Ultrafast Dynamics of a Solvatochromic Dye, Phenol Blue: Tautomerization and Coherent Wavepacket Oscillations.

Femtosecond time-resolved transient absorption spectroscopy was performed for a nonfluorescent solvatochromic dye, phenol blue, N-(4-dimethylaminophenyl)-1,4-benzoquinoneimine, which exhibits ultrafast nonradiative decay due to its flexible molecular structure. By exciting the molecule in ethanol (EtOH) solution with two excitation wavelengths located at shorter- and longer-wavelength sides of the visible absorption band, we observed ultrafast nonradiative decay from the excited state, followed by spectral evolution in the ground state. The nonradiative decay in the subpicosecond range creates a vibrationally hot ground state with the lifetime in the picosecond range. Subsequently, a tautomer that absorbs at shorter wavelengths is produced from the hot state, which causes a red shift of the ground-state bleach (GSB). The tautomerization presumably involves twisting of the benzoquinoneimine moiety induced by the breaking of the hydrogen bond (H-bond) between the solute and the solvent molecules. The recombination of the H-bond occurs with a time constant of ∼30 ps, and the system returns to its original state. We also observed low-frequency coherent wavepacket oscillations that modulate the GSB with dephasing times similar to the excited-state lifetime.

Ultrafast excited state dynamics of nonfluorescent cyclopheophorbide-a enol, a catabolite of chlorophyll-a detoxified in algae-feeding aquatic microbes.

For photosynthetic organisms that nourish the earth's biosphere, chlorophylls (Chls) are the major pigments utilized for light harvesting and primary charge separation. Although Chl molecules are effective photosensitizers, they are inevitably phototoxic to living organisms due to the facile generation of highly oxidative singlet oxygen (1O2) through triplet energy transfer from their photoexcited states to oxygen molecules. Such phototoxicity of Chls is a major problem for translucent microbes that feed on photosynthetic algae. Recently, it has been reported that the metabolic conversion of Chls-a/b to 132,173-cyclopheophorbide-a/b enols (cPPB-a/bEs) is the detoxification mechanism for algivorous protists. cPPB-a/bEs are colored π-conjugated cyclic tetrapyrroles but are nonfluorescent due to efficient nonradiative decay. In this study, femtosecond time-resolved transient absorption spectroscopy was applied to cPPB-aE with the aim of understanding its quenching mechanism. As a result, we have captured the ultrafast generation of an intermediate state (∼140 fs) that leads to the rapid internal conversion to the ground state (∼450 fs).