Exciton Dynamics in Anthracene Nanoaggregates - The Journal of

Article pubs.acs.org/JPCC

Exciton Dynamics in Anthracene Nanoaggregates Biswajit Manna, Rajib Ghosh, and Dipak Kumar Palit* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India S Supporting Information *

ABSTRACT: Anthracene nanoaggregates (NA), synthesized using a wellknown reprecipitation method, have shapes of nanodisks with the average diameter and height of about 260 and 50 nm, respectively. Following photoexcitation of the anthracene nanoaggregate, we have observed population of two different types of singlet excitonic states, namely, free exciton and self-trapped exciton or excimeric state. Our investigation reveals that the presence of defect conditions in the molecular aggregates reduces the free exciton lifetime and slows the exciton diffusion process marginally. Both the exciton diffusion coefficient and exciton diffusion length in the nanoaggregate and in crystals are comparable. Comparable values of the diffusion lengths in the case of the singlet exciton in the nanoaggregate and crystals suggest that the nanoaggreagates may also be considered as materials for efficient optoelectronic and photovoltaic devices. However, drastic reduction in triplet exciton diffusion coefficient in the nanoaggregate as compared to that in the crystalline form is explained by considering the triplet energy transfer mechanism via spin exchange, which is a slow process. thin films for a few of these polycyclic aromatic compounds have also been reported earlier.18−22 However, to the best of our knowledge, no information about the exciton dynamics in the nanoaggregates of these compounds is available in the literature. Anthracene, in its crystalline form, has been found to display many attractive optoelectronic properties, such as high charge carrier mobility,23 anisotropy,24,25 aromagnetism,26 etc. Moderate band gap of 3.1 eV 27 and high emission quantum yield28 make anthracene a potential material for application in OLED,27 organic photovoltics,29 optoelectronic devices,30 etc. Knowledge about the photophysical and diffusion properties of excitons populated in various kinds of materials is important in designing the optoelectronic devices with improved performance. Although exciton dynamics in the crystalline phase and in thin films of anthracene have been extensively studied by several groups,31−34 to the best of our knowledge, these properties have not been discussed in nanoaggregates. Preparation of anthracene nanoaggregate by reprecipitation technique has been reported recently, and these particles have been shown to be quite stable in ambient conditions.16,17,35−39 Herein, we have investigated the basic photophysical properties and exciton dynamics in anthracene nanoaggregates using steady state and time-resolved absorption and fluorescence spectroscopic techniques. We have determined the excitation intensity dependence of the exciton−exciton annihilation rates to estimate the diffusion constants and diffusion lengths for both the singlet and triplet excitons.

1. INTRODUCTION During the past two decades, nanoscience has become a fascinating field of research for the scientists from different disciplines. Preparation and properties of the nanomaterials have received huge attention because of their wide scale applications in multiple fields.1−7 Among them, nanomaterials prepared using organic molecules have shown immense potentiality due to their low cost, ease of syntheses, and excellent optical properties. The quantum confinement effect is less pronounced for organic nanoaggregates, but it has an additional advantage because of the possibility of producing different monomers by substitution,8,9 which helps to tune the properties of the nanoaggregates in desired manner. Organic nanocomposites are found to be useful for application in OLEDs,10 field effect transistors,11 organic solar cells,12 wave guides,13 etc. Different kinds of interactions, e.g., hydrophobic, π−π interaction, H-bonding, etc., lead to formation of nanoaggregates, and these interactions are weak in nature. Therefore, the thermodynamic stability is, of course, a point of concern for these materials. Materials consisting of polycyclic aromatic hydrocarbons are known to be good organic semiconductors (OSC), having excellent optical and electronic properties. They form crystals due to π−π intermolecular interactions.14 These molecules are extremely hydrophobic and form nanoaggregates, which are quite stable in ambient condition.15−17 It is important to understand the properties of these materials for their applications in preparing devices. Methods of synthesis of the nanoaggregates of the polyacene compounds have been reported earlier by several groups, and their basic photophysical properties have also been explored in recent years.15−17 Exciton dynamics in bulk crystal form and in © XXXX American Chemical Society

Received: January 16, 2015 Revised: April 23, 2015

A

DOI: 10.1021/acs.jpcc.5b00469 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Size distribution of the nanoaggregates determined using DLS (A) and AFM (B) techniques.

2. EXPERIMENTAL SECTION 2.1. Materials. Scintillation grade anthracene from Fluka (purity >99%) was further purified by recrystallization from toluene followed by vacuum sublimation. Purity of the sample was verified using NMR spectroscopy as well as comparing the absorption and fluorescence excitation spectra recorded in cyclohexane. Perfect matching between these absorption and excitation spectra ensured the high purity of the sample used for the experiments (Figure 1 in Supporting Information). Spectroscopic grade acetonitrile from Spectrochem (India) Pvt. Ltd. was used as received. Millipore water was used for preparation of nanoaggregates. Polyvinyl alcohol (PVA; 99+% hydrolyzed), which was used as stabilizer in preparation of the nanoaggregate, was purchased from Sigma-Aldrich and was used without any further purification. 2.2. Methods. Anthracene nanoaggregates have been prepared using reprecipitation method, where acetonitrile and water have been used as the good and poor solubility solvents, respectively. 10 mM anthracene solution in acetonitrile and 1 mg/mL PVA solution in Millipore water were prepared. 200 μL aliquot of the former solution was injected into the 10 mL PVA solution under vigorously stirring conditions. After 15 min of stirring, anthracene nanoaggregate dispersed in water was obtained. PVA acts as the stabilizer for the nanoaggregates and also limits the size distribution of the nanoaggregates. Average size of the nanoaggregates was determined using dynamic light scattering (DLS) technique with a Malvern 4800 Autosizer, which employs 7132 digital correlator. He−Ne laser with the output wavelength at 632.8 nm and 15 mW power was used for DLS measurements. All the measurements were performed at 130° angle. Atomic force microscopy (AFM, NTMDT, Solver model) was used to confirm the size of the nanoaggregates. Aqueous dispersion of the nanoaggregate was drop-cast on mica sheets and dried under IR lamp for AFM measurements. The X-ray diffraction (XRD) pattern of the nanoaggregate sample was recorded on a PANalytical X-Pert Pro X-ray diffractometer using Cu Kα radiation (λ = 1.5406 and 1.5444 Å) over the 2θ range of 10−50°. The UV−visible absorption spectra were recorded using Jasco V-670 model spectrophotometer. Fluorolog-3 spectrofluorometer (Horiba-Jobin-Yvon) was used for recording corrected room temperature photoluminescence (PL) spectra of the nanoaggregate samples. All the absorption and emission spectra were recorded with the aqueous dispersion of the nanoaggregates. Time correlated single photon counting (TCSPC) technique was used for the time-resolved emission

studies by using Horiba-Jobin-Yvon, IBH (U.K.) system. Samples were excited at 374 nm by using a diode laser having pulse duration (fwhm) of about 100 ps. All PL decay profiles were recorded with the emission polarizer set at the magic angle (54.7°) and analyzed by using IBH DAS 6.2 software. Iterative deconvolution method was used for the bi- or triexponential fitting of the data, and reduced χ2 ≤ 1.2 was considered as the good fit to the experimental data. The time-resolved transient absorption spectra were recorded with subpicosecond time resolution (∼120 fs) using a femtosecond pump−probe spectrometer, which has been described in detail elsewhere.36,40 Briefly, our setup consists of a Ti:sapphire laser system, supplied by Thales Optronique SA, Elancourt, France, which provides the pulses of about 40 fs and 1 mJ energy per pulse at a repetition rate of 1 kHz. Pump pulses of 400 nm were generated for excitation of the samples by frequency-doubling of one part (about 150 μJ) of the 800 nm output of the amplifier in a 0.5 mm thick BBO crystal, and about 1 μJ energy from the other part of the amplifier output was used to generate the white light continuum (450−1000 nm) probe in a 2 mm thick sapphire plate. The direction of polarization of the pump beam was fixed at 54.7° with respect to that of the probe beam, and the pulse energy of the pump beam could be varied in the range 1−14 μJ/pulse. The pump beam was focused to a diameter of approximately 0.5 mm at the probing region of the sample. The sample solutions were kept flowing through a quartz cell of 2 mm path length. To record the temporal profiles, wavelength regions of 10 nm width were selected using pairs of interference filters. Nanosecond laser flash photolysis experiments were performed using the spectrometer model LP-920, (Edinburgh Instruments, U.K.). Third harmonic output of the Nd:YAG laser (355 nm) having fwhm of 10 ns was used for photoexcitation of the samples in a quartz cell of 10 mm path length and the white light from a pulsed 450 W Xe lamp as the probe for this experiment. Temporal profiles were recorded using excitation energy of 10−22 mJ/pulse. All the measurements were performed after 10−15 min of N2 purging to eliminate the presence of oxygen molecules (efficient triplet quencher) in the solution.

