Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 6029−6034
pubs.acs.org/JPCL
Excited-State Symmetry Breaking in a Quadrupolar Molecule Visualized in Time and Space Bogdan Dereka,† Arnulf Rosspeintner,† Rafał Stęzẏ cki,⊥ Cyril Ruckebusch,‡ Daniel T. Gryko,⊥ and Eric Vauthey*,† †
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland ‡ Université de Lille, CNRS, UMR 8516, LASIR, Laboratoire de Spectrochimie Infrarouge et Raman, Lille 59000, France ⊥
S Supporting Information *
ABSTRACT: The influence of the length of the push−pull branches of quadrupolar molecules on their excited-state symmetry breaking was investigated using ultrafast timeresolved IR spectroscopy. For this, the excited-state dynamics of an A-π-D-π-A molecule was compared with those of an ADA analogue, where the same electron donor (D) and acceptor (A) subunits are directly linked without a phenylethynyl π-spacer. The spatial distribution of the excitation was visualized in real time by monitoring CC and CN vibrational modes localized in the spacer and acceptor units, respectively. In nonpolar solvents, the excited state is quadrupolar and the excitation is localized on the π-D-π center. In medium polarity solvents, the excitation spreads over the entire molecule but is no longer symmetric. Finally, in the most polar solvents, the excitation localizes on a single D-π-A branch, contrary to the ADA analogue where symmetry breaking is only partial.
O
does not only increase the total length of the molecule by a factor of 1.8 (Figure S2 and Table S2) but also provides an additional IR marker, namely, the CC stretching vibration located in the center of both arms. The presence of two different IR markers will allow SB to be resolved both temporally and spatially using time-resolved IR (TRIR) spectroscopy. Whereas the lengthening of the two arms has a negligible impact on the absorption solvatochromism, which is dominated by dispersion interactions (Figure 1B, Figures S3−S5), it has a spectacular effect on the fluorescence solvatochromism. Indeed, upon going from cyclohexane (CHX) to acetonitrile (ACN), the emission band of Q1 and Q2 downshifts by 5150 and 1630 cm−1, respectively (Figure 1A). Moreover, Figure 1B reveals that the slope of the solvatochromic plots of the emission band becomes steeper as solvent polarity increases, especially for Q1 (Table S4). This points to an increase in the permanent dipole moment of the excited state with solvent polarity,25,26 as expected for excited-state SB. The larger solvatochromism of Q1 shows that lengthening the charge-transfer branches allows the excited state to adopt a stronger dipolar character in the most polar solvents. The absence of a mirror-image relationship between the absorption and emission spectra in nonpolar solvents (Figure 1A) can be ascribed to torsional disorder in the ground state
ver the past few years, excited-state symmetry breaking (SB) in quadrupolar molecules has been attracting considerable attention.1−20 During this process, the electronic excitation, originally distributed evenly over the two chargetransfer branches of the molecule, localizes, at least partially, on one of them, conferring the excited state some dipolar character. Although the concept was formulated theoretically several years ago, 21,22 a real-time observation of this phenomenon was missing until very recently.7,12 Excited-state SB was shown to be due to the asymmetry of the instantaneous orientation of the surrounding solvent and thus to differences in dipolar, quadrupolar, H-bonding, and halogen-bonding interactions with the two charge-transfer arms of the molecule.7,12,15 These investigations revealed that the extent of SB can be controlled by tuning the strength of these solute− solvent interactions. However, the influence of the nature of the quadrupolar molecule itself on the amplitude of the SB was not addressed. Here we show how the length of the individual charge-transfer branches affects the extent of SB and the spatial distribution of the excitation. For this, we chose the quadrupolar A-π-D-π-A molecule Q1 (Figure 1A),23 which features a potent pyrrolo[3,2-b]pyrrole24 electron donor (D) flanked by two cyanophenyl electron acceptors (A) connected via phenylethynyl π-bridges. SB in this molecule will be compared with that in the analogue Q2 (Figure 1A), where the same A and D subunits are directly linked. Excited-state SB in Q2 was already reported and was detected by monitoring the CN stretching vibration localized on the acceptor ends.12,15 Introduction of the π-spacers in Q1 © XXXX American Chemical Society
Received: November 6, 2017 Accepted: November 30, 2017 Published: November 30, 2017 6029
DOI: 10.1021/acs.jpclett.7b02944 J. Phys. Chem. Lett. 2017, 8, 6029−6034
Letter
The Journal of Physical Chemistry Letters
