Ruling
over the surface chemistry of metal halide perovskite nanocrystals
(NCs) is crucial to access reliable luminophores. Here, we provide
an atomic-level description of the surface of colloidal CsPbBr3 NCs, achieving an effective passivation strategy that leads
to near-unity photoluminescence quantum yield. To this end, we used
two different types of CsPbBr3 NCs, which had been synthesized
with an outer shell of either oleylammonium bromide ion pairs or Cs-oleate
complexes. We perturbed the dynamic equilibria at the NCs’
surface with ligands from a comprehensive library, including amines
(and their conjugated acids) with different basicities, chain lengths,
and steric encumbrances. We demonstrate that control of both ligand
binding affinity and ligand-to-NC molar ratio is essential to attain
thermodynamically stable coordination of the NC surface. We thus present
a reliable protocol for managing the surface chemistry of colloidal
CsPbBr3 NCs and for selectively addressing their ligand-induced
morphological (and structural) transformations.
Functional
electrochromic materials that allow energy modulation
both in the visible and in the near-infrared (NIR) spectral ranges
are attracting increasing interest both for the fundamental scientific
aspects related to their spectroelectrochemistry and for their technological
applications. Vis-NIR dimmable windows based on these materials are
very promising for tunable shading, thus allowing lighting and heat
energy use saving. Organic mixed valence compounds (MVs) are an interesting
class of small molecules with NIR electrochromism arising from optically
induced intervalence charge transfer transitions (IVCT). Here, we
report the synthesis and vis-NIR electrochromic properties of new
organic mixed valence systems, with two and three amino redox centers
bridged by a dibenzofulvene (DBF) unit. We studied the neutral and
charged state characteristics of these MVs in solution by spectroelectrochemical
experiments, theoretical TD-DFT investigations, and, in the solid
state, through electrochromic devices (ECDs). We show that a fine-tuning
of the electro-optical properties of these MVs can be obtained by
different functionalization on the exocyclic fulvene bond of the DBF
moiety, including the introduction of a third redox center, leading
to compounds where all three redox centers participate in the electron
transfer processes as a function of the applied voltage. As a proof-of-concept,
the above MVs were used to form solid thermoplastic laminable films
in order to fabricate transmissive-to-black switching electrochromic
devices, with intermediate color switching characteristics, enabling
us to cover all the color palette. Beyond this important exploitability
in the vis region, useful in many applications, the most important
characteristic of these devices is their absorption in almost the
whole NIR range (800–2200 nm) through the excitation of highly
charged radical species, which show intense IVCTs. Importantly, all
the devices show high optical contrast, response times of a few seconds,
and excellent switching stability over 10 000 cycles.
We show here that the recently reported surprisingly large association constant (K = 7.6 x 10(4) M(-1)) between azulene and [60]fullerene is due to experimental artifacts, pointing out potential errors in the characterization of association equilibria by fluorescence spectroscopy, and suggesting the best experimental practices.
Herein we present organic mixed-valence compounds with an innovative H-shape design, where four redox centres are bridged "vertically" via a dibenzofulvene backbone and "horizontally" via a bis-(dibenzofulvene)-thiophene bridge. These compounds are easily oxidized to stable highly charged radical species which show intense intervalence charge transfer transitions in the near infrared region. Interestingly, depending on the position of the arylamine substituents on the bridge, both vertical and horizontal electron transfer pathways can be optically induced.
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