This review puts into perspective the present state and prospects for controlling quantum phenomena in atoms and molecules. The topics considered include the nature of physical and chemical control objectives, the development of possible quantum control rules of thumb, the theoretical design of controls and their laboratory realization, quantum learning and feedback control in the laboratory, bulk media influences, and the ability to utilize coherent quantum manipulation as a means for extracting microscopic information. The preview of the field presented here suggests that important advances in the control of molecules and the capability of learning about molecular interactions may be reached through the application of emerging theoretical concepts and laboratory technologies.

Coherent light sources have been widely used in control schemes that exploit quantum interference effects to direct the outcome of photochemical processes. The adaptive shaping of laser pulses is a particularly powerful tool in this context: experimental output as feedback in an iterative learning loop refines the applied laser field to render it best suited to constraints set by the experimenter. This approach has been experimentally implemented to control a variety of processes, but the extent to which coherent excitation can also be used to direct the dynamics of complex molecular systems in a condensed-phase environment remains unclear. Here we report feedback-optimized coherent control over the energy-flow pathways in the light-harvesting antenna complex LH2 from Rhodopseudomonas acidophila, a photosynthetic purple bacterium. We show that phases imprinted by the light field mediate the branching ratio of energy transfer between intra- and intermolecular channels in the complex's donor acceptor system. This result illustrates that molecular complexity need not prevent coherent control, which can thus be extended to probe and affect biological functions.

Pump−deplete−probe and transient absorption spectroscopy are applied to carotenoids with N = 11 conjugated double bonds in solution to study the origin of recently observed transient features that have been previously assigned to new electronic states. The depletion pulse pumps the transient near-IR band, whose lifetime coincides with the fluorescence lifetime, and is hence attributed to the S2 state. The subsequent signal of any lower-lying dark excited-state populated by internal conversion from S2 should be affected by the depletion pulse. Correspondingly, the signal in the S1 deactivation channel is diminished by the depleted excited population. In contrast, the Ssol* signal, purportedly reflecting an intermediate state on a competing deactivation pathway, is not affected by the depletion pulse. When comparing our results with literature data for other carotenoids, we find that the Ssol* lifetime is constant at 6.2 ± 0.4 ps for any N ≥ 11 carotenoid; for shorter chain lengths, it is equal to the S1 lifetime. To explain this puzzle, Ssol* is identified as a vibrationally excited ground state (Ssol* = hot S0), populated by a combination of impulsive Raman scattering of the pump pulse and internal conversion (S1−S0), and decaying by vibrational relaxation. The Ssol* state is not identical to the ST* state, which appears in the same spectral region when the carotenoid is embedded in light-harvesting complexes.

A new channel of excitation energy deactivation in bacterial light harvesting was recently discovered, which leads to carotenoid triplet population on an ultrafast timescale. Here we show that this mechanism is also active in LH2 of Rhodopseudomonas acidophila through analysis of transient absorption data with an evolutionary target analysis. The algorithm offers flexible testing of kinetic network models with low a priori knowledge requirements. It applies universally to the simultaneous fitting of target state spectra and rate constants to time-wavelength-resolved data. Our best-fit model reproduces correctly the well-known cooling and decay behavior in the S(1) band, but necessitates an additional, clearly distinct singlet state that does not exchange with S(1), promotes ultrafast triplet population and participates in photosynthetic energy transfer.

The influence of the carbon to nitrogen substitution on the photoinduced dynamics of TIPS-pentacene was investigated by ultrafast transient absorption measurements on spin-coated thin films in the visible and in the near-infrared spectral region. A global target analysis was performed to provide a detailed picture of the excited-state dynamics. We found that the chemical modification has a high impact on the triplet formation and leads to shorter dynamics; hence it speeds up the singlet fission process. A faster relaxation from the singlet into the triplet manifold implies a higher efficiency because other relaxation channels are avoided. The air-stable aza-derivatives have the potential to exceed the energy conversion efficiency of TIPS-pentacene.

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A series of all-trans-carotenoids with N=9, 13, and 15 conjugated bonds has been studied by pump-probe and pump-deplete-probe spectroscopies to obtain a systematic analysis of the energy flow between the different electronic states. The ultrafast dynamics in the carotenoids are initialized by excitation to the S2 state and subsequently manipulated by an additional depletion pulse in the near-IR spectral range. The changes in the dynamics after depletion of the excited state population allowed differentiation of the excited state absorption into two components, a major one corresponding to the well known S1 state and the small contribution on the red wing of the S0-S2 absorption band originating from the hot ground state. We found no evidence for an additional electronically excited state, usually called S*. Instead, a deactivation mechanism that includes the hot ground state supports the observed results nicely in the framework of a simple three state model (S2, S1, and S0).