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Excited state electron transfer from aminopyrene to graphene: a combined experimental and theoretical study† Himadri Chakraborti,a Kommula Bramhaiah,b Neena Susan Johnb and Suman Kalyan Pal*a The quenching of the fluorescence of 1-aminopyrene (1-Ap) by reduced graphene oxide (rGO) has been investigated using spectroscopic techniques. In spite of the upward curvature in the Stern–Volmer plot, the unchanged spectral signature of the absorption of 1-Ap in the presence of rGO and the decrease in fluorescence lifetime with increasing rGO concentration point toward the dynamic nature of the quenching. Detailed analysis of steady state and time-resolved spectroscopic data has shown that the quenching arises due to the photoinduced electron transfer from 1-Ap to rGO. This is again sup-
Received 11th August 2013, Accepted 25th September 2013 DOI: 10.1039/c3cp53416b
ported by estimating the Gibb’s free energy change for the ground as well as excited state electron transfer. Ab initio calculations under the density functional theory (DFT) formalism reveal that the possibility of p–p stacking is very slim in the 1-Ap–rGO system and the electron density resides completely on 1-Ap in the highest occupied molecular orbital (HOMO) and on graphene in the lowest unoccupied molecular orbital (LUMO), supporting the experimental findings of the intermolecular electron transfer
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between 1-Ap and rGO in the excited state.
Introduction Graphene has become a philosophers’ stone since its discovery. This fascinating allotropic form of carbon has drawn much attention of concurrent research due to its remarkable thermal, electrical and optical properties.1–4 Graphene is regarded as a more promising material than the other nanocarbons; fullerene and carbon nanotubes; for applications in sensors,5–8 photovoltaics9–11 and optoelectronics.12–14 The photophysical processes occurring between various nanocarbons (other than graphene) and organic molecules have been investigated intensively.15–18 However, attention should be paid to understanding the interaction of graphene with organic molecules upon illumination. Like a photon in vacuum, the electrons can move ballistically through graphene at room temperature without generating any heat by collision.19 Such an exclusive carrier transport property along with good electron accepting capabilities makes graphene a great electron acceptor. Moreover, the size of the graphene is comparably very large with respect to the usual a
School of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175001, Himachal Pradesh, India. E-mail: [email protected]; Fax: +911905 237924 b Centre for Soft Matter Research, P.B. No.1329, Jalahalli, Bangalore-560013, India † Electronic supplementary information (ESI) available: Additional results for ‘‘Excited state electron transfer from aminopyrene to graphene: a combined experimental and theoretical study’’. See DOI: 10.1039/c3cp53416b
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acceptor molecules which will facilitate more donor attachments to its surface and result in a big supramolecular colony. The colony can absorb more photons and efficiently generate huge number of charge-separated states by multiple electron transfer. The out of the ordinary nature of such a multi-electron transfer process is much more interesting than the traditional processes involving single-electron transfer. A range of polycyclic aromatic molecules have been explored in combination with graphene since they are known to interact non-covalently with its surface.20–22 The quenching of the excited state of such aromatic molecules was also investigated in the presence of graphene.23–26 Zhang et al. reported mixed quenching (both static and dynamic) of the phthalocyanine fluorescence by chemically reduced graphene due to complexation in the ground state via strong p–p interactions.27 Similar mixed quenching was observed for molecules like rhodamine 6G,23 rhodamine B, eosin, methylene blue26 etc. In most of these cases the quenching was proposed to occur via both photo-induced inter- and intra-molecular electron transfer,23 although quenching due to energy transfer28 was also reported. However, no general consensus about the nature and the origin of the quenching of dye fluorescence by graphene has been established yet. Here, we have investigated the photophysical properties of 1-aminopyrene (1-Ap) in the presence of reduced graphene oxide (rGO) to get to the actual essence of the underlying processes.
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Steady state and time-resolved experiments reveal that fluorescence quenching is dynamic in nature ruling out the possibility of complex formation in the ground state. Negative changes in the Gibb’s free energy suggests the quenching of dye fluorescence via electron transfer in the excited state. Furthermore, ab initio calculations using density functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is fully localized on the 1-Ap and the lowest unoccupied molecular orbitals (LUMO) is on the rGO, indicating electron transfer from excited 1-Ap to rGO.