3. RESULTS AND DISCUSSION 3.1. Morphological Characterization. Morphological analysis is considered as the primary step in characterization of the nanomaterials. Herein, DLS and AFM techniques have been used to determine the size distribution of the nanoB


Article

The Journal of Physical Chemistry C aggregates. Size distribution of the nanoaggregates determined using DLS technique has been shown in Figure 1A. It is observed that the average size of the nanoaggregates is about 260 ± 25 nm. AFM image of the nanoaggregates is also shown in Figure 1B. We find that the average height of the nanoaggregates is about 50 ± 10 nm and the diameter of the nanoaggregates varies in the 50−400 nm range. AFM analysis also indicates that the particles of these nanoaggregates are not spherical in shape but the aspect ratio varies in the range 0.05− 0.1. Therefore, they may be better described as nanodisks. Arrangement of the anthracene molecules in the nanoaggregates has been understood from the XRD analysis. After vacuum drying of the nanoaggregate sample, it was brought to powder form and XRD measurement was performed. The XRD pattern has been compared with those of the crystalline forms reported earlier.41,42 In the case of the nanoaggregate sample, we observe a weak but broad featureless XRD band in the range of 15−35° of the diffraction angle with a few intense lines superimposed on it (Figure 2). Those sharp lines are perfectly

Figure 3. UV−visible absorption spectra of anthracene dissolved in acetonitrile and anthracene nanoaggregate dispersed in water.

spectrum of the nanoaggregate sample is assigned to Mie scattering. Absence of this extended absorption in the red region of the fluorescence excitation spectrum (inset of Figure 4) confirms its assignment to Mie scattering. All these new

Figure 2. XRD pattern recorded for anthracene nanoaggregate and bulk solid.

coincident with those observed in the XRD pattern of the crystalline state. This observation leads us to describe the nanoaggregated form of anthracene as a crystalline substance with the presence of a large number of defect sites, providing it partial amorphous nature. The defect sites may have remarkable effect on the photophysical properties of the excitons populated in the nanoaggregate as compared to those in anthracene crystal or thin film. 3.2. Steady State Absorption and Emission Spectra. In Figure 3, we have compared the steady state absorption spectrum of anthracene dissolved in acetonitrile with that of the aqueous dispersion of the nanoaggregate. It is well-known that the absorption spectrum of anthracene in organic solvents shows distinct and sharp vibrational progressions. Similar vibrational progressions have also been found in the absorption spectrum of the nanoaggregate sample. However, although the positions of each of these progressions are nearly the same as those observed in the case of the molecule in solution, the lowest energy one (corresponding to the 0−0 transition) appears at a position that is red-shifted by about 20 nm as compared to that in solution. In addition, the relative heights of the individual vibronic bands are significantly changed and the widths of these bands arising due to Devydov splitting are also increased.43 The long tail in the red region of the absorption

Figure 4. Steady state fluorescence spectra of anthracene in acetonitrile solution and anthracene nanoaggregate dispersed in water. Inset: Excitation spectrum recorded for anthracene nanoaggregate for 445 nm emission.

features observed in the absorption spectrum of the nanoaggregate as compared to that of the molecule in solution may be attributed to strong intermolecular interaction between the different monomeric anthracene units in the closely packed nanoaggregated state. Room temperature PL spectra recorded for the anthracene molecule in acetonitrile solution as well as for the aqueous dispersion of the nanoaggregate sample are presented in Figure 4. We find that the emission spectrum of the nanoaggregate also consists of the features of sharp vibrational progressions. However, like in the case of the absorption spectrum of the nanoaggregate, the largest energy vibronic band in the emission spectrum is also found to be red-shifted by about 20 nm as compared to the 0−0 vibronic band of the monomer in C


Article

The Journal of Physical Chemistry C acetonitrile solution appearing at 380 nm. Absence of this vibronic band in the emission spectrum of the nanoaggregate sample ensures the absence of any isolated monomer molecule in aqueous dispersion of the nanoaggregate sample. We have determined the emission quantum yield (ϕF) of the nanoaggregate sample relative to that of the monomer in acetonitrile, and the former is found to be reduced by about one-half as compared to the latter. Increased rate of the intersystem crossing (ISC) process may be one of the possible reasons for the reduced ϕF in the nanaoaggregate sample. However, in the crystalline form of anthracene, the rate of the intersystem crossing (ISC) process has been found to decrease significantly as compared to that of the monomer in solution.44,45 The reason has been explained as follows: In the case of anthracene molecule in solution, the energy level of the T2 state lies just below the S1 energy level. This leads to efficient coupling between these two states, and hence, the rate of the ISC process is very fast. In solid crystalline form, because of stronger intermolecular interactions, the relative positions of these energy levels are changed and the energy level of the T2 state is lifted up with respect to that of the S1 excitonic state by about 73 meV. In addition, the S1−T1 energy gap being large (1.3 eV), coupling between these two states is poor.43,44 Hence, in the crystalline solid form, the rate of the ISC process is likely to decrease as compared to that of the molecule in solution. In the case of anthracene nanoaggregates, the absorption spectrum possesses good similarities with that of the anthracene crystal. In addition, our XRD analysis has revealed that the nanoaggregate also possesses good crystalline character. Therefore, these characteristics of the nanoaggregate sample suggest that the energy levels associated with these two kinds of materials may be quite similar.46 We have also discussed earlier that quantum confinement effect in the nanoaggregate is small and hence similarities in the energy levels in the nanoaggregate and in crystalline solid are a good possibility. Therefore, we may assume that the rate of the ISC process in the nanoaggregate is not larger than that of the monomer molecule in solution and hence the ISC process cannot be a responsible factor for smaller ϕF in the nanoaggregate. We predict that one or both of other two photophysical processes, namely, faster rate of the internal conversion process and trapping of excitons in the defect sites, are responsible for lower ϕF in the nanoaggregate (vide infra). 3.3. Time-Resolved Studies Using TCSPC Technique. To resolve the deactivation pathways for the excitonic states of the nanoaggregate populated following photoexcitation, timeresolved fluorescence experiments have been performed by using TCSPC technique. Temporal emission profiles have been recorded at regular intervals of 10 nm in the 400−650 nm region with a time resolution of about 200 ps following photoexcitation of the aqueous dispersions of the nanoaggregate sample at 374 nm. Each of these decay profiles could be fitted with a multiexponential function consisting of two or three components by using iterative deconvolution method. Three of them, along with the best-fit functions, are shown in Figure 5 and the lifetimes (τ) and the relative amplitudes (a) thus estimated are presented in Table 1. Multiexponential decay of the excitonic emission suggests population of multiple excitonic states following photoexcitation of the nanoaggregate sample. The temporal profile recorded at 400 nm could be fitted by using a biexponential decay function with the lifetimes of 0.52 and 1.8 ns. However, each of the temporal profiles recorded at longer wavelengths

Figure 5. Temporal florescence profiles recorded at three different wavelengths along with the multiexponential best-fit functions for the anthracene nanoaggregate sample. The lifetimes (τ) and the relative amplitudes (a) associated with the best-fit functions are given in Table 1.

