a)Recorded in CHCl3 solution; b)The numbers correspond to the Stokes shift calculated for the DPP emitter; c)Energy levels were determined by cyclic voltammetry from the onsets of the first oxidation and reduction waves; the data are referenced against the half-wave potential, E1/2, of the Fc/Fc+ redox couple, which has a HOMO energy level of −4.8 eV.
Well-Defined and Monodisperse Linear and Star-Shaped Quaterfluorene-DPP Molecules: the Significance of Conjugation and Dimensionality
Article first published online: 4 APR 2011
Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Volume 23, Issue 18, pages 2093–2097, May 10, 2011
How to Cite
Kanibolotsky, A. L., Vilela, F., Forgie, J. C., Elmasly, S. E. T., Skabara, P. J., Zhang, K., Tieke, B., McGurk, J., Belton, C. R., Stavrinou, P. N. and Bradley, D. D. C. (2011), Well-Defined and Monodisperse Linear and Star-Shaped Quaterfluorene-DPP Molecules: the Significance of Conjugation and Dimensionality. Adv. Mater., 23: 2093–2097. doi: 10.1002/adma.201100308
- Issue published online: 2 MAY 2011
- Article first published online: 4 APR 2011
- Manuscript Received: 24 JAN 2011
- energy transfer;
Linearly extended and monodisperse π-conjugated oligomers are materials of high interest, since they are functional organic semiconductors and serve as useful models for higher molecular weight polymers. Moreover, the molecules are well-defined, discrete structures with 100% synthetic reproducibility, and possess high purity and excellent solubility in common organic solvents. Expanding the dimensionality of oligomers into two or three dimensions through a central core leads to materials with physical properties that can be markedly different from their simple, linearly conjugated analogues. These so-called star-shaped conjugated oligomers have been the subject of intense research in recent years.1 In many examples, there is fascinating interplay between the conjugated arms and the central aromatic unit, leading to antennae-type light harvesting from the arms followed by energy transfer to the core.1
The compound 1,4-diketo-2,3,5,6-tetraphenyl-pyrrolo[3,4-c]pyrrole (DPP) represents an ideal structure for application as a central core unit in star-shaped conjugated structures. The DPP unit has been incorporated into conjugated materials that have been studied as components in solar cells2 and as efficient light emitters.3 However, despite the recent attention that DPP derivatives have received, there are only a few examples of structures tetrasubstituted with conjugated chains4 and certainly no examples of macromolecular single molecules. Herein, we report on the synthesis and properties of oligofluorene-substituted DPP macromolecules. The largest molecule in the series is defined (Scheme 1) as Star, in which the DPP core has four quaterfluorene arms. Twofold substitution of the core with quaterfluorenes is represented by Linear-c and Linear-nc; the terminology here refers to the conjugated pathway, which propagates throughout the molecule in the former and is restricted to quaterfluorenylphenylenes in the latter. The comparative properties of the compounds possessing these different features is the focus of this study (Scheme 1), which provides a definitive account of the significance and dimensionality of dissimilar architectures in closely related conjugated structures.
The oligomers were synthesized using a convergent strategy with standard and modified Suzuki protocols as coupling procedures. The coupling of octahexyl quaterfluorenylboronic acid with DPPBr4 in the presence of potassium carbonate as a base yielded the Star compound (56% yield). Using barium hydroxide and potassium phosphate as a base, the syntheses of Linear-c (63%) and Linear-nc (45%) were performed by coupling of DPPBr2c and DPPBr2nc, respectively, with the corresponding octahexyl quaterfluorenyl dioxaborolane. The full synthetic procedures and characterization for the Star, Linear-c, and Linear-nc compounds are given in the Supporting Information.
The DPP materials were studied using cyclic voltammetry, absorption and emission spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and optical gain measurements (see data in Table 1 and figures in the Supporting Information).