ground-state bleach at 2213 (CC) and 2230 cm−1 (CN) are hardly visible (Figures S7 and S8). The TRIR data were analyzed globally assuming a series of sequential exponential steps with increasing time constants. This provides relevant time scales and the corresponding evolution-associated difference absorption spectra (EADS).28 Three successive steps were sufficient to properly reproduce the time evolution of transient absorption. The EADS obtained from the measurements in cyclohexane (CHX), diethyl ether (DEE), CHCl3, and dimethyl sulfoxide (DMSO) are shown in Figure 2, whereas those in di-n-butyl ether (DBE), tetrahydrofuran (THF), benzonitrile (BZN), and acetonitrile (ACN) are depicted in Figure S13. In CHX, the TRIR spectra exhibit a single intense band, initially at 2030 cm−1, that increases, narrows and undergoes a 3 cm−1 upshift during the first ca. 10 ps after excitation. Afterward, it remains unchanged up to 1.9 ns, the upper time limit of the experiment. This band can be assigned to the antisymmetric CC stretching vibration of Q1 in the S1 state, and its initial spectral dynamics can be attributed to the dissipation of ∼0.5 eV excess excitation energy by vibrational relaxation7,12,29 and also possibly to the planarization of the molecule.30 The presence of a single CC stretching band is evidence that the S1 state of Q1 remains symmetric during its whole lifetime. The same conclusion was reached with Q2 from the presence of a single intense CN ESA band in CHX.12 Surprisingly, apart from a hardly detectable band around 2210 cm−1, no significant spectral feature can be observed in the CN region. This weak 2210 cm−1 band is tentatively assigned to the CN stretching mode of Q1 in the S1 state. Both the weak intensity of this band and its frequency close to that in the ground state, that is, 2230 cm−1 (Figure S7), indicate that the electronic density on the cyano groups remains almost unchanged upon excitation, suggesting that, in this solvent, the electronic excitation is mostly localized on the π-D-π center of Q1. Upon going to the low-polar DEE, two main differences can be observed. First, the CC ESA band is significantly broader and exhibits only an initial 6 cm−1 downshift. Considering that the same amount of excess excitation energy is deposited in the molecule independently of the solvent and that planarization also occurs,30 the vibrational/structural relaxation dynamics observed in CHX is hidden here by another process that leads to a decrease in intensity and a downshift of the CC band.
Figure 1. (A) Absorption spectrum of Q1 and Q2 in THF and fluorescence spectra in cyclohexane (blue), tetrahydrofuran (green), and acetonitrile (red) (B) Solvent polarity dependence of the absorption and emission maxima of Q1 (orange) and Q2 (blue).
but not in the excited state (see SI for details).27 Despite this, the ground state is essentially apolar as testified by the absence of significant solvatochromism in absorption (Figure 1B). Direct insight into excited-state SB in Q1 was obtained by performing TRIR measurements in the CC and CN stretching regions, namely, between 1950 and 2250 cm−1 in eight solvents of varying polarity (Figures S8−S12). In general, the transient spectra are dominated by intense excited-state absorption (ESA) bands, whereas negative bands due to
Figure 2. Evolution-associated difference spectra (EADS) and corresponding time constants obtained from global analysis of the transient infrared data measured with Q1 assuming a series of three successive exponential steps (A → B → C →). (pink: CC stretch; blue: CN stretch; CHX: cyclohexane; DEE: diethyl ether; CHCl3: chloroform; DMSO: dimethyl sulfoxide). 6030
DOI: 10.1021/acs.jpclett.7b02944 J. Phys. Chem. Lett. 2017, 8, 6029−6034
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associated with the less-excited branch upon full localization of the excitation. In summary, these TRIR data reveal that the electronic distribution in the equilibrated S1 state of Q1 depends considerably on the solvent as schematized in Figure 3. In
Second, two small bands can be observed at 2153 and 2178 cm−1. At early time, the low-frequency band is weaker, but it increases during the first 10 ps to reach an intensity close to that at higher frequency. Afterward, all three ESA bands decay on the ∼1.2 ns time scale. According to their frequency, the two weak bands are assigned to CN stretching modes. The evolution from the early spectrum with essentially a single CN band to a spectrum with two CN bands can be attributed to symmetry breaking. SB in Q1 should also lead to the appearance of a second CC stretching band.7 TRIR measurements with Q3,23 an analogue of Q1 with the cyano functionalities replaced by CF3 groups, show that SB indeed results in the emergence of a second, much