5 research outputs found
Solution Processed, Versatile Multilayered Structures for the Generation of Metal-Enhanced Fluorescence
We present an all-solution processed
multilayered structure completely
obtained via spin-coating, which can be used to study and optimize
the phenomenon of metal-enhanced fluorescence. Indeed, the electromagnetic
interactions occurring between fluorescent probes and localized surface
plasmons typical of metal nanoparticles (NPs), which influence the
fluorescence quantum yield, are strongly dependent on the nanoparticle/molecule
distance. The platform proposed here offers unique advantages in terms
of processability, allowing a fine-tuning of such a distance in a
single deposition step. Fluorescence versus fluorophore/AuNP spacing
curves are shown for two organic systems, namely, a perylene-based
dye dispersed in a polymer matrix and a polyconjugated polymer (polyÂ(3-hexylÂthioÂphene)),
interacting with a nanostructured gold thin film. In both cases, optimal
distances and enhancement factors have been measured
π‑Conjugation and End Group Effects in Long Cumulenes: Raman Spectroscopy and DFT Calculations
We
have investigated the structure and spectroscopic properties of cumulenic
carbon chains, focusing on the peculiar π-conjugation properties
and end-group effects that influence their behavior. With support
from Density Functional Theory (DFT) calculations, we have analyzed
the IR and Raman spectra of cumulenes characterized by different end-capping
groups and we have related them to the bond length alternation (BLA)
pattern and local spectroscopic parameters associated with the CC
bonds along the sp-carbon chain. For cumulenes we observe a breakdown
of the correlation existing in polyynes among frequencies, Raman intensities
of the R line (longitudinal
CC stretching modes), and BLA. While the low R line
frequency and equalized CC bonds would indicate the “metallic”
character of cumulenic species, we obtain an unusually strong Raman
intensity, which is typical of bond-alternated (semiconductive) structures.
DFT calculations reveal that this is a consequence of π-electron
conjugation, which markedly extends from the sp-carbon chain to the
aryl rings belonging to the end groups. These findings suggest the
existence of a strong electronic, vibrational and structural coupling
between sp-carbon chains and sp<sup>2</sup>-carbon species, which
could play a key role in nanostructured sp/sp<sup>2</sup>-hybrid carbon
materials (e.g., linear carbon chains coupled to graphene domains).
Within this context, Raman spectroscopy is a valuable tool for the
detailed characterization of the molecular properties of this kind
of materials
Chiral Peropyrene: Synthesis, Structure, and Properties
Herein we describe the synthesis,
structure, and properties of
chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne
derivatives as precursors, chiral peropyrenes were formed after a
4-fold alkyne cyclization reaction promoted by triflic acid. Due to
the repulsion of the two aryl substituents within the same bay region,
the chiral peropyrene adopts a twisted backbone with an end-to-end
twist angle of 28° that was unambiguously confirmed by X-ray
crystallographic analysis. The chiral peropyrene products absorb and
emit in the green region of the UV–visible spectrum. Circular
dichroism spectroscopy shows strong Cotton effects (Δε
= ±100 M<sup>–1</sup> cm<sup>–1</sup> at 300 nm).
The Raman data shows the expected D-band along with a split G-band
that is due to longitudinal and transversal G modes. This data corresponds
well with the simulated Raman spectra of chiral peropyrenes. The chiral
peropyrene products also display circularly polarized luminescence.
The cyclization reaction mechanism and the enantiomeric composition
of the peropyrene products are explained using DFT calculations. The
inversion barrier for racemization was determined experimentally to
be 29 kcal/mol and is supported by quantum mechanical calculations
Chiral Peropyrene: Synthesis, Structure, and Properties
Herein we describe the synthesis,
structure, and properties of
chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne
derivatives as precursors, chiral peropyrenes were formed after a
4-fold alkyne cyclization reaction promoted by triflic acid. Due to
the repulsion of the two aryl substituents within the same bay region,
the chiral peropyrene adopts a twisted backbone with an end-to-end
twist angle of 28° that was unambiguously confirmed by X-ray
crystallographic analysis. The chiral peropyrene products absorb and
emit in the green region of the UV–visible spectrum. Circular
dichroism spectroscopy shows strong Cotton effects (Δε
= ±100 M<sup>–1</sup> cm<sup>–1</sup> at 300 nm).
The Raman data shows the expected D-band along with a split G-band
that is due to longitudinal and transversal G modes. This data corresponds
well with the simulated Raman spectra of chiral peropyrenes. The chiral
peropyrene products also display circularly polarized luminescence.
The cyclization reaction mechanism and the enantiomeric composition
of the peropyrene products are explained using DFT calculations. The
inversion barrier for racemization was determined experimentally to
be 29 kcal/mol and is supported by quantum mechanical calculations
Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism, in Circularly Polarized Luminescence, and in Raman Spectra of Some Simple Optically Active Hexahelicenes
Four
different hexahelicenes, 5-aza-hexahelicene (<b>1</b>), hexahelicene
(<b>2</b>), 2-methyl-hexahelicene (<b>3</b>), and 2-bromo-hexahelicene
(<b>4</b>), were prepared and their
enantiomers, which are stable at r.t., were separated. Vibrational
circular dichroism (VCD) spectra were measured for compound <b>1</b>; for all the compounds, electronic circular dichroism (ECD)
and circularly polarized luminescence (CPL) spectra were recorded.
Each type of experimental spectrum was compared with the corresponding
theoretical spectrum, determined via Density Functional Theory (DFT).
Following the recent papers by Nakai et al., this comparison allowed
to identify some features related to the helicity and some other features
typical of the substituent groups on the helical backbone. The Raman
spectrum of compound <b>1</b> is also examined from this point
of view