In Situ Preparation of Metal Halide Perovskite Nanocrystal Thin Films for Improved Light-Emitting Devices.

Hybrid organic-inorganic halide perovskite semiconductors are attractive candidates for optoelectronic applications, such as photovoltaics, light-emitting diodes, and lasers. Perovskite nanocrystals are of particular interest, where electrons and holes can be confined spatially, promoting radiative recombination. However, nanocrystalline films based on traditional colloidal nanocrystal synthesis strategies suffer from the use of long insulating ligands, low colloidal nanocrystal concentration, and significant aggregation during film formation.

Ultrafast exciton dynamics in 2D in-plane hetero-nanostructures: delocalization and charge transfer.

In this article we study the ultrafast dynamics of excitons and charge carriers photogenerated in two-dimensional in-plane heterostructures, namely, CdSe-CdTe nanoplatelets. We combine transient absorption and two-dimensional electronic spectroscopy to study charge transfer and delocalization from a few tens of femtoseconds to several nanoseconds. In contrast with spherical nanocrystals, the relative alignment of the electron and hole states of CdSe and CdTe in thin 2D nanoplatelets does not lead to a type-II heterostructure.

Cooperative Subunit Refolding of a Light-Harvesting Protein through a Self-Chaperone Mechanism.

The fold of a protein is encoded by its amino acid sequence, but how complex multimeric proteins fold and assemble into functional quaternary structures remains unclear. Here we show that two structurally different phycobiliproteins refold and reassemble in a cooperative manner from their unfolded polypeptide subunits, without biological chaperones. Refolding was confirmed by ultrafast broadband transient absorption and two-dimensional electronic spectroscopy to probe internal chromophores as a marker of quaternary structure.

Photovoltaic concepts inspired by coherence effects in photosynthetic systems.

The past decade has seen rapid advances in our understanding of how coherent and vibronic phenomena in biological photosynthetic systems aid in the efficient transport of energy from light-harvesting antennas to photosynthetic reaction centres. Such coherence effects suggest strategies to increase transport lengths even in the presence of structural disorder. Here we explore how these principles could be exploited in making improved solar cells.

Dynamic Exchange During Triplet Transport in Nanocrystalline TIPS-Pentacene Films.

The multiplication of excitons in organic semiconductors via singlet fission offers the potential for photovoltaic cells that exceed the Shockley-Quiesser limit for single-junction devices. To fully utilize the potential of singlet fission sensitizers in devices, it is necessary to understand and control the diffusion of the resultant triplet excitons. In this work, a new processing method is reported to systematically tune the intermolecular order and crystalline structure in films of a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn), without the need for chemical modifications.

Broad-Band Pump-Probe Spectroscopy Quantifies Ultrafast Solvation Dynamics of Proteins and Molecules.

In this work, we demonstrate the use of broad-band pump-probe spectroscopy to measure femtosecond solvation dynamics. We report studies of a rhodamine dye in methanol and cryptophyte algae light-harvesting proteins in aqueous suspension. Broad-band impulsive excitation generates a vibrational wavepacket that oscillates on the excited-state potential energy surface, destructively interfering with itself at the minimum of the surface.

Anisotropic Conjugated Polymer Chain Conformation Tailors the Energy Migration in Nanofibers.

Conjugated polymers are complex multichromophore systems, with emission properties strongly dependent on the electronic energy transfer through active subunits. Although the packing of the conjugated chains in the solid state is known to be a key factor to tailor the electronic energy transfer and the resulting optical properties, most of the current solution-based processing methods do not allow for effectively controlling the molecular order, thus making the full unveiling of energy transfer mechanisms very complex. Here we report on conjugated polymer fibers with tailored internal molecular order, leading to a significant enhancement of the emission quantum yield.

Estimation of damped oscillation associated spectra from ultrafast transient absorption spectra.

When exciting a complex molecular system with a short optical pulse, all chromophores present in the system can be excited. The resulting superposition of electronically and vibrationally excited states evolves in time, which is monitored with transient absorption spectroscopy. We present a methodology to resolve simultaneously the contributions of the different electronically and vibrationally excited states from the complete data.

Slow Intramolecular Vibrational Relaxation Leads to Long-Lived Excited-State Wavepackets.

Broadband optical pump and compressed white light continuum probe were used to measure the transient excited-state absorption, ground-state bleach, and stimulated emission signals of cresyl violet solution in methanol. Amplitude oscillations caused by wavepacket motion in the ground and excited electronic states were analyzed. It was found that vibrational coherences in the excited state persist for more than the experimental waiting time window of 6 ps, and the strongest mode had a dephasing time constant of 2.

Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms.

The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET).

Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission.

Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting.

Dark States in the Light-Harvesting complex 2 Revealed by Two-dimensional Electronic Spectroscopy.

Energy transfer and trapping in the light harvesting antennae of purple photosynthetic bacteria is an ultrafast process, which occurs with a quantum efficiency close to unity. However the mechanisms behind this process have not yet been fully understood. Recently it was proposed that low-lying energy dark states, such as charge transfer states and polaron pairs, play an important role in the dynamics and directionality of energy transfer.

Two-Dimensional Visible Spectroscopy For Studying Colloidal Semiconductor Nanocrystals.

Possibilities offered by 2D visible spectroscopy for the investigation of the properties of excitons in colloidal semiconductor nanocrystals are overviewed, with a particular focus on their ultrafast dynamics. The technique of 2D electronic spectroscopy is illustrated with several examples showing its advantages compared to 1D ultrafast spectroscopic techniques (transient absorption and time-resolved photoluminescence).

Methylene Blue Exciton States Steer Nonradiative Relaxation: Ultrafast Spectroscopy of Methylene Blue Dimer.

The photochemistry and aggregation properties of methylene blue (MB) lead to its popular use in photodynamic therapy. The facile formation of strongly coupled "face-to-face" H-aggregates in concentrated aqueous solution, however, significantly changes its spectroscopic properties and photophysics. The photoinitiated dynamics of the simplest MB aggregate, MB2, was investigated over femtosecond to nanosecond time scales revealing sequential internal conversion events that fully relax the excited population.

Structure-Tuned Lead Halide Perovskite Nanocrystals.

Colloidally stable suspensions of lead halide perovskite nanocrystals are prepared from high-quality lead halide nanocrystal seeds. Perovskite nanocrystals with different layered crystal structures are reported. These systems are well suited for investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide perovskites.

Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation.

Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet-triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes-the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported.