme 1
Summary
Multichromophore compounds, i.e. species combining two or more photofunctional units, offer wide possibilities to manipulate the energy of electronic transitions on the molecular level and within the bulk materials. In contrast to L1−L3, compounds L4 and L5 containing pendant amine groups display structureless emission bands of considerably lower energy (Table 2 and Figure 5) These variations in luminescence characteristics indicate crucial changes in the character of the electronic transitions, which are associated with ILCT arising from the donor−acceptor (R2N−π-imidazole) molecular structure of L4 and L5 (Figure 4 and Figure S10). 1440 cm−1) of the HE band, its position, and the corresponding excitation spectrum, which are close to those of L1 and L2, point to the ligand-centered 1LC nature of HE emissions (i.e., fluorescence) that is evidenced by short lifetimes of 2.1− In this respect it should be noted that Re(I) diimine complexes, dually emissive in solution under ambient conditions, are rare. Article data involves electron transfer from the electron-rich chromophore to the photoexcited {ReII(diimine)} motif; such a process is not exceptional for rhenium(I) luminophores, and its probability depends on the properties of the diimine ligand.[43,108] The described photophysical behavior of 4 and 5 clearly contrasts with optical characteristics of the majority of rhenium(I) diimine compounds, which predominantly exhibit triplet emission or the formation of the intraligand dark triplet states.[32−36]
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