292
T. Mori
17.2 Dimeric and Multiphoton Annihilators for Efficient
TTA
The triplet acceptors typically used for the TTA-UC are anthracene, 9,10dimethylanthracene, 9,10-diphenylanthracene, perylene, ruburene, pyrene, borondipyrromethene, polyfluorenes, and derivatives. Recently, the critical factors for the
efficient TTA, such as effect of the conjugation and geometry of acceptor, have been
investigated with a series of substituted 9,10-diphenylanthracene derivatives [30].
Accordingly, the intensity of the TTA-UC emission was directly dependent on the
ISC rate, which can be modulated by the alignment of the S 1 and T 2 energy levels
of acceptor. A relatively less attention has been paid for developing better triplet
sensitizers and metal complexes of porphyrin and phthalocyanines are typically used.
Among the various parameters to be adjusted in the TTA-UC processes, the
bimolecular (i.e., TTET and TTA) processes become continuously bottleneck, but, in
principle, can be circumvented through the synthetic modification of sensitizer and/or
acceptor. One of the simplest approaches is to directly connect either sensitizer and
acceptor or two or more acceptor molecules. A direct covalent link between the
sensitizer and the acceptor allows to improve TTET process that generally requires
encounter through the diffusion of the excited triplet state of sensitizer and acceptor
[31, 32].
For improving TTA process, dimer and oligomer of anthracene chromophores
have been investigated (Fig. 17.4). Recently, anthracene dimers with two different
geometries were employed as model emitters in the TTA-UC for fine-tuning of
twisted intramolecular charge-transfer intermediate states and thus the emission color
by solvent polarity. In addition, the UC quantum yields were found to be 13% with
platinum(II) octaethylporphyrin as a sensitizer [33], showing a considerable enhancement of the TTA-UC efficiency. Linear oligomers and two generation dendrimers of
covalently linked 9,10-diphenylanthracenes have also been employed as the TTA
acceptor, showing substantial positive effect for the UC process, both in solution
and in poly(methyl methacrylate) matrix [34, 35]. Further conjugation of 9,10diphenylanthracene oligomers with ruthenium tris-bipyridine sensitizer has been also
realized to examine the TTA-UC processes in solution [36]. In this system, however,
while an efficient intramolecular TTET occurs, the TTA process only proceeds in an
intermolecular manner, leading to a comparatively low quantum yield for the overall
UC process.
Theoretical aspects on the diffusion and encounter processes in TTA were recently
investigated [37]. Ab initio quantum chemical calculations have been also employed
to rationally design the electronic coupling matrix elements between the chromophores as well as the excitation energies in the doubly bridged tetracenes for
efficient exciton up- and down-conversion processes [38]. However, to fully understand detailed mechanisms and to develop design principle for the efficient TTA-UC
sensitizer and acceptor, more issues remain to be elucidated.
T. Mori
17.2 Dimeric and Multiphoton Annihilators for Efficient
TTA
The triplet acceptors typically used for the TTA-UC are anthracene, 9,10dimethylanthracene, 9,10-diphenylanthracene, perylene, ruburene, pyrene, borondipyrromethene, polyfluorenes, and derivatives. Recently, the critical factors for the
efficient TTA, such as effect of the conjugation and geometry of acceptor, have been
investigated with a series of substituted 9,10-diphenylanthracene derivatives [30].
Accordingly, the intensity of the TTA-UC emission was directly dependent on the
ISC rate, which can be modulated by the alignment of the S 1 and T 2 energy levels
of acceptor. A relatively less attention has been paid for developing better triplet
sensitizers and metal complexes of porphyrin and phthalocyanines are typically used.
Among the various parameters to be adjusted in the TTA-UC processes, the
bimolecular (i.e., TTET and TTA) processes become continuously bottleneck, but, in
principle, can be circumvented through the synthetic modification of sensitizer and/or
acceptor. One of the simplest approaches is to directly connect either sensitizer and
acceptor or two or more acceptor molecules. A direct covalent link between the
sensitizer and the acceptor allows to improve TTET process that generally requires
encounter through the diffusion of the excited triplet state of sensitizer and acceptor
[31, 32].
For improving TTA process, dimer and oligomer of anthracene chromophores
have been investigated (Fig. 17.4). Recently, anthracene dimers with two different
geometries were employed as model emitters in the TTA-UC for fine-tuning of
twisted intramolecular charge-transfer intermediate states and thus the emission color
by solvent polarity. In addition, the UC quantum yields were found to be 13% with
platinum(II) octaethylporphyrin as a sensitizer [33], showing a considerable enhancement of the TTA-UC efficiency. Linear oligomers and two generation dendrimers of
covalently linked 9,10-diphenylanthracenes have also been employed as the TTA
acceptor, showing substantial positive effect for the UC process, both in solution
and in poly(methyl methacrylate) matrix [34, 35]. Further conjugation of 9,10diphenylanthracene oligomers with ruthenium tris-bipyridine sensitizer has been also
realized to examine the TTA-UC processes in solution [36]. In this system, however,
while an efficient intramolecular TTET occurs, the TTA process only proceeds in an
intermolecular manner, leading to a comparatively low quantum yield for the overall
UC process.
Theoretical aspects on the diffusion and encounter processes in TTA were recently
investigated [37]. Ab initio quantum chemical calculations have been also employed
to rationally design the electronic coupling matrix elements between the chromophores as well as the excitation energies in the doubly bridged tetracenes for
efficient exciton up- and down-conversion processes [38]. However, to fully understand detailed mechanisms and to develop design principle for the efficient TTA-UC
sensitizer and acceptor, more issues remain to be elucidated.