Enhanced nonlinear optical properties of a pi-conjugated porphyrin dimer-graphene nanocomposite

Functional organic-inorganic nanocomposite materials displaying large optical nonlinearities are eagerly required for many advanced optoelectronic applications, such as data storage, optical switching, laser mode-locking, and optical limiting. Herein, a pyrazine-linked porphyrin dimer (1a)-functionalized graphene nanocomposite G-1a was successfully constructed by physical grafting of 1a onto the surface of graphene through pi-pi stacking. The structure of G-1a was thoroughly characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, steady-state absorption, and fluorescence analysis. Z-scan studies demonstrate that unprecedented enhancement of reverse saturable absorption (RSA) of porphyrin dimer 1a compared to that of the porphyrin monomer ZnTPP is observed under 12 ns (nanosecond) laser irradiation, which can be ascribed to its large pi-conjugated system, longer excited-state lifetime in the ns scale, and reorganized excited states. G-1a exhibits the strongest nonlinear optical (NLO) absorption properties under identical laser conditions when compared to its individual precursors, stemming from a synergistic effect of the strongly enhanced RSA of 1a, the nonlinear scattering of graphene and the effective photoinduced electron/ energy transfer from 1a to graphene. These results expand the applications of porphyrin dimers in developing novel NLO-active materials towards ns laser irradiation and reveal that G-1a is a very promising optical limiter candidate. This study provides a new paradigm to develop advanced optoelectronic devices with organic conjugated chromophores.