pi-Extended diketopyrrolopyrrole-porphyrin arrays: one- and two-photon photophysical investigations and theoretical studies

A complete one- and two-photon spectroscopic and photophysical characterization of three diketopyrrolopyrrole (DPP)-porphyrin conjugates is reported. The increased conjugation introduced by the incremental addition of one, two and four DPP units on the meso porphyrin positions strongly affects the optical properties of the systems. Ground and triplet excited state absorption spectra show a gradual broadening and bathochromic shift and a trend to lower energies is also observed for both fluorescence and phosphorescence emission. Interestingly, the fluorescence quantum yield increases along the series, leading to remarkable NIR emission properties for the larger derivatives. Unlike the model porphyrin, all derivatives exhibit high two-photon absorption activity. An increase in two-photon absorption cross-section in the regions 800-840 nm and 910-930 nm is observed moving from one DPP to two DPP appended units, with a value of the order of 4000 GM at 910 nm for the latter system. The four compounds show high efficiency in generating singlet oxygen, with yields ranging from 0.7 to 0.5, envisaging favourable applications in both one- and two-photon photodynamic therapies. A detailed theoretical exploration of both linear (absorption and emission) and non-linear (two-photon absorption) properties proposes an analysis of the experimental spectra and a comprehensive interpretation of the two-photon activity within the series of compounds.