An adamantane-linked tetracene dyad (Tc-Ad-Tc) undergoes exergonic intramolecular singlet fission (SF), producing longlived (t = 175 µs) and high-energy (2 ´ 1.03 eV) multiexciton. Time-resolved absorption, fluorescence decay, and electron paramagnetic resonance (EPR) spectroscopic analysis revealed that the long-lived triplet species is generated in this system via correlated triplet pair having singlet and quintet characteristics. Time-resolved EPR analysis revealed formation of syn-and anti-conformers in the quintet, i.e. 5 ( 3 Tc-Ad-3 Tc)*. The quintet generation requires small conformational motion to induce singlet-quintet spin relaxation. The presence of aliphatic linkages, like the rigid adamantane group, may enable effective conservation of intrinsic high S1 and T1 levels of the original monomers, moderate bridge-mediated s-p interaction leading to exergonic intramolecular SF involving 1 Tc*-Ad-Tc ® 1 ( 3 Tc-Ad-3 Tc)*, and prevention of undesirable triplet-triplet annihilation, finally result in long-lived and high-energy multiexciton.
Theoretical investigations were performed
for typical iridium complexes,
Ir(C∧N)3, Ir(C∧N)2(C’∧N’), Ir(C∧N)2(N ∧O) and Ir(C∧N)2(O∧O), at the MCSCF+SOCI+SOC//B3LYP/SBKJC+p
level of theory. For Ir(dfppy)2(pic) (so-called FIrpic) and its related complexes, the introduction
of a fluoride into ppy ligands provides a blue shift
of about 20 nm for emission spectra, while the replacement of a pic ligand by an acac ligand does not seriously
affect the emission spectra of these complexes. It is proposed that
the homo-cis,hetero-O-cis isomer
(HC-5f, see text) of FIrpic should be used as a brighter
blue-color material instead of the homo-N-trans isomer
(HNT-5f). The energy difference between these isomers
is less than 1 kcal/mol, and the energy barrier of the isomerization
between these isomers is calculated to be larger than 30 kcal/mol.
It was also found that the use of two ancillary ligands, such as Ir(C∧N)(N ∧O)2 and Ir(C∧N)(O∧O)2, is unfortunately
inappropriate to energetically lift the π* orbital.
Dendrimers are unique polymers with globular shapes and well-defined structures. We previously prepared poly(amidoamine) (PAMAM) dendrimers having phenylalanine (Phe) residues at every chain end of the dendrimer as efficient gene carriers. In this study, we found that Phe-derivatized PAMAM dendrimers change their water solubility depending on temperature. The dendrimers were soluble in aqueous solutions at low temperatures, but they became water-insoluble at temperatures above a specific threshold, which is termed the lower critical solution temperature (LCST). Although the LCST of Phe-modified dendrimers decreased with increasing dendrimer generation, these dendrimers exhibited an LCST of 20-30 degrees C under physiological conditions. In addition, the LCST of the dendrimers was controlled by introducing isoleucine (Ile) residues at chain ends of dendrimers at varying ratios with respect to Phe residues. The PAMAM dendrimers are known to encapsulate various drug molecules. For these reasons, temperature-sensitive dendrimers might be useful as efficient drug carriers with controlled size and temperature-responsive properties.
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