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Novel organic paramagnetic nanofibers and nanostructures: A spectroscopic investigation
Chemical Physics Letters 498 (2010) 129–133
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Novel organic paramagnetic nanofibers and nanostructures: A spectroscopic investigation D. Mastromatteo, G. Macchi ⇑, P. Campiglio, A. Sassella, F. Meinardi Università degli Studi di Milano Bicocca, Dipartimento di Scienza dei Materiali, Via R. Cozzi 53, I-20125 Milano, Italy
a r t i c l e
i n f o
Article history: Received 17 May 2010 In final form 17 August 2010 Available online 21 August 2010
a b s t r a c t Photoluminescence (PL) properties of crystalline samples of a new paramagnetic organic molecule deposited on different substrates have been studied. Supported by optical absorption and PL spectroscopy in dilute solution, confocal polarized fluorescence microscopy was applied to investigate the supramolecular organization of the obtained structures. Among others, needle-shaped crystals and nanofibers were obtained, showing a high crystalline order along with a strong emission anisotropy demonstrating a preferential direction of the transition dipole moment along their main axis. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Photonic and optoelectronic nanotechnologies have been grabbing headlines in recent years. In this context, plastic materials deserve consideration for their well known qualities as easy processability, low cost fabrication over large areas, possibility of integration with inorganic technology, and compatibility with flexible substrates [1–3]. An interesting possibility for new plastic technologic applications is that of organic magnets: by opening a link between traditional electronic behaviour and magnetic phenomena, this class of materials is a promising subject for, e.g., novel spintronic devices or other applications [4–6]. Moreover, the presence of a magnetic group inside the molecular structure can be exploited to control the supramolecular packing by the application of an external magnetic field during both thin film deposition and self-assembling processes [7–11]. On the other side, the conventional solid-state packing of the non-magnetic moiety allows to obtain a suitable supramolecular arrangement of the magnetic one, producing ordered magnetic crystalline materials [8–11]. Among the others, self-assembled, highly-crystalline organic nanofibers (nFs) on a solid support are as well emerging as promising candidates for the next generation of nano-scale photonic and optoelectronic devices. This extremely anisotropic molecular systems are indeed a challenging tool to realize two-dimensional optical confinement, non-linear optical effects, gain amplification, and lasing phenomena [12,13]. The morphology, molecular packing and photophysical properties of such nano-scale aggregates are determinant for their optical and charge transport behaviour. Within this context, polarized fluorescence microscopy is a powerful technique recently employed to study the supramolecular organization in nanoaggregates [14–19]. ⇑ Corresponding author. Fax: +390264485400. E-mail address: [email protected] (G. Macchi). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.08.046
In this work, we report a photophysical study of crystalline samples of a new organic molecule, based on a polyfluoroacridine centre connected with a paramagnetic group. Due to its selfassembling properties, this compound is a suitable material for obtaining organic nano-scale aggregates. We employed a laserscanning confocal microscope to collect fluorescence images of the samples and to perform spatially resolved spectroscopy down to the microscale. Polarized photoluminescence (PL) emission of single crystals and of the self-assembled nFs allowed to infer the relative orientation of molecules inside different structures.
2. Experimental 2.1. Materials preparation 1,3,4-Trifluoro-2-TEMPOL–7-(N,N)dimethylamino-acridine (TEMPOL–TDAA), an organic magnetic compound, has been obtained by reacting the n-type semiconductor 1,2,3,4,-tetrafluoro-7-(N,N)dimethylamino-acridine (TDAA) with the paramagnetic substituent 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), as described in literature [20]. Its molecular structure is shown in the inset of Figure 1. The optical and structural properties of TDAA, both in solution and in the solid state, have been recently deeply investigated [21–23]. Electron paramagnetic resonance spectroscopy in solution and magnetization measurements of TEMPOL–TDAA (as a function of temperature and magnetic field intensity) are reported in [20]. Needle-shaped crystals were grown by slow isothermal evaporation from methanol solution. X-ray diffraction measurements on crystals of TEMPOL–TDAA showed a monoclinic structure, space group P21/n, with unit cell parameters a = 7.940(2) Å, b = 43.050(13) Å, c = 20.138(5) Å, b = 94.89(3)° [20]. The position of atoms inside the unit cell has not been identified yet.
