ARTICLE pubs.acs.org/JPCA
Optical Excitations in Star-Shaped Fluorene Molecules Neil A. Montgomery,† Jean-Christophe Denis,‡ Stefan Schumacher,‡,§ Arvydas Ruseckas,† Peter J. Skabara,^ Alexander Kanibolotsky,^ Martin J. Paterson,|| Ian Galbraith,‡ Graham A. Turnbull,† and Ifor D. W. Samuel*,† †
)
Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, United Kingdom ‡ Department of Physics, SUPA, and Department of Chemistry, School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh, EH14 4AS, United Kingdom ^ WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL, United Kingdom ABSTRACT: A detailed study of the low-energy optical transitions in two families of star-shaped molecules is presented. Both families have 3-fold rotational symmetry with oligofluorene arms attached to a central core. In one family, the core of the molecule is a rigid meta-linked truxene, while the other is a meta-linked benzene moiety. The low-energy transitions were studied both experimentally and using time-dependent density functional theory (TD-DFT). The optical transitions of these new star-shaped molecules were compared with corresponding linear oligofluorenes. Both families of star-shaped molecules showed higher absorption and fluorescence dipoles and photoluminescence quantum yields than straight chain oligofluorenes. TDDFT calculations show that absorption takes place across the entire molecule, and after excited state relaxation, the emission results from a single arm. In both theory and experiment the transition dipole moments show an approximate n0.5 dependence on the number of fluorene units in each arm.
1. INTRODUCTION Conjugated organic materials are the subject of intensive research for a range of optoelectronic applications. These include light sources such as light-emitting diodes1 light-emitting fieldeffect transistors,2 and lasers,3 as well as for photodetectors,4 photovoltaics,5 and nonlinear optics.6-8 A key advantage of organic materials is that they can be processed from solution for printed optoelectronics. The most widely studied of these solution-processable materials are conjugated polymers, though several other molecular geometries, including dendrimers,9 spiro compounds,10 and star-shaped molecules,11 have also been studied. The influence of molecular shape on the light absorption and emission properties is therefore an important issue. Star-shaped molecules have several advantages over the established polymer materials, such as no polydispersity, increased solubility, and improved morphology in films. They have been shown to exhibit many desirable properties, such as high solidstate photoluminescence quantum yield (PLQY) or high nonlinear absorption coefficients.12-17 However, despite increasing technological interest in star-shaped molecules, little has been done to understand how their shape influences their photophysical properties, and previous experimental studies of truxene-cored molecules have been limited to basic spectral properties.18,19 The one previous theoretical study of the truxene-cored molecules with 0-2 fluorene units per arm focused on Raman spectroscopy and electronic structure in the ground-state geometry.20 r 2011 American Chemical Society
In this paper, we study the dependence of transition energies and transition dipole moments of truxene- and benzene-cored molecules with respect to the length of their arms (with up to four repeat units in each arm) by combining steady-state and time-resolved spectroscopy with time-dependent density functional theory calculations. The results are then compared with linear oligofluorenes21,22 to show the effect of the star shape on the properties of the material. The structures of all three families of materials are shown in Figure 1. One family consists of a truxene-cored molecule with fluorene arms.18 These materials have been used in applications such as laser gain materials,14,23 field effect transistors,24 light-emitting devices,25 and for fluorescence probes.26 The other family of star-shaped materials investigated are benzene-cored molecules19 with three oligofluorene arms attached at the meta positions of the core. The third family are linear oligofluorenes.
2. METHODS 2.1. Experimental Methods. The arms of the benzene-cored and truxene-cored molecules consist of oligofluorene functionalized with two hexyl side groups per repeat unit. The materials were synthesized as previously reported,18 except for the trimer and pentamer oligofluorenes, which were sourced from Received: November 15, 2010 Revised: January 31, 2011 Published: March 23, 2011 2913
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Figure 1. Structures and spectra of (a) truxene-cored, (b) benzene-cored, and (c) oligofluorene molecules. The molar extinction coefficient is given by the red lines and the emission by the black lines. Note the change in vertical scale for the molar extinction coefficient in panel c.
