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Cite This: J. Phys. Chem. Lett. 2019, 10, 2944−2948
Ground-State Structural Disorder and Excited-State Symmetry Breaking in a Quadrupolar Molecule Magnus Söderberg,† Bogdan Dereka,†,¶ Assunta Marrocchi,‡ Benedetta Carlotti,‡ and Eric Vauthey*,† †
Department of Physical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, CH-1211 Geneva, Switzerland Department of Chemistry, Biology and Biotechnology, University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy
‡
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S Supporting Information *
ABSTRACT: The influence of torsional disorder around the ethynyl π-bridges of a linear D−π−A−π−D molecule on the nature of its S1 excited state was investigated using ultrafast time-resolved infrared spectroscopy. By tuning the pump wavelength throughout the S1 ← S0 absorption band, subpopulations with different extents of asymmetry could be excited. In nonpolar solvents, the equilibrated S1 state is symmetric and quadrupolar independently of the initial degree of distortion. Photoexcitation of distorted molecules is followed by planarization and symmetrization of the S1 state. Excited-state symmetry breaking is only observed in polar environments, where the equilibrated S1 state has a strong dipolar character. However, neither the extent nor the rate of symmetry breaking are enhanced in an initially distorted molecule. They are only determined by the polarity and the dynamic properties of the solvent.
T
moment upon excitation, determined from the two-photon absorption cross section, was consistent with that calculated for molecules with a 90° twist angle of either the acetylene ligand32 or the phenyleneethynylene chain.33 Whereas phenyleneethynylene oligomers exist with a relatively broad distribution of torsional angles in the ground state, their S1 electronic excited state is more rigid because of conjugation, and thus, the amount of structural disorder is significantly smaller.34−36 As a consequence, their S1 ← S0 absorption band is generally broad and featureless, whereas their S1 → S0 emission band is narrower and exhibits a vibronic structure.34−37 Additionally, the vertical S1−S0 transition energy varies with the torsion angle, allowing for photoselection.38 Here, we explore the effect on ES-SB of torsional disorder and structural asymmetry in the ground state of an organic D−π− A−π−D type molecule. Such asymmetry can be expected to result in a slightly unbalanced electronic distribution on the two A−π−D branches already in the ground state. We want to find out whether this effect could lead to ES-SB already in apolar environments or accelerate ES-SB in polar solvents. For this, we study the dye 1, which consists of a central benzothiadiazole electron acceptor flanked by two alkoxyphenyls, acting as electron donors, linked through −C≡C− bridges,39 as well as its single-branch dipolar analogue, 2 (Chart 1). We perform TRIR measurements on these two compounds in the −C≡C− stretching region in solvents of varying polarity (Table S1,
he growing interest for quadrupolar dyes with A−π− D−π−A and D−π−A−π−D motives, where D and A are electron donor and acceptor subunits, is mainly related to the quest for strong two-photon absorbers,1−4 which are in demand for a broad variety of applications, including fluorescence imaging, 5,6 phototherapy,7,8 or photopolymerization. 9,10 Although these molecules are centrosymmetric in the ground state, as testified by their one- and two-photon absorption spectra,11,12 their fluorescence exhibits a pronounced solvatochromism,13−21 similar to that observed with their single-branch D−π−A analogues. This phenomenon is explained in terms of excited-state symmetry breaking (ES-SB), i.e., by the localization of the electronic excitation on one D−π−A branch of the molecule and, thus, by a transition from a quadrupolar to a dipolar excited state.22−24 Theoretical work by Painelli and Terenziani suggests that ES-SB is primarily triggered by antisymmetric vibrations,22,23,25 with solvent and/or structural fluctuations leading to a subsequent stabilization of the SB state. Provided the molecule is equipped with IR marker modes localized in the D−π−A branches, ES-SB can be directly monitored using time-resolved IR (TRIR) spectroscopy.26−29 Otherwise, ES-SB can be inferred from the decrease of the emission transition dipole moment.30,31 Such investigations, performed with several symmetric A−π−D−π−A and D−π− A−π−D dyes, revealed that ES-SB is mainly driven by solvent fluctuations, as it takes place on the same time scale as that of solvent motion and as it does not occur in apolar solvents.26−30 Recent investigations on platinum acetylides and ferrocene− phenyleneethynylenes pointed to the role of structural disorder in the electronic ground state as the origin of symmetry breaking in these molecules.32,33 The change of permanent electric dipole © 2019 American Chemical Society
Received: April 10, 2019 Accepted: May 13, 2019 Published: May 13, 2019 2944
DOI: 10.1021/acs.jpclett.9b01024 J. Phys. Chem. Lett. 2019, 10, 2944−2948
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The Journal of Physical Chemistry Letters
and a significantly lower barrier for torsion in the ground than in the excited state (Figure S2). Indeed, the dihedral angle between the D and A planes of 1 in the ground state can vary freely by ±40° at room temperature, whereas in the excited state, the potential along the torsional coordinate is steeper, and the dihedral angle is confined within ±20°. Consequently, red-edge excitation is predicted to interact with planar molecules, whereas shorter wavelengths should excite distorted molecules, which should then planarize in the S1 state. To monitor the excited-state dynamics of planar molecules, TRIR measurements with 1 and 2 were first performed upon red-edge excitation. Figure 2 shows TRIR spectra in the
Chart 1. Structure of Dye 1 and of Its Single-Branch Analogue 2
Supporting Information (SI)) using different pump wavelengths. By exciting on the red edge of the S1 ← S0 absorption band, we expect to monitor ES-SB in mostly planar dyes, whereas distorted molecules are predominantly excited at shorter wavelength. We find that no ES-SB occurs in apolar solvents, independently of whether the molecule is planar or not in the ground state. In fact, ground-state distorted molecules become more symmetric in the S1 state upon planarization. In polar solvents, ES-SB occurs independently of the degree of the ground-state distortion, and its dynamics remain the same. The electronic absorption and fluorescence spectra of 1 in solvents of varying polarity are depicted in Figure 1, whereas
Figure 2. TRIR spectra measured after 490 nm excitation of 1 in solvents of increasing polarity and after 450 nm excitation of 2 in DMF.
−C≡C− stretching region recorded at various time delays after 490 nm excitation of 1 in solvents of increasing polarity. In CHX, the spectra are dominated by a single excited-state absorption (ESA) band around 2075 cm−1. Apart from a
