Time-Resolved Twisting Dynamics in a Porphyrin Dimer Characterized by Two-Dimensional Electronic Spectroscopy.
Molecular conformational changes in electronic excited states play a key role in numerous light-activated processes. In the case of porphyrin oligomers intramolecular twisting influences energy and charge transport dynamics. Here we address the twisting reaction in both ground and excited states in a model porphyrin dimer, employing two-dimensional electronic spectroscopy (2D ES). By spreading the information over excitation and detection frequencies, cross-peaks reveal the twisting reaction in both the ground and excited states unambiguously and distinctly from other dynamics. A quasi-barrierless planarization reaction is observed in the excited state on a tens of picoseconds time scale. This is accompanied by a spectral narrowing, indicative of a reduction in conformational disorder. The reverse reaction is suppressed in the excited state due to a steep activation energy barrier. However, in the ground state the barrier is within the thermal energy distribution, and therefore contributions from reverse and forward reactions could be observed on the subnanosecond time scale. Crucially 2D ES enables simultaneous assessment of ground and excited state reactions through analysis of different spectral regions on the 2D spectral maps.
School of Chemistry, Norwich Research Park, University of East Anglia , Norwich NR4 7TJ, United Kingdom.|CAPES Foundation, Ministry of Education of Brazil, Brasilia DF 70040-202, Brazil.
Franco V A
FV
Camargo
Department of Chemistry, Chemistry Research Laboratory, University of Oxford , Oxford OX1 3TA, United Kingdom.
Harry L
HL
Anderson
School of Chemistry, Norwich Research Park, University of East Anglia , Norwich NR4 7TJ, United Kingdom.
Stephen R
SR
Meech
School of Chemistry, Norwich Research Park, University of East Anglia , Norwich NR4 7TJ, United Kingdom.
Ismael A
IA
Heisler
D019281:Dimerization; D011166:Porphyrins / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods*
D011166:Porphyrins