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Figure 1. Optical absorption (solid line) and PL emission (dashed line) spectra of TEMPOL–TDAA in CH2Cl2 solution. Inset: structure of TEMPOL–TDAA, with the direction of the lowest-energy transition dipole moment (grey arrow).
by the PL spectrum, also reported in Figure 1, which exhibits a structureless emission band with a maximum at about 2.25 eV. To obtain detailed information on the TEMPOL–TDAA transition dipole moment (TDM), from which to gain details about the orientation of the molecules in the solid state, we focused our attention on the PL emission. Indeed, accordingly to Kasha’s rule [24], the PL originates from a single and well defined state: the one with the lowest energy. Figure 2a shows an unpolarized confocal microscope image (2.33 eV excitation) of many needle-like crystals in a fan-shaped disposition around a bright and structure-less aggregate. The same image has been also collected with a vertical polarizer (with respect to the images) inserted into the detection path (Figure 2b). By comparing the PL intensity in the two measurements, the effect of polarization is remarkably clear: the light emitted by crystals with the long axis parallel to the polarization direction is preserved, whereas that coming from crystals perpendicularly oriented is damped down, with a gradual intensity variation for crystals in intermediate directions. These findings demonstrate a
TEMPOL–TDAA thin films were grown in UHV by organic molecular beam epitaxy (OMBE) on polycrystalline gold (pAu), on a single crystalline slab of potassium acid phthalate (KAP), and on highly-oriented pyrolythic graphite (HOPG). The deposition was carried out using a Knudsen-type effusion cell, with 170 °C deposition temperature and 0.2 nm/min deposition rate; the 10 nm film nominal thickness was monitored during growth by a quartz microbalance. After 1 month in ambient conditions, the deposited layers evolved into aggregates with various morphology, depending on the substrates. 2.2. Photophysical characterization For what it concerns TEMPOL–TDAA dilute solution, optical absorption spectra were acquired using a Cary 50 Scan UV–Visible spectrophotometer. Continuous-wave PL spectra were recorded using a custom apparatus consisting of a 75 W Xe lamp (coupled with Jobin Yvon Gemini monochromator) as excitation source and a N2(l)-cooled CCD sensor coupled with a Jobin Yvon Triax 190 spectrograph. To obtain spatially resolved PL spectra (l-PL measurements) on the solid samples, a Nikon Eclipse 80i fluorescence microscope equipped with a D-Eclipse C1 confocal scanning head was connected via a multi-mode optical fiber to the spectrograph. A high pressure Hg lamp was used as light source, with an appropriate filter set to select the excitation spectral range (typically 3.25– 3.75 eV). Reflected-light and fluorescence images were collected by exciting with the 2nd harmonic of a linearly polarized Ti-sapphire laser tuned at 3.06 eV or a linearly polarized doubled (2.33 eV) Nd–Yag laser, coupled to the confocal scanning head via a single-mode, polarization maintaining optical fiber. Reflected and/or emitted light were collected through a 100 oil-immersion (NA 1.3) or a 20 (NA 0.4) objective and detected by a photomultiplier tube. Insertion of suitable polarizers allowed for the study under polarized light. 3. Results and discussion As a reference for the following discussion, the photophysical properties of TEMPOL–TDAA in dilute solution (<10 5 M) were firstly investigated. The absorption spectrum of TEMPOL–TDAA CH2Cl2 solution (see Figure 1) shows a broad band centred at 2.75 nm, and the tail of a composite structure at higher energies extending down to 3.20 eV. Its shape is almost perfectly mirrored
Figure 2. Fluorescence microscopy images of TEMPOL–TDAA single crystals (needle-like structures) and amorphous aggregates (circled in bottom-right corner) under 2.33 eV excitation. (a) Without polarizers and (b) both incident and emitted light vertically polarized (see arrow).
