Graphene Oxide With Covalently Linked Porphyrin Antennae

Graphene Oxide With Covalently Linked Porphyrin Antennae Swetanshu Tandon Paper Title: Graphene oxide with covalently linked porphyrin antennae: Synthesis,characterization and photophysical properties. Relevant spectroscopic Techniques UV-visible spectroscopy ATR-IR spectroscopy Raman spectroscopy Fluorescence spectroscopy Transient absorption spectroscopy Introduction In this paper, the authors describe the synthesis, characterisation and properties of a hybrid material, GO-H2P, obtained by treating graphene oxide (GO) with 5-(4-aminophenyl)-10, 15, 20-triphenyl-21, 23H-porphyrin (H2P). The characterisation has been done by UV–visible, ATR-IR (Attenuated Total Reflection Infrared) and Raman spectroscopy. Steady state and time resolved fluorescence studies and transient absorption measurements were also conducted to study the electron transfer process from H2P to graphene oxide by photoexcitation. Morphological characterisation has been carried out with the help of transmission electron microscopy (TEM) and atomic force microscopy (AFM). Cyclic voltametry and differential pulsed voltametry were conducted to study the electrochemical characteristics like redox potentials. Choice of techniques Spectroscopic techniques have been carried out on the GO-H2P material dispersed in dimethylformamide at concentration not exceeding 1 mg/mL. UV-visible spectroscopy has been employed in this study due to the presence of porphyrin group which have a characteristic absorption around 420 nm (Soret band) corresponding to a1u(π)–eg*(π) transitions and a weak absorption near 550 nm (Q band).The Soret band involves transition from ground state to second excited state while Q band involves transition from ground state to first excited state. Graphene oxide also shows a characteristic transition at 230 nm again corresponding to π-π* transitions. The nature of linkage can be investigated with the help of this technique. ATR-IR spectroscopy has been used in the study in order to characterise the O–H, C=O, and C–OH bands present in graphene oxide which have characteristic vibrations. Also, in order to confirm the formation of GO-H2P hybrid by the formation of amide units between carboxylic group of graphene oxide and amine group of the porphyrin derivative, ATR-IR spectroscopy can be used. Raman spectroscopy has been used to follow the transformation of graphite to graphene oxide and further to GO-H2P. The sp2 hybridisation in all the three materials, i.e. graphite, graphene and graphene oxide, leads to the formation of different peaks due to first and higher order scattering process. The presence of π-electrons make the scattering process resonant thus making the response stronger. The fluorescence properties of porphyrin derivatives due to π- π* transitions between the two highest occupied molecular orbitals and the two lowest occupied molecular orbitals within the ring justifies the use fluorescence spectroscopy in the study. Fluorescence spectroscopy has been used to examine the nature of electronic interaction between graphene oxide and H2P. The photoexcitability of GO-H2P leads to the application of transient absorption spectroscopy. Transient absorption spectroscopy has allowed the examination of the phenomenon of quenching of luminescence in further detail. Information Obtained A broad monotonically decreasing signal on moving from ultraviolet to visible region was obtained in the UV-visible spectrum of GO-H2P which is characteristic of graphene oxide. The spectrum was obtained in a solution of dimethylformamide at concentration not exceeding 1 mg/mL as mentioned before. Broadening and shortening of the band at 420 nm, characteristic of H2P (Soret-band), is also observed besides its bathochromic shift (by ca. 2 nm) while the Q bands were flattened to the base line. Bathochromic shift signifies increased conjugation. So, it can be concluded that not only there is a linkage between GO and H2P units but also electronic interactions between the two. The ATR-IR spectra provided in the supporting information reveals a peak at 1715 cm-1 corresponding to C=O vibration. The fingerprints are at 3616 cm-1 and 3490 cm-1due to O–H stretching due to their high bond strength. The covalent linkage between GO and H2P moieties is supported by the presence of a peak at 1630 cm-1 which is characteristic of carbonyl group of amide units. The signal due to graphene layer appears in the region ~1650 – 1550 nm. In the Raman spectrum for graphite the G-band, characteristic of all sp2 hybridised carbons, is present at 1580 cm-1. The G/ (or 2D)-band – the first overtone of the D peak which is also a characteristic of all sp2 hybridised carbons – is located at 2725 cm-1 as a sharp and symmetric band. For GO, the Raman spectrum shows a D band, which is a characteristic of disorder for sp2 hybridised carbons, at 1345 cm-1. This accounts for the defects produced due to the oxidation of graphite. Also, the G/-band in GO appears broader and hypsochromically shifted thereby implying the presence of single and bilayers of GO sheets which is further supported by AFM analysis. The Raman spectrum of GO-H2P is almost identical to that of GO. This means that treatment with H2P perturbs the graphene layer of GO to a very small extent. IR as well as Raman spectroscopic techniques have been used in this study to follow the formation of GO-H2P from graphite. Both these techniques complement each other as one (infrared spectroscopy) is applicable to vibrational modes in which the dipole moment of the molecule is altered while the other (Raman spectroscopy) is applicable to modes involving change in the electric polarizability. ATR-IR spectroscopy