Harald
Ceymann
^{a},
Arnulf
Rosspeintner
^{b},
Maximilian H.
Schreck
^{a},
Carina
Mützel
^{a},
Andreas
Stoy
^{a},
Eric
Vauthey
*^{b} and
Christoph
Lambert
*^{a}
^{a}Institut für Organische Chemie, Universität Würzburg, Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Center for Nanosystems Chemistry, Am Hubland, D-97074 Würzburg, Germany. E-mail: christoph.lambert@uni-wuerzburg.de
^{b}Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
First published on 31st May 2016
The linear and nonlinear optical properties of a series of oligomeric squaraine dyes were investigated by one-photon absorption spectroscopy (1PA) and two-photon absorption (2PA) induced fluorescence spectroscopy. The superchromophores are based on two indolenine squaraine dyes with transoid (SQA) and cisoid configuration (SQB). Using these monomers, linear dimers and trimers as well as star-shaped trimers and hexamers with benzene or triphenylamine cores were synthesised and investigated. The red-shifted and intensified 1PA spectra of all superchromophores could well be explained by exciton coupling theory. In the linear chromophore arrangements we also found superradiance of fluorescence but not in the branched systems. Furthermore, the 2PA showed enhanced cross sections for the linear oligomers but only additivity for the branched systems. This emphasizes that the enhancement of the 2PA cross section in the linear arrangements is probably caused by orbital interactions of higher excited configurations.
The magnitude of the cross section δ_{2PA} in symmetric systems can be estimated by eqn (1), with Γ_{02} being the homogeneous linewidth of the 2PA transition, using a three-level model as depicted in Fig. 1.^{8,9}
(1) |
Fig. 1 Schematic representation of the 2PA process in the case of the three level model for symmetric systems. |
Calculations and experiments have shown that in substituted quadrupolar chromophores the donor end groups have a great influence on the transition moment between the S_{1} and S_{2} state μ_{12} while the extent of conjugation and the strength of the centred acceptor are correlated to μ_{01}.^{11,12} Webster et al. varied the strength of the acceptor bridge between two identical heterocyclic donor groups by using a polymethine dye, a squaraine dye and a tetraone core. They interpreted the large δ_{2PA} (8200 GM) of the squaraine dye being due to the increase of the density of final states and, thus, to the number of possible electronic transitions between the 1PA state and a state with twice that energy. In addition a large intermediate state resonance enhancement was observed due to the sharply rising low-energy side of the linear absorption spectra.^{13} These authors also used a squaraine chromophore as the bridge between two zinc porphyrin moieties, leading to very broad 2PA spectra with cross sections of up to 11000 GM over the region of 1580 to 850 nm.^{14} Moreshead et al. achieved a high 2PA cross section in molecules that consisted of two squaraines that were bridged by a fluorene moiety.^{15}
Another way to accomplish large 2PA responses is the coupling of several chromophores. The ratio between the 2PA cross section δ^{(n)}_{2PA} of a superchromophore consisting of n monomers and the 2PA cross section δ^{(1)}_{2PA} of the monomer is defined as F_{(n)} in eqn (2). In the literature additive enhancement (F_{(n)} = n), cooperative behaviour (F_{(n)} > n) and weakening (F_{(n)} < n) of the cross sections were reported.^{16,17} More specific, δ^{(n)}_{2PA} should scale with n^{2} if full cooperativity is observed.^{18,19} This is caused by the linear dependence of both μ_{01}^{2} and μ_{12}^{2} on the number of chromophores in eqn (1).^{20}
(2) |
In an older work Scherer et al. together with one of us^{25} observed an enhancement of the 2PA cross section in a series of thiophene-bridged oligosquaraines (up to pentamers). There, it was concluded that the low-lying 2PA allowed states are the result of conjugational effects which cannot be explained by purely electrostatic exciton coupling. The latter however deliver a reasonable explanation for the shifts of 1PA states.
