Electronic States of Tris(bipyridine) Ruthenium(II) Complexes in Neat Solid Films Investigated by Electroabsorption Spectroscopy

Abstract

We present the electric field-induced absorption (electroabsorption, EA) spectra of the solid neat films of tris(bipyridine) Ru(II) complexes, which were recently functionalized in our group as photosensitizers in dye-sensitized solar cells, and we compare them with the results obtained for an archetypal [Ru(bpy)3]2+ ion (RBY). We argue that it is difficult to establish a unique set of molecular parameter values by discrete parametrization of the EA spectra under the Liptay formalism for non-degenerate excited states. Therefore, the experimental EA spectra are compared with the spectra computed by the TDDFT (time-dependent density-functional theory) method, which for the first time explains the mechanism of electroabsorption in tris(bipyridine) Ru complexes without any additional assumptions about the spectral lineshape of the EA signal. We have shown that the main EA feature, in a form close to the absorption second derivative observed in the spectral range of the first MLCT (metal-to-ligand charge transfer) absorption band in Ru(bpy)3(PF6)2, can be attributed to a delocalized and orbitally degenerate excited state. This result may have key implications for the EA mechanism in RBY-based systems that exhibit similar EA spectra due to the robust nature of MLCT electronic states in such systems.

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“…In the second case, it was assumed that the amplitude D 0 equals the value of the total absorbance D at 22 kK in the global absorption spectrum of complexes B1, B2, or B3, respectively. Taking from the experiment, the average width of Gaussian bands, w = 2000 cm −1 , the electric field intensity F rms = 6 × 10 5 V/cm and using the applied field approximation almost the same values of Δ µ = 10 ± 1 D (the first method) and Δ µ = 5 ± 1 D (the second method) for B1, B2, and B3 complexes were obtained, which is consistent with available literature data for another bipyridine Ru complexes 22 – 25 . It should be noted, however, that despite the observed similarity in the EA behavior of various Ru(II) bipyridine complexes, the above estimation of Δ µ should be treated with caution due to the somewhat complicated pattern of interaction of excited states with the electric field involved in the EA response.…”

“…All mono- and biruthenium complexes of bipyridine, based on the [Ru(bpy) 3 ] 2+ ion, show a characteristic EA signal with a spectral shape similar to the absorption second derivative with a minimum of the negative lobe observed at about 22 kK. This common feature of EA was attributed to a highly delocalized and orbitally degenerate or quasi-degenerate excited state in [Ru(bpy) 3 ] 2+ based systems due to the robust nature of MLCT electronic states in such systems 22 . Based on Liptay formalism 21 for a single electronic transition at about 22 kK in an isotropic system, the dipole moment change Δ µ (in debye unit, 1D = 3.34 × 10 –30 C·m) upon this MLCT excitation can be evaluated according to the formula: where the amplitude D 0 and the width w (expressed in cm −1 ) refer to the Gaussian band and the electroabsorption signal EA max , measured in an electric field with the rms value F rms (in V/cm), refers to the minimum of EA spectrum observed at the exact wavenumber as the central position of the MLCT absorption band.…”

“…The standard analysis, based on derivatives of absorption spectra and discrete parametrization of EA spectra in the form of molecular dipole moments using the Liptay formalism 21 , is difficult to reliably perform in spectral ranges with densely packed excited states. However, the EA spectra can be successfully compared with those computed by the DKH2/DKH2 method without imposing any restrictions on the shape of the EA signal spectral line 22 . The same approach can be applied to biruthenium complexes B1, B2, and B3, where much more excited states are involved in the EA signal.…”

“…Firstly, D 0 = 0.04 was obtained for the Gaussian band at 22 kK derived from the global absorption spectrum deconvoluted into the sum of several Gaussian profiles, as shown in Ref. 22 for a solid film of the B1 complex. The given D 0 value corresponds to the linear absorption coefficient α = 1.5 × 10 4 cm −1 , which is a factor of 5 lower than the maximum value of this coefficient recorded in the appropriate spectral range.…”

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“…[10][11][12] Several search groups have successfully used the above mentioned basis set (see, e.g., Refs. [13][14][15][16][17][18][19][20][21]).…”