Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects

Micah Raab, Jeffrey C. Becca, Jeongyun Heo, Chang Keun Lim, Alexander Baev, Lasse Jensen, Paras N. Prasad, Luis Velarde

Research output: Contribution to journalArticle

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Abstract

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.

Original languageEnglish (US)
Article number114704
JournalJournal of Chemical Physics
Volume150
Issue number11
DOIs
StatePublished - Mar 21 2019

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Isomers
Conformations
Natural frequencies
isomers
Spectroscopy
Infrared radiation
spectroscopy
Carrier concentration
Wavelength
Adsorbates
Crystal orientation
Electronic properties
vibration
Stretching
Photons
electronics
wavelengths
broken symmetry
retarding
valence

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

Cite this

Raab, Micah ; Becca, Jeffrey C. ; Heo, Jeongyun ; Lim, Chang Keun ; Baev, Alexander ; Jensen, Lasse ; Prasad, Paras N. ; Velarde, Luis. / Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects. In: Journal of Chemical Physics. 2019 ; Vol. 150, No. 11.
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abstract = "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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Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects. / Raab, Micah; Becca, Jeffrey C.; Heo, Jeongyun; Lim, Chang Keun; Baev, Alexander; Jensen, Lasse; Prasad, Paras N.; Velarde, Luis.

In: Journal of Chemical Physics, Vol. 150, No. 11, 114704, 21.03.2019.

Research output: Contribution to journalArticle

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AU - Becca, Jeffrey C.

AU - Heo, Jeongyun

AU - Lim, Chang Keun

AU - Baev, Alexander

AU - Jensen, Lasse

AU - Prasad, Paras N.

AU - Velarde, Luis

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