Table 1. Lifetimes (τ) and Relative Amplitudes (a) Associated with the Best-Fit Functions for the Temporal Fluorescence Profiles Recorded Following Photoexcitation of the Nanoaggregate Dispersed in Water lifetime (ns) (relative amplitude) wavelength (nm) 400 450 510

τ1 (a1) 0.52 ± 0.06 (0.47 ± 0.05) 0.72 ± 0.08 (0.17 ± 0.04) 0.44 ± 0.05 (−0.01 ± 0.01)

τ2 (a2) 1.8 ± 0.4 (0.53 ± 0.05) 2.89 ± 0.3 (0.40 ± 0.05) 2.78 ± 0.3 (0.04 ± 0.01)

τ3 (a3)

11.72 ± 1.5 (0.43 ± 0.05) 16.7 ± 2 (0.97 ± 0.05)

(say, at λ ≥ 450 nm) could only be fitted by using a threeexponential function. For example, the temporal profile recorded at 450 nm has been fitted with a three-exponential decay function with the lifetimes of 0.72, 2.8, and 12 ns. We find that the amplitude associated with the longest lived component (i.e., a3) increases but those of the two shorter lived components (i.e., a1 and a2) decrease as the monitoring wavelength is tuned toward the longer wavelength region. Temporal profiles recorded at wavelengths longer than 500 nm show an initial rise of fluorescence intensity with the lifetime of 0.44 ns followed by a biexponential decay with the lifetimes of about 2.8 and 16 ns. Similarities in the lifetimes of the fastest decay component associated with the temporal profiles recorded at 500 nm possibly suggest a complementary relationship between them. We attribute the former one to the decay of the excitonic state populated following photoexcitation of the nanoaggregate and the latter to the population of an excimeric state of anthracene. Seko et al. have reported that excimer formation takes place predominantly in the surface region of anthracene microcrystal.17 In nanoaggregates, the surface area to volume ratio is large and the regular arrangement of the molecules in the crystal is interrupted at the surface. Therefore, possibility of the presence of surface defects is very large in the nanoaggregate sample. In this report, we have not been able to define the specific role of surface defects on the exciton dynamics. Instead we observe the overall effect of the defects present both in the D


Article


different research groups in bulk crystals having defect sites as well as at high pressure condition.58−64 Shapes of the excimer emission spectra have been shown to depend on the experimental conditions thus populating the excimeric states having different kinds of geometries or configurations. Temporal fluorescence profiles recorded at different wavelengths have been used to construct the “time-resolved area normalized emission spectra” (TRANES) of anthracene nanoaggregate.65,66 These have been shown in Figure 6,

bulk and at the surface. However, we can predict that because of the large surface area to volume ratio in the nanoaggregate, surface defects play a major role in exciton trapping. It may be noted that the lifetimes and the relative amplitudes associated with the best fit functions for the temporal fluorescence profiles show significant wavelength dependence. This possibly can be attributed to several factors. First, because of different size distribution of the particles in the nanoaggregate assembly, lifetimes of the excitonic states in different particles may vary to some extent. According to the exciton−polariton model, lifetimes of the excitonic states depend upon the size of the nanoaggregates.47 In addition, wavelength dependence may also arise because of the heterogeneities of the defect sites because of differences in orientations of the monomers and hence populating excimeric states of different geometries (vide infra).48,49 Most importantly, wavelength dependence of the temporal profiles may also arise because of reabsorption effect. Exciton diffusion and reabsorption of exciton emission have been extensively studied in anthrecene crystal earlier both experimentally and by MD simulation.50−54 Reabsorption of exciton emission has been reported to be significant only in the case of thick crystals (thickness of a few tens of micrometers).51 However, in our case, the average size of the nanoaggregates prepared by us is only about 0.26 μm. Negligible effect of reabsorption on exciton dynamics in the nanoaggregates has been evident from two observations. First, the absorption or excitation spectrum and the emission spectrum of the exciton populated in the nanoaggregate maintain a perfect mirror image relationship. Further, the exciton lifetime is extraordinarily short (0.5 ± 0.2 ns) as compared to that reported in crystals. Usually, thin crystals show a fairly short decay time of the exciton luminescence, while thick crystals show long decay time due to reabsorption effect. The short decay time in thin crystals has rather been explained by the trapping of the excitons at the crystal defects at the surface, the number of which is rather larger in thinner crystals.51 The short decay time has been commonly considered to be intrinsic. Therefore, we assign the component having lifetime of 0.5 ± 0.2 ns to the free S1 excitonic state, which is shorter than that of the singlet exciton populated in anthracene crystal.20 Trapping of excitons at the defect sites and more surface encounters due to smaller size of the nanoaggregates are the major factors responsible for this short exciton lifetime in the nanoaggregate.55 While τ3 may be assigned to the lifetime of the excimeric state, the assignment of τ2 is not unanimous. The latter may have contribution from the quasi-free excitonic state, which is populated in the anthracene crystal because of strong exciton− phonon interaction.56 Population of quasi-free excitonic state becomes significant if the coupling constant is greater than unity, a condition which may arise at high pressure (say 5−18 kbar). However, in anthracene crystal at an ambient condition, the exciton−phonon coupling constant has a value of 0.85, and hence population of the quasi-free excitonic state in the crystal at ambient condition as well as in the nanoaggregate is quite unlikely.56 Therefore, both the fluorescence decay components, τ2 and τ3, represent the components of a biexponential fit to the nonexponential decay of the self-trapped excitonic and/or excimeric emissions. Population of the excimeric states is generally not observed in the case of anthracene in solution phase at ambient condition. In solution phase, anthracene molecules, on photoexcitation using UV light, prefer to form photodimers.57 Excimer emission has been reported by

Figure 6. Time-resolved area normalized emission spectra (TRANES) of anthracene nanoaggregate.

which reveals that distinctly different kinds of excitonic and/ or excimeric states are populated at different delay times and they emit in different wavelength regions. The time-resolved emission spectrum recorded at 0.2 ns delay time is mainly dominated by the contributions from free excitons. As the delay time increases up to 4 ns, contribution from the free excitons decreases leading to decay of emission intensity of the vibrational progression appearing in the 380−410 nm region. This is accompanied by the concomitant rise of intensities of the vibration progressions appearing in the 410−440 nm region, which may be assigned to population of the self-trapped excitons. With further increase in delay time, we observe the development and evolution of a new broad emission band in the 450−600 nm region at the expense of the one with the wellresolved vibrational bands in the 380−440 nm. We assign the emission band appearing in the 440−600 nm region to the excimeric state. However, the shape of this new emission band changes significantly as the delay time increases. This may be attributed to the reorientation of the anthracene molecules in the defect sites stabilizing the excimeric state. This also explains the wavelength dependence of τ2 and τ3 (Table 1) and time evolution of the transient fluorescence spectra in the 450−600 nm region (Figure 6). 3.4. Transient Absorption Studies. Transient absorption (TA) spectroscopic technique has been employed to obtain information regarding the ultrafast dynamics of the excitonic state populated in the anthracene nanoaggregate. Timeresolved TA spectra recorded following photoexcitation of the anthracene nanoaggregate sample at 400 nm are shown in Figure 7. The spectrum recorded at 0.3 ps delay time consists of a broad excited state absorption (ESA) band in the 460−750 nm region with a maximum at ca. 580 nm. With increase in delay time, the entire ESA band decays leading to the development of a weak negative absorption band in the 450− E


Article

The Journal of Physical Chemistry C k2

S1 + S1 → Sn + S0

(2)

k3

S1 + D → E

(3)

Equation 1 represents the natural decay of the free S1 excitonic state to the ground state, S0, whereas eq 2 represents the second order decay of the S1 excitonic state due to exciton− exciton annihilation process, which populates Sn (n > 1) excitonic state and the S0 state. Sn excitonic state undergoes ultrafast vibrational relaxation and internal conversion process and produces back the S1 excitonic state. Equation 3 represents the trapping of the S1 excitonic states at defect sites (D) leading to population of the excimeric state (E). We write the rate equation for the S1 excitonic state as d[S1] k [S ]2 = −k1′[S1] − 2 1 dt 2

(4)

k1′ represents the annihilation-free decay of the S1 excitonic state, i.e., combination of natural decay (eq 1) and decay due to excimer formation (eq 3), i.e., k1′ = k1 + k3[D]. Here, we assume that the defect concentration is fixed for a particular sample of the nanoaggregate prepared for the experiment. k2 represents the exciton−exciton annihilation decay rate constant. Solution of eq 4 gives us the concentration of the S1 excitonic state at time “t” (eq 5) (see Supporting Information section, section SI-2). Figure 7. (A) Time-resolved transient absorption spectra recorded at different delay times following photoexcitation of the nanoaggregate sample by using 400 nm laser pulses. (B) Temporal profiles recorded at three different wavelengths, along with the best fit functions (they have been normalized at the maximum ESA value).

