Content area
Full Text
ARTICLES
PUBLISHED ONLINE: 21 FEBRUARY 2017 | DOI: 10.1038/NCHEM.2729
In multidimensional spectroscopy, dynamics of coherences between excited states report on the interactions between electronic states and their environment. The prolonged coherence lifetimes revealed through beating signals in the spectra of some systems may result from vibronic coupling between nearly degenerate excited states, and recent observations conrm the existence of such coupling in both model systems and photosynthetic complexes. Understanding the origin of beating signals in the spectra of photosynthetic complexes has been given considerable attention; however, strategies to generate them in articial systems that would allow us to test the hypotheses in detail are still lacking. Here we demonstrate control over the presence of quantum-beating signals by packing structurally exible synthetic heterodimers on single-walled carbon nanotubes, and thereby restrict the motions of chromophores. Using two-dimensional electronic spectroscopy, we nd that both limiting the relative rotation of chromophores and tuning the energy difference between the two electronic transitions in the dimer to match a vibrational mode of the lower-energy monomer are necessary to enhance the observed quantum-beating signals.
Ultrafast two-dimensional electronic spectroscopy (2DES) has afforded opportunities for detailed studies of energy-transfer pathways and coupling between chromophores16. Using this
femtosecond spectroscopy, we see oscillations appear in the spectra on excitation of superpositions of quantum states in the system7.
Formally, these time-dependent oscillations are coherences (off-diagonal elements) in the density matrix in the Hamiltonian eigenbasis7. Any non-degenerate pair of states can generate such oscillating signals; the microscopic states involved in these oscillating signals may be vibrational, electronic or vibronic (states of mixed vibrationalelectronic character that arise from non-BornOppenheimer coupling). Coherences observed in the 2DES data persist longer than the measured dephasing time between the ground state and the excited state in several photosynthetic complexes3,810 as well as in articial systems, including synthetic
small molecules11,12, polymers13 and J-aggregates14. Despite the generality of these prolonged coherences, assignment of their microscopic origin is still in question, but was originally assigned as purely electronic15. However, the systems studied to date have vibrational modes coincident with the energy differences between the electronic transitions of the constituent chromophores, which frustrates the assignment of the observed signals2,1619. Recently,
vibronic coupling has been proposed as a possible mechanism for the generation and survival of these coherences14,16,2022....