Multiscale Approach to the Structure and Spectra of Nile Red Adsorbed on Polystyrene Nanoparticles

Multiscale Approach to the Structure and Spectra of Nile Red Adsorbed on Polystyrene

Nanoparticles

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2012 IOP Conf. Ser.: Mater. Sci. Eng. 38 012036

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Multiscale Approach to the Structure and Spectra of Nile Red Adsorbed on Polystyrene Nanoparticles

Alexandra Freidzon, Vyacheslav Tikhomirov, Alexey Odinokov, Alexander Bagaturyants 1 Photochemistry Center, Russian Academy of Sciences, Moscow, Russia

e-mail: [email protected]

Abstract. The structure and absorption spectra of the Nile Red (NR) dye adsorbed on the surface of a polystyrene (PS) nanoparticle is studied by DFT/TDDFT based on the results of previous molecular dynamics (MD) simulations. From NR/PS surface structures obtained from MD simulations, relatively small clusters were constructed composed of the adsorbed NR molecule and the nearest PS fragments. These clusters were treated either explicitly by DFT/TDDFT or using the effective fragment potentials (EFP) model. The absorption and emission spectra of the NR/PS system calculated using the EFP model are very close to those calculated with the explicit environment, so that the EFP model can be recommended for use in multiple large-scale calculations. The calculated positions of the absorption and fluorescence bands are in good agreement with experiment.

1. Introduction Nile Red (NR, Scheme 1) is a lipophilic dye soluble in organic solvents and alcohols and poorly soluble in water. NR is a good solvatofluorochromic indicator, because its absorption and fluorescence spectra are highly sensitive to the environment.1–3 Polystyrene (PS) is used as a matrix for the hydrophobic NR in optical chemical sensors. 4–6

Scheme 1

O

N

O N

The NR solvatochromism and dual fluorescence observed experimentally is commonly explained

by the charge-transfer character of its excited state, which, probably, involves a twist of the diethylamino group.1, 7 In our recent study, it has been shown that the NR S1 excited-state potential energy surface has two minima: one corresponding to a locally excited (LE) state with a planar structure and another one corresponding to a twisted intramolecular charge transfer (TICT) state. 8

Four heteroatoms of the NR molecule can serve as potential hydrogen bond acceptors in its interaction with protic molecules. With aprotic molecules, NR can form intermolecular complexes through electrostatic and dispersion interactions.

International Conference on Functional Materials and Nanotechnologies (FM&NT2012) IOP PublishingIOP Conf. Series: Materials Science and Engineering 38 (2012) 012036 doi:10.1088/1757-899X/38/1/012036

Published under licence by IOP Publishing Ltd 1

mailto:[email protected]

PS is an irregular carbon-chain nonconjugated polymer, and its nanoparticles are used as hydrophobic matrices for indicator dyes. Previously, we have studied the adsorption of the NR molecule on a PS film or globule by molecular dynamics (MD).9 No preferential orientation of the dye in the polymer surface has been found. This suggests that the absorption and emission spectra of NR on the polymer surface should exhibit an environment-induced shift and a noticeable inhomogeneous broadening.

The goal of this study was to demonstrate the possibility of simulating the absorption and emission spectra of NR adsorbed on the PS surface using the effective fragment potential (EFP) model of environment and the TDDFT method for the transition energies.10, 11

2. Computational Procedure We modelled the fragments (links) of PS chains nearest to the NR molecule in the obtained surface structures by isolated toluene molecules. We suggested that the response of the local environment of the chromophore to excitation is fully electronic, while the nuclear displacements of the polymer links are negligible in the solid polymer matrix.

The details of the MD simulation of NR adsorbed on the PS surface were described in Ref.9

2.1. Constructing a cluster from a snapshot of the MD trajectory. A snapshot of an equilibrium part of an MD trajectory was taken (Figure). The chromophore (shown in balls-and-sticks) and the neighbouring C6H5CH units (shown by thick lines) were selected. The unit was included in the cluster if the nearest contact between the chromophore and the monomer unit did not exceed 4 Å. The selected C6H5CH units were completed to C6H5CH3 (toluene) molecules with a standard Caliphatic–H bond length of 1.09 Å.

