Effective enhancement of the emission efficiency of tetraphenylporphyrin in solid state by tetraphenylethene modification

Abstract

Tetraphenylporphyrin (TPP) is a typical red-emitting luminogen showing evident aggregation caused quenching (ACQ) effect. To enhance its emission efficiency in solid state, four tetraphenylethene (TPE) units were attached to the four meso-positions of TPP core via ester group through a facile and efficient route. The derived compound (4(TPE−COO)-TPP) emits red fluorescence (peak at 655 nm) with a good quantum efficiency (Φ) of 7.5%, which is much higher than that of TPP (Φ ∼ 0.1%). In molecular aggregate formed in tetrahydrofuran (THF) and water mixtures, 4(TPE−COO)-TPP has a relative high Φ of 12%. The evidently subdued ACQ behavior can be ascribed to the propeller shape and bulky size of the TPE units, which prevent the close packing and strong π-π interaction of TPP cores. The loose molecular packing and weak interchromophore interactions were validated by different characterization methods including UV-visible absorption, steady state and transient fluorescence spectroscope, X-ray diffraction and scanning electronic microscope observations. It is noted that 4(TPE−COO)-TPP has an emission efficiency of 14.4% in dilute THF solution. This is due to the conjugation break between the TPP and TPE moieties, the rotational and vibrational motions of the phenyl groups cannot quench the fluorescence of 4(TPE−COO)-TPP.