Morphology directing synthesis of 1-pyrene carboxaldehyde microstructures and their photo physical properties

Abstract

Morphologically interesting organic microstructures from 1-pyrene carboxaldehyde (1-PyCHO) have been synthesized using sodium dodecyl sulphate (SDS) as a morphology directing agent. Here we have presented a re-precipitation method to synthesize rod, bar and rectangular plate shaped microstructures from 1-PyCHO. The morphology of the materials has been characterized using optical microscopy, SEM and XRD measurements. UV-Vis absorption, steady state and time resolved fluorescence emission techniques are used to study the photophysical behaviour of the aggregated structures of 1-PyCHO. Computation of the second order Fukui parameter as a local reactivity descriptor (LRD) on each atomic centre of 1-PyCHO suggests that two nearest 1-PyCHO molecules are arranged in face to face slipped conformations in their aggregated structures. Our computational results are in conformity with the single crystal data of 1-PyCHO and this is the first time that LRD is used to explain the stacking pattern of similar kinds of molecules.

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Introduction

In comparison to their inorganic analogues, nanometer and micrometer size crystals based on functional organic molecules offer large variability in their composition and physical properties. Their chemical and photo physical stability differ widely compared to that of the isolated molecules and display characteristic optical and optoelectronic properties. Therefore, they are of considerable interest for various potential applications in the field of biological sensors, photocatalysts, OLEDs, optical devices etc.¹ Most of the organic choromophores, which are highly fluorescent in solution at low concentration, show a drastic decrease of their emission efficiency in the solid state. This behaviour is generally attributed to interactions that provide non-radiative decay routes, in particular, the frequently encountered plane to plane staking of fluorophores.

The size dependence of organic crystals has not been investigated as much as that of inorganic crystals. The strong effect of electron confinement on electron–hole pairs in all three directions results in the size-tunable optoelectronic properties of semiconducting quantum dots.² But, this is not expected in organic molecular crystals (OMCs), because of the small radius of the Frenkel exiton.³ The primary differences between inorganic and organic semi-conductors are in the band width and the degree of orbital overlap. In the case of OMCs, the electronic and optical properties are fundamentally different from those of inorganic semi-conductors, because of weak van der Waals intermolecular forces.⁴ The controlling of size, shape and hence the properties of OMCs is still a challenge and an important aspect in the development of materials science.

Much effort has been devoted to synthesize organic nano/micro particles having various size and shapes. This includes zero dimensional (0-D) spherical or tetrahedral quantum dots,⁵ one-dimensional (1-D) nanorods and wires from small organic compounds⁶ and two-dimensional (2-D) nanoplates,⁷ nanoribbons and nanotubes,⁸ microcapsules,⁹ organic nano flowers,¹⁰ sub-microtubes¹¹ etc. Various techniques were developed to prepare organic nano/micro particles, such as reprecipitation,¹² physical vapour deposition,¹³ microemulsion,¹⁴ ultra-sonication,¹⁵ template method,¹⁶ self-organization¹⁷ etc. Of the above-mentioned methods, reprecipitation is one of the most favored routes towards the cost-effective large-scale production of nano/micro building blocks. Reprecipitation is a rapid injection of micro amounts of the solution in a good solvent, into macro amounts of poor solvent. In this process, sudden changes of environment for organic molecules induce precipitation.

