Subpicosecond Transient Absorption Analysis of the Photophysics of 2,2‘-Bipyridine and 4,4‘-Bipyridine in Solution

Abstract

The transient absorption spectra of 2,2‘-bipyridine (22BPY) and 4,4‘-bipyridine (44BPY) following femtosecond excitation at 266 nm in a series of organic solvents have been measured in the 300−670-nm wavelength range with subpicosecond time resolution. The results have been correlated with those obtained in the microsecond time domain. The S1 states are characterized and attributed to nπ* levels in both molecules. The S1 state of 22BPY decays essentially via intersystem crossing to the T1 ππ* state in all solvents (τS 1 = 50−80 ps). The 44BPY S1 lifetime varies from 10 to 70 ps depending on the solvent. The shortest lifetimes are found in alcohols (10−20 ps) and alkanes (≈11 ps) where the fast formation of the N-hydro radical 44BPYH•, already evidenced in previous nanosecond experiments, is shown to occur with high yields from the S1 state in parallel to ISC. In alcohols this process is found to be activated by the protic character of the solvent and to take place within H-bonded solute/solvent complexes pre-existing in the ground state. A mechanism involving a surprisingly fast H-atom abstraction by the 44BPY S1 state via homolytic breaking of the alcohol OH bond is suggested.

Hybrid hard- and soft-modelling applied to analyze ultrafast processes by femtosecond transient absorption spectroscopy: Study of the photochromism of salicylidene anilines

“…The femtosecond transient absorption set-up was already described elsewhere [26,27]. Briefly, in our experiments, a 1-kHz Ti:sapphire laser system (Coherent oscillators and a BM Industries regenerative amplifier) delivers a 100-fs pulse duration (0.8 mJ).…”

“…To improve the signal-to-noise ratio, the experiments were achieved with two synchronized beams, the white light continuum was split into a probe (with pump) and a reference beam (without pump) resulting on an optical density variation accuracy of ±0.001 (before averaging) in the spectral range of the experiments. The transmitted light of the probe and reference beam was recorded on two different channels of a multichannel spectrograph equipped with a CCD camera (Princeton Instrument) and the transient spectra are computed [26]. Sample solution (∼10 −3 M) was circulating in a flow cell equipped of 1 mm thick CaF 2 windows and characterized by a 2-mm optical path length.…”

Electron transfer in N-butylpyridinium tetrafluoroborate ionic liquid by pulse radiolysis

“…When the system contained 1.2 mmol/L BPY and 0.2 Table 1 Rate constants for different electron-transfer reactions mmol/L AQS in i-PrOH 4% aqueous solution, decay and formation were observed at 580 and 510 nm, respectively, which were almost synchronous to that of Figure 5. Two main absorption bands of BPY were presented at 375 and 580 nm, which were assigned to the 4,4′-BPY radical [15]. The peak at 510 nm contributed to the AQS •− anion radical.…”

Deconvolution of femtosecond time‐resolved spectroscopy data in multivariate curve resolution. Application to the characterization of ultrafast photo‐induced intramolecular proton transfer

“…The femtosecond transient absorption set-up has been described elsewhere [35,36]. Briefly, in our experiments, a 1-kHz Ti:sapphire laser system (Coherent oscillators and a BM Industries regenerative amplifier) delivers 80 fs (0.8 mJ) pulses at 780 nm.…”