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very strong polarization ratio, allowing us to identify these aggregates as single crystals. It should be pointed out that the decrease of PL intensity near the end of each needle can be attributed to the smaller amount of material, due to the sharpening of the tips of each crystal. Conversely, the circled region in Figure 2a and b shows a bright PL emission regardless of the polarization, suggesting being a non-crystalline material or an assembly of randomlyoriented small crystals. Thin films grown on different substrates were investigated by confocal microscopy in order to have a deeper understanding of the interactions between molecules and the substrate. After 1 month in ambient conditions, all the deposited layers were observed to evolve by a spontaneous self-assembling process into aggregates with various morphologies. The samples deposited on pAu, KAP and HOPG show the final morphology reported in Figure 3a–c respectively. Samples on HOPG and KAP evolved into anisotropic aggregates a few lm in length and about 1 lm in width (the resolution limit for the images is 0.2 lm), with a well defined preferential growth direction in the case of KAP. The formation of such aggregates results from the balance the intermolecular interactions, driving self-assembly and crystallization, and the interaction with between the substrate, which may drive their shape and orientation (for KAP the effect is stronger, being organic and single crystalline). A very interesting, peculiar morphology results from the evolution of thin films deposited on pAu, where the formation of nanofibers (nFs) is observed. These are both straight and curved, reaching tens of lm in length and about 2 lm in width. A deeper insight into the optical properties of the molecular aggregates of TEMPOL–TDAA has been achieved through l-PL measurements. The emission spectrum of the are in which we suppose to have an amorphous or polycrystalline material (the circled region in Figure 2) shows a broad, poorly structured band (Figure 4, dashed line), with a maximum at about 2.20 eV. Both its shape and position are very similar to PL emission of TEMPOL–TDAA in dilute solution (dotted line), suggesting that molecules within these aggregates interact weakly, as in the case of a disordered material. The fact that the PL emission in this region appears to be brighter than in the needle-like single crystals further supports the hypothesis of a non-crystalline material in which each molecule behaves as a nearly isolated emitter. Indeed, the fast migration of the excitation towards non-radiative decay centres (e.g. impurities, defects), typical of ordered aggregates, is the primary cause for depleted emission of organic crystals [24]. Needle-shaped single crystals and nFs showed emission bands (Figure 4, dashed-dotted and solid line, respectively) with similar shape and position, centred at about 2.05 eV, i.e. it is red-shifted of about 200 meV compared to dilute solution emission. Such a spectral shift well accounts for excitonic band formation, as ex-
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Figure 4. PL spectra of TEMPOL–TDAA CH2Cl2 solution (10 5 M) (dotted line) compared to l-PL spectra of amorphous aggregates (dashed line), single crystals (dashed-dotted line) and nFs (solid line).
pected for molecular aggregates [24]. It is interesting to notice that the detected red shift is quite similar to that of the un-substituted TDAA [23] showing a PL centred at about 2.05 eV and 2.30 eV in dilute solution and solid state, respectively [21,23]. Since the extent of the red shift is directly correlated with the interaction strength between excited molecules in the solid state, we can conclude that TEMPOL substitution does affect significantly this excited–state interaction. On the sample grown on pAu, polarized PL imaging was performed to investigate the molecular arrangement within nFs. Panels (a) and (b) of Figure 5 are obtained from two measurements on the same area, with excitation and detection light polarized vertically and horizontally, respectively (see arrows). A remarkably polarized emission is detected parallel to the direction of nF long axis, a strong evidence of a highly crystalline order of the emitting species within the aggregate. Neither the orientation of TEMPOL–TDAA molecules within the unit cell, nor their arrangement within the nFs, is known. We can extract details about this point from the above discussed polarized l-PL images. To do that it is important to remember that the PL of organic crystals behave differently depending on the relative orientation of the molecules. In H-type aggregates, molecular TDMs from the bottom of excitonic band to the ground state are parallel to each other and cancel themselves out, resulting in a forbidden transition. Conversely, in a J-type aggregate molecular TDMs are arranged in an head-to-tail fashion, giving rise to a non-zero sum at the bottom of the excitonic band and therefore collecting most of oscillator strength in the transition from that state [24]. In all
Figure 3. Reflected-light images of TEMPOL–TDAA nFs on pAu (a), HOPG (b) and KAP (c). 3.06 eV, 20, 20 lm (a), 2.33 eV, 100, 50 lm (b and c) are incident light wavelength and objective magnification, respectively.
