Excited State Torsional Processes in Chalcogenopyrylium Monomethine Dyes

Abstract

Chalcogenopyrylium monomethine (CGPM) dyes represent a class of environmentally activated singlet oxygen generators with applications in photodynamic therapy (PDT) and photoassisted chemotherapy (PACT). Upon binding to genomic material, the dyes are presumed to rigidify, allowing for intersystem crossing to outcompete excited state deactivation by internal conversion. This results in large triplet yields and hence large singlet oxygen yields. To understand the nature of the internal conversion process that controls the activity of the dyes, femtosecond transient absorption experiments were performed on a series of S-, Se-, and Te-substituted CGPM dyes. For S- and Se-substituted species in methanol, rapid internal conversion from the singlet excited state, S1, occurs in ∼5 ps, deactivating the optically active excited state. The internal conversion produces a distorted ground-state species that returns to its equilibrium structure in ∼20 ps. For Te-substituted species, the internal conversion competes with rapid intersystem crossing to the lowest triplet state, T1, which occurs with a ∼ 100 ps time constant in methanol. In more viscous methanol/glycerol mixtures, the internal conversion to the ground state slows by 2 orders of magnitude, occurring in 500–600 ps. For Se- and Te-substituted species in viscous environments, the slower internal conversion rate allows a larger triplet yield. Using femtosecond stimulated Raman spectroscopy (FSRS) and time-dependent density functional theory (TD-DFT), the internal conversion is determined to occur by twisting of the pyrylium rings about the monomethine bridge. Evolution from the distorted ground state occurs by twisting back to the S0 equilibrium structure. The environmentally dependent photoactivity of CGPM dyes is discussed in the context of PDT and PACT applications.

Mapping electronic decoherence pathways in molecules

“…We first note that can be reconstructed from resonance Raman (RR) spectroscopy as this technique provides detailed quantitative structural information about molecules in solution. While RR experiments are routinely used to investigate the vibrational structure and photodynamics of molecules ( 69 – 71 ), their utility for decoherence studies had not been appreciated before. A leading technique used to reconstruct for molecules is difference fluorescence line narrowing spectra ( FLN) ( 56 – 60 , 72 ).…”

Shining light on the photophysics of polymethine dyes with chalcogenopyrylium termini

“…The effect of molecular motion on these pathways has described previously. [19][20][21] These dyes have been described as singlet oxygen sensitizers in air-saturated solutions, [7,22,23] but these prior studies do not rule out the generation of other reactive oxygen species. Herein and elsewhere, [24] we describe the electronic structure and excited-state dynamics of fourteen dyes, including six novel dyes.…”

Photonics of Some Monomethine Cyanine Dyes in Solutions and in Complexes with Biomolecules

“…There are a large number of works by different research groups devoted to the synthesis and spectral fluorescent study of various MCDs (symmetrical and unsymmetrical) in order to develop probes for nucleic acids [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. There are also some works directed to the study of primary photophysical and photochemical processes in MCD molecules, both free and noncovalently bound to nucleic acids [ 35 , 36 , 37 , 38 , 39 , 40 ] (see also the review [ 41 ]). Such studies can give a new impetus to the application of MCD in biological objects.…”