The ability to control the light–matter interaction in nanosystems is a major challenge in the field of innovative photonics applications. In this framework, plexcitons are promising hybrid light–matter states arising from the strong coupling between plasmonic and excitonic materials. However, strategies to precisely control the formation of plexcitons and to modulate the coupling between the plasmonic and molecular moieties are still poorly explored. In this work, the attention is focused on suspensions of hybrid nanosystems prepared by coupling cationic gold nanoparticles to tetraphenyl porphyrins in different aggregation states. The role of crucial parameters such as the dimension of nanoparticles, the pH of the solution, and the ratio between the nanoparticles and dye concentration was systematically investigated. A variety of structures and coupling regimes were obtained. The rationalization of the results allowed for the suggestion of important guidelines towards the control of plexcitonic systems.
Plexcitonic nanohybrids
are plasmonic–excitonic novel materials
whose peculiar properties are attracting considerable attention in
photonics, solar cells, and sensing. These materials can be synthesized
and characterized easily by assembling organic or inorganic dyes on
plasmonic nanoparticles as support. However, the main factors controlling
the assembly process and the occurrence of the plexcitonic coupling
are still unclear. To fill this gap, in this work, we studied the
plexciton coupling of 12 different dyes with a series of gold nanourchins
with various coatings and sizes. Among 24 combinations tested, we
observed the formation of plexcitonic hybrids only in five cases.
Most of them had cyanine J-aggregates as excitonic counterparts. Stronger
plexcitonic couplings were obtained when nanourchins were coated with
an exchangeable citrate capping layer rather than a strong ionic thiols
capping layer. We propose that the presence of a strongly bound capping
layer, as in this latter case, reduces the effective volume available
to the dyes.
Plexcitons, that
is, mixed plasmon-exciton states, are currently
gaining broad interest to control the flux of energy at the nanoscale.
Several promising properties of plexcitonic materials have already
been revealed, but the debate about their ultrafast dynamic properties
is still vibrant. Here, pump–probe spectroscopy is used to
characterize the ultrafast dynamics of colloidal nanohybrids prepared
by coupling gold nanoparticles and porphyrin dyes, where one or two
sets of plexcitonic resonances can be selectively activated. We found
that these dynamics are strongly affected by the presence of a reservoir
of states including plasmon resonances and dark states. The time constants
regulating the plexciton relaxations are significantly longer than
the typical values found in the literature and can be modulated over
more than 1 order of magnitude, opening possible interesting perspectives
to modify rates of chemically relevant molecular processes.
The influence of hydrogen bonds (H-bonds) in the structure, dynamics, and functionality of biological and artificial complex systems is the subject of intense investigation. In this broad context, particular attention has recently been focused on the ultrafast H-bond dependent dynamical properties in the electronic excited state because of their potentially dramatic consequences on the mechanism, dynamics, and efficiency of photochemical reactions and photophysical processes of crucial importance for life and technology. Excited-state H-bond dynamics generally occur on ultrafast time scales of hundreds of femtoseconds or less, making the characterization of associated mechanisms particularly challenging with conventional time-resolved techniques. Here, 2D electronic spectroscopy is exploited to shed light on this still largely unexplored dynamic mechanism. An H-bonded molecular dimer prepared by self-assembly of two boron-dipyrromethene dyes has been specifically designed and synthesized for this aim. The obtained results confirm that upon formation of H-bonds and the dimer, a new ultrafast relaxation channel is activated in the ultrafast dynamics, mediated by the vibrational motions of the hydrogen donor and acceptor groups. This relaxation channel also involves, beyond intra-molecular relaxations, an inter-molecular transfer process. This is particularly significant considering the long distance between the centers of mass of the two molecules. These findings suggest that the design of H-bonded structures is a particularly powerful tool to drive the ultrafast dynamics in complex materials.
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