Techniques Experimental Graphene oxide was prepared from highly oriented pyrolytic graphite flakes by Hummer’s method as described elsewhere.29 GO was purified by centrifugation and filtration for further use. A rGO dispersion was obtained by reducing GO with hydrazine hydrate at 90 1C in an aqueous medium. 1-Ap was purchased from Sigma Aldrich and used as received. Double distilled water was used for the preparation of the solutions. We dissolved 1.5 mg 1-Ap in 2 ml tetrahydrofuran (THF) and 20 ml of that 1-Ap solution was added into 2 ml pure water to prepare the stock solution of 1-Ap (D1-Ap). All experimental studies were performed by adding 20 ml of D1-Ap into 2.5 ml water–THF (99 : 1) mixed solvent having a pH B 6. The final concentration of 1-Ap was estimated to be 0.27 mM. Experiments were carried out by keeping the concentration of 1-Ap fixed and varying rGO concentration using 1 mg ml1 stock solution. A high resolution field emission scanning electron microscope (FESEM) from Nova NanoSEM600, FEI, The Netherlands, operating at 5 kV at high vacuum, was used to capture images of the GO and rGO films on the doped silicon substrate. Raman spectra were recorded from the films on SiO2/Si substrate using a LabRAM HR spectrometer (Horiba-JobinYvon) equipped with a He–Ne laser (l = 632.8 nm). The rGO dispersion was drop cast onto the Si surface and analyzed using an Agilent 5500 atomic force microscope (AFM). A Shimadzu UV-2450 spectrophotometer
Fig. 1
and Cary Eclipse fluorescence spectrophotometer (Agilent Technologies) were employed to record the UV-vis absorption and fluorescence spectra, respectively. Fluorescence lifetime measurements were carried out in an LED based time-correlated single photon counting (TCSPC) set up from ISS, USA (ChronosBH fluorescence lifetime spectrometer). Computational All the calculations were performed using density functional (DFT) theory under the Gaussian09 software package.30 The electronic properties of the 1-Ap–rGO system were explored in the solvent (water) phase using a polarizable continuum model (PCM)31,32 with the integral equation formalism (IEF). The ground state structural geometry of 1-Ap was optimized using a Beke-Lee–Yang–Parr (B3LYP) exchange–correlation hybrid function33,34 at the 6-31G35,36 level, while for the 1-Ap–rGO system, B3LYP/LanL2DZ37–40 was used for better accuracy. The absence of imaginary frequencies confirms the successful structure optimization in all the cases (ESI,† Fig. S1–S3). The vertical transitions were calculated using a time-dependent DFT41 method. GAUSSSUM software42 was employed to calculate the density of states (DOS).
Results and discussion Spectroscopic measurements The synthesized graphene oxide (GO) and reduced graphene oxide (rGO) have been characterized using scanning electron microscopy and Raman spectroscopy (ESI,† Fig. S4 and S5). Raman spectra of GO and rGO (ESI,† Fig. S5) show the characteristic G and D bands at 1600 cm1 and 1356 cm1, respectively. The ratio of the intensity of D and G bands is increased in rGO due to the decrease of the GO sp3 domains which thereby confirms the reduction of GO.43 The rGO sheets are clearly visible from the AFM image (Fig. 1). The average sheet thickness obtained from the height profile is 2–3 nm, which indicates the formation of the few-layer graphene. The absorption spectra of GO shows characteristic p–p* and n–p*
AFM image of rGO along with the height profile.
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Fig. 2
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Absorption spectra of GO and rGO dispersions in water.
bands at 235 nm and 305 nm, respectively, while the p–p* band was red shifted and appears at 266 nm in rGO (Fig. 2) indicating the restoration of conjugation in the carbon network.26 Steady state absorption and emission spectra of 1-Ap with increasing rGO concentration in water–THF (99 : 1) mixed solvent are depicted in Fig. 3. Unlike the absorption spectra (Fig. 3a), a remarkable change was observed in the fluorescence spectra of 1-Ap after adding rGO at excitation wavelength 350 nm (Fig. 3b).
Fig. 3
The fluorescence intensity of dye was quenched by nearly 90% with the addition of rGO. The Stern–Volmer plot (inset of Fig. 3b) shows an upward curvature indicating mixed (static and dynamic) quenching.44 But the lack of appreciable changes in the absorption spectra of 1-Ap even after the addition of rGO rules out the possibility of the formation of ground state complexes and hence static quenching.45 In order to get deeper insight into the interaction between the dye and rGO, time-resolved fluorescence experiments for 1-Ap were performed in the presence of rGO following excitation at 441 nm. Fig. 4a shows the fluorescence decays of 1-Ap in the absence and the presence of rGO, and the decay profile of rGO is given in the ESI† (Fig. S6). The decay profiles were well fitted with multi-exponential functions and the fitting results are tabulated in Table 1. The decay profile of 1-Ap shows two lifetime components (t1 = 247 ps and t2 = 5.7 ns) with a dominating contribution (98%) from the longer one. Shizuka et al. pointed out that 1-Ap can exist in both the neutral and the protonated forms and the fluorescence lifetime of the neutral one is 5.9 ns.46 Guided by this earlier report, we correlate the longer and the shorter lifetimes to the neutral and the protonated forms of 1-Ap, respectively. Furthermore, the pH condition at which the experiments were carried out is more favorable for the neutral form
(a) Steady state absorption spectra and, (b) fluorescence spectra of 1-Ap (0.27 mM) in the presence of reduced graphene oxide. Inset: Stern–Volmer plot.