Pyrene and its derivatives are widely used as environmental probes due to the polarity sensitive vibronic emission of pyrene and this stimulated researchers to use pyrene and its derivatives as a probe for microenvironments like micelles, vesicles and other biologically interesting microenvironments.¹⁸ Though there are plenty of reports on the emission behavior of pyrene and 1-PyCHO monomer in different solvent environments, little work has been done to study the morphology of aggregated micro/nano crystals of pyrene and its derivatives like 1-PyCHO and also their emission behavior in aggregated forms. Zhang et al.^16a reported the synthesis of pyrene nanorods in SDS miceller medium and Sheng et al.^18a reported the ratiometric fluorescence behavior of 1-PyCHO in water. These works on pyrene and its derivatives motivated us to choose 1-PyCHO for the morphology directing synthesis of 1-PyCHO microstructures. Here we have reported the synthesis of rod, bar and rectangular plate shaped microstructures of 1-PyCHO using the re-precipitation method and SDS as the morphology directing agent. The morphologies and the structures of the as prepared microstructures are studied by optical microscope, field emission scanning electron microscope (FESEM) and X-ray diffraction study. Photo physical properties of the aqueous dispersed microstructures of 1-PyCHO are investigated using UV-vis absorption, steady state as well as time resolved fluorescence emission measurements. In order to understand the possible mode of arrangements of two 1-PyCHO molecules, we have computed the second order Fukui parameter as local reactivity descriptors (LRD) and our computational study revealed that the most favorable orientation of two 1-PyCHO molecules are the face to face slipped conformation. Our computational study is in conformity with the reported single crystal structure of 1-PyCHO,¹⁹ where 1-PyCHO molecules are present in a face to face slipped conformation in their triclinic unit cell.

Experimental

Materials

1-PyCHO and pyrene were purchased from Sigma-Aldrich Chemical Corp. Sodium dodecyl sulphate (SDS) was purchased from Merck India Ltd. Ethanol was obtained from S. D. Fine-Chem Ltd. All the chemicals were of analytical grade. Ethanol was distilled before use and the purity was checked spectrophotometrically. SDS was recrystallized from 1 : 1 ethanol–water mixture. Triply distilled deionized water was used throughout the experiments.

Synthesis of 1-pyrene carboxaldehyde microparticles

The microstructures of 1-PyCHO were synthesized by the re-precipitation method where SDS was used as a soft template. In a typical preparation, small volumes of 1-PyCHO (0.1 M) in EtOH were injected into 10 mL of continuously stirred aqueous SDS at room temperature (25 °C). The volume of 1-PyCHO and concentration of SDS were varied to synthesize different shaped 1-PyCHO microstructures.

1-PyCHO micro-rod (sample-a)

The rod shaped 1-PyCHO microcrystals were prepared by adding 0.1 mL 0.1 M 1-PyCHO in EtOH into 10 mL 10 mM SDS with continuous stirring. After 5 min of stirring the solution was kept undisturbed overnight at room temperature before characterization. The concentration of SDS and 1-PyCHO in the resulting solution was 9.9 mM and 0.99 mM respectively.

1-PyCHO micro-bar (sample-b)

0.5 mL 0.1 M 1-PyCHO in EtOH was injected into 10 mL 10 mM SDS with constant stirring. After 5 min of stirring the solution was aged overnight at room temperature before characterization. The concentration of SDS and 1-PyCHO in the resulting solution was 9.9 mM and 4.70 mM respectively.

1-PyCHO rectangular micro-plate (sample-c)

0.5 mL 0.1 M 1-PyCHO in EtOH was added to 10 mL 50 mM SDS with constant stirring for 5 min. The as-prepared solution was left standing overnight before characterization. The concentration of SDS and 1-PyCHO in the resulting solution was 47.60 mM and 4.70 mM respectively.

