Ultrafast Dynamics of a Fluorescent Tetrazolium Compound in Solution

Abstract

Tetrazolium salts are exploited in various fields of research by virtue of their low reduction potentials. Increasingly, associated applications also attend to the photochemical and luminescence properties of these systems. Here, we investigate the photoinduced dynamics of phenyl-benzo[c]tetrazolo-cinnolinium chloride (PTC), one of the very few known fluorescent tetrazolium compounds, by using time-correlated single-photon counting, femtosecond fluorescence upconversion, and ultrafast transient absorption spectroscopy. PTC is generated photochemically by ultraviolet illumination of 2,3,5-triphenyl-tetrazolium chloride (TTC) in various alcohols. Time-resolved fluorescence measurements on PTC with different excitation wavelengths disclose biphasic solvation and vibrational relaxation dynamics. Depending on the solvent, the emission behavior of PTC is characterized by quantum yields on the order of several tens of percent and corresponding excited-state lifetimes of several hundreds of picoseconds. The radiative rate is basically constant for the studied alcohols, whereas the rate of the competing non-radiative process is sensitive to the solvent polarity. Hence, we discuss the possible involvement of intermediate radicals and further presumptive reaction pathways pursued after photoexcitation of PTC.

Solvation Controlled Excited-State Planarization in a Push–Pull Pyrene Dye

“…It becomes evident that there are significant deviations in the first 200 ps, a period that cannot reliably be analyzed owing to the limited time resolution of the TCSPC. The kinetics recorded in the blue portion of the 540 nm is dominated by a fast decay, whereas that acquired in the long wavelength 620 nm concertedly exhibits picosecond fast growth which is characteristic for a dynamic red shift of fluorescence, affirming two emitting forms for EPP : the emission is generated somewhere below 540 nm (unrelaxed form), and it ends at 620 nm (fully relaxed form). ,, Notably, both of the time constants show a slight regular increase with the decrease of the solvent polarity from CH 3 CN (ε = 35.95) to 2-PrOH (ε = 19.92) (Table ); the time constants nicely correlate with the average solvation times of the solvents, as measured by Maroncelli . In light of these, the nonplanar EPP in the initially twisted vinyl bond leads us to propose the possibility of solvent coupled conformational planarization in the excited state, but it needs further investigation .…”

Bridge-Mediated Charge Separation in Isomeric N-Annulated Perylene Diimide Dimers

“…For comparison, monomeric NH-PDI was measured in different polar solvents to identify the spectral signatures. The fs-TA spectra of monomeric NH-PDI (Figure S2) show the ground-state bleach (GSB) at 480, 516, and 556 nm (match the ground-state absorption spectra shown in Figure ), the stimulated emission (SE) near 580 nm, and excited singlet state ( 1 *NH-PDI ) absorption (ESA) at 630 nm. ,, These TA data could be well analyzed through global fitting procedure by assuming two successive steps with the evolution-associated difference spectra (EADS) and the corresponding time constants shown in Figure S3 and Table S1 . The first relaxation process with time constant of about 3–8 ps is attributed to vibrational relaxation of hot 1* NH-PDI and the movement of the branched alkyl chains (R) upon photoexcitation, which is supported by the 10 nm red-shift of the stimulated emission and small changes in the main ESA band .…”

“…20,45,46 These TA data could be well analyzed through global fitting procedure by assuming two successive steps with the evolution-associated difference spectra (EADS) and the corresponding time constants shown in Figure S3 and Table S1. 47 The first relaxation process with time constant of about 3−8 ps is attributed to vibrational relaxation of hot 1 *NH-PDI and the movement of the branched alkyl chains (R) upon photoexcitation, which is supported by the 10 nm red-shift of the stimulated emission and small changes in the main ESA band. 48 The second time constant associated with a >1 ns lifetime is assigned to the decay of the 1 *NH-PDI back to the ground state.…”

Solvation-Dependent Excited-State Dynamics of Donor–Acceptor Molecules with Hybridized Local and Charge Transfer Character

“…Recently, some molecules with hybridized local and charge transfer (HLCT) character were introduced to enhance Φ IQE . ,− In a molecule with HLCT character, the lowest excited state possesses the local excitation (LE) and charge transfer (CT) characters simultaneously, where the LE component is a more efficient radiative state compared with CT because of the larger overlap of HOMO and LUMO, while the energy gap and the emission wavelength can be easily adjusted by controlling the CT properties via donor–acceptor (D–A) variations. , It is well known that the CT energy level decreases gradually with the increase of solvent polarity due to the strong interaction between the solvent and the solute, , while the LE energy level is almost unaffected by solvent polarity because of the small dipole moment of the LE state, which means that the CT character dominates the lowest excited state in high-polar solvents and the LE character mainly influences the decay pathways in low-polar solvents. , …”