weaker CC band around 2190 cm−1 (Figure S14). In the case of Q1, this second band, which is relatively broad, should overlap with the two CN bands. In the more polar CHCl3 and THF, the early spectral dynamics are similar but more pronounced than in DEE: The CC band shows a distinct decrease with a larger downshift (up to 9 cm−1), whereas the low-frequency CN band surpasses that at higher frequency. Excited-state SB in Q2 leads to the appearance of a second, weaker, CN band at higher frequency.12,15 This second band was attributed to the symmetric CN stretching mode that is no longer IR-inactive upon SB. In the case of Q1, the two cyano groups are probably too far apart for their vibrations to be coupled and should thus be considered as two independent vibrators. The overall larger intensity of both CN bands compared with CHX points to a higher density of the electronic excitation on the two acceptor ends. The density on the πbridges increases as well, as testified by the downshift of the CC band. These results reveal that going from nonpolar to medium polarity solvents leads not only to SB but also to an expansion of the excitation from the core to the periphery of the molecule. At early time, the S1 state is symmetric with the same electronic density on both arms, and thus a single CN band is observed at 2178 cm−1. Upon spatial expansion of the excitation and SB, the density on the cyano groups increases, but not equally for both. The CN stretching frequency is well known to be highly sensitive to the electronic density and to downshift significantly upon reduction.31−33 As a consequence, the CN band with the highest density of excitation is that at lower frequency, that is, at 2150 cm−1. Further ultrafast spectral dynamics can be observed in the most polar solvents BZN, DMSO, and ACN. After early dynamics similar to that in medium-polar solvents, the intense CC band at ca. 2030 cm−1 decreases almost entirely, whereas a new narrower band, also attributed to the CC stretch, grows around 2105 cm−1. In parallel, the high-frequency CN band decays almost completely. These dynamics accelerate when going from BZN to DMSO and to ACN. The presence of only two ESA bands in DMSO and ACN after a few picoseconds can be accounted for by a full localization of the electronic excitation on a single D-π-A branch of Q1. This implies that the S1 state is purely dipolar with a large chargetransfer character, in agreement with the nonlinear solvatochromic plot (Figure 1B). Upon increasing the charge-transfer character of the S1 state, the electronic density on the ethynyl group decreases and the CC band upshifts, as also observed for other push−pull molecules with ethyne bridges.7,34 The partial decrease in the low-frequency CN band at early time can be attributed to the decay of the overlapping CC band
Figure 3. Schematic representation of the excited-state symmetry breaking and excitation distribution in Q1 and Q2.
nonpolar solvents, the electronic distribution is symmetric and localized on the π-D-π core, and thus the S1 state is purely quadrupolar (Q state). In medium-polar solvents, the Franck− Condon S1 state is also quadrupolar, but it rapidly relaxes to a symmetry-broken state with the excitation unevenly delocalized over both branches, including the CN groups. Here the relaxed S1 state possesses a net dipole moment but keeps some quadrupolar character (intermediate I state). Finally, in the most polar solvents, the asymmetry of the solvent field is strong enough to fully localize the charge-transfer excitation on one branch, and the relaxed S1 state becomes purely dipolar (D state). The TRIR data in these solvents show that the spectral features associated with both I and D states coexist up to 1.9 ns (Figure S13). This points to equilibrium between these two states, which shifts toward the D state when going from BZN to DMSO and ACN. In the latter two solvents, the excitation is almost irreversibly trapped on one branch. By comparison, the S1 state of Q2 evolves from Q to I when going from apolar to polar solvents. However, full localization on a single branch (D state) is never achieved even in the most polar solvents.12,15 The transitions between the Q, I, and D states occur on very similar time scales as those of solvent motion.35 This, together with the quadrupolar nature of the S1 state in nonpolar solvents, indicates that SB in Q1 is driven by the environment, as also found with Q2 and with a quadrupolar D-π-A-π-D molecule.7,12 Although the S1 state is initially symmetric and quadrupolar, fluctuations of the orientation of the surrounding polar solvent molecules cause weak transient asymmetry of the reaction field that can favor an asymmetric distribution of the excitation, conferring the solute a weak dipolar character. The