TY - JOUR
T1 - Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects
AU - Raab, Micah
AU - Becca, Jeffrey C.
AU - Heo, Jeongyun
AU - Lim, Chang Keun
AU - Baev, Alexander
AU - Jensen, Lasse
AU - Prasad, Paras N.
AU - Velarde, Luis
PY - 2019/3/21
Y1 - 2019/3/21
N2 - Doubly resonant infrared-visible sum-frequency generation (DR-IVSFG) spectroscopy, encompassing coupled vibrational and electronic transitions, provides a powerful method to gain a deep understanding of nuclear motion in photoresponsive surface adsorbates and interfaces. Here, we use DR-IVSFG to elucidate the role of vibronic coupling in a surface-confined donor-acceptor substituted azobenzene. Our study reveals some unique features of DR-IVSFG that have not been previously reported. In particular, vibronic coupling resulted in prominent SFG signal enhancement of selective stretching modes that reveal electronic properties of coexisting photochromic isomers. Our analysis explores two concepts: (1) In partially isomerized azobenzene at the surface, coupling of the fundamental vibrations to the S 0 → S 1 transition is more prominent for the cis isomer due to symmetry breaking, whereas coupling to the S 0 → S 2 transition was dominant in the trans isomer. (2) A strong coupling between the fundamental vibrations and the valence π-electron density, promoted by the initial absorption of an infrared photon, may result in suppression of the intensity of the hot band vibronic transition. This may translate into a suppressed sum-frequency generation signal at sum frequency wavelengths resonant with the S 0 → S 2 transition of the trans isomer. The weaker coupling of the fundamental vibrations to the non-bonding electron density localized on the azo group can therefore produce detectable sum-frequency generation at the resonance wavelength of the weaker S 0 → S 1 transition in the cis form. These results are explained in the framework of a linear coupling model, involving both Franck-Condon and Herzberg-Teller coupling terms. Our theoretical analysis reveals the important role played by molecular conformation, orientation, and vibronic interference in DR-SFG spectroscopy.
AB - Doubly resonant infrared-visible sum-frequency generation (DR-IVSFG) spectroscopy, encompassing coupled vibrational and electronic transitions, provides a powerful method to gain a deep understanding of nuclear motion in photoresponsive surface adsorbates and interfaces. Here, we use DR-IVSFG to elucidate the role of vibronic coupling in a surface-confined donor-acceptor substituted azobenzene. Our study reveals some unique features of DR-IVSFG that have not been previously reported. In particular, vibronic coupling resulted in prominent SFG signal enhancement of selective stretching modes that reveal electronic properties of coexisting photochromic isomers. Our analysis explores two concepts: (1) In partially isomerized azobenzene at the surface, coupling of the fundamental vibrations to the S 0 → S 1 transition is more prominent for the cis isomer due to symmetry breaking, whereas coupling to the S 0 → S 2 transition was dominant in the trans isomer. (2) A strong coupling between the fundamental vibrations and the valence π-electron density, promoted by the initial absorption of an infrared photon, may result in suppression of the intensity of the hot band vibronic transition. This may translate into a suppressed sum-frequency generation signal at sum frequency wavelengths resonant with the S 0 → S 2 transition of the trans isomer. The weaker coupling of the fundamental vibrations to the non-bonding electron density localized on the azo group can therefore produce detectable sum-frequency generation at the resonance wavelength of the weaker S 0 → S 1 transition in the cis form. These results are explained in the framework of a linear coupling model, involving both Franck-Condon and Herzberg-Teller coupling terms. Our theoretical analysis reveals the important role played by molecular conformation, orientation, and vibronic interference in DR-SFG spectroscopy.
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U2 - 10.1063/1.5081726
DO - 10.1063/1.5081726
M3 - Article
C2 - 30902002
AN - SCOPUS:85063355627
VL - 150
JO - Journal of Chemical Physics
JF - Journal of Chemical Physics
SN - 0021-9606
IS - 11
M1 - 114704
ER -