Characterization

UV-Vis spectroscopic measurements were carried out in a 1 cm quartz cuvette with a Shimadzu UV-2450 spectrophotometer. Powdered XRD was recorded in a ‘Rigaku Miniflex-II’ X-ray diffractometer using CuK_α radiation (λ = 0.154056 nm). Scans were collected on dry samples in the range 10–80°. Fluorescence spectra were recorded using a Hitachi F-7000 Fluorescence Spectrophotometer. A FESEM study of the samples was performed using a Supra 40, Carl-ZEISS Pvt Ltd Field Emission Scanning Electron Microscope (FESEM) with an accelerating voltage of 5 kV. Samples for the FESEM study were prepared by placing a drop of the aqueous suspension of particles on a small glass slide followed by solvent evaporation under vacuum. To minimize sample charging, the dried samples were coated with a thin gold layer (<5 nm) right before SEM study. Time-resolved fluorescence measurements were carried out under ambient conditions using a time-correlated single-photon counting (TCSPC) spectrometer [a picosecond diode laser (IBH, UK)] with the detection wavelength at 430 nm for the mother solution (1-PyCHO in EtOH) and for sample-a, b & c two edges (500 nm & 600 nm) of the broad emission band were monitored. The excitation wavelength was 370 nm for all the samples. The signal was detected at magic angle (54.7°) polarization using a Hamamatsu MCP PMT (3809U). The time resolution of the experimental setup was ∼90 ps. Optical microscopy images were taken using a NIKON ECLIPSE LV100POL upright microscope equipped with a CCD camera (model no. DS-Fil), polarizer-analyzer assembly and 12 V–50 W halogen lamp as the excitation source for the emission study. The samples for the optical microscopic study were prepared by placing a drop of colloidal solution onto a clean glass slide.

Computational study

In the aggregated structures of molecules, where weak interactions between different atomic centre of similar kinds of molecule take place, a second-order local reactivity descriptor (LRD) called the second order Fukui function²⁰ may be used instead of electronic density. The Fukui function is defined in terms of the derivative of ρ(r), electronic charge density with respect to the total number of electrons (N), at a constant external potential, v(r).

The function f(r) reflects the ability of a molecular site to accept or donate electrons. High values of f(r) are related to a high reactivity at point r.^20a Since the number of electrons N is a discrete variable,²¹ right and left derivatives of ρ(r) with respect to N have emerged. By applying a finite difference approximation to eqn (1), two definitions of Fukui functions depending on total electronic densities are obtained:

where ρ_N+1(r), ρ_N(r), and ρ_N−1(r) are the electronic densities at point ‘r’ for the system with ‘N + 1’, ‘N’, and ‘N − 1’ electrons, respectively. The first one, f⁺(r), has been associated to reactivity for a nucleophilic attack so that it measures the intramolecular reactivity at site r toward a nucleophilic reagent. The second one, f⁻(r), has been associated to reactivity for electrophilic attack so that this function measures the intramolecular reactivity at site r toward an electrophilic reagent.^20a

Morell et al.²² have proposed a local reactivity descriptor (LRD), which is called the dual descriptor (DD) f⁽²⁾(r) ≡ Δf(r). Morell and co-workers used the notation Δf(r), but currently it has been replaced by the modern notation f⁽²⁾(r) in order to highlight the fact that this is a Fukui function of second order. Its physical meaning is to reveal nucleophilic and electrophilic sites on a molecular system at the same time. Mathematically, it is defined in terms of the derivative of the Fukui function, f(r), with respect to the number of electrons, ‘N’. Through a Maxwell relation, this LRD may be interpreted as the variation of ‘η’ (the molecular hardness which measures the resistance to charge transfer²³) with respect to υ(r), the external potential. The definition of f⁽²⁾(r) is shown as indicated by Morell et al.^22a,b

As mentioned above, DD allows one to obtain simultaneously the preferable sites for nucleophilic attacks (f⁽²⁾(r) > 0) and the preferable sites for electrophilic attacks (f⁽²⁾(r) < 0) into the system at point ‘r’.

Density Function Theory (DFT) based hybrid functional (B3LYP) and 6-31G(d) basis set were used to compute the second order Fukui function, f_k⁽²⁾(r) as a local reactivity descriptor. All the computations on 1-PyCHO in this present study were carried out using Gaussian-09 package programs.²⁴

Results and discussion

FE-SEM study

A FESEM study performed on all the three samples (Fig. 1) distinctly shows numerous small crystals whose shapes are reminiscent to that of the crystals observed by fluorescence microscopy (Fig. 2). Microcrystals of ‘sample-a’ prepared using 9.9 mM SDS and 0.99 mM 1-PyCHO are perfectly rod shaped, where as that of ‘sample-b’ (synthesized using 9.9 mM SDS and 4.70 mM 1-PyCHO) and ‘sample-c’ (synthesized using 47.60 mM SDS and 4.70 mM 1-PyCHO) are bar and rectangular plate like in shape.