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Figure 6. (a) Face-to-face arrangement and co-facial stacking of TDAA molecules. (b) Head-to-tail interaction and co-linear orientation of TEMPOL–TDAA chromophores. In both cases, growth direction and transition dipole moment orientation (along with the phase of related HOMO–LUMO transitions) are depicted (hydrogen atoms have been omitted for clarity).
Figure 5. Polarized fluorescence microscopy images of TEMPOL–DMAA nFs on a pAu substrate. The arrows indicate both polarization of incident light (3.06 eV) and direction of polarizer in the detection path. Excitation and emission horizontally (a) and vertically (b) polarized, as indicated by the arrows.
the intermediate cases, the components of TDMs perpendicular to the line connecting the centres of the moments produce a zero contribution, whereas the ones co-linear with it result in a nonvanishing overall TDM. TDMs of TEMPOL–TDAA in the gas phase has been evaluated with MOPAC package [25]. After geometry optimization with semi-empirical PM6 method [26], calculations with a configuration interaction including all singly-excited configurations (10 eV limit between the occupied and unoccupied orbitals) revealed that TDM between ground and first excited state of TEMPOL–TDAA lies on the plane of its conjugated backbone, roughly 30° tilted with respect to the long axis of the molecule (see inset of Figure 1). Acridines (such as TDAA) usually pack into a co-facial arrangement and the preferred face-to-face interaction results in a growth direction of crystals parallel to the stacking (Figure 6a) [23]. In this case, the small total TDM is normal to the growth direction (i.e. normal to the crystal long axis). This is not compatible with our results, where nFs brightly emit with a polarization parallel to their major axis. This phenomenology could be easily explained by assuming that TEMPOL–TDAA molecules are arranged in a co-linear orientation (Figure 6b), where a head-to-tail interaction is dominant and the resulting TDM is therefore parallel to the growth
direction. Indeed, a very similar chromophore, TEMPOL–Br–acridine [20], is known to possess such a kind of solid-state structure. Moreover, this arrangement well accounts also for the decrease in the strength of interaction between molecules, which results in a PL emission shifted towards low energy on passing from TDDA to TEMPOL–TDAA. In summary, combined l-PL and polarized PL imaging measurements demonstrate a co-linear solid-state packing of TEMPOL–TDAA, which seems to be a general trend for TEMPOL-substituted acridines, contrary to the behaviour of unsubstituted ones. 4. Conclusions TEMPOL–TDAA films grown on different substrates evolve into structures whose morphology is driven by the interaction between molecules and the underlying interface. For a suitable choice of the substrate (pAu), highly-crystalline nFs have been obtained. In this case, TEMPOL–TDAA molecules show a preferred head-to-tail interaction and are therefore oriented in a co-linear arrangement with their long molecular axis mainly parallel to nF major axis. Such a solid-state packing, markedly different from the one expected for un-substituted acridines, seems to be a general trend for TEMPOL-substituted ones. Acknowledgements A.S. and F.M. thank Fondazione Cariplo (Grants 2007/5205 and 2007/5385, respectively) for partial financial support. The authors also thank A. Papagni for fruitful discussions and assistance in TEMPOL–TDAA synthesis.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Novel organic paramagnetic nanofibers and nanostructures: A spectroscopic investigation D. Mastromatteo, G. Macchi ⇑, P. Campiglio, A. Sassella, F. Meinardi Università degli Studi di Milano Bicocca, Dipartimento di Scienza dei Materiali, Via R. Cozzi 53, I-20125 Milano, Italy
a r t i c l e
i n f o
Article history: Received 17 May 2010 In final form 17 August 2010 Available online 21 August 2010