Fig. 4 (a) Time-resolved fluorescence decay profiles of 1-Ap in the presence of rGO (solid curves are the fitting results), (b) the plot of t0/t vs. [rGO] for the second lifetime component of 1-Ap.
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Table 1 rGOa
Sample
rGO (mg ml1) t1 (ns) a1 (%) t2 (ns) a2 (%) t3 (ns) a3 (%) w2
1-Ap 1-Ap 1-Ap 1-Ap 1-Ap rGO
0 0.004 0.012 0.020 0.040 0.020
a
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Fluorescence lifetime components for 1-Ap, the 1-Ap–rGO systems and
+ + + + +
rGO rGO rGO rGO rGO
0.247 2 0.195 3 0.193 6 0.168 8 0.155 12 0.170 12
5.7 5.37 4.98 4.75 4.21 3.03
98 83 66 54 35 10
— 20 20 20.7 21.5 21.7
— 14 28 38 53 78
1.10 1.11 1.02 1.01 1.09 1.13
Time-resolved data were fitted with the following equation: n t P ai e ti , where ti is the ith component of the lifetime and ai is
I ¼ I0
i¼1
inner filter effect due to the insignificant absorption of rGO at the excitation wavelength (350 nm). The change in fluorescence lifetime of 1-Ap again excludes the possibility of an inner filter effect. The dynamic quenching of fluorescence is supposed to occur due to either energy transfer or electron transfer or both. Due to a negligible overlap between the emission spectra of 1-Ap and the absorption spectra of rGO, the energy transfer cannot lead to such giant quenching. Therefore, electron transfer (ET) could be the main underlying process behind the fluorescence quenching of 1-Ap. The feasibility of the electron transfer process from 1-Ap to rGO is further surveyed thermodynamically as well as through the use of quantum chemical calculations (vide infra).
its contribution to the total decay.
Thermodynamics of ET of the dye. The addition of rGO to the dye solution yielded three lifetimes: 195 ps (t1), 5 ns (t2) and 20 ns (t3). It should be noted that the shortest component is within the range of our instrument response. The longest lifetime component of 1-Ap (in the presence of rGO) and the lifetime t3 of pristine rGO are very much similar, but their contributions (a3 values) are different. Moreover, t2 decreases and approaches to the second lifetime component of rGO with further addition of rGO. The contributions of the lifetime components also move towards that for the pristine rGO. The second component of the 1-Ap lifetime (t2) is mostly affected by rGO. We plotted the change in that component (t0/t) with rGO concentration in Fig. 4b, where t0 and t are the lifetimes in the absence and presence of rGO, respectively. These results clearly unveil the presence of an excited state interaction between 1-Ap and rGO. The appearance of the positive deviation in the Stern– Volmer plot (inset of Fig. 3b) could be explained by introducing a transient component into the dynamic quenching.44,47 In that case, quencher molecules in the vicinity of the fluorophore cause instantaneous quenching at the moment of excitation, before a steady state is reached. Therefore, 1-Ap interacts not with all rGO sheets, but only with those which are in close proximity. The rest of the rGO sheets remain unaffected and contribute to the emission. With the increase of rGO concentration a situation is arrived at where the dye emission is quenched completely and the emission comes only from free rGO. Pyrene as well as its derivatives were used earlier to monitor the change of solvent pH48,49 because the shape of their absorption spectra is highly sensitive to the environment pH. However, no evolution in the steady state absorption spectral profile of 1-Ap (Fig. 3a) ruled out the possibility of pH change in the solvent with the addition of rGO. Furthermore, the actual fluorescence intensity change is much higher than the estimated
The electrochemical response of 1-Ap was investigated using a cyclic voltammetry arrangement with saturated calomel as the working electrode (SCE) (ESI,† Fig. S7). The scanning was done between 2 to +2 V using Ag/AgCl as a reference electrode in the mixed solvent. The oxidative scanning shows one irreversible peak at 0.83 V (E(Ox/Ox +) = +0.83 V) which resembles the electro-oxidation of pyrene.50 The peak at B1.2 V arises due to the solvent response. Due to its large 2D expanse and huge surface area, rGO can efficiently accept more than one electron and exhibit multiple reduction peaks yielding reduction potential, E(Re k/Re (k+1)) (k = 0,1,2,. . .), values ranging from 0.60 to 0.85 V.51,52 The possibility of an occurrence of the ground state ET process was verified using the following equation: DGET = E(Ox/Ox +) E(Re/Re ) 0.06
(1)
For the dye–rGO system: (0.83 (0.60) 0.06) Z DGET Z (0.83 (0.85) 0.06) i.e. 1.37 Z DGET Z 1.62 The positive change in the Gibb’s free energy (DGET) thermodynamically forbids the ground state electron transfer from 1-Ap to rGO. However, excited state ET from the S1 state of 1-Ap to rGO is thermodynamically allowed. The energy of the S1 state (E00) for 1-Ap is 3.14 eV as calculated from the intersection of the normalized absorption and the emission spectra (ESI,† Fig. S8). The Gibb’s free energy change for the excited state ET (DGPET = DGET E00; (1.62 3.14) Z DGPET Z (1.37 3.14); 1.52 Z DGPET Z 1.77) yields negative value and hence photoinduced electron transfer (PET) is favorable. These results suggest that (1Ap) + and (rGO) are formed as a result of electron transfer from photoexcited 1-Ap to ground state rGO as shown in Scheme 1.
