Nonlinear Optical Detection of Electron Transfer Adiabaticity in Metal Polypyridyl Complexes

Abstract

Nonlinear optical signatures of electron transfer (ET) adiabaticity are investigated in a prototypical metal polypyridyl system, Os(II)(bpy)(3), known to possess large interligand couplings. Together with a theoretical model, transient absorption anisotropy (TAA) experiments show that field-matter interactions occur with diabatic basis states despite these large couplings. In addition, activated and activationless interligand ET mechanisms are distinguished with a series of TAA experiments in which the pump pulse frequency is tuned over a wide range. At lower pump frequencies, activated interligand ET, which occurs with a time constant of approximately 600 fs, is the dominant mechanism. However, an activationless mechanism becomes most prominent when the pump pulse is tuned by only 800 cm(-1) to higher frequency. This sensitivity of the ET mechanism to the pump frequency agrees with earlier experimental work that estimated an activation energy barrier of 875 cm(-1). The premise of signal interpretation in this paper is that the basis states appropriate for modeling nonradiative relaxation also govern the optical response. Model calculations suggest that optical nonlinearities corresponding to diabatic and adiabatic bases are readily distinguished with TAA experiments. In the diabatic basis, field-matter interaction sequences are restricted to terms in which the pump and probe pulses interact with the same transition dipoles, whereas the adiabatic basis imposes no such restriction and supports a class of coherent cross terms in the nonlinear response function. It is suggested that TAA should be preferred to alternative methods of studying ET adiabaticity that vary solvents and/or temperature. Altering the solvent, for example, generally also impacts solvent reorganization energies and the free energies of the donor and acceptor states. Parallels are discussed between the present work and research aimed at understanding energy transfer mechanisms in molecular aggregates.

Quantum process tomography of excitonic dimers from two-dimensional electronic spectroscopy. I. General theory and application to homodimers

“…On the other hand, femtosecond timescales can be easily accessed with ultrafast optical techniques. Examples of phenomena studied via MDOS are vast and include molecular reorientation processes and solvation dynamics [8,9], electron transfer [10], vibrational coherences in organometallic complexes [11][12][13] or halogens in rare gas matrices [14,15], phonon dynamics in carbon nanotubes [16], protein unfolding kinetics [17], excitonic dynamics in light-harvesting systems [18][19][20][21][22][23] and organic polymers [24,25], as well as many-body physics in semiconductor quantum wells [26][27][28] and quantum dots [29].…”

Synthesis and Helical Structure of Spiroborate‐Based Double‐Stranded Helicate with Oligophenol Strands Bearing Bipyridine Units

“…[14b] This study has clearly demonstrated that the linkers connecting the two biphenol units can be exchanged for other units with a shape similar to that of the biphenlyene linker. Bipyridines are obvious candidates for the substitution and of particular importance since their transition metal complexes, especially those of ruthenium, [17] osmium, [18] and rhenium, [19] exhibit interesting photophysical and photochemical properties. We now describe the design and synthesis of a new spiroborate-based helicate (DH3 BNa2B ) from a tetraphenol strand bearing a bipyridine unit in the middle (3, H 4 L3), along with the elucidation of the double helical structure and its optical resolution by chiral HPLC.The bipyridine-containing tetraphenol (3) was prepared according to Scheme S1 (in the Supporting Information).…”

Ultrafast Spectroscopic Signatures of Coherent Electron-Transfer Mechanisms in a Transition Metal Complex

“…16,45−48 Frenkel exciton-like models have been adapted to transition metal complexes in recent work. 49,50 We opt for a delocalized basis set in [Ti(cat) 3 ] 2− , because estimates of the local (interligand) electronic couplings obtained with density functional theory are consistent with an intermediate regime, where valid arguments can be made for both basis sets (Supporting Information). In the intermediate regime, the extent of delocalization is distributed in the ensemble.…”

“…The best choice of a basis set in a transition metal complex is not necessarily obvious and can even be dynamic in some cases. − Energy level fluctuations induced by solute–solvent interactions tend to localize the excited states in transition metal complexes, whereas electronic coupling between ligands promotes delocalization. − Charge-transfer states should be treated as localized if the interligand electronic couplings are much smaller than the energy level fluctuations. Such general rules of thumb have been applied extensively to distinguish localized electronic excitations from Frenkel excitons in molecular aggregates, molecular crystals, and photosynthetic complexes. ,− Frenkel exciton-like models have been adapted to transition metal complexes in recent work. , We opt for a delocalized basis set in [Ti­(cat) 3 ] 2– , because estimates of the local (interligand) electronic couplings obtained with density functional theory are consistent with an intermediate regime, where valid arguments can be made for both basis sets (Supporting Information). In the intermediate regime, the extent of delocalization is distributed in the ensemble .…”