9 research outputs found
Ultrafast time-resolved photoluminescence from novel metalâdendrimer nanocomposites
We report the first results of ultra-fast enhanced light emission from goldâ and silverâdendrimer nanocomposites. There is a fast (70 fs) fluorescence decay component associated with the metal nanocomposites. Anisotropy measurements show that this fast component is depolarized. The enhanced emission is suggestively due to local field enhancement in the elongated metalâdendrimer nanoparticles. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71043/2/JCPSA6-114-5-1962-1.pd
Structure and Dynamics of the <sup>1</sup>(TT) State in a Quinoidal Bithiophene: Characterizing a Promising Intramolecular Singlet Fission Candidate
Tetracyanoquinodimethane
bithiophene (QOT2) has a long-lived (57
ÎŒs) photoinduced excited state that may correspond to triplets
resulting from intramolecular singlet fission (SF). Since SF usually
occurs through intermolecular processes, a detailed description of
the excited states involved and their evolution is needed to verify
this hypothesis. The photoresponse of QOT2 is investigated using high-level
electronic structure methods and quantum dynamics simulations, which
show ultrafast passage through a conical intersection from the bright
1<sup>1</sup>B<sub>u</sub> state to the dark 2<sup>1</sup>A<sub>g</sub> surface. Characterization of QOT2âs 2<sup>1</sup>A<sub>g</sub> wave function found it to be composed of two strongly coupled triplets,
leading to the first detailed electronic structure description of
an intramolecular <sup>1</sup>(TT) state. The population of such a
state upon excitation of QOT2 raises the possibility of SF through
conformational changes that decouple the triplets. However, reaching
an appropriate geometry for decoupled triplets appears unlikely given
the energy cost of 1.76 eV. Consequently, the hypothesis that the
long-lived excited state corresponds to 2<sup>1</sup>A<sub>g</sub>, a spin singlet, strongly interacting double triplet, was explored.
Transition moment calculations to assign excited-state absorption
signals and investigations into internal conversion and intersystem
crossing decay pathways indicate that a long-lived 2<sup>1</sup>A<sub>g</sub> state with <sup>1</sup>(TT) character is consistent with
the available experimental data
Optical Properties and Structural Relationships of the Silver Nanoclusters Ag<sub>32</sub>(SG)<sub>19</sub> and Ag<sub>15</sub>(SG)<sub>11</sub>
The recent discovery
of stable Ag nanoclusters presents new opportunities
to understand the detailed electronic and optical properties of the
metal core and the ligands using ultrafast spectroscopy. This paper
focuses on Ag<sub>32</sub> and Ag<sub>15</sub> (with thiolate ligands),
which are stable in solution. The steady state absorption spectra
of Ag nanoclusters show interesting quantum size effects, expected
for this size regime. Using a simple structural model for Ag<sub>32</sub>, TDDFT calculations show absorption at 480 nm and 680 nm that are
in reasonable correspondence with experiments. Ag<sub>32</sub>(SG)<sub>19</sub> and Ag<sub>15</sub>(SG)<sub>11</sub> have quantum yields
up to 2 orders of magnitude higher than Au nanoclusters of similar
sizes, with an emission maximum at 650 nm, identified as the metalâligand
state. The emission from both Ag nanoclusters has a common lifetime
of about 130 ps and a common energy transfer rate of <i>K</i><sub>EET</sub> â„ 9.7 Ă 10<sup>9</sup> s<sup>â1</sup>. A âdark stateâ competing with the emission process
was also observed and was found to be directly related to the difference
in quantum yield (QY) for the two Ag clusters. Two-photon excited
emission was observed for Ag<sub>15</sub>(SG)<sub>11</sub>, with a
cross-section of 34 GM under 800 nm excitation. Femtosecond transient
absorption measurements for Ag<sub>32</sub> recorded a possible metal
core state at 530 nm, a metalâligand state at 651 nm, and ground
state bleaches at 485 and 600 nm. The ground state bleach signals
in the transient spectrum for Ag<sub>32</sub> are 100 nm blue-shifted
in comparison to Au<sub>25</sub>. The transient spectrum for Ag<sub>15</sub> shows a weak ground state bleach at âŒ480 nm and a
broad excited state centered at 610 nm. TDDFT calculations indicate
that the electronic and optical properties of Ag nanoclusters can
be divided into core states and metalâligand states, and photoexcitation
generally involves a ligand to metal core transition. Subsequent relaxation
leaves the electron in a core state, but the hole can be either ligand
or core-localized. This leads to emission/relaxation that is consistent
with the observed photophysics