Ultrafast time‐resolved fluorescence for probing vibrational wave packets in excited‐state dynamics

Advances in time‐resolved spectroscopy have greatly enhanced our ability to observe ultrafast processes in chemical and physical systems. Femtosecond spectroscopies enable the initiation and real‐time observation of coherent nuclear motions. The coherent vibrational spectrum (CVS), obtained from the propagation of nuclear wave packets (NWPs), serves as a powerful tool for probing the nature of chemical processes, including potential energy surfaces and structural dynamics. Time‐resolved fluorescence (TF) is a classical technique for studying excited‐state dynamics. By recording spontaneous emission, TF offers the unique advantage of selectively detecting emitting species. Continued improvements in time resolution have expanded the capabilities of TF beyond simple population kinetics to include direct observation of ultrafast coherent phenomena and structural changes. Modern TF apparatus based on fluorescence upconversion can now achieve time resolutions of 30 fs, allowing detection of NWP motions. Moreover, because TF‐derived CVS reflects only excited‐state processes, its integration with quantum chemical calculations enables more accurate and detailed interpretations of excited‐state molecular dynamics. In this review, we present the development of high time‐resolution TF, including techniques and apparatus for achieving time resolutions as short as 30 fs, suitable for capturing NWP dynamics. We also provide a brief theoretical overview of TF, along with approaches for calculating NWPs using quantum chemical and molecular dynamics simulations. Two application examples are discussed. First, in the photoexcitation of coumarin 153 to the Franck–Condon region of the S 1 state, the feasibility of recording NWP motions by TF is demonstrated. The experimental and calculated CVS from vibrational displacements show good agreement. Second, TF is used to investigate the excited‐state intramolecular proton transfer (ESIPT) dynamics in 10‐hydroxybenzo[h]quinoline (HBQ). Because HBQ undergoes ultrafast ESIPT in <20 fs, TF captures the CVS of the product, whose amplitudes and phases reflect the potential energy surfaces associated with the chemical reaction. Combined with quantum chemical calculations and molecular dynamics simulations, TF provides a detailed picture of the ESIPT process. Further development of TF toward even higher time resolution of ~20 fs, extending into the fingerprint region, holds great promise for capturing comprehensive vibrational spectra and time‐resolved structural changes during chemical reactions.