[S1] = [S1]0

k 2[S1]0 2

(6)

Decay of the ESA band represents the decay of the S1 excitonic state. Therefore, we write

650 nm region. The entire ESA band is assigned to the excitonic state populated upon photoexcitation. Considering its signature in the TRANES, the negative absorption band appeared after 30 ps delay time is assigned to stimulated emission from the excimeric state. However, because of limitation of our femtosecond spectrometer (having provision of delay time up to about 1 ns), we could not measure complete evolution of the SE band. Figure 7B shows the temporal evolution of the transient species recorded at three selective wavelengths. Each of them reveals instrument response time limited rise of ESA followed by a nonexponential decay. We also observe that the temporal profiles show dependence on excitation intensity, and this suggests that the nonexponential decay of the excitonic state in subpicosecond time domain is the result of exciton−exciton annihilation reaction (vide infra).20,36,67 In addition, since the ESA band is overlapped with the stimulated emission spectrum of the excimeric state, the temporal profiles should also be associated with a component representing the rise of stimulated emission (negative absorbance). Therefore, we adopt the following kinetic model to analyze the temporal profiles recorded in the transient absorption experiments. 3.4.1. Singlet−Singlet Exciton Annihilation. The S 1 excitonic state populated following photoexcitation of the anthracene nanoaggregate sample undergoes the following processes: k1

(5)

Here, k 2′ =

S1 → S0

[exp( −k1′t )] (1 + k 2′t )

[ΔOD]ESA = [ΔOD]0

[exp( −k1′t )] (1 + k 2′τ )

(7)

Population of the excimeric state is associated with the rise of stimulated emission or negative absorption signal. Following eq 3, the rate equation for evolution of the stimulated emission signal can be written as dE = k 3[D][S1] dt

(8)

Using the value of [S1] estimated in eq 5 and making an assumption that [D] is constant for the particular sample of nanoaggregate investigated, we can solve the above equation to obtain the expression for [E], [E] = [S1]0 k 3[D]t[exp(−k1t )]]

(9)

Since stimulated emission from the excimeric state contributes as a negative signal to the decay of the transient absorption signal, we write [ΔOD]SE = −[ΔOD]0 k 3′t[exp( −k1t )]]

(10)

Here,

k 3′ = k 3[D]α

(11)

where α is a constant, which is the ratio of the stimulated absorption cross section of the S1 state and the stimulated

(1) F


Article

The Journal of Physical Chemistry C emission cross section of the excimeric state at a particular wavelength and hence is wavelength dependent. The excimeric state has a very long lifetime (>15 ns), and hence, the negative absorption signal due to this state, following its rise, will remain constant as a residual emission signal (R). Therefore, the transient absorption signal has three major contributions, which may be represented as [ΔOD]Total = [ΔOD]ESA + [ΔOD]SE + R [ΔOD]Total = [ΔOD]0

(12)

[exp( −k1′t )] − [ΔOD]0 k 3′ (1 + k 2′t )

t[exp( −k1′t )]] + R

(13)

We find that all the experimentally recorded decay profiles could be fitted well with eq 13 (Figure 7B) and Table 2 Table 2. Parameters Best-Fitting the Temporal Profiles Recorded at 470, 570, and 670 nm Using the Excitation Pulse Energy of 6 μJ/Pulse parameter

470 nm

570 nm

670 nm

[ΔOD]0 (mOD) k1′ (109 s−1) k′2 (1011 s−1) k3′ (108 s−1) R (10−3)

11.1 ± 1 2.9 ± 0.3 2.17 ± 0.1 8.0 ± 1 −5.1 ± 1

6.46 ± 0.7 3.1 ± 0.3 1.52 ± 0.08 2.0 ± 0.5 −0.2 ± 0.05

6.31 ± 0.06 2.9 ± 0.4 7.55 ± 0.08 1.0 ± 0.2 −0.1 ± 0.02

Figure 8. Temporal TA profiles recorded at 590 nm following photoexcitation of anthracene nanoaggregate using 400 nm light of different pump intensities, along with the best-fit functions following eq 13. Inset: plot of k2 vs [S1]0.

optoelectronic devices. The exciton diffusion constant, D, is related to the annihilation rate constant, k2, by eq 14

k 2 = 8πR aD

(14)

where Ra is the annihilation radius. The value of Ra can be estimated by determining the threshold value of the exciton density or [S1]th for on-setting the exciton−exciton annihilation process. [S1]th has been estimated from the plot of k2′ vs the exciton density at zero delay time or [S1]0. In turn, [S1]0 has been calculated for a given excitation intensity using the percent absorption of light by the sample, the excitation volume, and the average number of anthracene molecules per aggregate (see the Supporting Information, section SI-4). k2′ should have a nonzero value if the excitation intensity is large enough to create a population density of exciton, which is larger than the threshold exciton density. In the case of anthracene nanoaggregate studied here, we estimate the value of [S1]th as 0.4 × 1020 cm−3, which represents the fact that for this value of exciton density, there is only one exciton within a sphere of volume of about 25 nm3. The approximate value of the radius of this sphere is about 1.8 nm, which may be considered as the annihilation radius for the singlet exciton in the anthracene nanoaggregates. We could also determine the value of k2 (3.7 × 10−9 cm3 s−1) from the slope of the plot of k2′ vs [S1]0 (inset of Figure 8 and Table 3). Using these values of k2 and Ra in eq 14, we determine the value of exciton diffusion coefficient, D, as 0.83 × 10−3 cm2 s−1. It has been observed that for the anthracene crystal the

presents the parameters best-fitting the temporal profiles recorded at 470, 570, and 670 nm using the excitation pulse energy of 6 μJ/pulse. The inverse of k′1[τ = (k′1)−1 = 0.35 ns], which is close to the lifetime τ1 (Table 1) estimated from the TCSPC measurements, represents the lifetime of the free S1 excitonic state. The value of k2′ has been observed to be dependent on the monitoring wavelength, and it also depends upon the value of [ΔOD]0. We have performed the intensity dependent measurements of the temporal profiles to find out the annihilation rate constant and other exciton diffusion parameters for the nanoaggregate. At high excitation intensities, exciton−exciton annihilation process is considered to be a dominating decay process. Rate of this process is directly dependent on the rates of the diffusive motion of the excitons in the nanoaggregate and hence has been rightly used for estimation of the exciton diffusion parameters.20,36,67 We have recorded the temporal profiles at 590 nm by varying the excitation pulse energies in the range of 3−12.5 μJ/pulse. These temporal profiles have been fitted using eq 13 (Figure 8), and best-fit parameters are provided in Table SI-3 (in Supporting Information). In this analysis, we have only considered the contribution from the singlet excitons. Population of the triplet excitons occurs at relatively longer time domain, and therefore, we neglect the contribution of the triplet excitons to the temporal profiles of the transients recorded in the subpicosecond time domain (vide infra). We mentioned earlier that exciton−exciton annihilation is a diffusion controlled process. Therefore, the rate constant of the exciton−exciton annihilation process has been frequently used to calculate the exciton diffusion coefficient as well as the exciton diffusion length. Exciton diffusion length is one of the important parameters, which need to be known in the case of applications of the material in photovoltaic and other

Table 3. Estimation of k2 from the Excitation Energy Dependence of [S1]0 (Inset of Figure 8) excitation energy (μJ/pulse)

