Remarkable two-photon absorption in open-shell singlet systems

Abstract

Remarkable enhancement of two-photon absorption (TPA) peak is theoretically predicted in symmetric open-shell singlet diradical systems with intermediate diradical character as compared to closed-shell and pure diradical systems. It is revealed that the largest TPA peak intensities occur for open-shell singlet diradicals having a ferromagnetically coupled ground state and strongly depend on the ratio between damping factors of the excited states. This result confirms that open-shell singlet conjugated molecules with intermediate diradical characters have precedence over conventional closed-shell conjugated systems in resonant third-order nonlinear optical properties.

Effect of the diradical character on static polarizabilities and two-photon absorption cross sections: A closer look with spin-flip equation-of-motion coupled-cluster singles and doubles method

“…We analyze the results by constructing truncated sum-over-states models for these response properties. Consistent with the previous studies, [13][14][15][17][18][19]21 we find a correlation between the nonlinear response and the diradical character. An approximate measure of the diradical character is given by the ratios of leading wave function amplitudes.…”

“…where h 1 |x| 1 i ⇡ X 1 denotes the position of nucleus 1, X 2 X 1 = R 12 denotes the distance between nuclei 1 and 2, and we assume that h 1 |x| 2 i = 0 if 1 and 2 belong to different centers. Equation (18) implies that for a fixed , the transition dipole moment between wave functions (D) and (E) increases as the distance between the radical centers (on which the s are located) increases. For a fixed distance between the radical centers, this transition dipole moment increases when decreases ( 1 p 1+ 2 is a decreasing function for 0   1).…”

“…Thus, Eqs. (18) and (20) connect the transition dipole moments between diradical states with the (i) distance between the centers hosting the unpaired electrons and (ii) diradical character.…”

“…10,11 Static polarizability can also be used as a descriptor of molecular conductance in molecular junctions. 12 While the bulk of nonlinear materials has utilized closed-shell systems, a novel design strategy based on open-shell systems such as diradicals and polycyclic aromatic hydrocarbons (PAHs) with open-shell ground states has been explored recently, [13][14][15][16][17][18][19][20][21] motivated by theoretical predictions that open-shell systems have larger nonlinear responses relative to their closed-shell counterparts. Another example of utilizing open-shell states to enhance nonlinear responses was proposed by Marks and Ratner, who have considered twisted ⇡-system chromophores featuring increased diradical character.…”

Analysis of nonlinear optical properties in donor–acceptor materials

“…[1][2][3][4] Typical are so-called donor-π -acceptor (D−π -A) molecules, where a conjugated aromatic or polyene system makes up the π -electron core, and an electron donor, e.g., an amino group, is bonded at one end, and an electron acceptor group, such as a nitro group, is attached at the opposite end. However, although interest in NLO properties for such compounds has resulted in a relatively large number of theoretical studies at various levels of approximation, 5-23 even some investigating open-shell singlet systems with diradical character, [24][25][26][27] as well as of experimental work, 5, 28-33 a consistent comparison with experiment for validation remains elusive. This is important for application to newly developed materials, such as those that could be attached to gold clusters.…”