Synthesis, Solvatochromism and Fluorescence Quenching Studies of Naphthalene Diimide Dye by Nano graphene oxide

A naphthalene diimide dye with two side amine arm was prepared. Uv–Vis and fluorescence spectroscopic techniques are used for their photophysical and solvatochromic characteristics in different solvents. The Lippert-Mataga plot for naphthalene diimide demonstrated a negative linear dependence by increasing polarity. Results showed naphthalene diimide is more polar in the ground than in the excited state. A quenching study was conducted for interacting the naphthalene diimide as a fluorophore and graphene oxide as a quencher. Fluorescence quenching-based platforms in nanoscale have been used in sensing systems. Raman, FTIR, Uv–Vis and fluorescence spectroscopic techniques were used to study the quenching mechanism. The results indicated that graphene plays an effective quencher against the naphthalene diimide, with a quenching efficiency 91%. The Stern–Volmer analysis results show a mix of static and dynamic quenching mechanisms. The binding constant of the quencher-fluorophore and the number of binding sites have been reported. Thermodynamic parameters of their interaction were evaluated. The negative values of the Gibbs free energy confirm that the complexation process is spontaneous. Meanwhile, the positive entropy value confirms that the favorable pathway process.


Introduction
The two-dimensional, flat-structured, layered graphene sheets have extensively been applied in sensor detection due to their unique optical and physicochemical properties.Their excellent electrical conductivity, fast electron mobility, tunable optical properties, high mechanical strength, and functionalization capability make them an efficient nanomaterial for developing different sensor categories [1][2][3][4][5][6][7][8].Reported quenching mechanisms of graphene oxide as a quencher are based on one of these models, the photoinduced electron transfer (PET) process or Förster resonance energy transfer (FRET).The fluorophore directly affected by the graphene quencher makes it suitable for optical sensing platforms [9][10][11][12][13][14].When GO and fluorophore are close, the energy or excited electron will transfer from fluorophore to GO, reducing the fluorescence signal.
Among fluorescence dyes, naphthalene diimides have been applied in different areas.They are used in supramolecular chemistry, sensors, laser diodes, molecular switching devices, and automotive paint [15,16].Their excellent stabilities, such as photo, thermal and chemical stability, attracted much attention.Their derivatives showed high fluorescence efficiency and good conductivity [17,18].These properties make it to be a good candidate for sensitive fluorescent sensors.Easy modification of the fluorescence core achieved through imide or core position resulted in a desirable design for specific interaction and an efficient fluorescent probe [18].The design and development of novel platforms based on graphene as the energy acceptor have been a remarkable success in recent years.Our team is developing the design and application of fluorochrome dyes for probe applications [19][20][21][22].Recently we worked on the effect of graphene oxide as a quencher on the different naphthalimide derivatives.Studies on the quenching mechanism between graphene oxide and naphthalene diimide dyes have yet to be reported.
This work investigated the synthesis and solvatochromism properties of a water-soluble naphthalene diimide dye.The mechanism of quenching was studied by Uv-Vis and fluorescence spectroscopic techniques.Also, the thermodynamic pathway of interactions was investigated.The quenching parameters were obtained, and involved mechanisms were reported.Schematic of a naphthalene diimide (NDN2) quenching by graphene oxide (GO) is illustrated in Scheme 1.

Apparatus
1 HNMR spectra were recorded with Brucker spectrophotometer at 500 MHz (solvent: DMSO).FTIR spectra were recorded on a SPECTRUM ONE spectrometer.A Perkin-Elmer LS55 fluorescence spectrophotometer was used for all fluorescence measurements.UV-VIS absorption spectra were measured on a CECIL-CE9200 spectrophotometer.The atomic force microscope (AFM) images were obtained on operating in tapping mode.

