TIME-RESOLVED, STEP-SCAN FTIR STUDIES OF EXCITED STATES OF d 6 METAL POLYPYRIDINE COMPLEXES

Time-resolved infrared spectroscopy using step-scan FTIR has been developed as an approach to study transient inorganic species with lifetimes ranging from ca. 200 nanseconds to several microseconds. This approach has been applied to a variety of problems arising following laser excitation of d polypyridine complexes in solution, and several examples are shown to demonstrate the utility of this experiment.


INTRODUCTION
Time-resolved infrared spectroscopy compliments the more often used time-resolved resonance Raman approach in the study of excited states of inorganic complexes. With the advances made recently in transient infrared spectroscopy, spectra can now be measured on the time scales previously only obtained by the transient Raman method. The transient infrared technique is particularly valuable in the study of metal complexes containing CO or CNthat have intense u(CO) and u(CN) bands [1]. A new and potentially valuable approach to transient infrared studies of excited states on the nanosecond time scale uses step-scan interferometry [2,3]. Examples demonstrating the utility time-resolved, step-scan FTIR (TRssFTIR) spectroscopy in the study of excited states in d 6 metal polypyridyl complexes are presented. EXPERIMENTAL Experiments were carried out on a modified BioRad FTS 60A/896 step-scan FTIR spectrometer. In the optical arrangement, the IR beam from the interferometer is directed to an external optical train to a lens that focuses the IR radiation onto the sample to a size of less than 3 mm. The transmitted IR beam is collected by a second lens that focuses it onto an element of a photoconductive mercury cadmium telluride (MCT) detector (Graseby 1710117, rise time 200 ns). A lowpass germanium filter coupled with the CaF2 cell windows and optics provides a spectral window of 1250 to 2250cm-. The signal is amplified (Graseby DP-8000-4 amplifier) and processed by a BioRad Fast TRS board (200ns time-resolution) installed in a Pentium PC. The third harmonic (354.7 nm) of a Q-switched Nd:YAG laser was used as the pump source (Spectra Physics DCR-11). The laser timing was controlled by a digital delay generator (Stanford Research Systems Model DG535) which was triggered at the beginning of each interferometer step.

RESULTS AND DISCUSSIONS
The time-resolved, step-scan FTIR (TRssFTIR) approach has been applied to the study of electronic structure of the excited states of Re(CO)3 complexes. A complex interplay between metal-to-ligand charge transfer (MLCT) and ligand-based excited states is known to exist in these complexes and TRssFTIR has been utilized to distinguish between them.
An example of an MLCT excited state is fac-[Re(phen)(CO)3 (4-Mepy)] + (phen is 1,10-phenanthroline 4-Mepy is 4-methylpyridine) [3]. The TRIR and FTIR spectra for fac-[Re(phen)(CO)3(4-Mepy)] + in the u(CO) region are shown in Figure 1. In this spectrum, the broad, ground-state u(CO) band at 1931 cmappears as a bleach with new excited-state bands appearing at 1965 and 2015 cm-.T he third u(CO) band at 2036cmalso shifts to higher ene.rgy (2065cm-1) in the excited state. The relatively large shifts to higher energy are consistent with partial oxidation of Re(I) to Re(II) and formation of the MLCT state. The shift to higher energy is due to a decrease in Re-CO backbonding to Re(II) causing an increase in the triple bond character of the CO ligands.
In contrast, fac-[Re(dppz)(CO)3(PPh3)] +* (dppz is dipyrido [3,2-a:2',2'-c]phenazine; PPh3 is triphenylphosphine) is classified as having LC lowest excited states (a lowest-lying, dppz-based 37rTr* state) [4]. The transient infrared difference spectra for this complex (Fig. 1) shows only slight shifts for the three carbonyl bands to lower energy (< 5cm-1) [3,5]. The shifts are consistent with a ligandlocalized excited state which is slightly more electron donating at the metal relative to the ground state. This monitoring of the carbonyl shifts is very useful in distinguishing excited state character and is being extended to complicated excited state behavior including dual or mixed excited states and charge-separated states.
A common structural motif in many molecular assemblies based on polypyridyl complexes is ligand bridging. The use of ligand bridges provides a basis for creating complex oligomeric assemblies. Several molecular assemblies which undergo intramolecular energy transfer following excitation have been studied with TRssFTIR spectroscopy. These include cyano-bridged ReRu complexes and polypyridylbridged ReRu and ReRe complexes.
[(phen)(CO)3Re(NC)Ru(bpy)z(CN)] + is an example of a polynuclear complex designed to produced vectorial energy transport. Energy transfer is expected from the higher energy Re-based MLCT state to the Ru-based MLCT state [6]. The time-resolved resonance Raman and TRssFTIR spectra of this complex following 354.7-nm excitation are shown in Figure  The bridging ligand can provide control over intercomponent processes, serving not only as a structural link but also as a means to modulate electron or energy transfer. The complex [(bpy)zRu ABRe(CO)3CI] 2+ (AB is 2,2':3'2":6",2"'-quaterpyridine, Fig. 3) is an example where an asymmetric bridge (AB) contains two inequivalent metal binding sites, one more sterically hindered than the other [7]. The difference in the A and B sites is expected to perturb the MLCT energy level of the attached Re(I) or Ru(II) chromophores, thereby controlling the direction of energy transfer in the ReABRu and RuABRe isomers.