Scheme 1
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Kinetics of intermolecular PET
Theoretical calculations
The intermolecular dynamic quenching phenomenon follows the relation
A detailed theoretical investigation was performed to learn more about the interaction between the dye and graphene. To start with, the structure of 1-Ap was optimized in the ground state and the results are presented in the ESI† (Fig. S1–S3). In spite of the presence of an amine group, the ground state optimized structure of 1-Ap is planar like pyrene. The vertical transition energies of 1-Ap were calculated and found to match well with the absorption spectra (ESI,† Table S1). The HOMO and LUMO of 1-Ap are shown in Fig. 5. It is apparent from this figure that the amine group contributes to both the HOMO and the LUMO of 1-Ap. But the electron density on the nitrogen atom is sufficiently reduced in the LUMO in comparison to the HOMO due to the electron donating property of the amine group.
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t0 ¼ 1 þ k q t0 ½rGO t
(2)
where, kq is the (dynamic) quenching rate constant. A value of 1.6 1016 M1 s1 was obtained for the kq by fitting the data in Fig. 4b using eqn (2) and assuming the average size of the rGO sheet is 100 nm (ESI,† Fig. S9). Bimolecular collisions are responsible for the dynamic quenching in a system of small molecules and kq is in the range of the diffusion rate, kd (= 8RT/Zr), where R is the ideal gas constant, T is the temperature, eta is the coefficient of viscosity (Z) and rho is the density of the solvent (r), which is 109 to 1010 M1 s1 for water.23 In the case of collisions between rGO and 1-Ap, rGO can be assumed to be at rest because of its large mass and thereby reduce the kd value by a factor of 2. Nevertheless, the rGO sheet can simultaneously interact with a large number (N) of 1-Ap molecules. Therefore, the effective rate constant in a graphene-small molecule system becomes kq = N (kd/2). Comparing this expression with the experimentally observed kq, the value of N turns out to be 3.26 106. Such a high value of N shows the great quenching capability of a rGO sheet compared to a normal quencher molecule and thus suggests that multiple electron transfer is occurring.
Fig. 5
HOMO and LUMO of 1-Ap.
Fig. 6
Frontier molecular orbitals of the 1-Ap–GO system. Black: nitrogen; gray: carbon; white: hydrogen.
Fig. 7
Partial density of state of the 1-Ap–rGO system.
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A small 2D carbon cluster (C24H12) was used as a model for graphene during the study of its interaction with the dye to improve the ease of calculation. The ground state optimized structure of 1-Ap in the presence of graphene shows the nearly vertical inclination of 1-Ap on the model graphene surface suggesting a very poor p–p type interaction between them. Frontier HOMO and LUMO levels of 1-Ap and rGO system are depicted in Fig. 6. The HOMO is fully localized onto the 1-Ap entity, whereas the LUMO is solely restricted on the graphene surface. This provides a clear hint for the formation of the charge-separated state of (1-Ap)+–rGO following photo irradiation. A partial density of state (PDOS) was determined for the 1-Ap–rGO system and is presented in Fig. 7. At the valance band edge (B5 eV) of the system, the electron density is fully distributed over 1-Ap, but when the whole system is excited to the conduction band via light illumination, the electron density at the edge of the conduction band (B2.8 eV) is mostly transferred to the graphene skeleton, suggesting photoinduced electron transfer from dye to graphene.
Conclusions The fluorescence of 1-Ap is found to be quenched by rGO without affecting its absorption spectra. Steady state and time-resolved measurements rule out the possibility of ground state complex formation and confirm the dynamic nature of the quenching due to the electron transfer. The negative value of the Gibb’s free energy change further supports the ET from excited 1-Ap to rGO. The estimated value of the quenching constant (kq) is very high compared to the diffusion rate (kd), suggesting the occurrence of multiple ET. Quantum chemical calculations forbid complex formation via p–p interactions in the 1-Ap and rGO system. Moreover, the calculations of frontier molecular orbitals and PDOS for the system reveal that electron transfer is favorable from 1-Ap to rGO under illumination.
Acknowledgements Financial support from Indian Institute of Technology Mandi (Grant No. IITM/SG/SUG/004) aided the carrying out of this work. HC is thankful to IIT Mandi for his fellowship.