Fig. 1 FE-SEM images of 1-PyCHO microcrystals: sample-a (a-i & a-ii), sample-b (b-i & b-ii) and sample-c (c-i & c-ii).

Fig. 2 Optical fluorescence microscopy images with blue excitation filter of (a) sample-a, (b) sample-b and (c) sample-c.

Optical microscopic study

The intense solid state luminescence property of 1-PyCHO suggests that the optical fluorescence microscopic study will be a useful tool to investigate the morphology of microstructures (Fig. 2 and Fig. S1†). Distinct morphologies for sample-a to c are clearly observed using fluorescence microscopy. Upon excitation with blue light sample-a shows rod shaped particles with Olive Drab luminescence, sample-b shows bar shaped microcrystals with yellow green luminescence and sample-c exhibits rectangular plate like microcrystals having clear edges with orange luminescence. Again the dark field view of 1-PyCHO microcrystals using polarizer-analyzer assembly shows different colors depending on the direction of incident radiation and it suggests that the synthesized microcrystals are anisotropic in nature (Fig. 3).

Fig. 3 Polarized optical microscopic images of 1-PyCHO microstructures: (a) sample-a, (b) sample-b and (c) sample-c.

Role of concentration of SDS and 1-PyCHO

Small angle neutron scattering experiments on sodium dodecyl sulphate and other ionic micelles support the basic Hartley model of a spherical micelle.²⁵ However, as the concentration is increased, the shape of ionic micelles changes in sequence spherical–cylindrical–hexagonal–lameller.²⁶ In our present study, concentrations of SDS in sample-a & b are a little higher than its CMC and it will favor a cylindrical shaped micelle. On the other hand, sample-c has a much higher SDS concentration and it will result in lamellar structures of the micelle. Again, micelles of ionic surfactants are aggregates composed of a compressive core surrounded by a less compressive surface structure²⁷ and with a rather fluid environment (η= 8–17 cP). Aromatic hydrocarbons such as 1-PyCHO will sit both at the micellar core and the micellar surface layer.²⁸ The fraction of 1-PyCHO occupying each of these two sites depends on its concentration in the intermicellar bulk phase. The fraction in the inner core increases with increasing concentration in the bulk phase because the increase in the chemical potential of 1-PyCHO enables 1-PyCHO to move towards the micellar core.

A comparison of our sample-a & b shows that the concentrations of SDS are same in both the samples, but the concentration of 1-PyCHO is five times higher in sample-b than sample-a. 1-PyCHO being polar, it will prefer the less compressive polar surface layer of the cylindrical SDS micelle and it will help the one dimensional growth of crystals to form rod shaped microcrystals. Now with the increasing concentrations of 1-PyCHO in sample-b, 1-PyCHO will enter into the core layer of the micelle. This compressive core layer of the micelle will hinder the 1-dimensional growth and within this small core region, it will attain thermodynamically stable bar shaped micro structures. On the other hand in sample-c where the micelle has a lamellar structure, at this higher 1-PyCHO concentration, 1-PyCHO deposits on the core of the micelle to form a rectangular plate shaped morphology of microcrystals. Thus, though the concentrations of 1-PyCHO in sample-b & c are the same, this markedly different morphology of microcrystals is due to a different shape of the micelle in these two samples.

