1. Detection of Intramolecular Charge Transfer and Dynamic Solvation in Eosin B By Femtosecond Two Dimensional Electronic Spectroscopy
Soumen Ghosh, Jerome D. Roscioli, Warren F. Beck
Michigan State University
Support: DOE/BES Photosynthetic Systems Program, DE-SC

2. Motivation Carotenoids (B)Chls S2 S1 S0 11Bu+ 21Ag− 11Ag− A F T1 Qx Qy
Understanding the charge and energy transfer dynamics using Two dimensional (2D) electronic spectroscopy.
Carotenoids
(B)Chls S2 S1 S0
11Bu+
21Ag−
11Ag− A F T1 Qx Qy
The motivation behind this work actually stems from our interest to understand how the photosynthetic systems work, particularly the Peridinin-Chlorophyll Protein Complex. Peridinin is actually a long-chain polyene molecule, with a carbonyl and allene group in it. It absorbs in the visible part of the solar spectrum and transfer energy very quickly and efficiently to the nearby Chlorophyll. As peridinin absorbs light, because of conjugated allene and carbonyl group, the S1 state develops a strong CT character. We want to employ to 2D electronic spectroscopy to study the charge and energy transfer dynamics in such systems. In order to learn about how 2D will help us in this, we have used eosin Y and eosin B as model systems. Eosin B shows CT character, whereas eosin Y does not.
Peridinin
Birge, R. R. Acc. Chem. Res. 1986, 19,

3. The Bulk Behaviors of Eosin B and Eosin Y
Strongly fluorescent.
Long excited state lifetime.
Both Eosin Y and Eosin B are from xanthene family. The only difference between the two is the presence of nitro group. The two Br atoms in eosin Y is replaced by two –No2 groups in eosin B. And that makes a big difference in their properties. Eosin Y is fluorescent, has long lifetime ( few nanoseconds) and small Stokes shift ( ~ 900 cm-1). Eosin B is non-fluorescent in protic solvents, has short lifetimes ( 90 ps in methanol). Has large Stokes shift (2200 cm-1).
Non-fluorescent.
Short lifetime in protic solvents.

4. Steady State Spectra of Eosin Y and Eosin B
The fluorescence spectrum of Eosin Y
is mirror symmetric. Stokes shift is small (~ 900 cm-1).
The fluorescence spectrum of Eosin B
is not mirror symmetric and has large Stokes shift ( 2200 cm-1).
The emitting state of Eosin B is not the same state that was initially excited.
Fluorescence
Absorbance
Fita et al, J. Phys. Chem. A, 2011, 115, 2465–2470
This plot shows the absorbnce and fluorescence spectra of eosin B and eosin Y in methanol. The dotted one is for the eosin Y. we can see that fluorescence spectra of eosin Y looks same as the abosrbance spectrum. For eosin B, they are different. That tells you that the excited state is more displaced in eosin B and the emission does not occur from the same state which we excited. This is supported by the greater Stokes shift in eosin B.

5. Twisted Intramolecular Charge Transfer (TICT) in Eosin B
H-Bond assisted nonradiative deactivation.
The –NO2 group imparts charge transfer character in the molecule.
In a recent paper, Vauthey group showed that nitro group of eosin B is involved in H-bonding with the solvents and helps in nonradiative deactivation back to the ground state. You can see that the LUMO of the molecule has substantial electron density around the –NO2 grp indicating CT character in the molecule. What we think is that the charge transfer in eosin B is controlled by the torsional motion of the nitro group. One way to test the hypothesis would be t
Hypothesis: The torsional motion of –NO2 group controls the formation of charge transfer character in eosin B.
The charge transfer will be diminished in more viscous solvents.
Fita, P.; Fedoseeva, M.; Vauthey, E., J. Phys. Chem. A, 2011, 115, 2465–2470

