Pingyun Chen

Photophysical properties of polypyridyl carbonyl complexes of rhenium(I)

The photophysical properties of the metal to ligand charge transfer (m.l.c.t.) excited states of the complexes [Re(4,4′-X2-bipy)(CO)3Cl](X = NH2, NEt2, NHCOCH3, OCH3, CH3, H, Ph, Cl, CO2Et or NO2; bipy = bipyridine) vary systematically as the substituent X is varied. For the cases where m.l.c.t. states are lowest lying a quantitative correlation exists between ln (knr× 1 s)(knr is the rate constant for non-radiative decay) and the Franck–Condon factor calculated from parameters obtained by emission spectral fitting. The solvent reorganizational energy for [Re(bipy)(CO)3Cl] has been determined to be 1100 cm–1 in EtOH–MeOH (4:1 v/v) and 650 cm–1 in 2-methyltetrahydrofuran by a temperature dependent bandwidth study. Based on a comparative analysis of properties with related polypyridyl complexes of RuII and OsII it has been concluded that: (1) the extent of distortion at the 4,4′-X2-bipy acceptor ligand correlates with the energy gap between the excited and ground states; these results are in agreement with an earlier correlation found for polypyridyl complexes of OsII; (2) the unusually large Stokes shift and the broadening of the vibronic components in absorption and emission spectra arise from a combination of increased solvent reorganizational energies and greater distortions in the low-frequency modes between the excited and ground states; and (3) the relatively short lifetimes for the complexes of ReI have as a major contributing factor the participation of a ν(CO) mode at ca. 2020–2040 cm–1 as an energy acceptor in non-radiative decay.

Ten-Nanosecond Step-Scan FT-IR Absorption Difference Time-Resolved Spectroscopy: Applications to Excited States of Transition Metal Complexes:

Ten-nanosecond time resolution has been achieved with step-scan FT-IR absorbance difference spectroscopy (S2FT-IR ΔA TRS) and demonstrated by measuring ΔA spectra of fac-[Re(bpy)(CO)3Cl] and cis-[Os(bpy)2(CO)(4,4′-bpy)]2+ (bpy = 2,2′-bipyridine; 4,4′-bpy=4,4′-bipyridine) in CH3CN solution, following 355-nm laser excitation. In both complexes, the large shifts in (CO) to higher energy are consistent with the assignment that the lowest-energy excited states are metal-to-ligand charge transfer in nature. For [Os(bpy)2(CO)(4,4′-bpy)]2+, it is also possible to measure the excited-state decay kinetics, again with 10-ns resolution. In addition, ΔA bands are observed that are related to excited-state vibrations of the bipyridine ligands. ΔA spectra of good signal-to-noise ratio can be obtained for complexes with lifetimes as short as 10 ns.

Stabilization of Polymer Dispersed Liquid Crystal Systems Using Surface Active Agents

Abstract In this study, surface active agents were used in the preparation of polymer dispersed liquid crystals (PDLCs) by temperature induced phase separation. The surfactants Tween 20, a mixture of long chain carboxylic acids, and oleic acid were used to stabilize the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) in a poly(isobutyl methacrylate) matrix. The PDLCs formed using the surfactants exhibited more uniform particle sizes and greater particle size stability than comparable systems which contained no surfactant. The stabilized systems maintained their properties after being switched over 100,000 times. Also, the response to an applied electric field was shown to be faster for the stabilized PDLC systems.

