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Ultrafast photoisomerization of pinacyanol : Watching an excited state reaction transiting from barrier to barrierless forms

Ma, Fei LU and Yartsev, Arkady LU (2016) In RSC Advances 6(51). p.45210-45218
Abstract

Photoisomerization of 1,1′-diethyl-2,2′-carbocyanine iodide (pinacyanol) in alcohols was investigated by means of femtosecond time-resolved absorption spectroscopy. Only one out of two possible types of photoisomer was found. Viscosity-dependent kinetics indicate that the excited state dynamics consist of two parts, associated with a barrier-crossing observed in the blue side of stimulated emission (SE) and the barrierless isomerization observed in the red side of SE. These two processes occur sequentially. The blue barrier SE part is characterized by a decay lifetime (τ1, 5.2 ps in methanol) that is constant over the observed spectral region. Dependence of τ1 on solvent viscosity (η) is linear and follows Kramer's... (More)

Photoisomerization of 1,1′-diethyl-2,2′-carbocyanine iodide (pinacyanol) in alcohols was investigated by means of femtosecond time-resolved absorption spectroscopy. Only one out of two possible types of photoisomer was found. Viscosity-dependent kinetics indicate that the excited state dynamics consist of two parts, associated with a barrier-crossing observed in the blue side of stimulated emission (SE) and the barrierless isomerization observed in the red side of SE. These two processes occur sequentially. The blue barrier SE part is characterized by a decay lifetime (τ1, 5.2 ps in methanol) that is constant over the observed spectral region. Dependence of τ1 on solvent viscosity (η) is linear and follows Kramer's law for barrier-crossing reactions. The red barrierless part is faster and is characterized by a rise in time constants, which increases with the wavelength (τ2 λ, from 0.2 to 0.6 ps for 720-1000 nm range in methanol). τ2 λ depends on η as a fractional power function and follows BFO theory for barrierless photoisomerization. The overall photoisomerization rate is limited by the barrier-crossing rate, τ1 -1. A two-state two-mode model with a saddle-type of potential energy surface (PES) was used to explain the observed spectral dynamics. Along the stretching mode it is barrierless while along the torsion mode there is a very small barrier. As a result, the PES becomes ridge-like after displacement along the stretching coordinate. Comparing pinacyanol and 1,1′-diethyl-2,2′-cyanine iodide (1122C) with similar excited state lifetimes (5.2 vs. 4.9 ps) but quite different photoisomer quantum yields (QY, 3.6% vs. 26%, respectively), we conclude that excited state PES properties play the most important role in determining when the torsion mode couples with the stretching mode, and as a result the photoisomer QY.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
RSC Advances
volume
6
issue
51
pages
9 pages
publisher
Royal Society of Chemistry
external identifiers
  • Scopus:84971006520
ISSN
2046-2069
DOI
10.1039/c6ra03299k
language
English
LU publication?
yes
id
5bb1198c-cb1d-4ced-907c-8c12b95fbaee
date added to LUP
2016-06-23 08:37:15
date last changed
2016-06-23 08:37:15
@misc{5bb1198c-cb1d-4ced-907c-8c12b95fbaee,
  abstract     = {<p>Photoisomerization of 1,1′-diethyl-2,2′-carbocyanine iodide (pinacyanol) in alcohols was investigated by means of femtosecond time-resolved absorption spectroscopy. Only one out of two possible types of photoisomer was found. Viscosity-dependent kinetics indicate that the excited state dynamics consist of two parts, associated with a barrier-crossing observed in the blue side of stimulated emission (SE) and the barrierless isomerization observed in the red side of SE. These two processes occur sequentially. The blue barrier SE part is characterized by a decay lifetime (τ<sub>1</sub>, 5.2 ps in methanol) that is constant over the observed spectral region. Dependence of τ<sub>1</sub> on solvent viscosity (η) is linear and follows Kramer's law for barrier-crossing reactions. The red barrierless part is faster and is characterized by a rise in time constants, which increases with the wavelength (τ<sub>2</sub> <sup>λ</sup>, from 0.2 to 0.6 ps for 720-1000 nm range in methanol). τ<sub>2</sub> <sup>λ</sup> depends on η as a fractional power function and follows BFO theory for barrierless photoisomerization. The overall photoisomerization rate is limited by the barrier-crossing rate, τ<sub>1</sub> <sup>-1</sup>. A two-state two-mode model with a saddle-type of potential energy surface (PES) was used to explain the observed spectral dynamics. Along the stretching mode it is barrierless while along the torsion mode there is a very small barrier. As a result, the PES becomes ridge-like after displacement along the stretching coordinate. Comparing pinacyanol and 1,1′-diethyl-2,2′-cyanine iodide (1122C) with similar excited state lifetimes (5.2 vs. 4.9 ps) but quite different photoisomer quantum yields (QY, 3.6% vs. 26%, respectively), we conclude that excited state PES properties play the most important role in determining when the torsion mode couples with the stretching mode, and as a result the photoisomer QY.</p>},
  author       = {Ma, Fei and Yartsev, Arkady},
  issn         = {2046-2069},
  language     = {eng},
  number       = {51},
  pages        = {45210--45218},
  publisher    = {ARRAY(0x80e0b48)},
  series       = {RSC Advances},
  title        = {Ultrafast photoisomerization of pinacyanol : Watching an excited state reaction transiting from barrier to barrierless forms},
  url          = {http://dx.doi.org/10.1039/c6ra03299k},
  volume       = {6},
  year         = {2016},
}