|λabs [nm] (Fl4), CH2Cl2||368||368||369|
|λabs [nm] (Fl4), Film||365||366||366|
|λabs [nm] (DPP), CH2Cl2||517||491||486||491||480a)||475|
|λabs [nm] (DPP), Film||506||506||473|
|λPL [nm], CH2Cl2||568||567||522||527||520a)||535|
|(Stokes shift [nm])||(51)b)||(76)b)||(36)b)||(36)||(40)||(60)|
|(Stokes shift [eV])||(0.22)b)||(0.34)b)||(0.18)b)||(0.18)||(0.20)||(0.29)|
|λPL [nm], toluene||568||567||522|
|λPL [nm], Film||634||633||575|
|ϕPL (Fl4 excitation) [%], Film||6||4||2|
|ϕPL (DPP excitation) [%], Film||20||11||7|
|TGA [°C] (5% loss)||440||440||420|
|Oxidation potential, E1ox1/2 [V]||+0.82||+0.82||+0.80|
|Oxidation potential, E2ox1/2 [V]||+0.96||+0.96||+0.93|
|Reduction potential, Ered1/2 [V]||−1.85irrev||−1.90irrev||−1.59|
|HOMO–LUMO gap (echem) [eV]||2.2||2.3||2.2|
|HOMO–LUMO gap (optical) [eV]||2.2||2.2||2.4|
Thermal analysis shows that for the dendritic Star molecule the glass transition temperature, Tg, (74 °C), is significantly lower than that of Linear-nc (101 °C) due to its increased free volume. A glass transition is absent in the DSC of Linear-c and the presence of a well-defined melting point, Tm, (168 °C) and crystallization point, Tc, (101 °C and 127 °C) indicates that Linear-c is crystalline and not amorphous in nature (Supporting Information, Figures SI17–19). All three compounds have good thermal stability with a decomposition temperature range of Td = 420–440 °C (Table 1 and Supporting Information Figures SI20–22).
The absorption spectra of the fluorene derivatives in dichloromethane solution all gave a peak at 368–369 nm and this was assigned to the π–π* transition of the quaterfluorene arms (Fl4 in Table 1).5 A second, weaker band was seen at longer wavelengths and the peak maximum for this transition varied significantly between the three compounds (486–517 nm). Given that the longest wavelength absorption maxima for DPPBr2c, DPPBr2nc, and DPPBr4 are in the range 475–491 nm (Table 1 and Supporting Information Figures SI10,11), the lower energy bands in the absorption spectra for the fluorene derivatives were ascribed to the DPP core in these compounds. The longest wavelength absorption, as well as emission of Linear-nc, are similar to those of DPPBr4 and almost identical to the corresponding absorption and emission bands of DPP (λabs = 484 nm, λPL = 520 nm in CHCl3)6 and DPPBr2nc (Table 1 and Supporting Information Figure SI11), confirming that there is no significant influence on the absorption of the DPP unit from the oligofluorene substituents at the 2- and 5-positions. On the contrary, the Star compound exhibits a significant bathochromic shift for the absorption (Δλ = 33 nm) and emission (Δλ = 48 nm) wavelength of the DPP unit compared to DPP. Hence, the absorption of the core unit is heavily influenced by substituent effects at the 3- and 6-positions, showing typical intramolecular charge transfer (ICT) behavior. We postulate that ICT is a dominant feature in the fluorene-DPP compounds, arising from a push of electrons from substituents at the 3,6-positions to the 1,4-carbonyls. The wavelength for ICT, λICT, is known to increase with a greater conjugation length and this would account for the large difference between Star and Linear-nc.
Linear-c has the same push-pull components as Star, but the former has N-alkyl groups that reduce the electron-withdrawing effects of the carbonyls through a positive inductive effect (the phenylene ring in Star will have the opposite effect). The different electronic effects of the hexyldecyl and phenyl groups can be confirmed by a higher value of the lowest unoccupied molecular orbital (LUMO) level of Linear-c compared to those of Star and Linear-nc (Table 1). A reduction of donor/acceptor strength in push-pull systems causes a blue-shift in λICT, which would explain the difference between the longest wavelength maxima for Star and Linear-c. Also, a more twisted ground state conformation of the 3-Ph-DPP-6-Ph fragment, where Ph is a phenylene group, might reduce the degree of ICT in Linear-c.
Interestingly, both Linear-nc and the Star compounds, which share a common core substitution pattern (i.e., tetraphenyl DPP), exhibit a non-symmetrical DPP unit absorption band with obvious shoulders at the short wavelength side of the peak. In hexane these compounds show more resolved absorption spectra and their longest wavelength absorption bands reveal vibronic structure in the form of two clear maxima (499 nm, 522 nm for Star (Supporting Information Figures SI12,13) and 465 nm, 491 nm for Linear-nc (Supporting Information Figure SI13)). The Linear-c compound on the other hand exhibits a symmetrical and featureless band for the DPP unit in both hexane and dichloromethane solutions, similar to the featureless absorption of its core precursor DPPBr2c. For the closely related compound 2,5-dimethyl-3,6-diphenylpyrrolo[3,4-c]pyrrole-l,4-dione, a loss of vibrational structure has been explained on the basis of poor Frank–Condon overlap between the ground and first excited states caused by non-planarity of the molecules.7 Different equilibrium conformations in ground and excited states of DPPBr2c also lead to a more pronounced Stokes shift in its UV-photoluminescence (PL) spectra compared to that of DPPBr4 (Table 1 and Supporting Information Figure SI10).
