Photostability of D–π–A nonlinear optical chromophores containing a benzothiazolium acceptor

Abstract

For the practical application of second‐order NLO materials, not only a high molecular quadratic hyperpolarizability β but also good thermal, chemical, and photochemical stabilities are required. Most of the state‐of‐the‐art chromophores with high NLO response cannot be put to use because they are photochemically highly unstable. Good thermal and photochemical stabilities with preserved high hyperpolarizabilities can be achieved by replacement of an aromatic ring with easily delocalizable heteroaromatics, e.g., with benzothiazole. Furthermore, desirable modifications of the benzothiazole fragment lead to improvement in β values. Here we report results of a comprehensive investigation of the photochemical stability of seven D–π–A push–pull molecules based on a N‐methylbenzothiazolium acceptor and a N,N‐dimethylaminophenyl donor with a different length of conjugated bridge and different acceptor strength. The quantum yield (Φ) and the kinetic parameters of photoreactions were determined for existing photodegradation pathways on irradiation at 300–850 nm in MeOH. Trans–cis photoisomerization is proposed as a fast but inefficient photobleaching mechanism for these irradiation wavelengths. Self‐sensitized photooxidation by 1O2 makes very slow parallel photodegradation pathway and, albeit to small value of Φ, plays a dominant role in the photodegradation of the compounds investigated. Both structural modifications (extension of conjugated bridge and an additional acceptor group bonded to heterocycle) resulting in an increase of NLO response led to a decrease in photostability due to the self‐sensitized 1O2 photooxidative attack. Thus a compromise should be found between an increase in NLO response and a decrease in photostability to make a choice of studied compounds for practical applications. Copyright © 2010 John Wiley & Sons, Ltd.

Photobleaching of organic fluorophores: quantitative characterization, mechanisms, protection

“…Thus, the decreased photostability is commonly expected with the presence in fluorophore structure of heteroatoms with the electron accepting or donating properties. Also, the dyes that isomerize in the excited states demonstrate decreased photostability [199].…”

Application of spectroscopic and theoretical methods in the studies of photoisomerization and photophysical properties of the push—pull styryl-benzimidazole dyes

“…The long lifetime component may instead be assigned to the perpendicular quinoid (Q ⊥ ) or ICT form, respectively. 17,18,[40][41][42][43][44] However, because of the small percentage of long-lived components only the main component of the fluorescence decay curve (above 90%) was selected for further analysis.…”

“…The degree of the charge transfer can influence the properties of dyes including the bond order and thus the bond rotational barrier. 16,17,30,[34][35][36][37]40,41 Trans → cis photoisomerization Due to the presence of the carbon-carbon double bond, the hemicyanine dyes may undergo photoreactions typical of alkenes. The most characteristic reaction is the trans-cis isomerization.…”

Arylthienyl-vinyl-benzothiazoles as Efficient Second Harmonic Generators (SHG) for Nonlinear Optics

“…Organic molecules comprising strong electron donor and acceptor groups connected by a πconjugated system (often designated as donor-π-acceptor or "push-pull" chromophores) are of fundamental importance in materials chemistry due to their numerous applications in modern technology, such as nonlinear optical (NLO) devices, poled polymers, photovoltaic cells, organic light-emitting diodes (OLEDs), semiconductor materials, and optical data storage devices [1][2][3][4]. Several push-pull molecules containing benzothiazole as an (auxiliary) electron-withdrawing group have already been reported, and typically exhibit favorable fluorescence, electrochemical, solvatochromic, photochromic, and NLO properties [5][6][7]. We have reported the synthesis and characterization of the UV-vis, solvatochromic, thermal, and second-order NLO properties of benzothiazole derivatives containing bithienyl [8][9], arylthienyl [10], and thienylpyrrolyl [11][12] heterocyclic π-spacers in order to evaluate the effect of different π-excessive donor moieties/πbridges on their optical properties.…”