Step-scan Fourier transform infrared absorption difference time-resolved spectroscopy studies of excited state decay kinetics and electronic structure of low-spin d6 transition metal polypyridine complexes with 10 nanosecond time resolution

Step-scan Fourier transform absorption difference time-resolved spectroscopy (S2FTIR ∆A TRS) has been used to collect mid-IR time-resolved infrared spectra of the transient electronic excited states of polypyridine transition metal complexes with 10 ns time resolution. The time-resolved data can be used for kinetic analysis or to generate “snapshots” of the lowest lying excited state. Shifts of vibrational bands in the excited state relative to the ground state can be used to infer significant details of the electronic structure of the excited state. The multiplex advantage of the FTIR technique allows a wide variety of vibrational bands to be analyzed for this purpose. In the example illustrated, the shift of the ester ν(CO) band in {Ru(bpy)[4,4′-(COOEt)2bpy]2}2

Step-scan FTIR absorption difference time-resolved spectroscopy studies the excited state electronic structures and decay kinetics of d6 transition metal polypyridine complexes

Step-scan FTIR absorption difference time-resolved spectroscopy (S2 FTIR ΔA TRS) has been used to study the photo-excited states of several low-spin d6 transition metal polypyridine complexes. Insight into the distribution of electron density in the excited states is obtained by comparing the ground and excited state vibrational frequencies of various bands sensitive to electronic structure. The multiplex, registration, and IR throughput advantages of this interferometric technique are significant in comparison with other methods currently used to probe photo-excited processes on the nanosecond time scale. The S2 FTIR ΔA TR spectra were obtained by use of a step-scan modified Bruker IFS 88 FTIR spectrometer equipped with an AC/DC-coupled photovoltaic Kolmar Technologies MCT detector with a 20 ns rise time and a 100/200 MHz PAD82a transient digitizer. The complexes were excited with frequency-tripled pulses from a Q-switched Quanta-Ray DCR1A Nd:YAG laser (355 nm, 10 ns, 10 Hz, 3 mJ/pulse). Data were collec...

Excited State Structure and Relaxation Dynamics of Polypyridyl Complexes of Low Spin d6 Metal Ions by Means of Step-Scan FTIR Time-Resolved Spectroscopy (S2FT-IR TRS)

Time-resolved infrared spectroscopy has been applied to study the excited state dynamics of many biological molecules and transition metal complexes following laser-flash excitation [1–6]. The short lifetimes of the excited states involved in these processes require very high time resolution. Until recently, fast time-resolved IR measurements have typically utilized tunable IR diode lasers as the probe source. The limited tunability of these IR lasers limited the studies to systems that contain CO or CN groups as a “reporter” ligand.

Step-Scan FT-IR Time-Resolved Spectroscopy (S2 FT-IR ΔA TRS) of the Photodynamics of Carbonmonoxy-myoglobin

The kinetics of protein response and of CO recombination after photolysis of the Fe-CO bond in carbonmonoxy-myoglobin have been monitored by step-scan FT-IR absorption difference time-resolved spectroscopy (S2 FT-IR ΔA TRS) in D2O solution at ambient temperature. From simultaneous measurement of changes in vCO and in the amide I band it has been possible to correlate the CO recombination kinetics with protein secondary structural changes with μs time resolution. The spectral and kinetic data corroborate and confirm previously published single frequency infrared studies indicating that the rebinding process is accomplished with only minimal change in the protein secondary structure. Data of higher temporal resolution (to 20 ns) have also been obtained for the CO recombination process.

Step-Scan FT-IR Time-resolved Spectroscopy (S2FT-IR TRS) of the Electro-Reorientation of Liquid Crystalline Materials

Frequency domain and time domain step-scan FT-IR time-resolved spectra (S2FTIR TRS) of the homogeneous/homeo-tropic transition in several nematic liquid crystals are presented and discussed. These include the pure mesophases 5CB and 5PCH and the eutectic mesophase 7A. In agreement with previous studies, both frequency domain and time domain data indicate a difference in response of the rigid and “floppy” segments of the 5CB and 5PCH molecules. The time domain data show clearly that it is in the recovery (voltage off) phase that the pentyl “tails” of the molecules respond more quickly than the rigid cores. For 7A, frequency domain data collected at twice the AC field frequency and with zero DC offset, indicate a slight difference in phase of response of the two major components.