a b s t r a c t Photoluminescence (PL) properties of crystalline samples of a new paramagnetic organic molecule deposited on different substrates have been studied. Supported by optical absorption and PL spectroscopy in dilute solution, confocal polarized fluorescence microscopy was applied to investigate the supramolecular organization of the obtained structures. Among others, needle-shaped crystals and nanofibers were obtained, showing a high crystalline order along with a strong emission anisotropy demonstrating a preferential direction of the transition dipole moment along their main axis. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Photonic and optoelectronic nanotechnologies have been grabbing headlines in recent years. In this context, plastic materials deserve consideration for their well known qualities as easy processability, low cost fabrication over large areas, possibility of integration with inorganic technology, and compatibility with flexible substrates [1–3]. An interesting possibility for new plastic technologic applications is that of organic magnets: by opening a link between traditional electronic behaviour and magnetic phenomena, this class of materials is a promising subject for, e.g., novel spintronic devices or other applications [4–6]. Moreover, the presence of a magnetic group inside the molecular structure can be exploited to control the supramolecular packing by the application of an external magnetic field during both thin film deposition and self-assembling processes [7–11]. On the other side, the conventional solid-state packing of the non-magnetic moiety allows to obtain a suitable supramolecular arrangement of the magnetic one, producing ordered magnetic crystalline materials [8–11]. Among the others, self-assembled, highly-crystalline organic nanofibers (nFs) on a solid support are as well emerging as promising candidates for the next generation of nano-scale photonic and optoelectronic devices. This extremely anisotropic molecular systems are indeed a challenging tool to realize two-dimensional optical confinement, non-linear optical effects, gain amplification, and lasing phenomena [12,13]. The morphology, molecular packing and photophysical properties of such nano-scale aggregates are determinant for their optical and charge transport behaviour. Within this context, polarized fluorescence microscopy is a powerful technique recently employed to study the supramolecular organization in nanoaggregates [14–19]. ⇑ Corresponding author. Fax: +390264485400. E-mail address: [email protected] (G. Macchi). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.08.046
In this work, we report a photophysical study of crystalline samples of a new organic molecule, based on a polyfluoroacridine centre connected with a paramagnetic group. Due to its selfassembling properties, this compound is a suitable material for obtaining organic nano-scale aggregates. We employed a laserscanning confocal microscope to collect fluorescence images of the samples and to perform spatially resolved spectroscopy down to the microscale. Polarized photoluminescence (PL) emission of single crystals and of the self-assembled nFs allowed to infer the relative orientation of molecules inside different structures.
2. Experimental 2.1. Materials preparation 1,3,4-Trifluoro-2-TEMPOL–7-(N,N)dimethylamino-acridine (TEMPOL–TDAA), an organic magnetic compound, has been obtained by reacting the n-type semiconductor 1,2,3,4,-tetrafluoro-7-(N,N)dimethylamino-acridine (TDAA) with the paramagnetic substituent 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), as described in literature [20]. Its molecular structure is shown in the inset of Figure 1. The optical and structural properties of TDAA, both in solution and in the solid state, have been recently deeply investigated [21–23]. Electron paramagnetic resonance spectroscopy in solution and magnetization measurements of TEMPOL–TDAA (as a function of temperature and magnetic field intensity) are reported in [20]. Needle-shaped crystals were grown by slow isothermal evaporation from methanol solution. X-ray diffraction measurements on crystals of TEMPOL–TDAA showed a monoclinic structure, space group P21/n, with unit cell parameters a = 7.940(2) Å, b = 43.050(13) Å, c = 20.138(5) Å, b = 94.89(3)° [20]. The position of atoms inside the unit cell has not been identified yet.