The concentration dependence of the absorption spectra for all three oligofluorene compounds has been studied in hexane solution (Supporting Information Figure SI13). It was found that both linear compounds (Linear-c and Linear-nc) exhibit spectra that follow the Lambert–Beer law. On the contrary, the shape of the absorption band corresponding to the DPP unit for Star was heavily dependent on the concentration. In more concentrated solutions the short-wavelength maximum of the band mentioned above (499 nm) becomes dominant, whereas the long-wavelength maximum (522 nm) appears as a shoulder. Such behavior might be explained by the aggregation of the Star compound in more concentrated solutions, followed by a change in the conformation of the central tetraphenyl DPP unit; i.e., a planar 3-Ph-DPP-6-Ph fragment in the non-aggregated state changes to a propeller-like conformation with equal twisting of all four phenyl fragments after aggregation. The aggregation due to π–π stacking of an oligophenylenevinylene star-shaped system in low polarity solvent has been observed before.8 Alkyl chains at the 9,9-positions of a fluorene unit usually hinder this type of aggregation. Nevertheless the unique arrangement of phenylene spacers between the DPP core and the oligofluorene arms, as well as the restriction on the longitudinal displacement of the Star molecule in the aggregate due to hydrophobic interactions between alkyl chains, makes aggregation possible in hexane.
In the solid state, the materials possess similar absorption spectra (Figure 1), with the quaterfluorenes absorbing at 365–366 nm and clearly visible lower energy absorption arising from the DPP units. These long wavelength absorption features broadly follow the same relationships as those observed in hexane solution. The Star compound exhibits the absorption pattern of the aggregated tetraphenyl DPP unit (the dominance of the short-wavelength maximum and the appearance of a long-wavelength feature as a shoulder), while the Linear-c material possesses a featureless absorption band (although not symmetrical), with a hint of a shoulder at the long-wavelength side of the peak. In the case of the Linear-nc film, the long- wavelength absorption band shows vibronic splitting with two features at 473 and 491 nm, similar to the spectrum of the hexane solution although the short-wavelength feature becomes more intense.
Although we did not observe any deviation from the Lambert–Beer law in the absorption of Linear-c hexane solutions, DSC measurements indicated that in the condensed state this compound formed a crystalline phase (see above). The higher degree of crystallinity for Linear-c in comparison with Linear-nc and Star oligomers suggests the cooperative effect of quadrupole–quadrupole interactions (Figure 2a) in the condensed phase, which makes aggregation favorable and leads to a bathochromic shift of the longest wavelength absorption band. This type of aggregation has intrinsically 1D character, with all quadrupole ellipsoids aligned in one direction.
On the contrary, aggregation of the Star macromolecules involves 2D interactions between adjacent molecules between the more electron-rich 2,5-phenylene rings (colored blue in Figure 2b) and the 3,6-phenylene spacers (colored red), which are involved in ICT. The long axes of the quadrupole ellipsoids will be lying in one plane but in different directions so that the quadrupole–quadrupole interactions will affect the absorption in a different way, by diminishing the degree of ICT due to π–π 3,6-phenylene–2,5-phenylene interactions between adjacent molecules. As a consequence of the latter interaction, the molecules in the aggregate adopt a propeller-like conformation with equal twisting angles of 3,6- and 2,5-phenylene rings.
A small hypsochromic shift of the longest wavelength absorption band in the solid is observed for Linear-nc and may be accounted for by a similar weakening of ICT due to the aforementioned interactions. However in this case the change is not as pronounced as for the Star system because of: i) the linear shape of the Linear-nc molecule and ii) the weaker initial degree of charge transfer to the core from the 3,6-aryl substituents in the Linear-nc molecule compared to that in the Star system.
A Kramers–Kronig9 analysis was used to evaluate the refractive index at the PL maximum for each of the materials (Table 1) from the difference between the absorption of the DPP materials (Figure 1) and a star-shaped oligofluorene truxene.10 It can be seen that the Star material has the highest index, 1.71.