Figure 1. Constructing a cluster from a snapshot of the MD trajectory. The chromophore is shown by balls-and-sticks, thick lines denote PS links adjacent to the chromophore, and wire lines show the rest

of the polymeric nanoparticle.


2

Next, the EFPs corresponding to toluene molecules were placed instead of the C6H5CH3 units into their positions. A special care was taken to ensure that the distances between the effective fragments were no less than 1 Å.

2.1.1. Generation of an effective fragment potential. The EFP of toluene was generated using the MAKEFP procedure implemented in the GAMESS-US program package.12, 13 The geometry of the effective fragments was optimized by B3LYP/PC1. Only monopoles, multipoles, and local static polarisable points generated by the MAKEFP procedure were used in the further TDDFT calculations, while the other data were discarded.

2.2. Quantum-chemical calculations. All the ground-state structures were optimized by PBE0/6-31G(d,p), and the excited-state structures were optimized by TD-PBE0/6-31G(d,p) using GAMESS-US. We considered both NR and its complex with methanol coordinated to the carbonyl oxygen NR…MeOH on the surface.

Several clusters were treated completely by quantum mechanics (denoted by QM PS). The environment (toluene molecules) was frozen, and the dye structure was optimized by PBE0/6-31G(d,p). The difference in the dye structure optimized in the frozen environment and in vacuo was negligible. Later on, this allowed us to replace the dye structure obtained by molecular dynamics with that optimized in vacuo by PBE0/6-31G(d,p) in the clusters with effective fragments.

The absorption spectrum of the cluster treated by quantum mechanics was calculated by TD-PBE0/6-31G(d,p) and compared with the absorption spectrum of the same cluster with toluene molecules replaced by the effective fragments (denoted by EFP PS). In addition, the absorption spectrum of the chromophore was calculated by TD-PBE0/6-31G(d,p) in the polarisable dielectric continuum environment (D-PCM) using the Firefly program.14

2.3. Calculations of absorption and emission spectra. The absorption spectrum was constructed by the following procedure: the spectra were calculated

for 40 clusters constructed as described above. Each spectrum was presented as a Gaussian peak with full width at half maximum (FWHM) equal to 0.29 eV (homogeneous broadening)15 and area corresponding to its oscillator strength. Finally, the spectra were summed and normalized to the average oscillator strength.

Our previous calculations8 revealed two excited-state structures of NR: a planar locally excited (LE) structure and a structure with twisted amino group called twisted intramolecular charge transfer (TICT) state. These (fully optimized by TD-PBE0/6-31G(d,p)) structures were also used for calculating the emission spectra in the clusters with effective fragments. For the emission spectra, only average positions of the bands and standard deviation of the band maxima were calculated.

3. Results and Discussion The absorption spectra of the clusters with NR and NR–methanol complex (see Figure 2) treated fully by quantum mechanics are compared with the absorption spectra of the same clusters with toluene


3

molecules replaced by the effective fragments. The difference is rather small

(

a b

Figure 2. Example clusters: Nile Red (a) and Nile Red–methanol complex (b) surrounded by EFPs mimicking the polystyrene units.

Table 1). The effect of the polarisable dielectric continuum environment is also similar to either that of QM PS or that of EFP PS. Therefore, the local effects of electrostatics included through the point charges, multipoles, and polarisabilities are substantially the same as those included explicitly or those caused by the dielectric continuum. The calculated absorption spectrum agrees well with the experimental spectrum of NR on PS nanoparticles.5, 6

a b

Figure 2. Example clusters: Nile Red (a) and Nile Red–methanol complex (b) surrounded by EFPs mimicking the polystyrene units.

