Tian-Qing Ye

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the photochemistry of trans-azobenzene in n-hexane. Excitation to the S-1(n pi*) state results in transient absorption bands at ca. 400 nm (strong) and 550 nm (weaker) which decay with a lifetime 2.5 +/- 0.2 ps on excitation at 503 nm, close to the S-1 origin, and with an additional fast component of ca. 0.6 ps on excitation at 390 or 420 nm, both well above the S-1 origin. Excitation to the S-2(pi pi*) state results in transient absorption at 400 nm which decays with a dominant component of ca. 0.9 ps and a weaker component of ca. 15 ps; this 400 nm band itself is observed to rise synchronously as a transient band at 475 nm decays with a lifetime of < 200 fs. These results are discussed in terms of the dual mechanism proposed for azobenzene photoisomerization.

Photoisomerization of a Capped Azobenzene in Solution Probed by Ultrafast Time-Resolved Electronic Absorption Spectroscopy

Ultrafast time-resolved electronic absorption spectroscopy has been used to study the photochemistry of trans-azobenzene and trans-1, a derivative in which azobenzene is capped by an azacrown ether, on UV excitation to the S2(ππ*) state. Excitation of trans-1 results in transient absorption which decays with a dominant component of lifetime ca. 2.6 ps and in bleaching of the ground-state UV absorption band which recovers on a similar time scale. In contrast, excitation of trans-azobenzene results in transient absorption which decays with a dominant component with a shorter lifetime of ca. 1 ps, and in bleaching which recovers on a much longer time scale of ca. 18 ps. The recovery of the ground-state UV absorption band is not complete in either case, and the ultrafast data indicate that the quantum yield of trans-to-cis photoisomerization of 1 is approximately twice that of azobenzene. These observations demonstrate that the restricted rotational freedom of the phenyl groups in trans-1 has a significant ef...

Donor–π‐Acceptor Species Derived from Functionalised 1,3‐Dithiol‐2‐ylidene Anthracene Donor Units Exhibiting Photoinduced Electron Transfer Properties: Spectroscopic, Electrochemical, X‐Ray Crystallographic and Theoretical Studies

Steric interactions between the anthraquinoid core and the 1,3-dithiole and dicyanomethylene groups play a key role in determining the physical properties of system 1. The intramolecular charge transfer properties of this donor–π-acceptor species have been explored and cyclic voltammetric data, X-ray crystal structures and ab initio calculations are also reported.

An ultrafast time-resolved UV/visible absorption study of the UV photochemistry of [(η5-C5H5)Fe(CO)2]2

Abstract An ultrafast time-resolved UV/visible absorption study of the UV photochemistry of [CpFe(CO) 2 ] 2 (Cp = η 5 -C 5 H 5 ) in cyclohexane solution has shown that the long-lived CO-loss product, Cp 2 Fe 2 (μ-CO) 3 , is produced within 40 ps. The immediate precursor has a lifetime of 15 ps and may be assigned preliminarily to the coordinatively unsaturated CO-loss product, (Cp)(CO)Fe(μ-CO) 2 Fe(Cp). The initial transient absorption decays with a lifetime of 1.6 ps, which defines the upper limit for the timescale of CO loss.

DEVELOPMENT OF AN ULTRAFAST LASER APPARATUS FOR TIME-RESOLVED UV-VISIBLE AND INFRARED SPECTROSCOPY

An ultrafast laser apparatus has been developed that allows time-resolved UV-visible and infrared spectroscopy to be performed with a time resolution of 200 fs. The instrument design allows UV-vis and IR data to be collected either as spectra or as kinetic profiles. Solution flow and cell translation sampling systems have been developed that allow the handling of chemical and biochemical systems, including highly air-sensitive materials and volumes as small as 50 μL. The apparatus performance has been evaluated with the use of trans-stilbene in hexane (UV-vis) and silicon (IR) test samples, and the techniques have been applied in combination to study both native carbonmonoxymyoglobin and the novel H64L mutant.

Time-Resolved InfraredSpectroscopy of BinuclearRhenium (I) PolypyridylComplexes in Solution

A series of four binuclear rhenium (I) complexes of the general form [Re(CO)3Cl]2BL, where BL is a polypyridyl bridging ligand, have been studied using ultrafast time-resolved UV/visible (TRVIS) and infrared (TRIR) spectroscopies. Visible excitation produces a metal-to-ligand charge-transfer (MLCT) excited state. Kinetic measurements show that the lifetime of this MLCT state varies between 100 and 1900 ps, depending on the structure of the bridging ligand. TRIR difference spectra show that each complex forms a similar MLCT state which has mixed valence character.

TIME-RESOLVED RESONANCE RAMAN SPECTROSCOPY OF EXCITED-STATE PORPHYRINS

Time-resolved resonance Raman (TR3) and absorbance difference studies of the excited states of Cu(TPP) (TPP=5,10,15,20-tetraphenylporphyrin) have been carried out with < 10 ps times resolution in THF and pyridine solvents. In THF the distinctive transient Raman bands in the ν2 and ν4 regions, previously observed with ns laser pulses, grow in the first 55 ps before decaying in 100s of ps. The ∆A spectra also show biphasic decay. This behaviour is associated with attack by solvent on the 4-coordinate excited state to form the longer lived species observed in TR3 experiments.

ULTRAFAST REDUCTIVE ELIMINATION OF HYDROGEN FROM A METAL CARBONYL DIHYDRIDE COMPLEX; A STUDY BY TIME-RESOLVED IR AND VISIBLE SPECTROSCOPY

Laser flash photolysis of [Ru(PPh 3 ) 3 (CO)(H) 2 ] 1 in benzene solution yielded transient [Ru(PPh 3 ) 3 (CO)] which was observed by both microsecond UV/VIS and IR spectroscopy [λ max = 380 nm, ν(CO) = 1845 cm - ] and reacted with H 2 to reform 1 [k 2 = (8.4 ± 0.4) × 10 7 dm 3 mol - s -1 ]; photolysis of 1 with an ultrafast laser equipped with IR detection demonstrates that reductive elimination of H 2 and formation of [Ru(PPh 3 ) 3 (CO)] is complete within 6 ps.

Ultrafast Photodissociation ofHydrogen From Ru(PPh3)3(CO)(H)2:A Time-Resolved Infrared Study

Microsecond and picosecond time-resolved infrared studies of Ru(PPh3)3(CO)(H)2 have demonstrated that reductive elimination of H2 is complete within 5 ps of UV photolysis.