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Figure 1. Optical absorption (solid line) and PL emission (dashed line) spectra of TEMPOL–TDAA in CH2Cl2 solution. Inset: structure of TEMPOL–TDAA, with the direction of the lowest-energy transition dipole moment (grey arrow).
by the PL spectrum, also reported in Figure 1, which exhibits a structureless emission band with a maximum at about 2.25 eV. To obtain detailed information on the TEMPOL–TDAA transition dipole moment (TDM), from which to gain details about the orientation of the molecules in the solid state, we focused our attention on the PL emission. Indeed, accordingly to Kasha’s rule [24], the PL originates from a single and well defined state: the one with the lowest energy. Figure 2a shows an unpolarized confocal microscope image (2.33 eV excitation) of many needle-like crystals in a fan-shaped disposition around a bright and structure-less aggregate. The same image has been also collected with a vertical polarizer (with respect to the images) inserted into the detection path (Figure 2b). By comparing the PL intensity in the two measurements, the effect of polarization is remarkably clear: the light emitted by crystals with the long axis parallel to the polarization direction is preserved, whereas that coming from crystals perpendicularly oriented is damped down, with a gradual intensity variation for crystals in intermediate directions. These findings demonstrate a
TEMPOL–TDAA thin films were grown in UHV by organic molecular beam epitaxy (OMBE) on polycrystalline gold (pAu), on a single crystalline slab of potassium acid phthalate (KAP), and on highly-oriented pyrolythic graphite (HOPG). The deposition was carried out using a Knudsen-type effusion cell, with 170 °C deposition temperature and 0.2 nm/min deposition rate; the 10 nm film nominal thickness was monitored during growth by a quartz microbalance. After 1 month in ambient conditions, the deposited layers evolved into aggregates with various morphology, depending on the substrates. 2.2. Photophysical characterization For what it concerns TEMPOL–TDAA dilute solution, optical absorption spectra were acquired using a Cary 50 Scan UV–Visible spectrophotometer. Continuous-wave PL spectra were recorded using a custom apparatus consisting of a 75 W Xe lamp (coupled with Jobin Yvon Gemini monochromator) as excitation source and a N2(l)-cooled CCD sensor coupled with a Jobin Yvon Triax 190 spectrograph. To obtain spatially resolved PL spectra (l-PL measurements) on the solid samples, a Nikon Eclipse 80i fluorescence microscope equipped with a D-Eclipse C1 confocal scanning head was connected via a multi-mode optical fiber to the spectrograph. A high pressure Hg lamp was used as light source, with an appropriate filter set to select the excitation spectral range (typically 3.25– 3.75 eV). Reflected-light and fluorescence images were collected by exciting with the 2nd harmonic of a linearly polarized Ti-sapphire laser tuned at 3.06 eV or a linearly polarized doubled (2.33 eV) Nd–Yag laser, coupled to the confocal scanning head via a single-mode, polarization maintaining optical fiber. Reflected and/or emitted light were collected through a 100 oil-immersion (NA 1.3) or a 20 (NA 0.4) objective and detected by a photomultiplier tube. Insertion of suitable polarizers allowed for the study under polarized light. 3. Results and discussion As a reference for the following discussion, the photophysical properties of TEMPOL–TDAA in dilute solution (<10 5 M) were firstly investigated. The absorption spectrum of TEMPOL–TDAA CH2Cl2 solution (see Figure 1) shows a broad band centred at 2.75 nm, and the tail of a composite structure at higher energies extending down to 3.20 eV. Its shape is almost perfectly mirrored
Figure 2. Fluorescence microscopy images of TEMPOL–TDAA single crystals (needle-like structures) and amorphous aggregates (circled in bottom-right corner) under 2.33 eV excitation. (a) Without polarizers and (b) both incident and emitted light vertically polarized (see arrow).