American Dye Source Inc. The materials are labeled here by the core and number of fluorene units in each arm: the benzenecored molecules are B1-B4, the truxene-centered molecules are T0-T4, and the oligofluorenes are O1-O5. Solutions were prepared in either HPLC-grade tetrahydofuran or toluene, both of which were obtained from Sigma Aldrich. All spectroscopic measurements were performed in 1 cm fused quartz cuvettes, with all samples having an absorbance below 0.1. Photoluminescence and absorption measurements were performed using a Horiba Jobin Yvon Fluoromax 2 fluorimeter and a Cary 300 UV-vis spectrometer, respectively. All measurements were performed at room temperature. The photoluminescence quantum yields were measured using27 Quinine Sulfate in 0.5 M sulfuric acid (PLQY 0.56 at 360 nm excitation) as a reference for all but the B1 and T0 molecules. For these molecules 2-aminopyridine in 0.1 M sulfuric acid was used as the measurement standard (PLQY 0.6 at 285 nm excitation). The oligofluorene PLQY was reported previously in ref 21. The fluorescence lifetimes were measured using a Hamamatsu streak camera with a 3 ps time resolution, and the sample was excited using a 100 fs pulse from a frequency-doubled Ti:sapphire laser at 375 nm, except for B1 and T0, which were excited at 266 nm with the third harmonic of the laser. To estimate the vertical transition energies Evert for the molecules in absorption (abs) and emission (em), the following formulas were used28 R EAðEÞ dE ð1Þ Evert ðabsÞ ¼ R AðEÞ dE
and R EIðEÞ dE Evert ðemÞ ¼ R IðEÞ dE
ð2Þ
where A(E) is the absorbance and I(E) is the fluorescence intensity (in photons) at the photon energy E and the integration is over the absorption and emission bands. Fluorescence spectra have been corrected with λ2 factor when converting from wavelength to energy dependence. 2.2. Theoretical Methods. The ground-state geometries of the molecules were optimized using density functional theory (DFT). Excitation energies and dipoles in absorption were calculated using time-dependent-DFT (TD-DFT), which was also used for geometry optimization in the lowest optically active excited state. For the optimized excited state geometry, the emission data is calculated using TD-DFT. The program Gaussian 09 was used for all quantum chemistry calculations presented here. In the calculations, all the fluorene arms are attached to one of the cores and are of helix type,21 that is the dihedral angle between each pair of adjacent fluorene units on each spoke has the same sign, which in the optimized ground-state geometries is found to be about 35. In their most symmetric conformations, the optimized ground-state geometries of the benzene dendrimers and of the truxenes both show C3 symmetry. Generally, the C3 point group supports singly and 2-fold degenerate irreducible representations. Thus, the excited-state potential gradient is nontotally symmetric, due to the Jahn-Teller theorem. 2914
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Table 1. Results for B1 and T1 (given in parentheses) with Different Exchange-Correlation Functionalsa B3LYP
CAM-B3LYP
M06-2X
6-31G
6-31G
6-31G
absorption energy (eV) absorption dipole (eÅ)
4.03 (3.62) 1.72 (2.37)
4.58 (4.25) 1.97 (2.61)
4.57 (4.23) 2.01 (2.63)
emission energy (eV)
3.51 (3.13)
3.79 (3.48)
3.83 (3.5)
emission dipole (eÅ)
2.05 (2.69)
2.04 (2.63)
2.07 (2.66)
All the transitions are the first electronic transitions, except for the absorption calculated with B3LYP, where it is the second (but lowest bright) transition. a
Consequently, in the excited-state geometry optimization, a slight initial symmetry breaking was introduced, such that geometry optimization to the lowest excited state is seeded. The B3LYP exchange-correlation functional together with the 6-31G basis set was used unless noted otherwise. Results obtained with this choice of functional and basis set have previously shown very good agreement with experiments for a series of fluorene oligomers.21,22,29 In agreement with previous studies,21,29 the use of a bigger basis set does not significantly change the results for the types of molecules investigated here. For instance, for B1 we have found that, in absorption, using an extra polarization function in the basis set 6-31G* or an extra diffusion function 6-31þG decreases the lowest transition energies by less than 2% and increases the dipole strength by slightly more than 1%, when compared to the 6-31G basis set. For some of the molecules studied in this work, the functionals CAM-B3LYP and M06-2X, with the basis set 6-31G, have been used for comparison. Both of these newly developed functionals take more rigorously into account the long-range contributions to the electronic exchange interaction, e.g., allowing for a more accurate description of charge-transfer contributions to the excitations. Whereas CAM-B3LYP is a derivative of the B3LYP functional,30 M06-2X belongs to the M06 range of functionals, which in the recent past have proven to deliver accurate results for a large variety of different systems and are increasingly gaining in popularity.31 In the comparisons, both the geometry optimizations and the transition energies were calculated with these functionals. For B1 and T1, we find that different functionals predict a different ordering of the two lowest excited singlet states. With both M06-2X and CAM-B3LYP, the first transition in absorption is bright in contrast to the B3LYP result. For all the other molecules, we find qualitatively similar results for all functionals. Quantitatively, however, the transition energies calculated with B3LYP are significantly lower and give overall better agreement with the experimental data than with CAMB3LYP and M06-2X (both give similar results). CAM-B3LYP and M06-2X also give a slightly larger transition dipole in absorption than B3LYP. A detailed comparison for B1 and T1 is given in Table 1. We have also checked in the polarizable continuum model (PCM) that inclusion of the solvent (tetrohydrofuran, dielectric constant of 6.97) in the calculations does not change this overall picture. In the calculations, the C6H13 side chains attached to the fluorene arms have been replaced by shorter CH3 chains. This has explicitly been validated for B2, for which we have carried out the calculation including the full C6H13 side groups. Compared to the calculation with the shorter CH3 groups, the change in absorption energy and dipole is 0.6% and 2.4%, respectively (at
Figure 2. Photoluminescence quantum yield (PLQY) (a) and radiative lifetime (b) for truxene-cored molecules (black circles), benzene-cored molecules (red squares), and oligofluorene (blue triangles). Lifetime and PLQY measurements for the oligofluorene were reported in ref 21.
