ULTRAFAST PHOTOPHYSICAL INVESTIGATION OF CRESYL VIOLET AGGREGATES ADSORBED ONTO NANOMETER-SIZED PARTICLES OF SNO2 AND SIO2

The relaxation dynamics of cresyl violet H-aggregate dimers adsorbed o nto SnO2 or SiO2 colloidal particles has been examined with ca. 200 fs time resolution. These experiments were performed by monitoring both the ground state recovery of the excited dye molecules and the transie nt absorption signal in the region of the dye radical cation. For cres yl violet-SiO2, the ground state recovery is a single exponential with a 2.9 +/- 0.2. ps time constant. Transient absorption measurements th at monitored the excited electronic state of the dye show a similar 2. 5 +/- 0.4 ps decay. The observed dynamics for cresyl violet-SiO2 is as signed to internal conversion followed by vibrational relaxation of th e adsorbed cresyl violet dimers. The similarity of the transient absor ption and bleach recovery time constants shows that vibrational relaxa tion is extremely rapid, i.e., internal conversion is the rate-limitin g step. For cresyl violet-SnO2, the ground state recovery is biexponen tial with time constants of 2.4 +/- 0.4 ps (similar to 80% of the ampl itude) and 11.3 +/- 0.5 ps (similar to 20%). Transient absorption meas urements that monitor both the electronically excited dye aggregates a nd the dye radical cation also show a biexponential decay with time co nstants of 2.3 +/- 0.3 and 12.3 +/- 0.5 ps. The 2.4 ps process is attr ibuted to internal conversion/vibrational relaxation of the excited dy e aggregates, analogous to the results for the cresyl violet-SiO2 syst em. The 12 ps process is assigned to the decay of the cresyl violet di mer radical cation that is produced by electron transfer to the SnO2 s emiconductor particles. The radical cation only contributes to the sig nal for the cresyl violet-SnO2 system because electron transfer to SiO 2 is not energetically allowed. The decay mechanism for the radical ca tion is back electron transfer from SnO2.