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 onto SnO 2 or SiO 2 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 transient absorption signal in the region of the dye radical cation. For cresyl violet−SiO 2, the ground state recovery is a single exponential with a 2.9 ± 0.2 ps time constant. Transient absorption measurements that monitored the excited electronic state of the dye show a similar 2.5 ± 0.4 ps decay. The observed dynamics for cresyl violet−SiO 2 is assigned to internal conversion followed by vibrational relaxation of the adsorbed cresyl violet dimers. The similarity of the transient absorption and bleach recovery time constants shows that vibrational relaxation is extremely rapid, i.e., internal conversion is the rate-limiting step. For cresyl violet−SnO 2, the ground state recovery is biexponential with time constants of 2.4 ± 0.4 ps (∼80% of the amplitude) and 11.3 ± 0.5 ps (∼20%). Transient absorption measurements that monitor both the electronically excited dye aggregates and the dye radical cation also show a biexponential decay with time constants of 2.3 ± 0.3 and 12.3 ± 0.5 ps. The 2.4 ps process is attributed to internal conversion/vibrational relaxation of the excited dye aggregates, analogous to the results for the cresyl violet-SiO 2 system. The 12 ps process is assigned to the decay of the cresyl violet dimer radical cation that is produced by electron transfer to the SnO 2 semiconductor particles. The radical cation only contributes to the signal for the cresyl violet−SnO 2 system because electron transfer to SiO 2 is not energetically allowed. The decay mechanism for the radical cation is back electron transfer from SnO 2 .