[S1]0 (×1020 cm−3)

k2′ (×1011 s−1)

k2 (cm−3 s−1)

± ± ± ± ±

2.66 ± 0.07 1.99 ± 0.04 1.5 ± 0.04 1.02 ± 0.04 0.73 ± 0.02

4.28 ± 0.26 2.95 ± 0.17 2.105 ± 0.14 1.2 ± 0.09 0.748 ± 0.06

(3.7 ± 1.6) × 10−9

12.1 9.05 6.85 4.64 3.31 G

0.3 0.2 0.2 0.2 0.1


Article


Table 4. Comparison of the Annihilation Rate Constants, Exciton Diffusion Coefficients, and Diffusion Lengths for Singlet Exciton in Anthracene Crystal and Nanoaggregate system a

crystal nanoaggregate a

k2 (cm3 s−1) −9

(5−20) × 10 (3.7 ± 1.6) × 10−9

D (cm2 s−1)

Ld (nm)

(1−10) × 10−3 (0.83 ± 0.3) × 10−3

25−100 13.2 ± 4

References 20, 68−73.

reported exciton diffusion constant has a wide range of values and the exciton diffusion process is anisotropic in nature. In this present work, we have assumed that the diffusion process is isotropic in the nanoaggregate samples. The value of the diffusion coefficient thus determined for the anthracene nanoaggregate is nearly similar to those determined in the solid crystals (Table 4).68,69 We estimate the exciton diffusion length, LD, in the nanoaggregate using eq 15,

L D = (6Dτ )1/2

(15)

By use of the value of the lifetime of the exciton (τ = 0.35 ps), the diffusion length has been calculated to be ∼13 nm, which is also nearly similar to that in crystalline anthracene (Table 4). However, shorter S1 exciton lifetime results in a slight decrease in the exciton diffusion length in the nanoaggregate sample.69 Comparison of these values in the anthracene crystal and nanaoaggregate further ensures that the matrices or molecular packings in these two kinds of materials are not very much different. The effect of the defect sites on the exciton diffusion length has already been reported by Cohen et al. They have estimated the diffusion length in smooth and rough crystal regions of anthracene.72 For the smooth crystal region, where the proper molecular packing is expected, the exciton diffusion length determined is about 49 nm. On the other hand, for the rough region with a lot of defect sites, the exciton diffusion length has been found to be shorter by about 10−15 nm.72 It has also been known that crystals formed by different methods possess different exciton diffusion lengths. According to the reports available in the literature, the exciton diffusion length in a crystal grown from the molecular vapor is about 105 nm whereas this value is shortened to about 40−50 nm in a crystal grown from solution and to about 25 nm in that grown from melt.72 This means that exciton diffusion length varies in crystals grown under different conditions. Therefore, the marginal reduction in exciton diffusion length as compared to that of the crystals is predicted considering the presence of partial amorphous nature in the nanoaggregates. 3.4.2. Triplet−Triplet Exciton Annihilation. The yield of the intersystem crossing (ISC) process originating from the S1 state of anthracene in hexane has been reported to be about 0.55.74 However, in solid thin films or in crystalline forms, the energy level of the T2 state lies above that of the S1 state. As a consequence of this, the yield of the triplet state is reduced by about 2 orders of magnitude.44,45 Yield of the triplet exciton in the nanoaggregates is also a point of interest to us. In Figure 9, we have presented the time-resolved transient absorption spectra recorded following photoexcitation of anthracene nanoaggregate using laser pulses of 355 nm wavelength and 10 ns duration. The absorption spectrum of the triplet exciton is very broad extending through the 300−750 nm region, on which the fine structures superimposed may have arisen because of ground state bleaching. Triplet absorption spectrum of anthracene in acetonitrile solution reported earlier shows the presence of a

Figure 9. Time-resolved transient absorption spectra recorded at different delay times following photoexcitation of the anthracene nanoaggregate sample using 355 nm laser light of 10 ns duration.

sharp band at ∼420 nm and another broad band in the 450− 650 nm region.75−77 We could observe the similar features in the spectrum recorded in the nanoaggregate, although the absorption band at 420 nm is overlapped on a strong bleaching band in the 300−400 nm region and the band in the 450−700 nm is much more broad, possibly because of strong interactions between the triplet exciton and the lattice. Relaxation of the triplet exciton and lattice interaction is quite evident from the time evolution of the shape of the triplet exciton spectrum. However, we are unable to comment on the possibility of triplet exciton trapping or population of the triplet excimeric state in the nanoaggregate. Long-lived triplet excitons in anthracene crystal have earlier been detected by simultaneous observation of both their weak red phosphorescence as well as delayed blue fluorescence due to exciton−exciton annihilation.76 While the free−free interaction, which follows bimolecular kinetic equations, is dominant at room temperature, interactions between free and shallowly trapped excitons have been reported to be much more efficient in yielding delayed blue fluorescence at low temperature.76 There are a few reports available in the literature about the triplet exciton trapping in anthracene crystal.78,79 Arnold et al. have artificially created crystal dislocation sites in anthracene crystal and trapped triplet excitons.78 In another report, Goode et al. reported population of the triplet excimeric state at imperfect crystal sites.79 In nanoaggregates, because of imperfect molecular packing, the presence of defect sites makes trapping of the triplet excitons highly probable and reduce the triplet exciton lifetime and exciton diffusion length. The broad excited state absorption band observed in the time-resolved spectra recorded at early time scale may possibly have some contribution from the triplet excimeric state too. We have estimated the triplet exciton yield in the anthracene nanoaggregate to be about 0.14 ± 0.03 by using a method reported by Bachilo and Weisman80 (see section SI-5 in the Supporting Information for detailed analysis). H


Article


populated in anthracene crystal.81,82 The values of the annihilation radius and the diffusion coefficient for the triplet exciton could be estimated as 1.26 nm and 7.0 × 10−8 cm2 s−1, respectively. The diffusion coefficient of the triplet exciton in the nanoaggreagte is thus smaller by 2 orders of magnitude as compared to that reported for the crystal (Table 5).83 Smaller diffusion coefficient and shorter triplet exciton lifetime are the responsible factors for reduction of the diffusion length of the triplet exciton by about 3 orders of magnitude.84 More significant effect on the triplet exciton diffusion, as compared to that of the singlet exciton, can be explained on the basis of the mechanism associated with the triplet exciton diffusion. The triplet exciton diffusion occurs through electronic spin exchange mechanism, which is a slow process.88

To understand the dynamics of the triplet exciton, we recorded the temporal profiles at 540 nm using different excitation intensities. Like in the case of the singlet exciton, we found strong intensity dependence of the temporal absorption profiles suggesting triplet−triplet exciton annihilation. These temporal profiles have also been analyzed following the mixed order decay scheme. Decay of the triplet excitonic state follows mainly two parallel processes: (1) natural radiative and/or nonradiative decay (eq 16) and (2) triplet−triplet exciton annihilation reaction (eq 17). k1(T)

T1 ⎯⎯⎯⎯→ S0

(16)

k 2(T)

T1 + T1 ⎯⎯⎯⎯→ Sn + S0

(17)

Solving the kinetic equations corresponding to these two processes, we derive eq 18, which represents the decay of the triplet excitons, [ΔOD]ESA = [ΔOD]0

[exp( −k1′(T )t )] [1 + k 2′(T )t ]

4. CONCLUSION Anthracene nanoaggregates (NAs) have been synthesized using well-known reprecipitation method in the presence of PVA stabilizer. The bulk solid-like crystal structure of the anthracene nanoaggregate consists of significantly large concentration of defect sites. The DLS and AFM analyses reveal that the particles have shapes of nanodisks with the average diameter and height of about 260 and 50 nm, respectively. Maxima of the 0−0 vibronic bands in the absorption and emission spectra of the NA are red-shifted by about 20 nm as compared to the corresponding bands of the monomeric species. Time-resolved emission study shows wavelength dependent dynamics, which originates from the distribution of particle size as well as the orientational heterogeneities in the aggregate leading to different types of defect sites and hence the excimeric states of different conformations. Transient absorption studies reveal nonexponential dynamics of both the singlet and triplet excitonic states due to exciton−exciton annihilation reactions at high excitation intensities. Exciton annihilation rate constants of the singlet and the triplet excitons have been determined as 3.7 × 10−9 and 2.2 × 10−13 cm3 s−1, respectively, and exciton diffusion lengths to be about 13.2 and 6.5 nm, respectively. The singlet exciton diffusion length is comparable to that reported earlier in the anthracene crystal, and this suggests that the nanoaggreagates may also be considered as materials for efficient optoelectronic and photovoltaic devices. However, drastic reduction in triplet exciton diffusion coefficient in the nanoaggregate form is explained considering slower triplet energy transfer mechanism via spin exchange.