In this work we again focus on the 2PA properties of superchromophores built up from squaraine dyes (see Fig. 2).^{26–34} Besides linear arrangements we also investigate branched systems with a triarylamine core or a benzene core. In addition to the well-investigated trans-squaraine parent chromophore SQA^{9,14,15,25,35,36} we also employ the cis-squaraine dye SQB. In the latter system one oxygen atom of the central squaric acid is replaced by a dicyanovinylene group leading to higher acceptor strength and, as a result, to a red shift of the 1PA. In the homodimers dSQA and dSQB and the homotrimers tSQA and tSQB we investigate the effect of direct coupling of dye monomers to yield oligomers on their 2PA properties. In the star-shaped compounds SQA-TAA, SQB-TAA, SQA-ben and dSQA-ben the branching effect is examined. We expect that in all oligomeric superchromophores, the nonlinear optical properties will change as the linear optical properties do because of exciton coupling effects. Thus, it is the goal of this work to examine whether exciton coupling of excited monomer states can be used to increase the 2PA of superchromophores. However, for all the above mentioned dyes one has to take into account that diverse conformers (rotational isomers via the biaryl axis) may be present in solution and the symmetry of compounds might be lower as indicated in Fig. 2.
For a dimer of SQA (=dSQA) with a head-to-tail orientation we may consider two idealised structures, a bent structure and a more stretched structure both with C_{2} symmetry. Both structures can be formed by torsion around the biaryl axis. In Fig. 4, the exciton coupling interaction of localised transition moments (blue arrows) with their phase relation (direction of the arrows) for both scenarios is depicted. This theory predicts two possible electronic transitions into two exciton states which are separated by twice the exciton coupling energy J. While for the bent case both transitions are allowed, only the lowest exciton state is an allowed transition in case of the linear structure. In practice, mixtures of at least these two conformers will be present in solution and we expect to observe the lowest exciton state (termed S_{1} in the following) as a more intense transition and the upper exciton state (termed S_{1}′ because it derives from the S_{1} state of the monomer) as a less intense peak (shoulder). Fig. 3a shows that for dSQA the more intense maximum at 14500 cm^{−1} is by 1000 cm^{−1} bathochromically shifted in comparison to SQA, and the weaker peak has an energy of 16100 cm^{−1}. The difference of these two absorption maxima can be used to estimate the exciton coupling energy J which is ca. 800 cm^{−1} (see Table 1).
_{abs}/cm^{−1} (/nm) | ε/M^{−1} cm^{−1} | μ _{abs} ^{2}/D^{2} | _{fl}/cm^{−1} | ϕ _{f} ^{ } (_{ex}/cm^{−1}) | τ _{f} ^{ }/ns (TCSPC) | /ns | μ _{f} ^{2}/D^{2} | |J|/cm^{−1} | |
---|---|---|---|---|---|---|---|---|---|
a Fluorescence quantum yield and excitation wavenumber used for measuring the fluorescence spectra in parentheses. The given error was determined by three independent measurements. b Multiexponential fit of fluorescence decays measured by TSCPC, excitation at 15200 cm^{−1}. Amplitudes are given in brackets. c Lifetimes acquired by stretched exponential analysis of fluorescence decays measured by TCSPC, excitation at 15200 cm^{−1}. Stretching exponent in brackets. d Lifetimes acquired by distribution analysis of fluorescence decays measured by TCSPC, excitation at 15200 cm^{−1}. The distribution is shown in Fig. S1 (ESI). e This quantum yield deviates from older measurements^{39} because of the use of an improved set-up. | |||||||||
SQA | 15500 (643) | 365000 | 127 | 15300 | 0.62 ± 0.033^{e} (16700) | 0.07 (0.03) | 1.64 (0.97)^{c} | 114 | |
0.20 (0.10) | |||||||||
1.72 (0.87) | |||||||||
dSQA | 14500 (690) | 466000 | 248 | 14200 | 0.82 ± 0.033 (16700) | 0.15 (−0.31) | 1.67^{d} | 172 | 800 |
0.25 (0.30) | |||||||||
1.82 (0.39) | |||||||||
tSQA | 14000 (714) | 663000 | 410 | 13700 | 0.85 ± 0.038 (16100) | 0.08 (−0.26) | 1.57^{d} | 219 | 743 |
1.14 (0.42) | |||||||||
1.94 (0.32) | |||||||||
SQA-TAA | 15000 (665) | 805000 | 363 | 14800 | 0.71 ± 0.009 (16700) | 0.05 (−0.13) | 1.79 (0.96)^{c} | 125 | |
