Photophysics of Thiacarbocyanine Dyes:  Relaxation Dynamics in a Homologous Series of Thiacarbocyanines

Abstract

Fluorescence quantum yields and emission lifetimes have been measured at room temperature for a series of thiacarbocyanine dyes in various alcohol solvents. All radiationless deactivation events are accounted for by a process, e.g., torsional relaxation, whose rate is viscosity-dependent. The relaxation rate constant (k η) and the exponent of viscosity dependence (α) in the Wirtz−Gierer analysis decrease with increasing polymethine chain length (n) in the dye series. Two dyes were further examined by time-resolved picosecond laser flash spectroscopy. Recovery of ground state in both cases is biphasic. In both cases, the short recovery time corresponds to the fluorescence lifetime. We infer that torsional relaxation of the vertical S1 state is rate determining and does not cross between the S1 and So potential surfaces in the relaxed S1 conformation. The slower recovery process is identified with a pathway that leads to photoisomerization of the cyanine chromophores and implies involvement of a species intermediate between torsionally relaxed S1 and the ground-state photoisomer. This intermediate is suggested to be an isomerized triplet state.

Visualizing overdamped wavepacket motion: Excited-state isomerization of pseudocyanine in viscous solvents

“…S 1 contour remains unaltered, suggesting similar shapes of the S 1 PES of 1122C in the initially excited Franck-Condon region in the series of n-alcohols used in this study. This is in line with the established result that the molar extinction of polymethine dyes changes only slightly upon going from methanol to higher unbranched n-alcohols [41,42]. Hence, the average value of e NF = 46900 ± 960 M À1 cm À1 was adopted.…”

“…S 1 absorption spectrum of 1122C undergoes a slight ($145 cm À1 ) bathochromic shift. This 'general red shift' was already observed for 1122C [38] as well as for many other open-chain polymethine dyes, and is due to dispersive interactions [39][40][41][42]. We note that the S 0 ?…”

“…2230 cm À1 ) is found to be large compared to the absorption of related dyes with a longer, unsubstituted polymethine chain [13,32,[39][40][41][42]. In addition, the observation of an unstructured and broad emission band with a large Stokes-shift (3390 cm À1 ) hints to significant geometrical rearrangements from the absorptive to the emissive excited-state geometry.…”

Influence on fluorescence properties of hyperbranched poly(amidoamine)s by nano golds

“…The luminescence maximum moves to lower energy or disappears with increased core size, and only the smaller nanodots were observed to luminesce 33. The luminescence from GNDs is thought to arise from transitions between the filled d band and sp conduction bands 35, 36…”

Vibrationally Coherent Preparation of the Transition State for Photoisomerization of the Cyanine Dye Cy5 in Water

“…19 Near the conical intersection, the biradicaloid character of the twisted polyene 20 enables intersystem crossing to the first excited triplet state, T 1 . 21,22 The activation energy required to access twisted conformations increases with the length of the conjugated region because the π* character of the S 1 state decreases. The impact of the transition state barrier on the dynamics in cyanines can be qualitatively judged from the line shape of the fluorescence spectrum.…”