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

Abstract

Photoisomerization of 1,1 0 -diethyl-2,2 0 -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 (s 1 , 5.2 ps in methanol) that is constant over the observed spectral region. Dependence of s 1 on solvent viscosity (h) 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 (s 2 l , from 0.2 to 0.6 ps for 720-1000 nm range in methanol). s 2 l depends on h as a fractional power function and follows BFO theory for barrierless photoisomerization. The overall photoisomerization rate is limited by the barrier-crossing rate, s 1 À1 . 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 0 -diethyl-2,2 0 -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.

Broadband visible two-dimensional spectroscopy of molecular dyes

“…76 This photoisomerization involves an overdamped wavepacket motion 77 along a combined stretching and torsion reaction coordinate of the methine C=C backbone double bond. 51 The two most prominent vibrational coherences observed in the 2D signal of pin, with 833 and 1360 cm −1 , shown in Fig. 5(b) fall within the so-called "fingerprint" region, 74,[78][79][80][81] indicating that they are highly sensitive to the molecular structure.…”

“…This is The Journal of Chemical Physics ARTICLE scitation.org/journal/jcp validated by the observation of vibrational coherences up to 1400 cm −1 , allowing us to extract beating maps for various vibrational modes of the molecules, and demonstrated for the case of pin, which serves as an interesting test case due to its light-induced photoisomerization. 51 While comparison to simple theoretical models 46 highlights the information that can be obtained from such an analysis, further work is needed to account for more complex photophysics and experimental considerations, such as non-uniform spectral intensity and finite pulse duration.…”

Identification of Nonradiative Decay Pathways in Cy3

“…After photoexcitation from the trans ground state, isomerization to the cis configuration begins through picosecond to nanosecond generation of a twisted 90° angle configuration. The twisted configuration has an optically dark state with charge-transfer character (twisted intramolecular charge-transfer state or TICT), which relaxes back to the trans ground state or to the cis ground state. − Although ultrafast spectroscopy has been used extensively to characterize the barrierless photoisomerization of cyanines such as the quinoline-based pseudoisocyanine and pinacyanol, , cyanines such as Cy3 and Cy5 must overcome an energetic activation barrier to populate the TICT state. Therefore, the barrierless dynamics previously characterized cannot be extrapolated to other cyanines.…”

One-Dimensionally Conjugated Carbocyanine Dyes Isolated under Cold Gas-Phase Conditions: Electronic Spectra and Photochemistry

“…The isomerization pathway in the S 1 state is barrierless for these dyes, in sharp contrast to the activation barriers for photoisomerization confirmed for longer-chain cyanine dyes, such as 1 (Scheme 1) and 1,1′-diethyl-2,2′-dicarbocyanine (DDI). 17 It is also significant that these dye molecules, in general, have a large cross section for multiphoton absorption, which often plays a key role in their photoisomerization, as revealed for 2 (Scheme 1) and related molecules. 12,18,19 The excited-state dynamics described beforehand should be reflected in the spectral profiles; therefore, reliable electronic spectra of these molecules are highly desired.…”