0.48 (0.12) | |||||||||
1.96 (0.75) | |||||||||
SQA-ben | 15100 (664) | 811000 | 319 | 14900 | 0.69 ± 0.001 (16700) | 0.08 (−0.21) | 1.71 (0.96)^{c} | 123 | 167 |
0.23 (0.17) | |||||||||
1.84 (0.61) | |||||||||
dSQA-ben | 14200 (705) | 1390000 | 800 | 14000 | 0.80 ± 0.015 (16100) | 0.06 (−0.28) | 1.51^{d} | 193 | 167/900 |
0.98 (0.27) | |||||||||
1.68 (0.45) | |||||||||
SQB | 14300 (700) | 202000 | 92.7 | 14000 | 0.75 ± 0.002^{e} (15400) | 3.45 | 3.46 (1.00)^{c} | 81 | |
dSQB | 13400 (747) | 362000 | 226 | 13100 | 0.69 ± 0.020 (15200) | 2.46 (0.83) | 2.56 (0.93)^{c} | 121 | 800 |
4.08 (0.17) | |||||||||
tSQB | 12900 (773) | 411000 | 339 | 12700 | 0.58 ± 0.023 (15200) | 2.10 (0.89) | 2.05 (0.88)^{c} | 142 | 636 |
4.30 (0.11) | |||||||||
SQB-TAA | 13900 (721) | 637000 | 332 | 13600 | 0.75 ± 0.020 (15200) | 3.06 | 3.02 (0.99)^{c} | 102 |
For a head-to-tail arrangement of three chromophores as in tSQA we disregard different conformers completely and give only the qualitative solution of exciton coupling theory for a zig-zag arrangement. This is sketched in Fig. 4. Thus, exciton theory predicts three exciton states where the lowest one is highly allowed, the highest one is slightly allowed but the intermediate state is forbidden. The energy splitting of the upper and lower exciton state is . Again, in practice we expect that these selection rules are lifted because of the presence of different conformers. Indeed, for the trimer tSQAFig. 3a shows three peak maxima with decreasing intensity from lower to higher energy. From the energy difference of the lowest (14000 cm^{−1}) and the highest energy peak (16100 cm^{−1}) we estimate an exciton coupling energy of ca. 740 cm^{−1} in good agreement with that of the dimer dSQA.
In the superchromophores derived from SQB the substitution of one oxygen atom by a dicyanomethylene group in the central squaric acid moiety leads to increased electron acceptor strength and a lowering of the LUMO energy. This shifts the lowest energy absorption by ca. 1100 cm^{−1} to the red compared to their SQA analogues (Table 1). The absorption spectra of the dimer dSQB and the trimer tSQB are similar to their SQA analogues but show some variation in peak intensities (Fig. 3b).
The star-shaped trimers SQA-TAA and SQB-TAA with a triarylamine as the connecting core unit essentially show a broadening of the lowest energy absorption band which is indicative of a weak exciton coupling effect (Fig. 3c). The red shift of the absorption relative to their monomers is primarily due to the addition of the propeller-like triarylamine donor to the quadrupole dyes, thus breaking the C_{2} symmetry of the squaraine moieties and inducing some CT character.^{40,41} In contrast the trimer SQA-ben with benzene as the connecting bridge displays a small shoulder at ca. 15700 cm^{−1} on the high energy side of the main absorption band which could be caused by a weak exciton coupling (Fig. 3d). This transition is forbidden for a truly C_{3} symmetric chromophore as indicated in Fig. 4 and is only visible because of some structural disorder.
A similar shoulder is observed for the larger dSQA-ben. Here, it is difficult to discriminate between the exciton coupling within the dimer branches and those between the three branches.
The squared transition moments (see Table 1) of the squaraine absorption bands of the superchromophores show nearly additivity to their smaller analogues and thus follow the Thomas–Reiche–Kuhn sum rule.^{42} This means, that no other electronic transitions with significant oscillator strength contribute to the exciton manifold. In all cases, two-exciton states^{43,44} can be anticipated as the result of the interaction of two excited chromophores (see Fig. 4). These two-exciton states cannot be reached by 1PA from the ground state but are visible as two consecutive absorption processes from the ground state to the 1PA state and from there to the two-exciton state in e.g. transient absorption spectroscopy experiments.
I = I_{0}exp(−(t/_{f})^{β}) | (3) |
The average lifetimes acquired in this way were used to calculate the transition moment of the fluorescence μ_{f} by the Strickler–Berg eqn (4)^{47} from the fluorescence quantum yield ϕ_{f}.