(18)

Temporal profiles along with the best-fit functions following eq 18 are shown in Figure 10, and the inset of this figure shows

Figure 10. Temporal TA profiles, along with the best-fit functions, recorded at 544 nm following photoexcitation of the anthracene nanoaggregate by using different pump intensities. Inset diagram represents the estimated k2′ (T) vs [T1]0 plot.

■

ASSOCIATED CONTENT

S Supporting Information *

the plot of k′2(T) vs density of triplet exciton, [T1]0. Procedures for calculation of the [T1]0 values at different excitation intensities and the values have been provided in the Supporting Information (section SI-6 and Table SI-7). Triplet−triplet exciton annihilation rate constant, k2(T), thus estimated is 2.22 × 10−13 cm3 s−1. This value is smaller by nearly 2 orders of magnitude as compared to that estimated for the triplet exciton

Absorption spectra and excitation spectra (for emission at 425 nm) of anthracene dissolved in cyclohexane; derivation of the equations for the fitting of TA temporal profiles; table listing parameters best-fitting to the temporal TA profiles recorded at 590 nm at different excitation intensities by using mixed order decay kinetics (eq 13); calculation of singlet exciton density, [S1]0; calculation of triplet exciton yield; triplet exciton density

Table 5. Comparison of the Triplet−Triplet Exciton Annihilation Rate Constant, k2(T), Diffusion Coefficient, and Diffusion Length of the Triplet Exciton in the Solid Crystal and the Nanoaggregate τ(T1-exciton)

system a

solid crystal nanoaggregate a

40 ms 1 ± 0.1 μs

k2(T) (cm3 s−1) −11

(0.5−5) × 10 (2.2 ± 0.8) × 10−13

D (cm2 s−1)

Ld (nm)

(5−50) × 10−6 (7.0 ± 2) × 10−8

10000−20000 6.5 ± 2

References 80−87. I


Article


(13) Zhao, Y. S.; Zhan, P.; Kim, J.; Sun, C.; Huang, J. Patterned Growth of Vertically Aligned Organic Nanowire Waveguide Arrays. ACS Nano 2010, 4, 1630−1636. (14) Hinoue, T.; Shigenoi, Y.; Sugino, M.; Mizobe, Y.; Hisaki, I.; Miyata, M.; Tohnai, N. Regulation of π-Stacked Anthracene Arrangement for Fluorescence Modulation of Organic Solid from Monomer to Excited Oligomer Emission. Chem.Eur. J. 2012, 18, 4634−4643. (15) Latterini, L.; Roscini, C.; Carlotti, B.; Aloisi, G. G.; Elisei, F. Synthesis and Characterization of Perylene Nanoparticles. Phys. Status Solidi A 2006, 203, 1470−1475. (16) Chung, H. R.; Kwon, E.; Oikawa, H.; Kasai, H.; Nakanishi, H. Effect of Solvent on Organic Nanocrystal Growth using the Reprecipitation Method. J. Cryst. Growth 2006, 294, 459−463. (17) Seko, T.; Ogura, K.; Kawakami, Y.; Sugino, H.; Tanaka, J. Excimer Emission of Anthracene, Perylene, Coronene and Pyrene Microcrystal Dispersed in Water. Chem. Phys. Lett. 1998, 291, 438− 444. (18) Fujino, T.; Tahara, T. Femtosecond Fluorescence UpConversion Microscopy: Exciton Dynamics in α-Perylene Microcrystal. J. Phys. Chem. B 2003, 107, 5120−5122. (19) Furube, A.; Murai, M.; Tamaki, Y.; Watanabe, S.; Katoh, R. Effect of Aggregation on the Excited-State Electronic Structure of Perylene Studied by Transient Absorption Spectroscopy. J. Phys. Chem. A 2006, 110, 6465−6471. (20) Yago, T.; Tamaki, Y.; Furube, A.; Katoh, R. Self-Trapping Limited Exciton Diffusion in a Monomeric Perylene Crystal As Revealed by Femtosecond Transient Absorption Microscopy. Phys. Chem. Chem. Phys. 2008, 10, 4435−4441. (21) Burdett, J. J.; Müller, A. M.; Gosztola, D.; Bardeen, C. J. Excited State Dynamics in Solid and Monomeric Tetracene: The Roles of Superradiance and Exciton Fission. J. Chem. Phys. 2010, 133, 144506. (22) Ahn, T.-S.; Müller, A. M.; Al-Kaysi, R. O.; Spano, F. C.; Norton, J. E.; Beljonne, D.; Brédas, J.-L.; Bardeen, C. J. Experimental and Theoretical Study of Temperature Dependent Exciton. J. Chem. Phys. 2008, 128, 54505−54511. (23) Briseno, A. L.; Aizenber, J.; Han, Y. J.; Penkada, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. Patterned Growth of Large Oriented Organic Semiconductor Single Crystals on Self-Assembled Monolayer Templates. J. Am. Chem. Soc. 2005, 127, 12164−12165. (24) Dutta, A. K. Characterization of Aggregates of Nonamphiphilic Anthracene Assembled in Ultrathin Supramolecular Langmuir− Blodgett Films. Langmuir 1997, 13, 5678−5684. (25) Clark, L. B.; Philpott, M. R. Anisotropy of the Singlet Transitions of Crystalline Anthracene. J. Chem. Phys. 1970, 53, 3790− 3801. (26) Fedotov, V. A.; Marinov, K.; Boardman, A. D.; Zheludev, N. I. On the Aromagnetism and Anapole Moment of Anthracene Nanocrystals. New J. Phys. 2007, 9, 95. (27) Meyer, Y. H.; Astier, R.; Leclerq, J. M. Triplet-Triplet Spectroscopy of Polyacenes. J. Chem. Phys. 1972, 56, 801−815. (28) Katoh, R.; Susuki, K.; Furube, A.; Kotani, M.; Tokumaru, K. Fluorescence Quantum Yield of Aromatic Hydrocarbon Crystals. J. Phys. Chem. C 2009, 113, 2961−2965. (29) Garnett, E.; Yang, P. Light Trapping in Silicon Nanowire Solar Cells. Nano Lett. 2010, 10, 1082−1087. (30) Gabor, N. M.; Zhong, Z.; Bosnick, K.; Park, J.; Mceuen, P. L. Extremely Efficient Multiple Electron-Hole Pair Generation in Carbon Nanotube Photodiodes. Science 2009, 325, 1367−1371. (31) Takahasi, Y.; Tomura, M. Diffussion of Singlet Excitons in Anthracene Crystals. J. Phys. Soc. Jpn. 1971, 31, 1100−1108. (32) Seefeld, K. P.; Muller, H.; Bassler, H. Energy Transfer in Amorphous Anthracene-Tetracene Films. J. Lumin. 1978, 16, 395− 404. (33) Galanin, M. D.; Khan-Magometova, S. D.; Chizhikova, Z. A.; Demchuk, M. I.; Chernyavskii, A. F. The Spectroscopic Investigation of the Fluorescence Decay Time of the Anthracene Crystal. J. Lumin. 1975, 9, 459−466.

calculation [T1]0; table listing estimation of k2(T) from the excitation energy dependence of [T1]0. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b00469.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: 91-22-2559 5091. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the kind help of Dr. V. Sudarshan and Dr. Rajib Ganguly of Chemistry Division, BARC, Mumbai, India, for AFM and DLS measurements of the nanoaggregate samples. We are also thankful to Dr. A. K. Tyagi and Mohsin Jafar of Chemistry Division, BARC, Mumbai, India, for their kind help in the XRD measurement of the samples.