XRD analysis

In view of the variety of microcrystals observed, a question comes in our mind: does this variety result from a change in crystal habit or the formation of different polymorphs? The problem is that the possibility of polymorphism is hard to study in our case. Because of small size of microcrystals, we did not succeed in growing a single crystal of sufficient size, so that the crystal structure of 1-PyCHO can be known. X-Ray powder diffraction pattern of sample-a, b & c are shown in Fig. 4a–c. A comparison of XRD spectra shows that the strong peak at 2θ = 12.6° is absent in sample-b and sample-c. Again the peak at 2θ = 37.3° is present in sample-b & c, but is absent in sample-a. On the other hand the simulated powder XRD spectra (show in Fig. 4d) from the single crystal (triclinic) data reported by Matsuzaki et al.¹⁹ shows peak at 2θ = 11.72°, 13.17°, 18.44°, 37.4° and 43.9° and these correspond to (0 −1 1), (1 0 0), (−1 1 1), (−2 2 2) and (0 −2 5) plane of crystal. Comparing the simulated powder XRD spectra with our experimental spectra, we presume that the peak at 2θ = 11.72°, 13.17°, 18.44°, 37.4° and 43.9° Correspond to (0 −1 1), (1 0 0), (−1 1 1), (−2 2 2) and (0 −2 5) plane of crystal lattice and our synthesized microcrystals have triclinic unit cell where 1-PyCHO molecules are arranged in parallel face to face slipped conformation.

Fig. 4 Powder XRD of 1-PyCHO microcrystals: (a) sample-a, (b) sample-b, (c) sample-c and (d) simulated from single crystal data (ref. 19).

Role of –CHO functional group

To confirm whether the –CHO functional group has any role in determining the morphology of 1-PyCHO microcrystals, we have carried out the same procedure of synthesis using pyrene instead of 1-PyCHO. SEM and optical microscopic photographs of the pyrene microcrystals synthesized using identical experimental conditions are shown in the ESI (Fig. S2–S4†) and they illustrate that the morphology of the particles is not regular in our present experimental conditions and rather quite uneven shapes microcrystals of pyrene are observed. So it suggests that the –CHO functional group plays a major role in directing the morphology of 1-PyCHO microcrystals.

The X-ray single crystal study of 1-PyCHO by Matsuzaki et al.¹⁹ illustrated that 1-PyCHO crystallizes in two modifications, one is triclinic and the other is orthorhombic. Comparison of simulated powder XRD with our observed powder XRD data suggests that our synthesized 1-PyCHO microcrystals have a triclinic structure. Matsuzaki et al. also showed that 1-PyCHO is present in a parallel face to face slipped conformation in its triclinic form and the density of a 1-PyCHO crystal is greater than that of a pyrene crystal i.e. the inter planer distance between the neighboring pyrene group is shorter in 1-PyCHO than pyrene. Though any intramolecular hydrogen bonding between the aldehyde group within the crystal is not observed by Matsuzaki et al., they suggested that the molecular packing of 1-PyCHO and pyrene crystals are mainly governed by the dipole–dipole and induced dipole–induced dipole (London dispersion) interactions. In order to make a comparison of dipole–dipole and London dispersion interactions, we have computed the dipole moment and average polarizability of pyrene and 1-PyCHO using a DFT based method with a B3LYP 6-31G (d) hybrid functional. Our computational study suggests that both dipole moment and average polarizability of 1-PyCHO (μ = 4.6263 D, α = 199.22 a.u) are greater than pyrene (μ = 0.0011D, α = 172.04a.u). This supports the larger dipole–dipole and induced dipole–induced dipole interaction in 1-PyCHO than pyrene and is responsible for the reduced distance between aromatic pyrene groups, resulting in higher density of 1-PyCHO crystals.

UV-Vis study

Fig. 5 shows the UV-Vis absorption spectra of the as prepared hydrosol of 1-PyCHO microparticles (curves a–c) and the diluted solution of 1-PyCHO in ethanol (curve m). The ultraviolet absorption spectrum of 1-PyCHO can be divided into two clearly distinguishable regions, associated with two groups of transition, one at 250–300 nm and the others at 320–410 nm. There are some early reports by R. N. Jones²⁹ suggesting that for condensed polyaromatic hydrocarbons like anthracene and pyrene, the group of bands with higher energy are associated primarily with excitation direction along the long (a, a′) axis and the group of band with lower energy are associated with the excitation process directed along the short (b, b′) axis (Scheme 1).