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Excited state electron transfer from aminopyrene to graphene: a combined experimental and theoretical study† Himadri Chakraborti,a Kommula Bramhaiah,b Neena Susan Johnb and Suman Kalyan Pal*a The quenching of the fluorescence of 1-aminopyrene (1-Ap) by reduced graphene oxide (rGO) has been investigated using spectroscopic techniques. In spite of the upward curvature in the Stern–Volmer plot, the unchanged spectral signature of the absorption of 1-Ap in the presence of rGO and the decrease in fluorescence lifetime with increasing rGO concentration point toward the dynamic nature of the quenching. Detailed analysis of steady state and time-resolved spectroscopic data has shown that the quenching arises due to the photoinduced electron transfer from 1-Ap to rGO. This is again sup-
Received 11th August 2013, Accepted 25th September 2013 DOI: 10.1039/c3cp53416b
ported by estimating the Gibb’s free energy change for the ground as well as excited state electron transfer. Ab initio calculations under the density functional theory (DFT) formalism reveal that the possibility of p–p stacking is very slim in the 1-Ap–rGO system and the electron density resides completely on 1-Ap in the highest occupied molecular orbital (HOMO) and on graphene in the lowest unoccupied molecular orbital (LUMO), supporting the experimental findings of the intermolecular electron transfer
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between 1-Ap and rGO in the excited state.
Introduction Graphene has become a philosophers’ stone since its discovery. This fascinating allotropic form of carbon has drawn much attention of concurrent research due to its remarkable thermal, electrical and optical properties.1–4 Graphene is regarded as a more promising material than the other nanocarbons; fullerene and carbon nanotubes; for applications in sensors,5–8 photovoltaics9–11 and optoelectronics.12–14 The photophysical processes occurring between various nanocarbons (other than graphene) and organic molecules have been investigated intensively.15–18 However, attention should be paid to understanding the interaction of graphene with organic molecules upon illumination. Like a photon in vacuum, the electrons can move ballistically through graphene at room temperature without generating any heat by collision.19 Such an exclusive carrier transport property along with good electron accepting capabilities makes graphene a great electron acceptor. Moreover, the size of the graphene is comparably very large with respect to the usual a
School of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175001, Himachal Pradesh, India. E-mail: [email protected]; Fax: +911905 237924 b Centre for Soft Matter Research, P.B. No.1329, Jalahalli, Bangalore-560013, India † Electronic supplementary information (ESI) available: Additional results for ‘‘Excited state electron transfer from aminopyrene to graphene: a combined experimental and theoretical study’’. See DOI: 10.1039/c3cp53416b
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acceptor molecules which will facilitate more donor attachments to its surface and result in a big supramolecular colony. The colony can absorb more photons and efficiently generate huge number of charge-separated states by multiple electron transfer. The out of the ordinary nature of such a multi-electron transfer process is much more interesting than the traditional processes involving single-electron transfer. A range of polycyclic aromatic molecules have been explored in combination with graphene since they are known to interact non-covalently with its surface.20–22 The quenching of the excited state of such aromatic molecules was also investigated in the presence of graphene.23–26 Zhang et al. reported mixed quenching (both static and dynamic) of the phthalocyanine fluorescence by chemically reduced graphene due to complexation in the ground state via strong p–p interactions.27 Similar mixed quenching was observed for molecules like rhodamine 6G,23 rhodamine B, eosin, methylene blue26 etc. In most of these cases the quenching was proposed to occur via both photo-induced inter- and intra-molecular electron transfer,23 although quenching due to energy transfer28 was also reported. However, no general consensus about the nature and the origin of the quenching of dye fluorescence by graphene has been established yet. Here, we have investigated the photophysical properties of 1-aminopyrene (1-Ap) in the presence of reduced graphene oxide (rGO) to get to the actual essence of the underlying processes.
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Steady state and time-resolved experiments reveal that fluorescence quenching is dynamic in nature ruling out the possibility of complex formation in the ground state. Negative changes in the Gibb’s free energy suggests the quenching of dye fluorescence via electron transfer in the excited state. Furthermore, ab initio calculations using density functional theory (DFT) show that the highest occupied molecular orbital (HOMO) is fully localized on the 1-Ap and the lowest unoccupied molecular orbitals (LUMO) is on the rGO, indicating electron transfer from excited 1-Ap to rGO.