provides information of the functional groups and thus helps in following and confirming the formation of GO-H2P from graphene oxide and the porphyrin derivative. Raman spectroscopy, on the other hand, supports the observations obtained by infrared spectroscopy. Also, it provides an insight of the extent of disorder of the graphene layer which keeps vibrating. The D band gives information about the extent of disorder in graphene layers. UV-visible spectroscopy provides information about the nature of interaction between GO and H2P units. It, in addition to the data provided b y IR and Raman spectroscopy, points out that transfer of electrons might be possible between the porphyrin units and graphene oxide layers. The nature of electronic interactions of the H2P units with the graphene oxide sheets has been further investigated by fluorescence spectroscopy. On excitation with 418 nm radiation, characteristic fluorescence emission H2P, in dimethylformamide, at 660 nm and 716 nm are observed. In GO-H2P, these emissions are significantly quenched. The emission at 660 nm is also shifted by 10 nm hypsochromically. Quantifying the quenching of the porphyrin emission in this hybrid material tends to be a little difficult because of interference from the absorption of graphene oxide at the excitation wavelength. However, the effective emission quenching of porphyrin in the GO-H2P hybrid indicates that electronic interactions between the singlet excited state of the porphyrin and graphene oxide are dominant. The fluorescence lifetime of photoexcited porphyrin in GO-H2P hybrid have been calculated to be 675 ps (50%) and 1600 ps (50%) which is significantly lower than that of the intact porphyrin, H2P (2900 ps (100%)). This further supports the efficient emission quenching by the graphene sheets. The effective quenching of the fluorescence emission due to H2P in GO-H2P implies electronic interaction between the singlet excited state of H2P with GO. So, H2P acts as an energy absorber unit and GO unit acts as an electron transporting unit. Transient absorption spectrum complements the information obtained from fluorescence spectroscopy using Nd-YAG laser. The Q band can be photoexcited by using a laser light source of 532 nm which led to the population of the singlet excited state of H2P. The band absorption, in the transient absorption spectrum, due to oxidised porphyrin species (H2P+) lies in the visible region at 610 nm. The band observed at 450 nm is the characteristic feature of porphyrin and is almost identical to the one observed in the transient absorption spectroscopy of intact H2P. It occurs due to triplet-triplet absorption of the porphyrin. The bands in the near infrared region are due to the electrons trapped within the reduced graphene sheets (GO) in which absorption is observed in the near infrared region due to the presence of GO species. This is further supported by the absorption spectrum of electrochemically reduced graphene oxide. Thus, transient absorption spectrum provides evidence in support of t he formation of charge separated radical ion pair GO H2P+. The depletion observed near 1100 nm is due to the fundamental YAG laser. The decline of transient absorption with time is a proof of the development of charge recombination which de-excites the radical ion pair back to its ground state. The decay profile for these transient species gave a charge-recombination rate constant (kCR) of 1.8 X 107 s1. This allows the evaluation of the lifetime of the radical ion pair GO H2P+ which comes out to be 56 ns in dimethylformamide. This further confirms that the charge separation is the cause of fluorescence quenching. The emission and transient absorption spectroscopy give information about the conductive nature of GO-H2P hybrid. As pointed out in the observations of UV-visible spectroscopy, the nature of bond in between graphene oxide and H2P if not purely covalent but involves electronic interaction too. Fluorescence spectroscopy helps in further examining the nature of interaction of the GO-H2P linkage. It confirms the observations of UV-visible spectroscopy and indicates the presence of charged species as inferred by the short fluorescence lifetime profile. Transient absorption spectroscopy further confirms this. Also, it confirms that the observations of cyclic voltammetry and differential pulsed voltametry which indicate the presence of radical ion pairs GO H2P+. It also indicates the development of charge recombination which drives the radical ion pair back to its ground state. Overall, it gets confirmed that H2P can easily get photoexcited and transfer electron to graphene oxide which is able to capture these electrons effectively. Additional spectroscopic information that may provide useful information The presence of porphyrin moiety makes this substance particularly interesting. This is because of the characteristic Soret band which is used for its identification. Also, the molar extinction coefficient for porphyrins is pretty high. 5-(4-aminophenyl)-10, 15, 20-triphenyl-21, 23H-porphyrin, which has been used in the analysis, is chiral in nature. So, circular dichroic spectroscopy can be conducted. The high value of the molar extinction coefficient would be helpful as it increases the sensitivity of the technique in this study carried out over this porphyrin derivative covalently linked to graphene oxide. The chiral nature would not only help in supporting the data obtained by various characterisation techniques (infrared, UV-visible, Raman spectroscopy) but would prove helpful in the conformational and configurational analysis of the porphyrin as well. It might also help in exploring the chiral nature of the GO-H2P hybrid. Spectra and Tables