Spectral and Photophysical Behavior of Cytisine in n-Hexane. Experimental Evidence for the S(n,π*) → S Fluorescence.

Spectral and photophysical properties of (-)-cytisine (the compound used as a smoking cessation aid and a potential drug in Alzheimer's and Parkinson's diseases) were investigated. The two conformers of cytisine, whose presence in the S state has been earlier proved by the NMR and IR methods as well as in theoretical calculation, in nonpolar n-hexane show a rarely observed prompt fluorescence from the S(n,π*) excited state. This observation is unambiguously evidenced by very small radiative rate constants of these two emitting conformers, k = 7.

Electron transfer in silicon-bridged adjacent chromophores: the source for blue-green emission.

Si-Bridged chromophores have been proposed as sources for blue-green emission in several technological applications. The origin of this dual emission is to be found in an internal charge transfer reaction. The current work is an attempt to describe the details of these processes in these kinds of substances, and to design a molecular architecture to improve their performance.

Lumisterol to Tachysterol Photoisomerization in EPA Glass at 77 K. A Comparative Study.

We present a comparative study of the photoisomerizations of lumisterol (Lumi), previtamin (Pre), and provitamin D (Pro) to tachysterol (Tachy) at 77 K in EPA (5:5:2 ether, isopentane, and ethanol by volume) glass. Fluorescence, fluorescence excitation, and UV spectra, measured in the course of these reactions, were analyzed using singular value decomposition with self-modeling (SVD-SM). This represents an extension of previous work that led to the conclusion that in the EPA glass Pre exists as an s-cis,s-cis-conformer (cZc-Pre) which gives, exclusively, an unstable s-cis,s-cis-conformer of Tachy (cEc-Tachy) and Pro gives mainly the tEc-Tachy, that corresponds to a stable s-trans,s-cis-conformer.

Fluorescent H-aggregates of an asymmetrically substituted mono-amino Zn(ii) phthalocyanine.

The photophysical properties of a newly synthesized unsymmetrically substituted zinc phthalocyanine derivative (1) bearing in its peripheral positions six n-hexylsulfanyl substituents and one amino-terminated n-hexylsulfanyl substituent were investigated. This mono-amino phthalocyanine exhibited a high tendency to form H-type aggregates in all of the investigated solvents: dichloromethane (DCM), tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO). Several species of H-aggregates were present together in relatively broad concentration ranges in THF and DCM, whereas in DMSO they were observed separately depending on the concentration used.

Intramolecular charge transfer photoemission of a silicon-based copolymer containing carbazole and divinylbenzene chromophores. Electron transfer across silicon bridges.

A new copolymer consisting of N-isopropylcarbazole/dimethylsilylene bridge/divinylbenzene units was synthesized and characterized. Dual fluorescence was observed in this copolymer in polar solvents. The absence of the second band at the lower transition energy of the two emission maxima in nonpolar solvents and the quantitative correlation of the lower-energy emission band maxima with solvent polarity indicate that the lower-energy emission band arises from an intramolecular charge transfer (ICT) state.

Photoisomerization of Pre- and Provitamin D3 in EPA at 77 K: One-Bond-Twist, Not Hula-Twist.

The photoisomerizations of previtamin D3 (Pre) and provitamin D3 (Pro) in EPA at 77 K were monitored using fluorescence spectroscopy. In the glassy EPA medium equilibrated tachysterol (Tachy) exists as a mixture of three conformers, the major of which we assign to s-trans,s-cis- and s-cis,s-cis-conformers (tEc- and cEc-Tachy). By contrast, Pre exists exclusively as the s-cis,s-cis-conformer (cZc-Pre) and undergoes cis → trans photoisomerization to cEc-Tachy.

Kinetics of reversible photoisomerization: determination of the primary quantum yields for the E-Z photoisomerization of silylenephenylenevinylene derivatives.

The kinetics scheme for directly excited, photoreversible reactions is solved exactly under the assumptions of no irreversible side reactions and constant excitation intensity for the duration of the reaction. The advantages of the methodology over the extrapolation-to-zero-time and the back-reaction correction methods are (i) that the quantum yields of both the forward and reverse photoreactions can be obtained starting from either pure reactant or pure product and (ii) the conversion percentage is not limited to a narrow domain in the neighborhood of small conversions. Examples of E-Z photoisomerizations are given to illustrate the fitting procedures required.