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very strong polarization ratio, allowing us to identify these aggregates as single crystals. It should be pointed out that the decrease of PL intensity near the end of each needle can be attributed to the smaller amount of material, due to the sharpening of the tips of each crystal. Conversely, the circled region in Figure 2a and b shows a bright PL emission regardless of the polarization, suggesting being a non-crystalline material or an assembly of randomlyoriented small crystals. Thin films grown on different substrates were investigated by confocal microscopy in order to have a deeper understanding of the interactions between molecules and the substrate. After 1 month in ambient conditions, all the deposited layers were observed to evolve by a spontaneous self-assembling process into aggregates with various morphologies. The samples deposited on pAu, KAP and HOPG show the final morphology reported in Figure 3a–c respectively. Samples on HOPG and KAP evolved into anisotropic aggregates a few lm in length and about 1 lm in width (the resolution limit for the images is 0.2 lm), with a well defined preferential growth direction in the case of KAP. The formation of such aggregates results from the balance the intermolecular interactions, driving self-assembly and crystallization, and the interaction with between the substrate, which may drive their shape and orientation (for KAP the effect is stronger, being organic and single crystalline). A very interesting, peculiar morphology results from the evolution of thin films deposited on pAu, where the formation of nanofibers (nFs) is observed. These are both straight and curved, reaching tens of lm in length and about 2 lm in width. A deeper insight into the optical properties of the molecular aggregates of TEMPOL–TDAA has been achieved through l-PL measurements. The emission spectrum of the are in which we suppose to have an amorphous or polycrystalline material (the circled region in Figure 2) shows a broad, poorly structured band (Figure 4, dashed line), with a maximum at about 2.20 eV. Both its shape and position are very similar to PL emission of TEMPOL–TDAA in dilute solution (dotted line), suggesting that molecules within these aggregates interact weakly, as in the case of a disordered material. The fact that the PL emission in this region appears to be brighter than in the needle-like single crystals further supports the hypothesis of a non-crystalline material in which each molecule behaves as a nearly isolated emitter. Indeed, the fast migration of the excitation towards non-radiative decay centres (e.g. impurities, defects), typical of ordered aggregates, is the primary cause for depleted emission of organic crystals [24]. Needle-shaped single crystals and nFs showed emission bands (Figure 4, dashed-dotted and solid line, respectively) with similar shape and position, centred at about 2.05 eV, i.e. it is red-shifted of about 200 meV compared to dilute solution emission. Such a spectral shift well accounts for excitonic band formation, as ex-
131
Figure 4. PL spectra of TEMPOL–TDAA CH2Cl2 solution (10 5 M) (dotted line) compared to l-PL spectra of amorphous aggregates (dashed line), single crystals (dashed-dotted line) and nFs (solid line).
pected for molecular aggregates [24]. It is interesting to notice that the detected red shift is quite similar to that of the un-substituted TDAA [23] showing a PL centred at about 2.05 eV and 2.30 eV in dilute solution and solid state, respectively [21,23]. Since the extent of the red shift is directly correlated with the interaction strength between excited molecules in the solid state, we can conclude that TEMPOL substitution does affect significantly this excited–state interaction. On the sample grown on pAu, polarized PL imaging was performed to investigate the molecular arrangement within nFs. Panels (a) and (b) of Figure 5 are obtained from two measurements on the same area, with excitation and detection light polarized vertically and horizontally, respectively (see arrows). A remarkably polarized emission is detected parallel to the direction of nF long axis, a strong evidence of a highly crystalline order of the emitting species within the aggregate. Neither the orientation of TEMPOL–TDAA molecules within the unit cell, nor their arrangement within the nFs, is known. We can extract details about this point from the above discussed polarized l-PL images. To do that it is important to remember that the PL of organic crystals behave differently depending on the relative orientation of the molecules. In H-type aggregates, molecular TDMs from the bottom of excitonic band to the ground state are parallel to each other and cancel themselves out, resulting in a forbidden transition. Conversely, in a J-type aggregate molecular TDMs are arranged in an head-to-tail fashion, giving rise to a non-zero sum at the bottom of the excitonic band and therefore collecting most of oscillator strength in the transition from that state [24]. In all
Figure 3. Reflected-light images of TEMPOL–TDAA nFs on pAu (a), HOPG (b) and KAP (c). 3.06 eV, 20, 20 lm (a), 2.33 eV, 100, 50 lm (b and c) are incident light wavelength and objective magnification, respectively.
