Excitation energy density dependent fluorescence behaviour of a regioselectively functionalized tetraphenylporphyrin–cellulose conjugate

Abstract

The fluorescence behaviour of tetraphenylporphyrin appended cellulose strands in chloroform is studied by weak cw excitation and intense picosecond laser pulse excitation (wavelength λ L=347.15 nm, duration Δt L=35 ps). With rising pump pulse energy density, a reduction of the fluorescence efficiency and a shortening of the fluorescence signal decay time are observed. The energy density dependent fluorescence quenching is analysed in terms of intra-strand singlet exciton–singlet exciton annihilation. The annihilation constant is determined and a critical singlet exciton number density ( N ex, ann =4.8×10 19 cm −3 ) as well as a critical fraction of excited repeat units ( f RU,ann=0.1) for annihilation are determined. Additionally the excited-state absorption cross-section, σ ex,L, at the pump laser wavelength, λ L, is determined by energy density dependent transmission measurements and is found to be approximately equal to the ground-state absorption, σ L, at λ L. The small-signal fluorescence quantum distribution, fluorescence quantum yield ( φ F=0.072), and degree of fluorescence polarisation ( P F=0.04) are determined by cw fluorimetry. The emitting exciton size is found to be equal to the repeat-unit size of the cellulose strands.