the B3LYP, 6-31G level of theory). This comparison clearly demonstrates that the C6H13 side chains are largely uninvolved in the electronic excitations of the molecules.
3. RESULTS AND DISCUSSION 3.1. Optical Spectra, Photoluminescence Quantum Yields, and Lifetimes. The photoluminescence and molar absorption
spectra, in solution for all three families of molecules are presented in Figure 1. The spectra show that for all families there is a red-shift in absorption and emission with increasing arm lengths. The molar absorption coefficient for all families increases with increasing arm length, and for a given length, the molar absorption coefficient is similar for the truxene-cored and benzene-cored star-shaped molecules. For molecules with two to four fluorene units in the arms, the molar extinction coefficient is approximately 3 times greater than that of the equivalent oligofluorene arm. This difference can be explained by the star-shaped molecules absorbing light across all three fluorene arms. The photoluminescence spectra for all families show clear vibronic peaks separated by ∼0.16 eV which is a characteristic energy for the vibrational modes of carbon-carbon bonds. As the arm length increases, the spectra red-shift and the 0-0 peak becomes dominant. Photoluminescence quantum yield (PLQY) and radiative lifetime data for all the molecules are presented in Figure 2, parts a and b, respectively. The PLQY for all families increases with arm length, with the truxene-cored molecules having consistently high PLQY across the entire family, with a slight increase from 78% for T1 to 87% for T4. Only T0 (truxene core) has a low 2915
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Table 2. Molar Extinction Coefficients, Vertical Transition Energies (Evert), Transition Dipole Moments (|d|), and Radiative Lifetimesa Evert (eV) absorption 3
-1
transition dipole |d|/e0 (Å) emission
absorption
emission
-1
arm length
molar extinction coefficient (10 M cm )
B1
126
4.16
4.03
3.31
B2 B3
176 197
3.73 3.54
3.53 3.33
3.07 2.95
B4
295
3.42
3.24
2.90
exp
theory
exp
theory
theory
τrad (ns)
exp
theory
exp
3.51
2.3
1.72
0.3
2.05
42
3.01 2.82
2.7 3.2
2.95 3.72
2.3 3.3
3.14 3.74
1.2 0.75
2.74
3.4
4.29
3.7
4.15
0.64
Benzene-cored
Truxene
a
T0
77
4.25
4.34
3.31
4.16
1.2
1.03
1.07
400
T1
186
3.66
3.62
3.14
3.13
2.5
2.37
2.0
2.69
1.9
T2
235
3.57
3.37
2.97
2.86
2.9
3.4
3.0
3.56
0.74
T3
256
3.49
3.25
2.91
2.75
3.2
4.08
3.3
4.05
0.7
T4
341
3.38
3.19 2.89 Oligofluorene
2.71
3.4
4.62
3.6
4.39
0.63
O1
11
4.64
4.7
3.91
4.18
0.9
0.8
0.2
1.07
23
O2
40
3.9
3.81
3.25
3.23
1.9
1.97
2.1
2.33
1.4
O3
80
3.65
3.47
3.00
2.91
2.5
2.63
2.7
3.08
1.1
O4
124
3.52
3.31
2.95
2.78
3.1
3.13
2.9
3.60
1.0
O5
128
3.41
3.24
2.93
2.72
3.2
3.55
3.4
4.00
0.75
Experimental emission dipoles and lifetimes of oligofluorenes were previously reported in ref 21.
