The common feature of these off-the-shelf photosensitizers is
weak or moderate absorption of visible light, and the difficulty
to modify the molecular structures of the photocatalysts to
optimize the catalytic property. For example, the molar
absorption coefficient ε of Ru(II) complexes is usually less
than 00000 M−0 cm−0 at ca. 000 nm,0−00 and the absorption
wavelength of the typical Ru(II) and Ir(III) complexes is below
000 nm. Triplet photosensitizers with absorption in a longer
wavelength region for photocatalysis were rarely reported.
Furthermore, the redox properties of these photosensitizers are
not readily tunable. Therefore, it is highly desired to develop
new organic photosensitizers that show easily derivatizable
molecular structures to promote the photoreactions. To date,
very few organic triplet photosensitizers have been used for
photocatalytic reactions, such as oxidative coupling of amines.0b
To the best of our knowledge, no photocatalytic organic
reaction was carried out with organic photocatalyts that show
strong absorption in the visible region and readily changeable
molecular structures.0,0,00
The organic photosensitizers for photocatalysis should have
the following properties: (0) strong absorption of visible light,
preferably in green and red regions of the spectrum; (0) the
triplet excited state should be efficiently populated upon
photoexcitation; (0) the lifetime of the triplet excited state
should be long so that the single electron transfer (SET, the
most popular mechanism for photocatalytic organic reactions)
will be improved; (0) the molecular structure of the triplet
photosensitizers (photocatalysts) should be readily tunable, so
that the photophysical and electrochemical properties can be
optimized.