A class of molecular electron transfer cascades—those based on PtII
complexes of 2-(1-pyrazole)-pyridine (pzpy) ligands—are reported. The synthesis
of a new electron-acceptor imide-modified pzpy ligands is reported, and their
application to transition-metal chemistry demonstrated by the synthesis of the
PtII chloride and acetylide complexes. These donor–acceptor
assemblies are promising models for investigation of photoinduced charge
separation. Accordingly, picosecond time-resolved infrared (TRIR) and
femtosecond transient absorption (TA) studies have been undertaken to elucidate
the nature and dynamics of the lowest excited states in
Pt(NAP-pyr-pyrazole)(–CC–Ph–C7H15)2. It has
been established that the initial population of an MLL'CT excited state in the
chromophoric [Pt(pyridine-pyrazole)(acetylide)] core is followed by an electron
transfer to the naphthalimide (NAP) acceptor, forming a charge-separated state.
This state is characterized by a large shift in ν(CO) vibrations of the NAP
acceptor, as well as by a very intense and broad [×10 times in comparison to
ν(CO)] asymmetric acetylide stretch which incorporates –CC–Pt–CC– framework and
occurs at approximately 300 cm–1 lower in energy than its
ground-state counterpart. In CH2Cl2 at room temperature,
the charge-separated state with the lifetime of 150 ps collapses into an almost
isoenergetic NAP-localized triplet state; the rate of this transformation
changes upon decreasing the temperature to 263 K. This final excited state,
3NAP-(pyr-pyrazole)Pt(–CC–Ph–C7H15)2,
has an unusually long, for PtII complexes, excited-state lifetime of
tens of microseconds. The work demonstrates the possibility of tuning
excited-state properties in this new class of PtII chromophores
designed for electron-transfer cascades.
Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin-Copper Corrole Donor-Acceptor Conjugates