PLQY of 7% (not shown). Benzene-cored molecules show a much greater change in PLQY, increasing from 36% for B1 to 86% for B4. The increasing PLQY of the benzene-cored molecules follow a similar trend as the oligofluorene molecules, but are always higher. The PLQY for the star-shaped molecules with three and four repeat fluorene units are similar, suggesting that the photophysics becomes dominated by the arms of the molecules for the larger molecules. The radiative lifetime is determined using τR ¼ τ=PLQ Y
ð3Þ
where PLQY is the photoluminescence quantum yield and τ is the measured fluorescence lifetime. The radiative lifetime decreases with increasing arm length. For star-shaped molecules with two to four fluorene units, the radiative lifetimes for both families lie between 550 and 700 ps, with the truxene-cored molecules having shorter lifetimes. Both star-shaped families have shorter radiative lifetimes than their corresponding linear oligofluorene molecules. The radiative lifetimes of all the one arm unit molecules are much greater than the longer arm length molecules for all materials, especially the benzene-cored molecules. The data is summarized in Table 2. 3.2. Calculated transition densities. Figure 3 shows TDDFT calculations of the change in the electron density by absorption of circularly polarized light and by fluorescence in B2. The change in the electron density during both transitions is greatest in the middle of the arms; however, absorption transitions are delocalized over the entire star-shaped molecule. While fluorescence transitions are localized on a single arm. The circularly polarized light selects the linear combination of the degenerate, optically bright transitions that reflect the circular symmetry. For B1 and B2, we have run a series of calculations with a variety of different initial (small) symmetry breakings in the excited state; however, only three different minima on the
excited state potential energy surface were found. In each of these three excited state geometries, the excitation is localized on one of the molecule’s three arms. The symmetry is broken due to the well-known Jahn-Teller effect, i.e., vibronic coupling between the degenerate components of an electronic state via nontotally symmetric vibrations (in this case the EXe Jahn-Teller effect), leading to three equivalent minima on a “tricorn” potential.32 In both benzene- and truxene-cored molecules there are three stable emission states that allow the emission to radiate from any one of the three arms. The excited state relaxation process occurs in a qualitatively similar way for both families of star-shaped molecules, and for a range of arm lengths. Physically, following the absorption process there is a change in the geometry such that the C3 symmetry is spontaneously broken and the excitation is localized on one of the arms. Similar to linear oligofluorenes,21 the equilibrium geometry of the arm on which the excitation localizes is planarized by the excitation. 3.3. Transition Energies and Dipoles. Figure 4 presents the optical transition energies for all three families of materials in solution. For each family, the transition energies in both absorption and fluorescence decrease with increasing arm length. For two to four fluorene units, the transition energies of linear oligofluorenes and both star-shaped molecules show a much smaller change in both fluorescence and absorption. For one fluorene unit, there is a substantial spread in transition energies, which is due to the cores, where present, being a relatively large fraction of the molecule. The energies in both star-shaped families are red-shifted with respect to the oligofluorenes, with the size of the red-shift dependent upon the size of the core. Truxene-cored molecules have absorption and emission characteristics similar to n þ 1 number of oligofluorene units when compared to the oligofluorene data. This is because the conjugation extends through an extra fluorene unit, which is part of the core. The benzene-cored molecules 2916
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Figure 3. Visualization of the calculated change in electron density for the B2 molecule in absorption (a) and emission (b). Shown are the three possible emission states for the B2 molecule. In absorption, circular polarization is assumed for light incident on the molecule from on top; the emission dipole is oriented along the respective fluorene arm from which it originates.
also show a slight red-shift from the corresponding arm length oligofluorene chains, because of conjugation to the central benzene unit at the core. Due to the smaller size of the benzene core, this shift is less pronounced. Figure 4b shows the corresponding calculated transition energies for all the molecules. The red-shift of the truxene-cored molecule from the oligofluorene is also clearly seen in the figure. Values obtained from experiment and TD-DFT show good agreement. In Figure 5 we present the transition dipole moments for all the molecules on a logarithmic scale. The experimentally measured molar decadic extinction coefficient includes the sum over all dipole allowed transitions, therefore33,34 m ~ 8π3 N 0υ εA ð~ υÞ ¼ jda ð~ υ Þj2 ð4Þ 3 hcn0 lnð10Þi ¼ 1
∑
where N0 is Avogadro’s constant divided by 1000, m is the ~) is the transition number of dipole allowed transitions, da(υ dipole strength density, h is Planck’s constant, c is the vacuum speed of light, and υ ~ is the wavenumber in cm-1.