■

REFERENCES

(1) Liu, H.; Xu, J.; Li, Y. Aggregate Nanostructures of Organic Molecular Materials. Acc. Chem. Res. 2010, 43, 1496−1508. (2) Cui, S.; Liu, H.; Gan, L.; Li, Y.; Zhu, D. Fabrication of Low Dimensional Nanostructures Based on Organic Conjugated Molecules. Adv. Mater. 2008, 20, 2918−2925. (3) Palmer, L. C.; Stupp, S. I. Molecular Self-Assembly into OneDimensional Nanostructures. Acc. Chem. Res. 2008, 41, 1674−1684. (4) Bhongle, C. J.; Chang, C. W.; Lee, C. S.; Diau, E. W. G.; Shu, C. Relaxation Dynamics and Structural Characterization of Organic Nanoparticles with Enhanced Emission. J. Phys. Chem. B 2005, 109, 13472−13482. (5) Yao, H.; Yamashita, M.; Kimura, K. Organic Styryl Dye Nanoparticles: Synthesis and Unique Spectroscopic Properties. Langmuir 2009, 25, 1131−1137. (6) Xiao, J.; Yang, H.; Yin, Z.; Guo, J.; Boey, F.; Zhang, H.; Zhang, Q. Preparation, Characterization and Photoswitching/Light-Emitting Behaviors of Coronene Nanowires. J. Mater. Chem. 2011, 21, 1423− 1427. (7) Xiao, D.; Xi, L.; Yang, W.; Fu, H.; Shuai, Z.; Fang, Y.; Yao, J. SizeTunable Emission from 1,3-Diphenyl-5-(2-anthryl)-2-pyrazoline Nanoparticles. J. Am. Chem. Soc. 2003, 125, 6740−6745. (8) Meng, H.; Sun, F.; Goldfinger, M. B.; Jaycox, G. D.; Li, Z.; Marshall, W. J.; Blackman, G. S. High-Performance, Stable Organic Thin-Film Field-Effect Transistors Based on Bis-5′-alkylthiophen-2′-yl2,6-anthracene Semiconductors. J. Am. Chem. Soc. 2005, 127, 2406− 2407. (9) Meng, H.; Sun, F.; Goldfinger, M. B.; Gao, F.; Londono, D. J.; Marshall, W. J.; Blackman, G. S.; Dobbs, K. D.; Key, D. E. 2,6-Bis[2(4-pentylphenyl)vinyl]anthracene: A Stable and High Charge Mobility Organic Semiconductor with Densely Packed Crystal Structure. J. Am. Chem. Soc. 2006, 128, 9304−9305. (10) Yu, J.-Y.; Chen, I.-W. P.; Chen, C.-H.; Lee, S.-J.; Chen, I.-C.; Lin, C.-S.; Cheng, C.-H. On the Nanoaggregated Emitter of All sp2Hybridized Bistriphenylenyl in the Device Layout of Organic LightEmitting Diodes. J. Phys. Chem. C 2008, 112, 3097−3102. (11) Chao, Y. C.; Chung, C. H.; Zan, H. W.; Meng, H. F.; Ku, M. C. High-Performance Vertical Polymer Nanorod Transistors Based on Air-Stable Conjugated Polymer. Appl. Phys. Lett. 2011, 99, 233308. (12) Liang, W.-W.; Chang, C.-Y.; Lai, Y.-Y.; Cheng, S.-W.; Chang, H.-H.; Lai, Y.-Y.; Cheng, Y.-J.; Wang, C.-L.; Hsu, C.-S. Formation of Nanostructured Fullerene Interlayer through Accelerated SelfAssembly and Cross-Linking of Trichlorosilane Moieties Leading to Enhanced Efficiency of Photovoltaic Cells. Macromolecules 2013, 46, 4781−4789. J


Article

The Journal of Physical Chemistry C (34) Horiguchi, R.; Iwasaki, N.; Maruyama, Y. Time-Resolved and Temperature-Dependent Fluorescence Spectra of Anthracene and Pyrene in Crystalline and Liquid States. J. Phys. Chem. 1987, 91, 5135−5139. (35) Oliveira, D.; Baba, K.; Teizer, W.; Kasai, H.; Oikawa, H.; Nakanishi, H. Stopped-Flow Studies of the Formation of Organic Nanocrystals in the Reprecipitation Method. In Nanocrystal; Masuda, Y., Ed.; InTech: Rijeka, Croatia, 2011. (36) Kakade, S.; Ghosh, R.; Palit, D. K. Excited State Dynamics of Zinc−Phthalocyanine Nanoaggregates in Strong Hydrogen Bonding Solvents. J. Phys. Chem. C 2012, 116, 15155−15166. (37) Wu, J. H.; Xu, T. Z.; Ang, S. G.; Xu, Q.-H.; Xu, G. Q. Radially Oriented Anthracene Nanowire Arrays: Preparation, Growth Mechanism, and Optical Fluorescence. Nanoscale 2011, 3, 1855−1860. (38) Liu, H.; Li, Y.; Xiao, S.; Gan, H.; Jiu, T.; Li, H.; Lei, J.; Zhu, D.; Yu, D.; Xiang, B.; et al. Synthesis of Organic One-Dimensional Nanomaterials by Solid-Phase Reaction. J. Am. Chem. Soc. 2003, 125, 10794−10795. (39) Zhao, M.; Wang, S.; Bao, Q.; Wang, Y.; Aug, P. K.; Loh, K. P. A Simple High Yield Method for the Synthesis of Organic Wires from Aromatic Molecules Using Nitric Acid as the Solvent. Chem. Commun. 2011, 47, 4153−4155. (40) Ghosh, R.; Mandal, J. A.; Palit, D. K. Ultrafast Dynamics of Excited States of Curcumin in Solution. J. Phys. Chem. B 2010, 114, 12129−12143. (41) Madhurambal, G.; Srinivasan, P. A. Growth of High Quality Crystals by a Simple Solution Technique. Cryst. Res. Technol. 2006, 41, 231−235. (42) DeHaven, S.; Wincheski, R.; Albin, S. Anthracene Fibers in a Microstructured Optical Fiber for X-ray Detection. Materials 2014, 7, 6291−6303. (43) Philpott, M. R. Dipole Davydov Splittings in Crystalline Anthracene, Tetracene, Naphthalene, and Phenanthrene. J. Chem. Phys. 1969, 50, 5117−5128. (44) Köhler, A.; Bässler, H. Triplet States in Organic Semiconductors. Mater. Sci. Eng. R 2009, 66, 71−109. (45) Kellogg, R. E. Second Triplet States of Anthracene. J. Chem. Phys. 1966, 44, 411−412. (46) Mulder, B. J. Symmetry of the Fluorescence and Absorption Spectra of Anthracene Crystals. J. Phys. Chem. Solids 1968, 29, 182− 184. (47) Nishimura, H.; Yamaoka, T.; Hattori, K.; Matsui, A.; Mizuno, K. Wavelength-Dependent Decay Times and Time-Dependent Spectra of Singlet-Exciton Luminescence in Anthracene Crystals. J. Phys. Soc. Jpn. 1985, 54, 4370−4381. (48) Kalinowski, J. Excimers and Exciplexes in Organic Electroluminescence. Mater. Sci.-Pol. 2009, 27, 735−756. (49) Kalinowski, J.; Giro, G.; Cocchi, M.; Fattori, V.; Zamboni, R. The Nature of Emitting States in Electroluminescence of Polymeric Films Doped with Anthracene and Anthracene-Based Supramolecules. Chem. Phys. 2002, 277, 387−396. (50) Bateman, R. J.; Chance, R. R.; Hornig, J. F. Fluorescence Reabsorption in Anthracene Single Crystals: Lifetime Variations with Emission Wavelength and Temperature. Chem. Phys. 1974, 4, 402− 408. (51) Nishimura, H. Effect of Exciton Diffusion and Reabsorption on Decay Curve of Exciton Luminescence. J. Phys. Colloq. 1985, 46 (C7), 55−59. (52) Nozue, Y.; Goto, T. Picosecond Studies of Excitonic Luminescence and Reabsorption Effect in Anthracene Crystals. J. Phys. Soc. Jpn. 1989, 58, 1831−1837. (53) Shimura, H.; Ohba, Y. Time-Resolved Reabsorption Effect on the Fluorescence Decay of Anthracene Crystal and Solution. Jpn. J. Appl. Phys. 1981, 20, 1683−1690. (54) Birks, J. B. The Influence of Reabsorption and Defects on Anthracene Crystal Fluorescence. Mol. Cryst. Liq. Cryst. 1973, 28, 117−129. (55) Nozue, Y. Spatial Distribution and Dynamics of Excitons in Anthracene Crystals. J. Phys. Soc. Jpn. 1988, 57, 3204−3219.