Techniques Experimental Graphene oxide was prepared from highly oriented pyrolytic graphite flakes by Hummer’s method as described elsewhere.29 GO was purified by centrifugation and filtration for further use. A rGO dispersion was obtained by reducing GO with hydrazine hydrate at 90 1C in an aqueous medium. 1-Ap was purchased from Sigma Aldrich and used as received. Double distilled water was used for the preparation of the solutions. We dissolved 1.5 mg 1-Ap in 2 ml tetrahydrofuran (THF) and 20 ml of that 1-Ap solution was added into 2 ml pure water to prepare the stock solution of 1-Ap (D1-Ap). All experimental studies were performed by adding 20 ml of D1-Ap into 2.5 ml water–THF (99 : 1) mixed solvent having a pH B 6. The final concentration of 1-Ap was estimated to be 0.27 mM. Experiments were carried out by keeping the concentration of 1-Ap fixed and varying rGO concentration using 1 mg ml1 stock solution. A high resolution field emission scanning electron microscope (FESEM) from Nova NanoSEM600, FEI, The Netherlands, operating at 5 kV at high vacuum, was used to capture images of the GO and rGO films on the doped silicon substrate. Raman spectra were recorded from the films on SiO2/Si substrate using a LabRAM HR spectrometer (Horiba-JobinYvon) equipped with a He–Ne laser (l = 632.8 nm). The rGO dispersion was drop cast onto the Si surface and analyzed using an Agilent 5500 atomic force microscope (AFM). A Shimadzu UV-2450 spectrophotometer
Fig. 1
and Cary Eclipse fluorescence spectrophotometer (Agilent Technologies) were employed to record the UV-vis absorption and fluorescence spectra, respectively. Fluorescence lifetime measurements were carried out in an LED based time-correlated single photon counting (TCSPC) set up from ISS, USA (ChronosBH fluorescence lifetime spectrometer). Computational All the calculations were performed using density functional (DFT) theory under the Gaussian09 software package.30 The electronic properties of the 1-Ap–rGO system were explored in the solvent (water) phase using a polarizable continuum model (PCM)31,32 with the integral equation formalism (IEF). The ground state structural geometry of 1-Ap was optimized using a Beke-Lee–Yang–Parr (B3LYP) exchange–correlation hybrid function33,34 at the 6-31G35,36 level, while for the 1-Ap–rGO system, B3LYP/LanL2DZ37–40 was used for better accuracy. The absence of imaginary frequencies confirms the successful structure optimization in all the cases (ESI,† Fig. S1–S3). The vertical transitions were calculated using a time-dependent DFT41 method. GAUSSSUM software42 was employed to calculate the density of states (DOS).
Results and discussion Spectroscopic measurements The synthesized graphene oxide (GO) and reduced graphene oxide (rGO) have been characterized using scanning electron microscopy and Raman spectroscopy (ESI,† Fig. S4 and S5). Raman spectra of GO and rGO (ESI,† Fig. S5) show the characteristic G and D bands at 1600 cm1 and 1356 cm1, respectively. The ratio of the intensity of D and G bands is increased in rGO due to the decrease of the GO sp3 domains which thereby confirms the reduction of GO.43 The rGO sheets are clearly visible from the AFM image (Fig. 1). The average sheet thickness obtained from the height profile is 2–3 nm, which indicates the formation of the few-layer graphene. The absorption spectra of GO shows characteristic p–p* and n–p*
AFM image of rGO along with the height profile.
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Fig. 2
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Absorption spectra of GO and rGO dispersions in water.
bands at 235 nm and 305 nm, respectively, while the p–p* band was red shifted and appears at 266 nm in rGO (Fig. 2) indicating the restoration of conjugation in the carbon network.26 Steady state absorption and emission spectra of 1-Ap with increasing rGO concentration in water–THF (99 : 1) mixed solvent are depicted in Fig. 3. Unlike the absorption spectra (Fig. 3a), a remarkable change was observed in the fluorescence spectra of 1-Ap after adding rGO at excitation wavelength 350 nm (Fig. 3b).
Fig. 3
The fluorescence intensity of dye was quenched by nearly 90% with the addition of rGO. The Stern–Volmer plot (inset of Fig. 3b) shows an upward curvature indicating mixed (static and dynamic) quenching.44 But the lack of appreciable changes in the absorption spectra of 1-Ap even after the addition of rGO rules out the possibility of the formation of ground state complexes and hence static quenching.45 In order to get deeper insight into the interaction between the dye and rGO, time-resolved fluorescence experiments for 1-Ap were performed in the presence of rGO following excitation at 441 nm. Fig. 4a shows the fluorescence decays of 1-Ap in the absence and the presence of rGO, and the decay profile of rGO is given in the ESI† (Fig. S6). The decay profiles were well fitted with multi-exponential functions and the fitting results are tabulated in Table 1. The decay profile of 1-Ap shows two lifetime components (t1 = 247 ps and t2 = 5.7 ns) with a dominating contribution (98%) from the longer one. Shizuka et al. pointed out that 1-Ap can exist in both the neutral and the protonated forms and the fluorescence lifetime of the neutral one is 5.9 ns.46 Guided by this earlier report, we correlate the longer and the shorter lifetimes to the neutral and the protonated forms of 1-Ap, respectively. Furthermore, the pH condition at which the experiments were carried out is more favorable for the neutral form
(a) Steady state absorption spectra and, (b) fluorescence spectra of 1-Ap (0.27 mM) in the presence of reduced graphene oxide. Inset: Stern–Volmer plot.
