Ultrafast Investigation of the Internal Conversion and Vibrational Relaxation in Excited States of Zinc Phthalocyanine Derivatives

An internal conversion（IC）process from the S₂ state to the S₁ state of tetra-tert-butylphthalocyanine（BuPc）,Zinc tetra-tert-butylphthalocyanine（ZnBuPc）and Zinc tetra-t-propoxyphthalocyanine（ZnPrPc）has been investigated respectively using femtosecond time-resolved fluorescence depletion method.The S₂ state of BuPc,ZnBuPc and ZnPrPc was prepared by the excitation of a 398nm laser pulse of 100fs duration.Via a fast IC process from S₂ to S₁,the excited molecule emitted fluorescence from the S₁ state.However,an ultrafast 796nm laser pulse depopulated the S₂ state and therefore weakened the fluorescence.The time scale of IC is estimated to be tens of femtoseconds.The vibrational relaxation in the S₁ state was also studied by two 796nm photon excitation followed with a 398nm dumping laser pulse.The decay rate was measured as 1.5ps for BuPc.For ZnBuPc and ZnPrPc,decay rate of 13.4ps and 7.6ps were also observed respectively.