Photophysical and Solvatochromic Properties
NDN2 is closely related to the 1,8-naphthalimides, which their systems photophysical properties strongly depend on the dicarboximide and amino-substituted groups.The amino-substituted groups with the ethylene space show a photoinduced solid charge transfer process (PET) to carbonyl groups.The electron charge transfer between the donor-acceptor (the imide moiety of the chromophoric core and the carbonyl groups) and resulting polarization is responsible for the spectral characteristics of these derivatives [15,16].
Figure 1 shows the absorption and emission spectra of NDN2 (10-5 M) in H 2 O.The dye shows a yellow-green color (λ a = 280,437 nm).The wavelength band around 420 nm (ε = 36,700 L mol −1 cm −1 ) confirms a charge transfer (CT) band due to (π → π*) transition of the S0 → S1 transition [23].The dye emission was shifted to the visible region with a maximum (λ em = 537 nm) with an intense yellow-green emission and a significant 100 nm stock shift.Solvent polarity and the local environment profoundly affect the emission spectral properties of fluorophores.Several organic solvents with different dielectric constants were chosen to study the solvent effect on the photophysical properties of the NDN2.
Figure 2 shows the absorption and emission spectra of NDN2 in different media.The relevant spectral data (absorption (λ a ) and fluorescence (λ e ) maxima, stokes shift (υ a -υ e ), solvent polarities and full width at half maximum (FWHM) of emission bands) are summarized in Table 1; the solvents are listed in order of their increasing the polarity.
The absorption spectrum was known to be less insensitive to the polarity of the environment [23].As shown in Fig. 2a, b, only a few redshifts were observed for absorption and emission bands from 437 nm in 1-BuOH to 433 nm in H 2 O and 518 to 536.5 nm, respectively.As shown in Fig. 2 insets, the absorption and emission vs solvent polarity slope represent the bond's sensitivity against solvent polarity, which is remarkedly different.Generally, absorption spectra are less sensitive to solvent polarity changes because the molecule in the ground and excited state is exposed to the same local environment.Light absorption occurs in about 10-15 s, a time too fast for the movement of the fluorophore.In contrast, the emitting fluorophore is exposed to solvent molecules oriented around the polarized excited state [23].
In general, the observation of solvatochromic shifts in absorption and emission indicates that these molecules' excited state dipole moments are more significant than ground states and involved with "charge-transfer transition" or solvent relaxation.Typically, such transitions are associated with considerable solvent reorganization energies, increasing nonradiative decay rates [23].Increasing the solvent polarity (Fig. 3) decreased the FWHM of NDN2 emission.Although the data in Table 1 reveal no clear correlation, there is still a slight trend.Energy changes with increasing solvent polarity are less affected by solvent relaxation.
A range of solvents with different polarities was chosen for a deeper understanding.
It is found that the spectral changes by solvent polarity can be stated in terms of the dipole moments in the ground and excited states.It has been proven that if the changes in fluorophore dipole moment are more extensive, it shows more sensitivity to solvent polarity.This effect can be estimated from a Lippert-Mataga plot (Eq. 1) [23][24][25], essentially a plot of the Stokes shift versus the solvent polarity.∆ν is the difference in the maximum absorption and emission wavelengths.

Scheme 2 Synthesis of NDN2
Herein µe-µg is the difference between the dipole moments of the excited and the ground states, respectively (∆µ), h is Plank's constant, c is the light velocity, and α is the radius of the Onsager cavity around the fluorophore.The ε and n are the solvent dielectric constant and refraction index, respectively, as ∆f (Eq.2).∆f called orientation polarizability.For a more detailed study, Lippert-Mataga was constructed.It shows the dependence of the fluorophore's stokes shift on the solvent's orientational polarizability (∆f).The Lippert-Mataga plot for NDN2 was illustrated in Fig. 4.This plot indicated that NDN2 demonstrated a negative linear dependence by increasing polarity (slope ∆µ = -586.33).
As can be seen in Fig. 4, increasing solvent polarity resulted in a decrease in energy.A negative slope confirms these compounds have more polar character in the ground state than in the excited state μ g > μ ex .On the other hand, a small solvatochromic can be observed in the excited state.It (1) results in less solvent reorientation around the exited state.
As shown in Fig. 4b, c, the Stokes shift of NDN2 was illustrated individually in a series of protic and aprotic solvents.
It shows a linear response to the solvent polarity and correlates with a decrease in the dipole moment at exited state.Lippert-Mataga plot slope for protic solvents have remarkedly smaller than apprentices and confirms the low interaction of exited state with solvent.Although these changes are insignificant and do not show a complete linear correlation, the results confirm the expected low polarity of NDN2 symmetric structure in the excited state.