Fig. 4 (a) Time-resolved fluorescence decay profiles of 1-Ap in the presence of rGO (solid curves are the fitting results), (b) the plot of t0/t vs. [rGO] for the second lifetime component of 1-Ap.
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Table 1 rGOa
Sample
rGO (mg ml1) t1 (ns) a1 (%) t2 (ns) a2 (%) t3 (ns) a3 (%) w2
1-Ap 1-Ap 1-Ap 1-Ap 1-Ap rGO
0 0.004 0.012 0.020 0.040 0.020
a
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Fluorescence lifetime components for 1-Ap, the 1-Ap–rGO systems and
+ + + + +
rGO rGO rGO rGO rGO
0.247 2 0.195 3 0.193 6 0.168 8 0.155 12 0.170 12
5.7 5.37 4.98 4.75 4.21 3.03
98 83 66 54 35 10
— 20 20 20.7 21.5 21.7
— 14 28 38 53 78
1.10 1.11 1.02 1.01 1.09 1.13
Time-resolved data were fitted with the following equation: n t P ai e ti , where ti is the ith component of the lifetime and ai is
I ¼ I0
i¼1
inner filter effect due to the insignificant absorption of rGO at the excitation wavelength (350 nm). The change in fluorescence lifetime of 1-Ap again excludes the possibility of an inner filter effect. The dynamic quenching of fluorescence is supposed to occur due to either energy transfer or electron transfer or both. Due to a negligible overlap between the emission spectra of 1-Ap and the absorption spectra of rGO, the energy transfer cannot lead to such giant quenching. Therefore, electron transfer (ET) could be the main underlying process behind the fluorescence quenching of 1-Ap. The feasibility of the electron transfer process from 1-Ap to rGO is further surveyed thermodynamically as well as through the use of quantum chemical calculations (vide infra).
its contribution to the total decay.
Thermodynamics of ET of the dye. The addition of rGO to the dye solution yielded three lifetimes: 195 ps (t1), 5 ns (t2) and 20 ns (t3). It should be noted that the shortest component is within the range of our instrument response. The longest lifetime component of 1-Ap (in the presence of rGO) and the lifetime t3 of pristine rGO are very much similar, but their contributions (a3 values) are different. Moreover, t2 decreases and approaches to the second lifetime component of rGO with further addition of rGO. The contributions of the lifetime components also move towards that for the pristine rGO. The second component of the 1-Ap lifetime (t2) is mostly affected by rGO. We plotted the change in that component (t0/t) with rGO concentration in Fig. 4b, where t0 and t are the lifetimes in the absence and presence of rGO, respectively. These results clearly unveil the presence of an excited state interaction between 1-Ap and rGO. The appearance of the positive deviation in the Stern– Volmer plot (inset of Fig. 3b) could be explained by introducing a transient component into the dynamic quenching.44,47 In that case, quencher molecules in the vicinity of the fluorophore cause instantaneous quenching at the moment of excitation, before a steady state is reached. Therefore, 1-Ap interacts not with all rGO sheets, but only with those which are in close proximity. The rest of the rGO sheets remain unaffected and contribute to the emission. With the increase of rGO concentration a situation is arrived at where the dye emission is quenched completely and the emission comes only from free rGO. Pyrene as well as its derivatives were used earlier to monitor the change of solvent pH48,49 because the shape of their absorption spectra is highly sensitive to the environment pH. However, no evolution in the steady state absorption spectral profile of 1-Ap (Fig. 3a) ruled out the possibility of pH change in the solvent with the addition of rGO. Furthermore, the actual fluorescence intensity change is much higher than the estimated
The electrochemical response of 1-Ap was investigated using a cyclic voltammetry arrangement with saturated calomel as the working electrode (SCE) (ESI,† Fig. S7). The scanning was done between 2 to +2 V using Ag/AgCl as a reference electrode in the mixed solvent. The oxidative scanning shows one irreversible peak at 0.83 V (E(Ox/Ox +) = +0.83 V) which resembles the electro-oxidation of pyrene.50 The peak at B1.2 V arises due to the solvent response. Due to its large 2D expanse and huge surface area, rGO can efficiently accept more than one electron and exhibit multiple reduction peaks yielding reduction potential, E(Re k/Re (k+1)) (k = 0,1,2,. . .), values ranging from 0.60 to 0.85 V.51,52 The possibility of an occurrence of the ground state ET process was verified using the following equation: DGET = E(Ox/Ox +) E(Re/Re ) 0.06
(1)
For the dye–rGO system: (0.83 (0.60) 0.06) Z DGET Z (0.83 (0.85) 0.06) i.e. 1.37 Z DGET Z 1.62 The positive change in the Gibb’s free energy (DGET) thermodynamically forbids the ground state electron transfer from 1-Ap to rGO. However, excited state ET from the S1 state of 1-Ap to rGO is thermodynamically allowed. The energy of the S1 state (E00) for 1-Ap is 3.14 eV as calculated from the intersection of the normalized absorption and the emission spectra (ESI,† Fig. S8). The Gibb’s free energy change for the excited state ET (DGPET = DGET E00; (1.62 3.14) Z DGPET Z (1.37 3.14); 1.52 Z DGPET Z 1.77) yields negative value and hence photoinduced electron transfer (PET) is favorable. These results suggest that (1Ap) + and (rGO) are formed as a result of electron transfer from photoexcited 1-Ap to ground state rGO as shown in Scheme 1.
