Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

CCCP induces vasorelaxation by a mechanism that does not involve K channel activation in smooth muscle cells of arteries.

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To optimize the optical and optoelectronic functionalities of two-dimensional (2D) covalent organic frameworks (COFs), detailed properties of emissive and nonradiative pathways after photoexcitation need to be elucidated and linked to particular structural designs. Here, we use transient absorption (TA) spectroscopy to study the colloidal suspension of the full sp2 carbon-conjugated sp2c-COF and characterize the spatial extent and diffusion dynamics of the emissive excitons generated by impulsive photoexcitation. The ∼3.5 Å stacking distance between 2D layers results in cofacial pyrene excitons that diffuse through the framework, while the state that dominates the emissive spectrum of the polycrystalline solid is assigned to an extended cofacial exciton whose 2D delocalization is promoted by CC linkages. The subnanosecond kinetics of a photoinduced absorption (PIA) signal in the near-infrared, attributed to a charge-separated exciton, or polaron pair, reflects three-dimensional (3D) exciton diffusion as well as long-range exciton–exciton annihilation driven by resonance interactions. Within our experimental regime, doubling the excitation intensity results in a 10-fold increase in the estimated exciton diffusion length, from ∼3 to ∼30 nm, suggesting that higher lattice temperature may enhance exciton mobility in the COF colloid.