Table 1. Calculated absorption spectra of NR and NR…MeOH in vacuo and in PS environment

free QM EFP D-PCM Exp.5, 6 eV nm eV nm eV nm eV nm eV nm

NR 2.76 449 2.63–2.66 467–472 2.69 462 2.67 465 2.59 480 NR…MeOH 2.71 458 2.60–2.62 473–477 2.69 462 2.64 470

The absorption spectrum of NR was constructed as described in the previous section. The resulting spectrum is shown in Figure 3. The small shift in the absorption band caused by the coordination of


4

methanol to the dye is completely concealed by the environment-induced shift and inhomogeneous broadening.

Figure 3. Calculated absorption spectrum of Nile Red in polystyrene including homogeneous and

inhomogeneous broadening.

For the emission spectra, no calculated homogeneous broadening data is available to date. Therefore, only the average transition energy and standard deviation of the sample were calculated (Table 3). One can see that the LE fluorescence exhibits less scatter than the TICT fluorescence. Large dipole moment of the twisted structure in the S1 excited state8 induces large dipole moments in the environment, which, in their turn, affect the transition energy in the TICT state. This qualitatively agrees with the fact that the observed TICT emission bands are weak and broad. One can also see that the TICT state is more sensitive to complexation with MeOH than the LE state: even inhomogeneously broadened, the TICT fluorescence band exhibits a more pronounced shift upon complexation.

Table 3. Calculated emission spectra of NR and NR…MeOH in vacuo and in PS environment

free EFPa eV nm (Δλ) eV (FWHMb) nm (Δλ)

NR LE 2.60 477 2.49 (0.12) 499 NR TICT 1.92 646 1.88 (0.35) 660

NR…MeOH LE 2.57 483 6 2.47 (0.13) 503 4 NR…MeOH TICT 1.77 701 55 1.77 (0.45) 700 40

a EFP PS data for NR are average of 30 clusters; for NR…MeOH, of 10 clusters. b FWHM due to inhomogeneous broadening: 4lnσ=FWHM ; σ is the standard deviation of the sample.

The agreement with the experimental emission data, however, is only semi-quantitative: the experimental emission spectra of NR in NR/PS nanoparticles strongly depend on the particle size. These spectra were recorded in polar solvents (water or alcohols). As a result, the emission spectrum was governed either by the NR molecules adsorbed on the surface (in small nanoparticles) or by those in the bulk (in large particles). The surface NR molecules can interact with the polar solvent, while the bulk ones interact only with nonpolar PS. The experimental emission spectrum of bulk NR


5

qualitatively agrees with the calculated LE emission spectrum, while the emission spectrum of NR on the surface qualitatively agrees with the calculated TICT emission spectrum.

4. Conclusions The indicator form of the dye is the TICT structure. Its emission spectrum is sensitive to the interaction with analytes because of its high dipole moment. The nearest environment of the chromophore can be described by EFPs to a reasonable accuracy. The distribution of transition energies and, thus, the shape and inhomogeneous broadening of absorption and emission bands can be obtained by sampling of the calculated MD trajectories.

5. Acknowledgments This work was supported by the Russian Foundation for Basic Research, project no. 12-03-01103, and Russian Ministry of Education and Science, state contract no. 16.523.11.3004. The calculations were performed using the facilities of the Joint Supercomputer Center of the Russian Academy of Sciences.

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[13] Gordon M S, Schmidt, M.W. in Theory and Applications of Computational Chemistry, the First Forty Years; Eds C E Dykstra, G Frenking, K S Kim, G E Scuseria 2005 (Amsterdam: Elsevier) p 1167.

[14] Granovsky A A Firefly version 8.0.0, http://classic.chem.msu.su/gran/gamess/index.html. [15] The procedure for calculating the homogeneous broadening will be described elsewhere.


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Multiscale Approach to the Structure and Spectra of Nile Red Adsorbed on Polystyrene Nanoparticles

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