Characterization of GO
TEM, AFM and UV-vis techniques were applied to characterize the used commercial GO, as shown in Fig. 5a-c.TEM image shows the planar morphology of GO.As shown in Fig. 5c, maximum absorption at 229 nm confirms the π-π* transition of aromatic C = C bonds.Also, a shoulder around 300 nm is related to the n-π* transition of C = O bonds.AFM technique was conducted to measure the thickness of the GO flakes, whereas the height of GO was 2.00 nm.All these results showed that the used GO was indeed a single-layer sheet.

Absorption Characteristics of NDN2 /GO
Figure 6 shows the absorption spectra of dye in the presence of various concentrations of GO.The intensities of maximum absorption at 435 nm increase by adding GO.The shape and shifts of UV − Vis absorption spectrum peaks can help to verify the involved interaction mechanisms.The UV − Vis absorption spectrum of the fluorophore would change during the static quenching due to the formation of a complex between the ground state of the fluorophore and the quencher, while no absorption spectrum change should be observed in the dynamic quenching [26].As can be seen in (Fig. 6), the peak at 364 nm decreases and a new peak grows up at 430 nm during addition of GO. Results clearly confirm the GO/ NGL complex formation, which indicate quenching was followed a static process in the ground state of chromophore.Our previous work obtained the same results for naphthalimide [21,22].
Raman analysis was also conducted for the absorption of NDN2 and GO, shown in Fig. 7.The Raman spectra of GO show GO's characterized D and G bands at (1357 cm − 1 ) and (1604 cm − 1 ), respectively, approving the presence of GO.The GO spectrum, after interaction with NDN2 remarkable intensity reduction at the D and G bands without any shift.These results can be further evidence of the interaction between GO and NDN2.

Quenching Mechanism
The fluorescence emission spectra of NDN2 are illustrated in Fig. 8a).It shows a maximum intensity at 538 nm, gradually decreasing with increasing GO concentration.
Changes indicate that GO act as a quencher of the naphthalene diimide fluorophore.Also, GO has no fluorescence at the NDN2 excitation wavelength.The observed quenching was due to the interaction between NDN2 and GO, not an inner filter effect or reabsorption.As previously reported in the literature, GO was the oxidized form of graphene with the carboxylic acid group, hydroxyl and epoxy groups [27,28].Therefore, GO could have π-π stacking interaction with aromatic rings of aromatic molecules, herein, naphthalene diimide dye.The quenching efficiency, defined as Ƞ = (F0-F)/F0 × 100, for which a high value of (91%), was obtained for the NDN2.
It was found that quenching goes through either static or dynamic interaction mechanisms [29][30][31].When a complex formation between an emissive molecule and a quencher occurs, it is called a Static quenching.The resulting complex has no emission.Therefore, its intensity gradually decreases by adding a quencher.Dynamic quenching is called collisional quenching because a collision with a quencher causes a loss of emission.The Stern-Volmer equation (Eq. 3) was used to study the dynamic quenching mechanism.F0 and F are the molecule's fluorescence intensity in the quencher's absence and presence, respectively.K sv is the binding constant of the quencher-fluorophore, [Q] is the concentration of the quencher; Ka is the rate constant of the bimolecular quenching and τ the average lifetime of fluorophore in the absence of a quencher.Figure 9 illustrate the Stern-Volmer plot.Results showed that dynamic quenching was not the primary quenching mechanism of NDN2.As can be seen, the dependence of F0/F on [Q] strayed from linearity, confirmed there is not only a dynamic quenching.
Generally, dynamic quenching involves the collision followed by the formation of a transient complex between an excited-state fluorophore and quencher.Recording the effect of temperature on the quenching efficiency helps recognize which mechanisms are involved.It is found that the higher temperature increases the possibility of collision.It resulted in a higher quenching efficiency, known as dynamic quenching.At the same time, less quenching was observed at a lower temperature [28,32].The fluorescence emission results of naphthalene diimide in the presence of GO at different temperatures (288, 298 and 308 ºk) was illustrated in Fig. 9. Based on these results it seems the quenching follows a static route as absorption specra of GO/NDN confirmed.
(3) The resulting curves in Fig. 9 show a nonlinear behavior with an upward curvature.It cannot be accurately fitted in the linear Stern-Volmer equation (Eq.3).The modified equation (Eq.4) has been introduced for a single species undergoing both dynamic quenching and ground-state complex static quenching [33].K sv and Ka need to be determined in an analysis of the nonlinear steady-state fluorescence quenching model [34].The measured K sv , K a and R 2 are evaluated from Fig. 9 and given in Table 2.
The quenching of NDN2 could occur through the following possible processes, an energy transfer or electron transfer.For an energy transfer via the FRET mechanism, an overlap between the absorption spectra of a quencher (acceptor) and the emission spectra of emissive species (4) (donor) is needed.However, there is no overlap between the absorption spectra of GO (acceptor) and emission spectra of NDN2(donor) (Fig. 10a).Therefore, energy transfer between NDN2 and GO was impossible via the FRET mechanism.The fluorescence quenching of NDN2 and GO seems to follow an electron transfer process.Further study was done to study the energy levels of the ground and the excited state of NDN2.Cyclic voltammetry and Uv-Vis spectra were used to calculate the band gap energy of GO and NDN2.The obtained energies are illustrated in Fig. 10b.-4.00 eV and -6.91 eV were obtained for LUMO and HOMO energy levels of NDN2, respectively.Therefore, the calculated band gap, ΔΕ, is -2.9 eV for NDN2.The reported valence bound of GO is to be -4.7 eV.Therefore, as shown in Fig. 10b, electron transfer is possible from the excited state of the NDN2 to the GO   plates.NDN2 acts as a donor in the exited state to the graphene surface as an acceptor.The same behavior for GO and coumarin dye was reported previously [34].