In our case, m = 2 and transitions d1 = d2, because of the C3 symmetry of the star-shaped molecules. This degeneracy is confirmed by the quantum chemistry calculations. On rearranging eq 3 and integrating over the absorption band we obtain Z 2 -3 υ Þ=~ υ d~ υ ð5Þ jda j ¼ 4:593 10 n0 ½εð~ where da is the frequency integrated dipole matrix element in units of debye for one of the degenerate electronic transitions. The fluorescence transition dipole |df| is determined using34,35 3πε0 p4 c3 ÆE-3 æ ð6Þ n0 τ R R R where ÆE-3æ = [ E-3I(E) dE]/[ I(E) dE], I(E) is the fluorescence intensity expressed in number of quanta, ε0 is the vacuum dielectric constant, p = h/2π is Planck’s constant, c is the speed of light in a vacuum, and τR is the radiative lifetime. jdf j2 ¼
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Figure 4. (a) experimental vertical transition energies and (b) theoretical calculated energies for the truxene-cored molecules (black circles), the benzene-cored molecules (red squares), and straight chain oligofluorenes (blue triangles). Solid shapes represent absorption and hollow shapes emission.
In fluorescence only a single optical transition needs to be considered, because the symmetry of the molecules is spontaneously broken in the excited state relaxation process, which lifts the degeneracy for the emission process. The TD-DFT results shown in Figure 5 correspond to the energetically lowest optically active transitions. For most of the molecules this is the transition from the ground state into one of the two lowest excited electronic singlet states. For B1 and T1 only, we have found that the lowest energy singlet transition in absorption is dark (in agreement with ref 20), so that the lowest bright transition is the transition from the ground state into the degenerate second and third excited singlet states. For T0 we find both in absorption and emission that the lowest bright transition is from the ground state into the third excited singlet state. All the families show an increase in the absorption and fluorescence transition dipole moments with increasing arm length. In both families of star-shaped molecules both experimental and theoretical transition dipole moments are larger than their corresponding straight chain oligofluorene molecules due to the extra conjugation between the oligofluorene arms and the core. The theoretically calculated transition dipole moments for both the benzene-cored and truxene-cored molecules show an approximate n0.5 dependence on the arm length. This same dependence was previously found for the linear oligofluorene molecules in frozen solution at 77 K.21 The experimental measurements in this work show a weaker increase of the transition dipole moment in both absorption and fluorescence, which is likely to be due to the effects of conformational disorder on these molecules at room temperature in solution.
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Figure 5. (a) experimental and (b) theoretical transition dipole moments vs number of fluorene units in each arm plotted double logarithmically for the truxene-cored molecules (black circles), benzene-cored molecules (red squares), and the oligofluorenes (blue triangles). The solid shapes represent the absorption and hollow shapes the emission. The solid line represents an n0.5 dependence on the number of fluorene units. Experimental fluorescence dipoles for oligofluorene were reported in ref 21.
4. CONCLUSION As the number of applications for star-shaped organic semiconductors increases, it is important to have an understanding of the underlying optical transitions and the effects of the core and the length of the arms upon the properties and efficiency of the molecule. Our experimental results show that the star-shaped molecules have higher PLQYs than oligofluorenes with equivalent arm lengths and that there is a red-shift in the emission and absorption energies due to the extra conjugation of the core. Experimental results also show that the transition dipole moments in both absorption and emission are slightly larger for the star-shaped molecules than for the linear oligofluorenes. These experimental results are complemented by the theoretical results which provide a good quantitative agreement of the absorption and emission energies and dipole moments. Our results confirm that when inferring the transition dipole moment from measurements it is important to correctly account for the number of involved transitions, which due to rotational symmetry is two for the star-shaped molecules in absorption. This is confirmed by the calculations, which also show that the absorption of light in both families of star-shaped molecules generates excitons delocalized over the whole molecule. In contrast, the calculations show that the emission is localized on a single arm due to an excited-state relaxation process. The transition dipole moments in the starshaped molecules increase with an n0.5 dependence on arm length, which is the same as for the oligofluorenes. The increase 2918
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Physics Department and CeOPP, Universit€at Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany.
’ ACKNOWLEDGMENT We are grateful to the Engineering and Physical Sciences Research Council for financial support.
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