Fig. 5 The UV-visible absorption spectra of 1-PyCHO monomer in ethanol (m) and aqueous suspensions of (a) sample-a (b) sample-b (c) sample-c.

Scheme 1 1-PyCHO with its long (a, a′) and short (b, b′) axis.

The absorption spectra (curves: a–c) of 1-PyCHO hydrosols are red shifted and broadened compared to its monomer absorption. The shift and broadening are likely originated due to strong π–π interactions between the neighbouring 1-PyCHO molecules in its aggregated structures. In addition the overall shift of the baseline of the spectra compared to the monomer is attributed to the scattering of light by the larger aggregated structures in solution.

Emission study

1-PyCHO has two types of low-lying excited states, (n, π*) and (π, π*). In nonpolar solvents, the emission is highly structured and weak arising from the (n, π*) excited state. The upper-lying (π, π*) state is solvent sensitive and can undergo relaxation in polar solvents. When the dielectric constant (ε) of the solvent exceeds 10, the emission is broad (400–500 nm) and moderately intense and it is coming exclusively from the (π, π*) excited state.³⁰ The emission spectra of 1-PyCHO (1 μM) in ethanol (ε = 24.5) shows a maximum at 440 nm typical from a solvent of high polarity (Fig. 6d) and that of concentrated solution (5 mM) has an emission maxima at 470 nm. On the other hand 1-PyCHO hydrosols show broad (475–650 nm) red shifted emission upon excitation at λ_max = 370 nm (Fig. 6a–c). These broad red shifted emission bands are due to 1-PyCHO excimer emission. It is also interesting to note that with the increased size of microcrystals the emission maxima shifted to the red, e.g. λ^max_em are at 520 nm for sample-a, 530 nm for sample-b and 550 nm for sample-c. The intensity and the red shift of excimer emission in the concentrated solution is less compared to the microcrystals’ emission band. Excimer formation in solution is a diffusion control process i.e. upon excitation of the monomer, another monomer must approach towards the excited monomer during its excited state lifetime to form an excimer. Again each collision may not be effective for excimer formation. Therefore the extent of staking between two monomer units to form an excimer in the solution phase is not as strong as in the aggregated structures. This causes decreasing intensity as well as less red shift of the excimer in solution compared to the microcrystals. On the other hand with the increasing size of crystals, the extent of staking i.e. overlap of π orbitals become more and more until it reaches the thermodynamically stable larger crystal structures. This extent of increasing staking interaction with the increasing size of microcrystals gives stability of the excimer state and is responsible for the red shift of excimer emission with increasing size.

Fig. 6 Fluorescence emission spectra of (d) 5 mM 1-PyCHO in ethanol (intensity × 60) (e) 1μM 1-PyCHO in ethanol and aqueous suspensions of 1-PyCHO microcrystal (a) sample-a (b) sample-b (c) sample-c. All emission spectra were taken with 370 nm excitation.

