Non‐covalent nanohybrids composed of cationic 5,10,15,20‐tetra(4‐trimethylammoniophenyl)porphyrin tetra(p‐toluenesulfonate) (TMAP) and the graphene oxide sheets were prepared under two pH values (6.2 vs. 1.8). The TMAP molecule was positively charged, regardless of the pH value during preparation. However, protonation of the imino nitrogens increased the overall charge of the porphyrin molecule from +4 to +6 (TMAP4+ and TMAP6+). It was found that at acidic pH, interaction of TMAP6+ with GO was largely suppressed. On the other hand, results of FTIR, Raman spectroscopy, thermogravimetric analysis, atomic force microscopy (AFM) and elemental analysis confirmed effective non‐covalent functionalization of graphene oxide with cationic porphyrin at pH 6.2. The TMAP4+‐GO hybrids exhibited well defined structure with a monolayer of TMAP4+ on the GO sheets as confirmed by AFM. Formation of the ground‐state TMAP4+‐GO complex in solution was monitored by the red‐shift of the porphyrin Soret absorption band. This ground‐state interaction between TMAP4+ and GO is responsible for the static quenching of the porphyrin emission. Fluorescence was not detected for the nanohybrid which indicated that a very fast deactivation process had to take place. Ultrafast time‐resolved transient absorption spectroscopy clearly demonstrated the occurrence of electron transfer from the photoexcited TMAP4+ singlet state to GO sheets, as proven by the formation of a porphyrin radical cation.

Two noncovalent nanohybrids between cationic porphyrin (free-base TMPyP and zinc(II) ZnTMPyP) bearing cationic ( N -methylpyridyl) groups and graphene oxide (GO) were constructed with the aim of generating a photocatalyst active for rhodamine B (RhB) degradation. The obtained materials were thoroughly characterized by steady-state and time-resolved absorption and emission methods, which indicated that metalation of the porphyrin with Zn(II) increases the affinity of the porphyrin toward the GO surface. Photocurrent experiment together with femtosecond transient absorption spectroscopy clearly showed the existence of electron transfer from the photoexcited porphyrin to GO. Both hybrid materials demonstrated higher photocatalytic activity toward RhB degradation as compared to GO; however, ZnTMPyP–GO exhibited more efficient performance (19% of RhB decomposition after 2 h of irradiation). Our data indicate that the presence of Zn(II) in the core of the porphyrin can promote charge separation in the ZnTMPyP–GO composites. The higher degradation rate seen with ZnTMPyP–GO as compared to the TMPyP–GO assemblies highlights the beneficial role of Zn(II)-metalation of the porphyrin ring.

We have demonstrated that the photocatalytic system, containing Eosin Y (EY) as a sensitizer, triethanolamine (TEOA) as a sacrificial electron donor, CoSO 4 as a catalyst, and graphene oxide (GO), exhibited a 9-fold increase in hydrogen production rate compared to the analogous system in the absence of graphene oxide. Interaction of Eosin Y (EY) with graphene oxide (GO) in the ground state and excited state was probed by steady-state and time-resolved absorption and emission measurements. Analysis of the emission quenching of EY by GO revealed that the measured decrease of the fluorescence in the presence of GO was solely attributed to inner filter effects I and II and an absorbance change of EY itself at the excitation wavelength in the presence of GO. Femtosecond and nanosecond transient absorption spectroscopy experiments pointed to a lack of electron transfer from the excited states of EY, neither singlet nor triplet excited states, to GO sheets. It was demonstrated that the electron donor triethanolamine (TEOA) participates in the primary photochemical reaction, and the electron transfer to GO occurs from the EY radical anion and not directly from the excited state of EY. GO sheets were photochemically reduced using EY and TEOA under ambient conditions. After illumination of GO in the presence of TEOA and EY, the formation of reduced graphene oxide (rGO) was confirmed through optical absorption, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopies (XPS). This indicates that the electron transfer to GO is not followed by back electron transfer and thus can be further transferred to hydrogen evolution catalysts. The existence of a stable charge-separation state explains the role of graphene in the improvement of photocatalytic efficiency in the Eosin Y-based systems.

Graphene-based nanohybrids are good candidates for various applications. However, graphene exhibits some unwanted features such as low solubility in an aqueous solution or tendency to aggregate, limiting its potential applications. On the contrary, its derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), have excellent properties and can be easily produced in large quantities. GO/RGO nanohybrids with porphyrins were shown to possess great potential in the field of photocatalytic hydrogen production, pollutant photodegradation, optical sensing, or drug delivery. Despite the rapid progress in experimental research on the porphyrin-graphene hybrids some fundamental questions about the structures and the interaction between components in these systems still remain open. In this work, we combine detailed experimental and theoretical studies to investigate the nature of the interaction between the GO/RGO and two metal-free porphyrins 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) and 5,10,15,20-tetrakis(4-hydroxyphenyl) porphyrin (TPPH)]. The two porphyrins form stable nanohybrids with GO/RGO support, although both porphyrins exhibited a slightly higher affinity to RGO. We validated finite, Lerf–Klinowski-type (Lerf et al. in J Phys Chem B 102:4477, 1998) structural models of GO ($$\hbox {C}_{59}\hbox {O}_{26}\hbox {H}_{26}$$ C 59 O 26 H 26 ) and RGO ($$\hbox {C}_{59}\hbox {O}_{17}\hbox {H}_{26}$$ C 59 O 17 H 26 ) and successfully used them in ab initio absorption spectra simulations to track back the origin of experimentally observed spectral features. We also investigated the nature of low-lying excited states with high-level wavefunction-based methods and shown that states’ density becomes denser upon nanohybrid formation. The studied nanohybrids are non-emissive, and our study suggests that this is due to excited states that gain significant charge-transfer character. The presented efficient simulation protocol may ease the properties screening of new GO/RGO-nanohybrids.

Cocrystals, solids composed of molecular and/or ionic compounds connected by noncovalent interactions, are objects of interest in crystal engineering. Theobromine, as an active pharmaceutical ingredient, was used in cocrystallization with dihydroxybenzoic acids.