Thermodynamic Parameters
Generally, the binding constants are affected by temperature.Therefore, we follow the quenching process in different temperatures (288,298, and 308 k).From the value of the stability constant at different temperatures, the enthalpy can be calculated from the Van't Hoff equation (Eq.5) based on the relationship between the binding constant (k) and temperature (T).
R is the gas constant, T is the temperature, and K is the binding constant [33].The enthalpy (ΔH) of the complexation process was obtained from the slope of the plot of log K vs 1/T using the graphical representation of Van't Hoff's equation, and the entropy (ΔS) could then be calculated as follows using Eq. ( 6).
The thermodynamic parameters of complex formation between NDN2 and GO are included in Table 3.
It seems that the enthalpy (ΔH) did not vary significantly over the studied temperature range, and cab assume it is constant.As can be seen, all the thermodynamic parameters are negative.The negative ΔG values confirm a spontaneous complexation process.The positive ΔS is evidence of a favorable entropy change during the complexation process.

Conclusions
The solvatochromic and photophysical properties of synthesized naphthalene diimide dye (NDN2) have been studied.The NDN2 behaves as a weak solvatochromic dye.It shows a slight bathochromic shift in emission with increasing solvent polarity (λ em = 518 and 536.5 nm in 1-BuOH and H 2 O, respectively).Its solvatochromic behavior may be explained by the significant change of its dipole moment between excited and ground states.The Lippert-Mataga plot for NDN2 demonstrated a negative linear dependence by increasing polarity (slope ∆µ = -586.33).Results showed NDN2 is more polar in the ground than in the excited state μg > μex.
GO efficiently interacts with naphthalene diimide dye (NDN2) via fluorescence quenching.Based on the Stern − Volmer plot and fluorescence study between NDN2 and GO, the primary quenching mechanism was suggested as a combined static-dynamic quenching.These experimental and theoretical results are potentially important in (5) ΔG = −2.303RT log K (6) ΔG = ΔH − TΔS understanding the interaction mechanism between graphene and naphthalene diimide dye.They would be helpful guides for designing and developing efficient graphene-based sensors.The negative values of the ΔG confirm that the complexation process is spontaneous, and the positive entropy value confirms the favorable pathway process.

Fig. 2 a
Fig. 2 a Absorption and b emission spectra of NDN2 in organic solvents.The insets show the maximum absorption and emission variation by solvent polarity

Fig. 10 a
Fig. 10 a Absorption spectra of GO and emission spectra of NDN2.b Energy levels of NDN2 and GO

Table 1
Spectral properties of

Table 2
Quenching parameters of graphene oxide and NDN2 at different temperatures.Temperature (K)