SPECTROSCOPIC AND PHOTOPHYSICAL INVESTIGATIONS OF THE UNSATURATED CYCLIC TRINUCLEAR CLUSTERS M3(DPPM)3CO2+ (M=PD, PT) - EVIDENCE FOR EXCITED-STATE GUEST-HOST CHEMISTRY

Evidence for the guest-host chemistry of the organometallic M3(dppm)3CO2+ clusters (M = Pd, Pt; dppm = ((C6H5)2P)2CH2)in the excited states has been demonstrated for the first time. The first part of the paper addresses the excited-state electronic and structural properties of the cavity, while the second part deals with the dynamics. First, the lowest energy excited states for both M = Pd and Pt are found to be in the 3A2* state according to approximate density function theoretical computations. When we monitor the UV-visible band at approximately 460 nm of the Pd3(dppm)3CO2+ compound (which corresponds to the nearly degenerate 17e --> 7a2 and 18e --> 7a2 electronic transitions) as a function of temperature, the moment band analysis enables us to confirm the M-M antibonding nature of the 7a2 MO (which indicates that the cavity in the excited state would increase with the M-M bond lengthening) and to estimate DELTAQ (in these 1E* states). During the course of these studies the nu(M-M) modes have been measured by Raman spectroscopy: M = Pd, nu(al) = 205 cm-1, nu(e) = 143 cm-1; M = Pt, nu(a1) = 149 cm-1, nu(e) = 125 cm-1. When DELTAQ is fixed to a reasonable value (0.7 angstrom) by comparison with ''d10-d10-d10'' Ag3 and Cu3 cluster systems, the structure of the cavity has been modeled using commercially available SYBIL 5.5. It is found that the dppm-phenyl groups (that described the cavity) gain a larger degree of rotational freedom upon the increase of DELTAQ and that the cavity is not perfectly cylindrical but rather exhibits a local C2v symmetry. Second, nanosecond and picosecond flash photolysis experiments demonstrate that molecular associations between the Pt3(dppm)3CO2+ clusters and the organic solvent molecules also occur in the excited states via the formation of tightly (major product) and loosely bonded complexes which deactivate in the picosecond and nanosecond time scale, respectively. The cavity is found to be hydrophobic in the ground state (according to the emission lifetime measurements at 77 K for Pt3(dppm)3CO2+ and also appear to be hydrophobic in the excited state despite the fact that the cavity size increases.