6. Two Dimensional Photon Echo SpectroscopyDO
Sample ND
WP (τ)
1 and 2
3 and LO
signal
In 2D spectroscopy, we focus four laser beam on the sample and the emitted signal is detected in a spectrograph. Two pump pulses are used to excite the sample. The time delay between these pulses (τ) is scanned to obtain information about the dynamics of the system. At some time T after the pump pulses, a third pulse probes the sample, and the emitted field provides information about how the system has evolved. We detect the emitted field by heterodyning with a local oscillator to amplify the signal and obtain information about its phase.
Miller and coworkers, Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 6110.
Fleming and coworkers, J. Chem. Phys. 2004, 121, 4221.
Scherer and coworkers, J. Chem. Phys. 2006, 124,

7. Obtaining 2D Spectrum FT Emission frequency Emission frequency
Eosin B FT
Emission frequency
The frequency of the echo signal can be determined in the time domain by scanning the time delay of the local oscillator and Fourier transforming the thus obtained signal wrt this time variable or spectrally dispersing both the beams in spectrograph. a spectral interferogram is obtained when the signal plus local oscillator fields are dispersed in a monochromator, a procedure that amounts to a Fourier transform of the signal along the t axis
Emission frequency
Delay (fs)
Excitation frequency

8. 2D Relaxation Spectra of Eosin B in Methanol
T=0 fs
T=100 fs
T=50 fs
Loss of Correlation
For waiting times shorter than the frequency correlation time (τ2⪡τc), frequency memory is largely preserved and the rephasing signal is emitted as an echo peaked at τ3 equal to τ1, as evident in the 2D time domain surface shown in Fig. 6a. The nonrephasing signal, plotted for negative τ1 delays, is symmetric in τ1 and τ3. Before the correlation function has decayed significantly, the amplitude of the rephasing signal outweighs the nonrephasing contribution. This imbalance of intensity occurs because inhomogeneity acts to damp the signal field during τ3 in the nonrephasing response, while its presence is necessary for the rephasing process. Double Fourier transformation of this time domain surface leads to a diagonally elongated absorptive line shape, indicating that the spectral holes burned during τ1 are largely unchanged when they are probed in τ3. For τ2⪢τc, frequency memory is destroyed, the signal maximum occurs near τ3 = 0 regardless of the value of τ1, and the amplitude and shape of the rephasing and nonrephasing signals are nearly equal [see Fig. 6b]. In the frequency domain the 2D line shape is no longer elongated. Instead its symmetric shape indicates that the spectral holes burned in τ1 have had enough time to sample the whole frequency distribution.
Inhomogeneous
Spectral Diffusion
Homogeneous
√2 τ
T2 increases

9. The Time-Correlation Function
Transition frequency
Time
The fluctuating energy gap:
Time Correlation function: a b
a= diagonal line width
B=anti-diagonal line width
M. Khalil et al, Phys. Rev. Lett.,2003, 90,

10. Correlation plot of Eosin Y in Methanol
Ultrafast Gaussian decay followed by slow diffusive dynamics.
Ellipticity

11. Correlation plot of Eosin B in Methanol
The 106 fs component indicates the formation of charge transfer state.
Ellipticity

12. Correlation in Eosin B decays faster than in Eosin Y
The initial correlation is lost more quickly in eosin B due to formation of charge transfer state from the initially excited state. CT
is the distribution of

13. Correlation Plots at different Glycerol Concentrations
Ellipticity
With increasing viscosity, charge transfer is diminished due to hindrance of
-NO2 group rotation.

14. Excited State Dynamics of Eosin B
Torsional motion of -NO2 group controls the excited state dynamics. Conical intersection is possibly involved.
Loss of correlation arises from adiabatic curve crossing.

15. Summary
Two dimensional electronic spectroscopy has been employed to detect the charge transfer in eosin B.
Charge transfer state is detected from the additional decay component of the correlation function.
The greater loss of frequency-frequency correlation in eosin B is due to propagation of the molecule from the local excited (LE) state to the charge transfer state (CT).

16. Acknowledgment Michael Bishop (NHMFL-FSU)
Prof. Andrew Moran (UNC Chapel Hill)
NSF MCB
DOE BES DE-SC

18. Solvation Parameters for Eosin B in different solvents