1 3
Top Curr Chem (Z) (2018) 376:24
hitherto unexplored. The complexity of the information contained in 2DES maps and
the involvement of high-energy electronic states calls for the implementation of computational tools based on a combination of ab initio electronic structure and nonlinear response formalism. Accurate simulation of 2DUV electronic spectroscopy may,
in fact, lead to both interpretation and prediction of nonlinear spectra, enhancing the
potential application of this technique in various fields. Here, we outline a route towards
accurate simulation of 2DUV spectra of fundamental biological systems. After describing the general 2DES technique and the target UV-active bio-chromophores in this
section, we present a basic background in Sect. 2, aimed at providing the theoretical
foundations of our simulation protocols and showing the approximations adopted. In
Sect. 3, the main results of our developments and applications are illustrated for both
nucleic acid and protein model systems, showing the potential for 2DUV in characterizing the ground-state (GS) conformational space and excited-state dynamics in these
target systems. Finally, an outlook summary and perspective are provided in Sect. 4.
2 The 2DES Technique
Two-dimensional electronic spectroscopy [29] is based on a sequence of three
ultrashort laser pulses interacting with the sample, generating a third-order nonlinear polarization and emitting signal fields in phase-matched directions (Fig. 1).
The major advantage of 2DES is the higher spectral resolution relative to 1D-PP
time-resolved techniques, with the nonlinear signal containing information on system dynamics and electronic couplings spread over two frequency axes (pump and
probe, Ω 1 and Ω 3 , respectively; see Fig. 1b). The spectral resolution in two dimensions enables accurate characterization of inhomogeneous and homogeneous broadening processes, with the ability to separate these two contributions (not possible
with 1D techniques), providing information about solvent reorganization timescales
as well as enabling the detection of signals associated with coupling between electronic excitations and charge/energy transfer processes.
The heterodyne-detected three-pulse photon echo (3PPE) non-collinear scheme [5,
30] is the experimental setup that can fully resolve (in amplitude and phase) the thirdorder nonlinear response, by collecting a four-wave mixing signal in a backgroundfree direction, which is heterodyned by a local oscillator (LO); see Fig. 1a. The signal
field emitted can be detected in the so-called rephasing K I (k LO = −k 1 + k 2 + k 3 ) and
non-rephasing K II (k LO = +k 1 − k 2 + k 3 ) phase-matching directions, as a function of
three controlled excitation-pulse time delays (t 1 , t 2 and t 3 ). Instead of 3PPE, the partially collinear pump–probe (PCPP) geometry [21, 31, 32] can be adopted by using
a pair of collinear pump pulses (k 1 , k 2 ) non-collinearly combined with a probe pulse
(k 3 ), providing a nonlinear response signal which is heterodyned by the probe pulse
itself (self-heterodyning). The PCPP experiment is disadvantaged by the strong background signal, and it intrinsically provides the combined K I + K II “quasi-absorptive”
response, with loss of information on specific rephasing and non-rephasing signals,
although removal of slowly decaying dispersive contributions due to phase cancellation of the concurring K I and K II signals might help in resolving weak signals.
65
Reprinted from the journal
Top Curr Chem (Z) (2018) 376:24
hitherto unexplored. The complexity of the information contained in 2DES maps and
the involvement of high-energy electronic states calls for the implementation of computational tools based on a combination of ab initio electronic structure and nonlinear response formalism. Accurate simulation of 2DUV electronic spectroscopy may,
in fact, lead to both interpretation and prediction of nonlinear spectra, enhancing the
potential application of this technique in various fields. Here, we outline a route towards
accurate simulation of 2DUV spectra of fundamental biological systems. After describing the general 2DES technique and the target UV-active bio-chromophores in this
section, we present a basic background in Sect. 2, aimed at providing the theoretical
foundations of our simulation protocols and showing the approximations adopted. In
Sect. 3, the main results of our developments and applications are illustrated for both
nucleic acid and protein model systems, showing the potential for 2DUV in characterizing the ground-state (GS) conformational space and excited-state dynamics in these
target systems. Finally, an outlook summary and perspective are provided in Sect. 4.
2 The 2DES Technique
Two-dimensional electronic spectroscopy [29] is based on a sequence of three
ultrashort laser pulses interacting with the sample, generating a third-order nonlinear polarization and emitting signal fields in phase-matched directions (Fig. 1).
The major advantage of 2DES is the higher spectral resolution relative to 1D-PP
time-resolved techniques, with the nonlinear signal containing information on system dynamics and electronic couplings spread over two frequency axes (pump and
probe, Ω 1 and Ω 3 , respectively; see Fig. 1b). The spectral resolution in two dimensions enables accurate characterization of inhomogeneous and homogeneous broadening processes, with the ability to separate these two contributions (not possible
with 1D techniques), providing information about solvent reorganization timescales
as well as enabling the detection of signals associated with coupling between electronic excitations and charge/energy transfer processes.
The heterodyne-detected three-pulse photon echo (3PPE) non-collinear scheme [5,
30] is the experimental setup that can fully resolve (in amplitude and phase) the thirdorder nonlinear response, by collecting a four-wave mixing signal in a backgroundfree direction, which is heterodyned by a local oscillator (LO); see Fig. 1a. The signal
field emitted can be detected in the so-called rephasing K I (k LO = −k 1 + k 2 + k 3 ) and
non-rephasing K II (k LO = +k 1 − k 2 + k 3 ) phase-matching directions, as a function of
three controlled excitation-pulse time delays (t 1 , t 2 and t 3 ). Instead of 3PPE, the partially collinear pump–probe (PCPP) geometry [21, 31, 32] can be adopted by using
a pair of collinear pump pulses (k 1 , k 2 ) non-collinearly combined with a probe pulse
(k 3 ), providing a nonlinear response signal which is heterodyned by the probe pulse
itself (self-heterodyning). The PCPP experiment is disadvantaged by the strong background signal, and it intrinsically provides the combined K I + K II “quasi-absorptive”
response, with loss of information on specific rephasing and non-rephasing signals,
although removal of slowly decaying dispersive contributions due to phase cancellation of the concurring K I and K II signals might help in resolving weak signals.
65
Reprinted from the journal