(4) |
The squared fluorescence transition moments of SQA and SQB (see Table 1) are within the experimental uncertainty equal to those of the absorption. Thus, we discuss only relative squared transition moments. Here we observed a less than additive behaviour on going from SQA to dSQA and tSQA but still an increase which proves some microscopic superradiance effect, i.e. collective emission from delocalised states of chromophore aggregates.^{48–51} A similar effect was observed for dSQB and tSQB but not for all star-like superchromophores which indicates weak exciton coupling in the latter cases and emission from more localised states, that is, symmetry breaking takes place in the excited state.^{52,53}
Fig. 6 Two photon absorption spectra (solid lines) and one photon absorption spectra (dashed lines) of the investigated squaraines in toluene at rt. |
σ ^{1PA}_{max}/10^{−15} cm^{2} | E _{1}/cm^{−1} | δ ^{2PA}_{1}/GM | E _{2}/cm^{−1} | δ ^{2PA}_{2}/GM | E _{3}/cm^{−1} | δ ^{2PA}_{3}/GM | E _{4}/cm^{−1} | δ ^{2PA}_{4}/GM | |
---|---|---|---|---|---|---|---|---|---|
a Could not be measured because of red-shifted fluorescence. b The fact that the 2PA cross sections are grouped in columns does not necessarily mean that they derive from similar electronic states. | |||||||||
SQA | 1.40 | 16700 | 133 | 25000 | 800 | ||||
SQA-TAA | 3.10 | 16500 | 470 | 22100 | 4890 | ||||
SQA-ben | 3.10 | 16400 | 370 | 23900 | 3580 | ||||
dSQA | 1.78 | 15900 | 340 | 18800 | 3430 | 21800 | 5780 | 25000 | 11400 |
tSQA | 2.54 | 15400 | 415 | 18600 | 8340 | 21800 | 12240 | 24000 | 17740 |
dSQA-ben | 5.31 | 15600 | 860 | 18600 | 10170 | 21900 | 16020 | 24100 | 16290 |
SQB | 0.903 | 15700 | 100 | 22800 | 400 | ||||
dSQB | 1.38 | 14800 | 300 | 17600 | 3830 | 20000 | 3450 | 22000 | 3880 |
tSQB ^{ } | 1.57 | — | — | — | — | — | — | — | — |
SQB-TAA | 2.44 | 15300 | 465 | 22000 | 6220 | 24100 | 16470 |
For a centrosymmetric chromophore such as SQA the selection rules for electronic 1PA processes allow transitions from the ground state (A_{g}) to the excited singlet state with u symmetry while the transitions to g symmetric states are dipole forbidden.^{54} Indeed, TD-DFT computations (see ESI†) assign B_{u} symmetry to the lowest singlet excited state. According to this method the two next higher energy excitations are n → π* excitation of the oxygen lone pair electrons into the LUMO. Because of minimal orbital overlap these transitions possess vanishing oscillator strength and can neither be observed by 1PA nor by 2PA spectroscopy. This type of excitation is disregarded in the following. According to the computations the next orbital-overlap allowed state is an A_{g} state which is 1PA forbidden but 2PA allowed. In our older work of SQA with R = ethyl in CHCl_{3}, the symmetry of this state was confirmed by polarisation dependent 2PAF measurements. This state is at 24200 cm^{−1} and is also visible by fluorescence excitation anisotropy spectroscopy in viscous media.^{9} 2PAF spectroscopy also showed a weakly allowed transition at 16700 cm^{−1} just above the lowest B_{u} state at 15700 cm^{−1} (for SQA with R = ethyl in CHCl_{3}). This transition is weakly 2PA allowed because of vibronic coupling to a b_{u} symmetric vibration (B_{u} × b_{u} = A_{g}).^{25} For SQA with R = dimethyloctyl in toluene we can confirm these older results (see Fig. 6) in our present work.
