Two-photon circular dichroism

Typically, two-photon absorption (TPA) takes place at twice the wavelength as one-photon absorption (OPA). This feature allows for the TPCD based study of chiral systems in the far to near ultraviolet (UV) region. ECD cannot be employed in this region due to interferences from strong linear absorption of typical buffers and solvents and also because of the scattering exhibited by inhomogeneous samples in this region. Several other advantages are associated with the use of non-linear absorption, i.e. high spatial resolution, enhanced penetration depth, improved background discrimination and reduced photodamage to living specimens.[2] In addition, the fact that TPA transitions obey different selection rules than OPA (even-parity vs. odd-parity) leads to think that in chiral molecules ECD and TPCD should present different spectral features, thus making the two methods complementary. TPCD is very sensitive to small structural and conformational distortions of chiral molecules, and therefore, is potentially useful for the fundamental study of optically active molecules. Finally, TPCD has the potential to penetrate into the far-UV region, where important structural/conformational information is typically obscure to ECD. This would enable the discovery of new information about molecular systems of interest such as, peptides, biological macromolecules (allowing for a deeper understanding of diseases like Alzheimer's and Parkinson's) and potential candidates for negative refractive index (for the developing of cloaking devices).

TPCD has been applied in experiments using pump-probe,[3] intensity dependent multiphoton optical rotation,[4] resonance-enhanced multiphoton ionization,[5][6] and polarization modulation single beam Z-scan.[7] The first experimental measurement of TPCD was performed in 1995 using a fluorescence based technique (FD-TPCD),[8] but it was not until the introduction of the double L-scan technique in 2008 by Hernández and co-workers,[9] that a more reliable and versatile technique to perform TPCD measurements became available. Since the introduction of the double L-scan several theoretical-experimental studies based on TPCD have been published, i.e. TPCD of asymmetric catalysts,[10][11][12] effect of the curvature of the π-electron delocalization on the TPCD signal,[13] fragmentation-recombination approach (FRA) for the study of TPCD of large molecules[14][15] and the development of an FD-TPCD based microscopy technique.[16] Additionally, Rizzo and co-workers have reported purely theoretical works on TPCD.[17][18][19][20][21][22][23]

TPCD was theoretically predicted by Tinoco[24] and Power[25] in 1975, and computationally implemented three decades later by Rizzo and co-workers,[26] using DALTON[27] and later[28] at the CC2 level in the TURBOMOLE package. The expression for TPCD, defined as, ${\displaystyle \Delta \delta (\lambda )=\delta _{L}^{TPA}(\lambda )-\delta _{R}^{TPA}(\lambda )}$ , was obtained by Tinoco in his 1975 paper as a semiclassical extension of the TPA formulae.[24] Quantum electrodynamical equivalent expressions were obtained by Power,[25] by Andrews[29] and, in a series of papers, by Meath and Power[30][31][32][33] who were able to generalize the approach to the case of n photons,[32] and considered also the modifications occurring in the formulae when elliptical polarization is assumed.[33]

TPCD can be obtained theoretically using Tinoco’s equation[24]

${\displaystyle \Delta \delta ^{TPCD}(\omega )={\frac {4}{15}}{\frac {(2\pi )^{3}}{c_{0}^{3}(4\pi \epsilon _{0})^{2}}}\times \omega ^{2}\sum _{f}g(2\omega ,\omega _{0f},\Gamma )\cdot R_{0f}^{TPCD}(\omega _{0f})}$

where ${\displaystyle \omega }$ is the circular frequency of the incident radiation, ${\displaystyle \omega _{0f}}$ is the circular frequency for a given 0→f transition, ${\displaystyle R_{0f}^{TPCD}(\omega _{0f})}$ is the TPCD rotatory strength, ${\displaystyle g(2\omega ,2\omega _{0f},\Gamma )}$ is a normalized lineshape, ${\displaystyle \epsilon _{0}}$ is the electric constant and ${\displaystyle c_{0}}$ is the speed of light in vacuum.

${\displaystyle R_{0f}^{TPCD}(\omega _{0f})}$, is obtained from

${\displaystyle R_{0f}^{TPCD}(\omega _{0f})=-b_{1}B_{1}^{TI}(\omega _{0f})-b_{2}B_{2}^{TI}(\omega _{0f})-b_{3}B_{3}^{TI}(\omega _{0f})}$

where the ${\displaystyle b_{n}}$ terms refer to the experimental relative orientation of the two incident photons. For the typical double-L scan setup, ${\displaystyle b_{1}=6}$, and ${\displaystyle b_{2}=-b_{3}=2}$, which corresponds to two left or right circularly polarized photons propagating parallel to each other and in the same direction. The molecular parameters are obtained from the following equations,

${\displaystyle B_{1}^{TI}(\omega _{0f})={\frac {1}{\omega ^{3}}}\sum _{\rho \sigma }M_{\rho \sigma }^{p,0f}(\omega _{0f})P_{\rho \sigma }^{p^{*},0f}(\omega _{0f})}$

${\displaystyle B_{2}^{TI}(\omega _{0f})={\frac {1}{2\omega ^{3}}}\sum _{\rho \sigma }T_{\rho \sigma }^{+,0f}(\omega _{0f})P_{\rho \sigma }^{p^{*},0f}(\omega _{0f})}$

${\displaystyle B_{3}^{TI}(\omega _{0f})={\frac {1}{\omega ^{3}}}\sum _{\rho \sigma }M_{\rho \sigma }^{p,0f}(\omega _{0f})P_{\sigma \sigma }^{p^{*},0f}(\omega _{0f})}$

where the molecular parameters are defined in function of the two-photon generalized tensors, ${\displaystyle M_{\rho \sigma }^{p,0f}(\omega _{0f})}$ (involving magnetic transition dipole matrix elements), ${\displaystyle P_{\rho \sigma }^{p^{*},0f}(\omega _{0f})}$ (involving electric transition dipole matrix elements in the form of the velocity operator) and ${\displaystyle T_{\rho \sigma }^{+,0f}(\omega _{0f})}$ (including electric quadrupole transition matrix elements, in the velocity formulation).