(56) Kobayaschi, M.; Mizuno, K.; Matsui, A. High-Pressure Study of Free Excitons and Self-Trapped Excitons in Anthracene Crystals at 1.5 K. J. Phys. Soc. Jpn. 1989, 58, 809−812. (57) Breton, G. W.; Vang, X. Photodimerization of Anthracene. J. Chem. Educ. 1998, 75, 81−82. (58) Jones, P. F.; Nicol, M. Excimer Emission of Napthalene, Anthracene, and Phenanthrene Crystals Produced by Very High Pressures. J. Chem. Phys. 1968, 48, 5440−5447. (59) Dreger, Z. A.; Balasubramaniam, E.; Gupta, Y. M.; Joly, A. G. High-Pressure Effects on the Electronic Structure of Anthracene Single Crystals: Role of Nonhydrostaticity. J. Phys. Chem. A 2009, 113, 1489− 1496. (60) Hofmann, J.; Seefeld, K. P.; Hofberger, W.; Bässler, H. Excimer in Solid Non-Crystalline Anthracene. Mol. Phys. 1979, 37, 973−979. (61) Fielding, P. E.; Jarnagin, R. C. “Excimer” and “Defect” Structure for Anthracene and Some Derivatives in Crystal, Thin Films, and Other Rigid Matrices. J. Chem. Phys. 1967, 47, 247−252. (62) Mataga, N.; Torihashi, Y.; Ota, Y. Studies on the Fluorescence Decay Times of Anthracene and Perylene Excimers in Rigid Matrices at Low Temperatures in Relation to the Structures of Excimers. Chem. Phys. Lett. 1967, 1, 385−387. (63) Dreger, Z. A.; Lucas, H.; Gupta, Y. M. High-Pressure Effects on Fluorescence of Anthracene Crystals. J. Phys. Chem. B 2003, 107, 9268−9274. (64) Nozue, Y. Spatical Distribution and Dynamics of Excitons in Anthracene Crystals. J. Phys. Soc. Jpn. 1988, 57, 3204−3219. (65) Koti, A. S. R.; Krishna, M. M. G.; Periasamy, N. Time Resolved Area-Normalized Emission Spectroscopy (TRANES): A Novel Method for Confirming Emission from Two Excited States. J. Phys. Chem. A 2001, 105, 1767−1771. (66) Koti, A. S. R.; Periasamy, N. Time Resolved Area Normalized Emission Spectroscopy (TRANES) of DMABN Confirms Emission from Two States. Res. Chem. Intermed. 2002, 28, 831−836. (67) Paillotin, G.; Swenberg, C. E.; Breton, J.; Geacintov, N. E. Analysis of Picosecond Laser Induced Fluorescence Phenomena in Photosynthetic Membranes Utilizing a Master Equation Approach. Biophys. J. 1979, 25, 513−534. (68) Katoh, R.; Kotani, M. Photoionization of a Singlet Exciton in an Anthracene Single Crystal through Two Color, Two Step Excitation. J. Chem. Phys. 1991, 94, 5954−5960. (69) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999. (70) Schwoerer, M.; Wolf, H. C. Organic Molecular Solids; WileyVCH Verlag Gmbh & Co. KGaA: Weinheim, Germany, 2007. (71) Jasny, J.; Sepiol, J. Nanosecond Transient Absorption Spectrophtometer with Dye Laser Probe. J. Phys. E: Sci. Instrum. 1981, 14, 493−497. (72) Cohen, M. D.; Klein, E.; Ludmer, Z. Micromeasurements of Exciton Diffusion Lengths in Single Crystals of Anthracene. Chem. Phys. Lett. 1976, 37, 611−613. (73) Meth, J. S.; Marshall, C. D.; Fayer, M. D. An Examination of Radiative and Nonradiative Excitation Transport in Thin Anthracene Crystals: Transient Grating Experiments. Solid State Commun. 1990, 74, 281−284. (74) Donati, D.; Williams, J. O. Exciton Diffusion Lengths for Pure and Doped Anthracene Single Crystals from Microscopic Measurements. Mol. Cryst. Liq. Cryst. 1978, 44, 23−32. (75) Windsor, M. W.; Dawson, W. R. Quantum Efficiencies of Triplet Formation in Aromatic Molecules. Mol. Cryst. 1968, 4, 253− 258. (76) Smith, G. C. Triplet Exciton Phosphorescence in Crystalline Anthracene. Phys. Rev. 1968, 166, 839−847. (77) Babenko, S. D.; Benderskii, V. A.; Goldanskii, V. I.; Lavrushko, A. G.; Tychinskii, V. P. Singlet Exciton Annihilation in Anthracene Crystal. Phys. Status Solidi B 1971, 45, 91−97. (78) Arnold, S.; Whitten, W. B.; Damask, A. C. Triplet Exciton Trapping by Dislocations in Anthracene. J. Chem. Phys. 1970, 53, 2878−2884. K


Article

The Journal of Physical Chemistry C (79) Goode, D.; Lupien, Y.; Siebrand, W.; Williams, D. F.; Thomas, J. M.; Williams, J. O. Triplet Excitons as Probes for Structural Imperfections in Crystalline Anthracene. Chem. Phys. Lett. 1974, 25, 308−311. (80) Bachilo, S. M.; Weisman, R. B. Determination of Triplet Quantum Yields from Triplet-Triplet Annihilation Fluorescence. J. Phys. Chem. A 2000, 104, 7111−7114. (81) Kostin, A. K.; Vannikov, A. V. Spectral and Electrical Investigations of Triplet Excitons on Pulsed Electron Excitation of Molecular Crystals. Radiat. Phys. Chem. 1980, 15, 613−616. (82) Avakian, P.; Merrifield, R. E. Triplet Excitons in Anthracene Crystal. Mol. Cryst. 1968, 5, 37−77. (83) Swenberg, C. E. Theory of Triplet Exciton Annihilation in Polyacene Crystals. J. Chem. Phys. 1969, 51, 1753−1765. (84) Drew, M.; Williams, D. F. Comment Concerning the Diffusion Coefficient of Triplet Excitons in Anthracene. J. Chem. Phys. 1971, 54, 1844−1845. (85) Avakian, P.; Merrifield, R. E. Experimental Determination of the Diffusion Length of the Triplet Excitons in Anthracene Crystals. Phys. Rev. Lett. 1964, 13, 541−544. (86) Moore, G. F. Effect of Temperature on the Triplet-Triplet Annihilation Rate in Anthracene Crystals. Nature 1966, 211, 1170− 1171. (87) Schwoerer, M.; Wolf, H. C. Organic Molecular Solids; WileyVCH Verlay GmbH & Co. KGaA: Weinheim, Germany, 2007. (88) Yost, S. R.; Hontz, E.; Yeganeh, S.; Voorhis, T. V. Triplet vs Singlet Energy Transfer in Organic Semiconductors: The Tortoise and The Hare. J. Phys. Chem. C 2012, 116, 17369−17377.

L


Exciton Dynamics in Anthracene Nanoaggregates - The Journal of

Recommend Documents