For SQB no 2PAF experiments are available in the literature, but, because of its C_{2v} symmetry the 1PA and 2PA allowed states do not show the above mentioned parity selection rules. Thus, in principle all 1PA states are also 2PA allowed. Nevertheless, the 2PA spectrum of SQB is very similar to the one of SQA, showing a weak 2PA (100 GM) at the vibronic shoulder of the S_{1} absorption at ca. 15700 cm^{−1} and a much higher 2PA of S_{2} at ca. 23000 cm^{−1}. According to TD-DFT computations both states have B_{2} symmetry and are polarised along the long axis of the molecule. S_{3} is an A_{1} state and polarised along the molecular C_{2} axis but is out of reach for the 2PA experiments. This agrees very well with the fluorescence excitation anisotropy (FEA) spectrum (see ESI†).
The 2PAF spectrum of the dimer dSQA shows dramatic differences in comparison to SQA: in addition to the vibronically allowed 2PA peak at the shoulder of the S_{1} ← S_{0} transition at 14500 cm^{−1}^{55} there are pronounced 2PA maxima at ca. 18800 cm^{−1} (δ_{2PA} = 3430 GM) and at 21800 cm^{−1} (δ_{2PA} = 5780 GM) which are absent in the monomer SQA. A comparison of the 1PA and the 2PA spectra on a logarithmic scale together with the fluorescence excitation anisotropy (FEA) spectrum allows a more detailed assignment of states (see Fig. 7). The FEA value r = 0.36 around the 0–0 peak of the S_{1} state proves this transition to be polarised parallel to the emission transition moment (ideally r = 0.4 in this case).^{56} The FEA drops to ca. 0.2 at the S_{1}(0–1) transition and finally to ca. −0.03 at the S_{1}′ state and to ca. 0.09 for its associated 0–1 transition. The latter values indicate a pronounced angle between the transition moments of these excitations and that of the fluorescence (S_{1} → S_{0}). Indeed, for the bent dimer case Fig. 4b shows a perpendicular orientation of the S_{1}′ ← S_{0} transition moment which would ideally give r = −0.2. The following states (termed S_{1}′′ and S_{1}′′′) are visible in the 2PA spectra as pronounced peaks, in the FEA as maxima (r = 0.2) and minima (r = 0.07) but also as weak shoulders and peaks in the 1PA spectrum at ca. 18800 cm^{−1} and at 21400 cm^{−1}. While the S_{1} and S_{1}′ states can easily be derived by exciton coupling theory, S_{1}′′ and S_{1}′′′ cannot. Therefore, assuming a linear dimer situation we construct an orbital interaction diagram as given in the ESI† in which the π–HOMO and the π–LUMO of a monomer interact and give in total four new dimer π-orbitals. With these four orbitals, four singly excited configurations can be generated. Because of their orbital symmetry (either g or u) this gives rise to two allowed and two forbidden transitions from the ground state.^{57} TD-DFT computations come to the same conclusion (see ESI†).^{58} Thus, four exited states may be derived from the orbital interaction of two monomers while simple excited state exciton interaction theory predicts only two states. Therefore, it appears that exciton coupling theory is intrinsically insufficient to cover all aspects of 2PA properties. We assume that the four electronic states that we observe experimentally up to 23000 cm^{−1} are the four states that we constructed from orbital mixing. Because these states are closely related to S_{1} of the monomer they are consequently numbered S_{1} through S_{1}′′′. The relative order of these states is difficult to assign as CI mixing may lead to energy shifts of the simple MO configurations and, furthermore, intensity borrowing between states of same symmetry may alter the observed intensities.^{59–61}
Fig. 7 1PA (black) and the 2PA (red) spectra of dSQA in toluene on a logarithmic scale together with the fluorescence excitation anisotropy (FEA) spectrum (blue) in polyTHF at 26 °C. |
In the trimer tSQA the 1PA and 2PA allowed states are shifted a little bit towards lower energy (see Fig. 6 and Table 2). More interesting is that the 2PA cross section for the S_{1}′′ and S_{1}′′′ band is about twice that of the dimer (at energies E_{2} and E_{3}, see Table 2), thus showing a significant cooperative enhancement (see eqn (2)) of δ_{2PA} at very similar state energies. Beyond these general statements it is difficult to draw more specific conclusions about possible n^{2} dependencies of the 2PA cross sections. This has to do with the fact that one can only compare 2PA cross sections which derive from similar electronic states, this is possible for e.g. the S_{1}′′ and S_{1}′′′ states of dSQA and tSQA but not for SQA where these states are missing. For the former, a relative 2PA cross section of 4:9.7 is obtained for S_{1}′′ at 18800 cm^{−1} and of 4:8.5 for S_{1}′′′ at 21800 cm^{−1}. Both these ratios are very close to the 4:9 ratio expected for a dimer and a trimer.
