Efficient Vibrational Energy Transfer through Covalent Bond in Indigo Carmine Revealed by Nonlinear IR Spectroscopy

Abstract

Ultrafast vibrational relaxation and structural dynamics of indigo carmine in dimethyl sulfoxide were examined using femtosecond pump-probe infrared and two-dimensional infrared (2D IR) spectroscopies. Using the intramolecularly hydrogen-bonded C═O and delocalized C═C stretching modes as infrared probes, local structural and dynamical variations of this blue dye molecule were observed. Energy relaxation of the vibrationally excited C═O stretching mode was found to occur through covalent bond to the delocalized aromatic vibrational modes on the time scale of a few picoseconds or less. Vibrational quantum beating was observed in magic-angle pump-probe, anisotropy, and 2D IR cross-peak dynamics, showing an oscillation period of ca. 1010 fs, which corresponds to the energy difference between the C═O and C═C transition frequency (33 cm). This confirms a resonant vibrational energy transfer happened between the two vibrators. However, a more efficient energy-accepting mode of the excited C═O stretching was believed to be a nearby combination and/or overtone mode that is more tightly connected to the C═O species. On the structural aspect, dynamical-time-dependent 2D IR spectra reveal an insignificant inhomogeneous contribution to time-correlation relaxation for both the C═O and C═C stretching modes, which is in agreement with the generally believed structural rigidity of such conjugated molecules.

Solvent Mediated Excited State Proton Transfer in Indigo Carmine

“…In particular, there are two positive peaks at 1642 and 1610 cm –1 corresponding to the ground state bleach (GSB) and one broad negative peak at 1530 cm –1 corresponding to the excited state absorption (ESA). The two GSB peaks have previously been assigned to the asymmetric stretching of two C=O groups and ring C=C bonds of InC in the ground electronic state (S 0 ), whereas the ESA peak was assigned to the C=O asymmetric stretching in the excited (S 1 ) electronic state. , These assignments agree with our calculated IR spectra (Figure e), discussed later. In D 2 O, both GSB (1645 and 1616 cm –1 ) and ESA (1525 cm –1 ) peaks appear (Figure c) at a very similar spectral region as in dDMSO.…”

Effect of Solvation Shell Structure and Composition on Ion Pair Formation: The Case Study of LiTDI in Organic Carbonates

“…In the case of the sample containing almost all of CIP, such as LiTDI in DMC, the peak at 2230 cm –1 shows a faster dynamics than the free ion and it has an offset (Table ). While the difference in the dynamics evidences that the CN transitions might have a different mechanism for the frequency fluctuation, the offset in the CLS is related to the presence of multiple vibrational transitions associated with different configurations of the CIP, which are likely to exchange with much slower time constants than the investigated time window, as previously seen for other CIPs. , Moreover, the residual of the CLS fitting of the 2230 cm −1 band shows the presence of an oscillation with a period of 9−30 cm −1 , which could be assigned to the CLS signature of coherent vibrational energy transfer. , In this particular case, the oscillation frequency agrees well with the two CN mixed modes of TDI − being involved in the coherent vibrational energy transfer, since they are separated by ∼20 cm −1 in DMC. Interestingly, the CLS of the high-frequency peak, located at 2247 cm –1 and assigned to one transition of the CIP, exhibits a nearly constant behavior in both DMC and BC samples.…”

Ultrafast Structure and Vibrational Dynamics of a Cyano-Containing Non-Fullerene Acceptor for Organic Solar Cells Revealed by Two-Dimensional Infrared Spectroscopy

“…Figure a reveals the diagonal anharmonicity of mainly the ss–as mode (Δ ss–as ≈ 8.7 cm –1 ). It also shows the as–as mode (Δ ss–as ≈ 8.6 cm –1 ), which appears due to ultrafast energy transfer (i.e., intramolecular vibrational energy redistribution, IVR) from the ss–as mode to the as–as mode in the form of off-diagonal signal, actually has the diagonal anharmonicity of the energy-accepting mode (as–as). Figure a also reveals another type of off-diagonal anharmonicity that is due to AVC , between the ss–as mode and as–as mode (Δ ss–as,as–as ≈ 22.8 cm –1 ), which differs from and is significantly larger than both of the diagonal anharmonicities.…”