Synthesis and comparative charge transfer studies in porphyrin-fullerene dyads: mode of attachment effect

Fullerene-porphyrin dyad system has gained increasing interest because of not only their interesting optoelectronic properties but also their applications in single-material organic solar cells. In the present work, two non-metallated fullerene-porphyrin dyads are prepared, viz, H2P1-C60 (dyad I) and H2P2-C60 (dyad II), where the modes of attachment on fullerene are different, i.e. via 1,3-dipolar cyclopropanation reaction in dyad I and Prato reaction in dyad II. We compare the photophysical and thermally activated conducting properties of these two non-metallated dyads in solution as well as in film to establish the better mode of attachment for dyad preparation with exotic properties. Both the dyads are structurally characterized by FT-IR, NMR and HRMS. Photophysical studies reveal that for dyad II, the reaction rate of charge separation is much higher than for dyad I (0.62 x 10(9) s(-1) vs. 0.32 x 10(9) s(-1)) in nonpolar solvent. Moreover, dyad II shows higher quenching of porphyrin emission than dyad I (90% vs. 70%) due to faster deactivation of singlet excited state by photoinduced charge transfer between porphyrin and fullerene. The reaction rate for charge transfer (k(ET(CS))) for dyad II is found to be maximum with lowest activation energy value as compared to dyad I. Excited-state electronic interactions in solution as well as in film have been studied by transient absorption spectroscopy to identify charge separated states. Finally, both the dyads are tested for their thermally activated electrical conductivity where dyad II shows an increase in conductivity with increase in temperature, which is reversible. Thus, dyad II is found to be an excellent material with interesting photophysical and electronic properties that can be beneficial in various applications such as organic solar cells and sensors.