The emission spectra for the fluorene-DPPs show large Stokes shifts with very little overlap with the absorption spectra. Polymeric DPPs have shown similar behavior3d,11 and these characteristics are promising for high-gain materials in lasing applications.12 The results from solution-state spectra are consistent with the dominant effect of ICT on the electronic properties of Star and Linear-nc because the common core emits at wavelengths that are 31 nm apart. For these two compounds, emission follows the trend of the ICT absorption wavelength. Star and Linear-c have nearly identical emission wavelengths (568 and 567 nm). Linear-c in its structurally relaxed, first excited state contains a more planar 3-Ph-DPP-6-Ph fragment than in its ground state, which increases the Stokes shift for Linear-c (compared to its analogues) and coincidentally leads to an almost identical emission maximum as that for Star. Similar behavior is found for spin-coated thin films. The solid-state PL exhibits a considerable red-shift when compared to the toluene solution emission. However the overall trend is similar, with Star and Linear-c emitting at 633–634 nm, while Linear-nc is blue-shifted with emission peaking at 575 nm.
We also note that, in contrast to the emission from solution, the film photoluminescence can only be observed from the DPP core and indicates complete energy transfer from the quaterfluorene arms.
Cyclic voltammetry was used to determine the redox behavior of the molecules and their highest occupied molecular orbital (HOMO)–LUMO energies (Supporting Information Figure SI8). Each compound gave two reversible oxidation processes, approximately 130–140 mV apart. The oxidation potentials for Linear-nc were slightly lower than those for the other two compounds (20 and 30 mV). Linear-nc was also the only material to feature a reversible reduction process, which was observed at a significantly lower potential than for Star and Linear-c. The HOMO and LUMO energies were obtained from the onsets of the first oxidation and reduction waves, respectively. The electrochemical and optical HOMO–LUMO gaps are in good agreement.
Measurements were carried out to study the effect of the different configurations of conjugation through the DPP core on the optical gain for each of the compounds. At high excitation densities, a line-narrowed peak (627 nm) appears in the spectrum of the Linear-c compound (Figure 3), while the Star and Linear-nc materials do not show any significant changes to their emission profile. The intensity-dependent peak at 627 nm can be attributed to the appearance of amplified spontaneous emission (at high pump fluence) from the spontaneous emission spectrum of the material. The Star material displayed slight modification of its spectrum at increased intensities, but no spectral narrowing could be observed up to the maximum excitation energy density possible with the experimental setup. In the case of the Linear-c sample, the film was pumped at both 355 nm and 532 nm, selectively exciting the quaterfluorene arms and DPP core, respectively. It was found that when the excitation density was corrected for absorption, the energy density required for the observation of gain was 106 μJ cm−2 for the 355 nm pumped sample and 42 μJ cm−2 for the 532 nm pumped sample. The behavior of this material with pump wavelength is similar to that of complex red-emitting conjugated polymers,13 showing that the loss due to the transfer of energy from the quaterfluorene arms to the DPP core plays a role in increasing the apparent onset of amplified spontaneous emission. This trend can also be seen from measurements of the quantum yield, ϕPL, (see Table 1) where for all materials in this study, the emission efficiency is greater when exciting the DPP core directly compared with quaterfluorene excitation.
The comparison of optical characteristics of Star and Linear-c in solution and in the solid film demonstrates the influence of the molecular dimensionality and substituent effect on the property of the material in the bulk. The linear shape of the Linear-c system and conjugation pathway between donor arms and acceptor core provide the opportunity to increase ICT in the solids. The opposite effect on ICT is observed due to the aggregation of Star and to a lesser degree for that of the Linear-nc compound. The 2D character of the Star system and the alternating position of phenylene spacers between the core and the arms make the aggregating propensity of this compound very strong and notable not only in the solids but also in hexane solutions. The different types of aggregation in the solid phase provided by ICT, the substitution pattern, and the dimensionality of the molecular system must be taken into account during the design of new materials.
The DPP unit is clearly an interesting and exciting unit for use as a core system in star-shaped conjugated architectures. While amplified spontaneous emission from neat films has only been observed in Linear-c, blends of the new compounds reported here with photoresists will provide promising emissive materials without the complexity of aggregating behavior. Such blends can be solution-processed and patterned by inkjet printing14 or direct laser writing15 and manipulated into distributed feedback structures for lasing applications.
Supporting Information is available from the Wiley Online Library or from the author.
The authors thank the EPSRC for funding via grants EP/F061609, EP/F05999X (HYPIX project), EP/E027431, and an Advanced Fellowship award to P.N.S. (EP/C539494). D.D.C.B. thanks the Lee-Lucas endowment for support.
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