In order to understand the nature of aggregates in 1-PyCHO microcrystals, we did a time resolved fluorescence study of 1-PyCHO in ethanol as well as the aggregated 1-PyCHO hydrosols. All the samples were excited using a 370 nm pulsed diode laser. The emission wavelength for concentrated (5 mM) and diluted (1 μM) 1-PyCHO was 470 nm and 440 nm respectively. On the other hand for sample-a to c, the fluorescence emission decay was measured by monitoring the two extreme positions i.e. 500 nm & 600 nm of the broad eximer band. The fluorescence lifetime of the samples is measured by deconvoluting the response function from the decay curves. The obtained best-fit data are tabulated in Table 1 and the fluorescence decay traces including the lamp profile are shown in Fig. 7. Our measured fluorescence lifetime of 1-PyCHO (1 μM) in ethanol is 0.80 ns and the emission decay of the concentrated solution of 1-PyCHO (5 × 10⁻³M) is fitted with a bi-exponential fit with measured lifetimes 0.75 ns and 4.1 ns respectively (Table 1). It seems to us that at a concentration of 5 mM or less, formation of a diffusional excimer is negligible and it is also confirmed by the absence of rise time in the fluorescence decay. Thus, 0.8 ns and 0.75 ns components are due to the 1-PyCHO monomer in EtOH and the 4 ns component 5 mM 1-PyCHO in EtOH is due to the monomer plus ground state dimers or oligomers at this high concentration. The measured lifetime at 500 nm for each of the sample (a–d) shows two components having values calculated as ∼1.5 ns and ∼4–4.5 ns. The longer component is the major contributor for each decay profile. Since the monomer emission tail has significant intensity at 500 nm, this 1.5 ns comes from the monomer present in micellar medium and the 4 ns component is due to the foregoing aggregates in suspension, which strongly emit at this wavelength. On the other hand emission monitored at 600 nm also fitted with two components, the shorter one having lifetime ∼4 ns for each and the longer component with lifetime 13 ns for sample-a, ∼17.72 ns for sample-b and ∼17.77 ns for sample-c. This longer component i.e. 13–17 ns can be assigned to the crystal fluorescence life time and the ∼4 ns components for each aggregate 1-PyCHO hydrosol is due to the foregoing aggregates in suspension, which emit weakly at this wavelength.

Table 1 Fluorescence lifetime of 1-PyCHO monomer in EtOH and its different shaped microcrystals in water

Sample	λmax (nm)	τ1 (ns)	τ2 (ns)
1 μM 1-PyCHO in EtOH	440	0.80 (100%)	—
5 mM 1-PyCHO in EtOH	470	0.75 (30%)	4.01 (70%)
Sample-a	500	1.58 (15%)	4.5 (85%)
	600	4.58 (20%)	13.77 (80%)
Sample-b	500	1.4 (18%)	4.1 (82%)
	600	4.6 (22%)	17.72 (78%)
Sample-c	500	1.5 (16%)	3.98 (84%)
	600	4.15 (21%)	17.77 (79%)

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Fig. 7 The fluorescence decay profiles of (d) 5 mM 1-PyCHO in ethanol (e) 1 μM 1-PyCHO in ethanol and hydrosol of 1-PyCHO microcrystals (a) sample-a (b) sample-b (c) sample-c. All emission spectra were taken with 370 nm excitation.

Computation of Fukui parameter as LRD

An earlier report by Matsuzaki et al.¹⁹ suggests that the single crystal of 1-PyCHO has a triclinic unit cell geometry and the neighbouring 1-PyCHO molecules are arranged in face to face slipped geometry. In order to understand the possible driving force for attaining such a slipped face to face conformation, we have computed the second order Fukui parameter as a local reactivity index for each atomic centre of 1-PyCHO. It has been discussed in section ‘Computational study’ that a positive value of f⁽²⁾(r) at a particular atomic centre is a measure of its electrophilicity and the negative value indicates its nucleophilicity. Our computed f⁽²⁾(r) for each atomic center of 1-PyCHO is shown in Table 2. The crystal structure of 1-PyCHO shows that the carbon atom no. 1C, 3C, 5C, 8C, 16C, and 22H of one 1-PyCHO molecule are projected perpendicularly to carbon atom no. 8C, 5C, 3C, 1C, 26C, and 10C of its nearest neighbour 1-PyCHO.

Table 2 Electrophilic f⁺ and nucleophilic f⁻ condensed Fukui functions and second order Fukui parameter, f⁽²⁾(r) of 1-PyCHO molecule calculated using DFT B3LYP 6-31G (d) level of theory

Atom no.	f^+ × 10^−3	f^-− × 10^−3	f^(2)(r) × 10^−3
1C	106	10	96
2C	−40	−108	68
3C	0	5	−5
4C	−55	−3	−52
5C	20	4	16
6C	68	−118	186
7C	121	−73	194
8C	−88	2	−90
9C	52	−9	61
10C	65	−60	125
11C	53	−109	162
12H	−166	−24	−142
13C	108	2	106
14C	54	−107	161
15C	50	−18	68
16C	61	−49	110
17C	108	−68	176
18H	−169	−23	−146
19H	−171	−25	−146
20H	−151	−18	−133
21H	−161	−27	−134
22H	−173	−23	−150
23H	−166	−24	−142
24H	−158	−29	−129
25H	−168	−24	−144
26C	−265	8	−273
27O	−279	376	−655
28H	−539	349	−888

