EARLY EXCITED-STATE DYNAMICS OF W(CO)5(4-CYANOPYRIDINE) AND W(CO)5(4-FORMYLPYRIDINE) - OBSERVATION OF AN ULTRAFAST INTERSYSTEM CROSSING

Picosecond time-resolved absorption spectra of W(CO)5(4-cyanopyridine) and W(CO)5(4-formylpyridine) complexes were measured following excita tion into the ligand-field, LF (355 nm), and, for the latter complex, also into the metal to ligand charge-transfer, MLCT (532 nm), absorpti on bands. In each case, formation of the transient absorption spectrum of the long-lived 3MLCT excited state was observed. Its population fo llows biphasic kinetics. The faster component is a sub-picosecond proc ess (probably hundreds of femtoseconds) whereas the lifetimes of the s lower component in toluene solution under 355-nm excitation were estim ated as 45 and 36 ps for W(CO)5-(4-cyanopyridine) and W(CO)5(4-formylp yridine), respectively. These results were interpreted as two intersys tem crossings: one is ultrafast and takes place from a (near) Franck-C ondon excited state; the other is slower, occurring from a vibrational ly relaxed 1MLCT state.