The dimer dSQB shows a surprisingly similar 2PA spectrum as dSQA with the individual peaks a little bit shifted to lower energies (Fig. 8). The 2PA cross sections are also very similar. This demonstrates that the principal components of the 1PA and 2PA allowed states are in both dimers formed by interaction of the transitions polarised along the molecular long axis and that those polarised perpendicular to the long axis do not play a significant role.^{62}
Fig. 8 Two photon absorption spectra (solid lines) and one photon absorption spectra (dashed lines) of the dimers dSQA and dSQB in toluene at rt. |
The star-like superchromophores SQA-TAA and SQA-ben display qualitatively similar 2PA spectra as SQA. In particular there is a pronounced 2PA at the energy of the vibronic progression at ca. 17000 cm^{−1} (see Table 2). Because of three squaraine chromophores in the star-like systems the 2PA is about three times that of SQA in this energy region. The 2PA at higher energy (>22000 cm^{−1}) also rises for the star-like superchromophores, in particular for SQA-TAA. This effect cannot be caused by a double resonance effect with the 1PA state because the lowest 1PA energy is almost the same in SQA, SQA-ben and SQA-TAA. We suppose that it is caused by an increase of density of states because of the enlargement of the π-system, possibly also because of states with CT character in the case of SQA-TAA. This effect is even stronger in the case of SQB-TAA.
Much in contrast, the 2PA of dSQA-ben at the S_{1} to S_{1}′′′ energies is almost exactly three times that of the corresponding 2PA values of dSQA thus showing additivity of the 2PA cross section. Consequently, while there is a cooperative enhancement extending dSQA by one squaraine chromophore to tSQA there is additivity on going from dSQA to dSQA-ben. This demonstrates that the interactions between the branches of the 1,3,5-substituted benzene are negligible but they are strong in the linear arrangement of squaraine dyes.
However, the fluorescence quantum yield is one of the major sources of error in the determination of the 2PA cross section.^{63} Particularly in case of SQA-ben and dSQA-ben the quantum yields may possess a systematic error because of surface adsorption effects to the cuvette walls. Therefore, the conclusions concerning those chromophores should be taken with care.
For all squaraine superchromophores the 2PA cross section rises strongly when the excitation energy approaches the 1PA energy. This is, as mentioned in the introduction, the consequence of a double resonance effect because for all exciton coupled chromophores, there are biexciton states at about twice the energy of the 1PA states (see Fig. 4). Although the biexciton states cannot simply be characterised as such by 2PA spectroscopy, they may have an indirect influence via the double resonance effect.
Besides these findings we found enhancement of 2PA cross sections in linear arrangements of squaraines (dSQA, tSQA, dSQB) but not in those with star-like arrangement (SQA-ben, dSQA-ben) where merely additivity of 2PA cross section is found. The latter is in accord with observations made by Blanchard-Desce et al.^{22} for other superchromophores with a benzene core. This is surprising given the high oscillator strength of the squaraine dyes. This should lead to a sizable Coulomb coupling between squaraine chromophores even though they are connected by a benzene ring in 1,3,5-position as is evident by comparing the 1PA spectra of SQA and SQA-ben where moderate excitonic couplings were found. Obviously, this is not the case for higher excited states and only additivity of 2PA cross section is therefore observed. Similarly, we found no superradiance effect for branched chromophores but for all linear arrangements. All these observation suggest that the enhancement of 2PA cross section in the linear arrangements of dSQA and tSQA is probably caused by orbital interactions of higher excited configurations.
The squares of the transition dipole moment were obtained by integration of the main low energy absorption band and calculated as follows^{64}
(5) |
The two photon cross section at a given wavenumber, δ^{(2)}_{s}(), was calculated as follows^{66}
(6) |
The power dependence of the fluorescence intensity was monitored throughout the measurement (Fig. S3, ESI†).
Footnote |
† Electronic supplementary information (ESI) available: Synthetic protocols, fluorescence excitation anisotropy spectrum, power dependence of 2PAF, lifetime distributions and TD-DFT calculations. See DOI: 10.1039/c6cp02312f |
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