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Atoms for each of 1-PyCHO unit involved in interaction with their f⁽²⁾(r) values are listed in Table 3. Computed f⁽²⁾(r) values (Table 3) suggest that (1C–8C), (3C–5C), (16C–26C), and (22H–10C) atom pairs will interact strongly to give a stable structure of the 1-PyCHO dimer.

Table 3 Computed value of f⁽²⁾(r) of possible interacting atomic centers in 1-PyCHO dimer

1-PyCHO (upper)		1-PyCHO (lower)
Atom no.	f^(2)(r) × 10^−3	Atom no.	f^(2)(r) × 10^−3
1C	96	8C	−90
3C	−5	5C	16
5C	16	3C	−5
8C	−90	1C	96
16C	110	26C	−273
22H	−150	10C	125

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Our computed value of f⁽²⁾(r) for the atomic centre 1C, 5C, 10C and 16C have positive f⁽²⁾(r) and atomic centre 3C, 8C, 26C and 22H have negative f⁽²⁾(r) values. The electrophilic centre of one molecule will show a strong affinity towards the nucleophilic centre of its nearest neighbour. When the two 1-PyCHO molecules approach each other, they will arrange in such a way that the atomic centre of one having +ve f⁽²⁾(r) value will face to towards the atomic centre having –ve f⁽²⁾(r) value of the other (Scheme 2). This type of arrangement is in conformity with the face to face slipped conformation of 1-PyCHO molecules in their crystal structure.

Scheme 2 Staked conformer of 1-PyCHO dimer.

Conclusions

The present synthesis using SDS as a morphology directing soft template is very much useful in synthesizing a large quantity of morphologically interesting 1-PyCHO microcrystals. Our study reveals that the concentration of both 1-PyCHO and SDS play a major role in directing the morphology of 1-PyCHO microcrystals. It has been observed that a uniform two dimensional growth for each crystal takes place with the increasing concentration of 1-PyCHO. XRD, UV-vis absorption, steady state and time resolved fluorescence emission studies reveal that the extent of perturbation to the emissive species increases until it reaches the thermodynamically stable structure. Our extensive computation of the second order Fukui parameter for each atomic center as a local reactivity parameter suggests that adjacent 1-PyCHO molecules are present in a face to face slipped conformation in their aggregated structures. We also observed that our computation results are in agreement with the single crystal data of 1-PyCHO, which showed that adjacent 1-PyCHO molecules are present in a face to face slipped conformation. An extensive survey of the literature shows that this is the first time that the second order Fukui parameter is used as a local reactivity parameter to explain the possible conformation of organic molecules in its aggregated structure.

Acknowledgements

We gratefully acknowledge the financial support received from CSIR (ref. no. 01 (2443)/10/EMR-II), New Delhi for carrying out this research work. D. Das and P S.Sheet thank to CSIR and UGC, New Delhi respectively for their individual fellowship. We thank to Prof. N. Sarkar, IIT KGP for extending help in doing TCSPC measurements. We also gratefully acknowledge the help render by CRF, IIT, Kharagpur and USIC, Vidyasagar University for doing FESEM and fluorescence measurements respectively.

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Footnote

† Electronic supplementary information (ESI) available: Optical micrographs of (a) sample-a, (b) sample-b and (c) sample-c of 1-PyCHO; fluorescence microscopy images of pyrene, optical micrographs of pyrene, SEM images of (a) sample-a, (b) sample-b and (c) sample-c of pyrene and synthetic methodology of pyrene microparticles. See DOI: 10.1039/c3ra47203e

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