Scheme 1
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Kinetics of intermolecular PET
Theoretical calculations
The intermolecular dynamic quenching phenomenon follows the relation
A detailed theoretical investigation was performed to learn more about the interaction between the dye and graphene. To start with, the structure of 1-Ap was optimized in the ground state and the results are presented in the ESI† (Fig. S1–S3). In spite of the presence of an amine group, the ground state optimized structure of 1-Ap is planar like pyrene. The vertical transition energies of 1-Ap were calculated and found to match well with the absorption spectra (ESI,† Table S1). The HOMO and LUMO of 1-Ap are shown in Fig. 5. It is apparent from this figure that the amine group contributes to both the HOMO and the LUMO of 1-Ap. But the electron density on the nitrogen atom is sufficiently reduced in the LUMO in comparison to the HOMO due to the electron donating property of the amine group.
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t0 ¼ 1 þ k q t0 ½rGO t
(2)
where, kq is the (dynamic) quenching rate constant. A value of 1.6 1016 M1 s1 was obtained for the kq by fitting the data in Fig. 4b using eqn (2) and assuming the average size of the rGO sheet is 100 nm (ESI,† Fig. S9). Bimolecular collisions are responsible for the dynamic quenching in a system of small molecules and kq is in the range of the diffusion rate, kd (= 8RT/Zr), where R is the ideal gas constant, T is the temperature, eta is the coefficient of viscosity (Z) and rho is the density of the solvent (r), which is 109 to 1010 M1 s1 for water.23 In the case of collisions between rGO and 1-Ap, rGO can be assumed to be at rest because of its large mass and thereby reduce the kd value by a factor of 2. Nevertheless, the rGO sheet can simultaneously interact with a large number (N) of 1-Ap molecules. Therefore, the effective rate constant in a graphene-small molecule system becomes kq = N (kd/2). Comparing this expression with the experimentally observed kq, the value of N turns out to be 3.26 106. Such a high value of N shows the great quenching capability of a rGO sheet compared to a normal quencher molecule and thus suggests that multiple electron transfer is occurring.
Fig. 5
HOMO and LUMO of 1-Ap.
Fig. 6
Frontier molecular orbitals of the 1-Ap–GO system. Black: nitrogen; gray: carbon; white: hydrogen.
Fig. 7
Partial density of state of the 1-Ap–rGO system.
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A small 2D carbon cluster (C24H12) was used as a model for graphene during the study of its interaction with the dye to improve the ease of calculation. The ground state optimized structure of 1-Ap in the presence of graphene shows the nearly vertical inclination of 1-Ap on the model graphene surface suggesting a very poor p–p type interaction between them. Frontier HOMO and LUMO levels of 1-Ap and rGO system are depicted in Fig. 6. The HOMO is fully localized onto the 1-Ap entity, whereas the LUMO is solely restricted on the graphene surface. This provides a clear hint for the formation of the charge-separated state of (1-Ap)+–rGO following photo irradiation. A partial density of state (PDOS) was determined for the 1-Ap–rGO system and is presented in Fig. 7. At the valance band edge (B5 eV) of the system, the electron density is fully distributed over 1-Ap, but when the whole system is excited to the conduction band via light illumination, the electron density at the edge of the conduction band (B2.8 eV) is mostly transferred to the graphene skeleton, suggesting photoinduced electron transfer from dye to graphene.
Conclusions The fluorescence of 1-Ap is found to be quenched by rGO without affecting its absorption spectra. Steady state and time-resolved measurements rule out the possibility of ground state complex formation and confirm the dynamic nature of the quenching due to the electron transfer. The negative value of the Gibb’s free energy change further supports the ET from excited 1-Ap to rGO. The estimated value of the quenching constant (kq) is very high compared to the diffusion rate (kd), suggesting the occurrence of multiple ET. Quantum chemical calculations forbid complex formation via p–p interactions in the 1-Ap and rGO system. Moreover, the calculations of frontier molecular orbitals and PDOS for the system reveal that electron transfer is favorable from 1-Ap to rGO under illumination.
Acknowledgements Financial support from Indian Institute of Technology Mandi (Grant No. IITM/SG/SUG/004) aided the carrying out of this work. HC is thankful to IIT Mandi for his fellowship.
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