solute field polarizes the solvent, leading to further asymmetry of the reaction field and, in turn, to further asymmetry of the electronic distribution in the solute occurring on the time scale of solvent relaxation. The Q, I, and D states of Q1 are characterized by distinct vibrational spectra. To find out whether these states could also be differentiated through their electronic spectra, the excitedstate dynamics of Q1 was also monitored by transient electronic absorption spectroscopy (Figures S15 and S16). No clear spectral signature of SB could be detected in the resulting spectra, in agreement with previous studies on other multipolar molecules.3,7,10,36,37 To determine the transient electronic absorption spectra associated with the Q, I, and D states, both electronic and vibrational transient data were combined and analyzed in a multiset using the multivariate 6031
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as well be accounted for by the stabilization of a quadrupolar state upon solvent relaxation.43 In DMSO, the Sn ← S1 ESA band exhibits significant changes with time. However, here again, this change is mostly related to the red shift of the overlapping stimulated emission band, whose maximum is at ∼650 nm after solvent relaxation. This multiset data analysis reveals that the main effect of SB on the electronic spectra is a pronounced red shift of the emission. However, such red shift is not specific to SB, and thus the occurrence of SB in Q1 cannot be inferred from the transient electronic absorption spectra. Comparison between Q1 and Q2 shows that increasing the length of the quadrupolar branches leads to a higher extent of SB. This can be rationalized using the Ivanov SB model,14 where the extent of SB is given by the dipolar parameter D
curve resolution-alternating least-squares (MCR-ALS) approach.38−42 A hard model scheme consisting of three sequential exponential steps was applied as a constraint on the time-dependent concentration profiles. The resulting spectral contributions in both visible and infrared regions obtained in nonpolar and weakly and highly polar solvents are presented in Figure 4. The spectral
D=
1−
4V 2 (λ + γ )2
(1)
where V is the electronic coupling between the branches, λ is the solvation energy, and γ is the Coulombic repulsion of the charges on the acceptor units. As discussed in the Supporting Information, increasing the distance between the D and A units results in smaller coupling and larger solvation energy. As a consequence, the extent of SB in a given solvent is larger for Q1 than for Q2. In ACN, D is predicted to be 0.98 for Q1 and 0.84 for Q2. The presence of two different vibrational markers allows indepth insight into symmetry breaking in centrosymmetric molecules to be gained through TRIR. Different markers at specific locations in the molecule enhance the spatial resolution of the distribution of the excitation. The above results reveal how the extent of SB in quadrupolar molecules can be tuned by changing the distance between the D and A units. π-Expansion of pyrrolo[3,2-b]pyrrole has a significant effect on the magnitude of symmetry breaking; that is, excitation in polar solvents is localized on a single D-π-A branch.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02944. Details of experiments and data analysis, stationary absorption and emission spectra, time-correlated single photon counting data, additional TRIR and TA data, and details of the Ivanov SB model (PDF)
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Figure 4. Spectral contributions in the mid-IR (left) and UV−visible (right) regions and corresponding time constants obtained from the multiset analysis using the MCR-ALS approach with a hard model constraint (A → B → C →) applied on the time-dependent concentration profiles.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bogdan Dereka: 0000-0003-2895-7915 Arnulf Rosspeintner: 0000-0002-1828-5206 Daniel T. Gryko: 0000-0002-2146-1282 Eric Vauthey: 0000-0002-9580-9683
contributions in the IR region as well as the associated time constants coincide with those obtained from the analysis of the TRIR data alone (Figure 2). Vibrational relaxation and planarization results in a slight intensity increase of the Sn ← S1 ESA band around 700 nm as well as a red shift and an increase in the vibronic structure of the S1 → S0 stimulated emission around 500 nm.30 In CHCl3, the main change is a red shift of the stimulated emission band. However, without knowledge of the TRIR data, this shift cannot be unambiguously ascribed to SB (Q → I transition) but could
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Fonds National Suisse de la Recherche Scientifique through project Nr. 200020-165890, the 6032
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