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Figure 6. (a) Face-to-face arrangement and co-facial stacking of TDAA molecules. (b) Head-to-tail interaction and co-linear orientation of TEMPOL–TDAA chromophores. In both cases, growth direction and transition dipole moment orientation (along with the phase of related HOMO–LUMO transitions) are depicted (hydrogen atoms have been omitted for clarity).
Figure 5. Polarized fluorescence microscopy images of TEMPOL–DMAA nFs on a pAu substrate. The arrows indicate both polarization of incident light (3.06 eV) and direction of polarizer in the detection path. Excitation and emission horizontally (a) and vertically (b) polarized, as indicated by the arrows.
the intermediate cases, the components of TDMs perpendicular to the line connecting the centres of the moments produce a zero contribution, whereas the ones co-linear with it result in a nonvanishing overall TDM. TDMs of TEMPOL–TDAA in the gas phase has been evaluated with MOPAC package [25]. After geometry optimization with semi-empirical PM6 method [26], calculations with a configuration interaction including all singly-excited configurations (10 eV limit between the occupied and unoccupied orbitals) revealed that TDM between ground and first excited state of TEMPOL–TDAA lies on the plane of its conjugated backbone, roughly 30° tilted with respect to the long axis of the molecule (see inset of Figure 1). Acridines (such as TDAA) usually pack into a co-facial arrangement and the preferred face-to-face interaction results in a growth direction of crystals parallel to the stacking (Figure 6a) [23]. In this case, the small total TDM is normal to the growth direction (i.e. normal to the crystal long axis). This is not compatible with our results, where nFs brightly emit with a polarization parallel to their major axis. This phenomenology could be easily explained by assuming that TEMPOL–TDAA molecules are arranged in a co-linear orientation (Figure 6b), where a head-to-tail interaction is dominant and the resulting TDM is therefore parallel to the growth
direction. Indeed, a very similar chromophore, TEMPOL–Br–acridine [20], is known to possess such a kind of solid-state structure. Moreover, this arrangement well accounts also for the decrease in the strength of interaction between molecules, which results in a PL emission shifted towards low energy on passing from TDDA to TEMPOL–TDAA. In summary, combined l-PL and polarized PL imaging measurements demonstrate a co-linear solid-state packing of TEMPOL–TDAA, which seems to be a general trend for TEMPOL-substituted acridines, contrary to the behaviour of unsubstituted ones. 4. Conclusions TEMPOL–TDAA films grown on different substrates evolve into structures whose morphology is driven by the interaction between molecules and the underlying interface. For a suitable choice of the substrate (pAu), highly-crystalline nFs have been obtained. In this case, TEMPOL–TDAA molecules show a preferred head-to-tail interaction and are therefore oriented in a co-linear arrangement with their long molecular axis mainly parallel to nF major axis. Such a solid-state packing, markedly different from the one expected for un-substituted acridines, seems to be a general trend for TEMPOL-substituted ones. Acknowledgements A.S. and F.M. thank Fondazione Cariplo (Grants 2007/5205 and 2007/5385, respectively) for partial financial support. The authors also thank A. Papagni for fruitful discussions and assistance in TEMPOL–TDAA synthesis.
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