Evolution of the Dynamics of As-Deposited and Annealed Lead Halide Perovskites

The rapid rise of organolead trihalide perovskites as solar photovoltaic materials has been followed by promising developments in light-emitting devices and lasers due to their unique and promising optical properties. Evolution of the photophysical properties in as-deposited or annealed CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ perovskite films processed through the interdiffusion method has been investigated. Absorption spectra showed broad band edge saturation in the as-deposited films in contrast to sharp excitonic absorption in the annealed films. Fluorescence emission of the perovskite films showed strong dependence on the halogen type with a very high quantum yield of ∼90% for the annealed CH₃NH₃PbBr₃ film. An explanation for this was provided based on its crystallinity and quantum confinement of the excitons. The emission showed weakly Stokes shifted bands. Time-resolved spectroscopic measurements were carried out to probe the ultrafast dynamics for the perovskites for the as-deposited or annealed films. We classified the evolution in the absorption features in the excited state of CH₃NH₃PbBr₃ perovskite films for the first time and compared them to CH₃NH₃PbI₃. We suggest a bleach feature below 400 nm as the charge transfer band, which results in the photoinduced absorption in the CH₃NH₃PbBr₃ perovskite film, a charge-separated band gap state, and the existence of intermediate excited-state species that regenerate the ground state

Two-Photon Fluorescence Spectroscopy and Imaging of 4‑Dimethylaminonaphthalimide Peptide and Protein Conjugates

We report detailed photophysical studies on the two-photon fluorescence processes of the solvatochromic fluorophore 4-DMN as a conjugate of the calmodulin (CaM) and the associated CaM-binding peptide M13. Strong two-photon fluorescence enhancement has been observed which is associated with calcium binding. It is found that the two-photon absorption cross-section is strongly dependent on the local environment surrounding the 4-DMN fluorophore in the CaM conjugates, providing sensitivity between sites of fluorophore attachment. Utilizing time-resolved measurements, the emission dynamics of 4-DMN under various environmental (solvent) conditions are analyzed. In addition, anisotropy measurements reveal that the 4-DMN–S38C–CaM system has restricted rotation in the calcium-bound calmodulin. To establish the utility for cellular imaging, two-photon fluorescence microscopy studies were also carried out with the 4-DMN-modified M13 peptide in cells. Together, these studies provide strong evidence that 4-DMN is a useful probe in two-photon imaging, with advantageous properties for cellular experiments

Optical Properties and Structural Relationships of the Silver Nanoclusters Ag₃₂(SG)₁₉ and Ag₁₅(SG)₁₁

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 Ag₃₂ and Ag₁₅ (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 Ag₃₂, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag₃₂(SG)₁₉ and Ag₁₅(SG)₁₁ 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 metal–ligand 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>_EET ≥ 9.7 × 10⁹ s^–1. A “dark state” 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 Ag₁₅(SG)₁₁, with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag₃₂ recorded a possible metal core state at 530 nm, a metal–ligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag₃₂ are 100 nm blue-shifted in comparison to Au₂₅. The transient spectrum for Ag₁₅ shows a weak ground state bleach at ∼480 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 metal–ligand 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

Synthesis and Ultrafast Time Resolved Spectroscopy of Peripherally Functionalized Zinc Phthalocyanine Bearing Oligothienylene-ethynylene Subunits

Two new soluble tri-<i>tert-</i>butyl zinc­(II) phthalocyanines, <b>1</b> and <b>2</b>, bearing dendritic oligothienylene-ethynylene (DOT) groups as one of the peripheral substituents, have been prepared. The conjugated DOT moieties were introduced to cover the spectral window between 380 and 550 nm, where the ZnPc does not exhibit a strong absorption, in order to improve light harvesting. For their preparation, a convergent approach has been used starting from the corresponding iodoPc as precursor. Further transformation of the iodo groups by a Pd-catalyzed Sonogashira reaction with the appropriate DOT-functionalized terminal alkyne allowed the easy preparation of extended π-conjugated compounds <b>1</b> and <b>2</b>. The compounds have been characterized by standard spectroscopic methods, and their photophysical behaviors have been established by using ultrafast time-resolved techniques. Femtosecond upconversion measurements showed an ultrafast energy transfer from the DOT to zinc phthalocyanine in a time scale of 300 fs. As the number of thiophene groups increases in the dyads, the extent of ultrafast energy transfer was found to increase. Compounds <b>1</b> and <b>2</b> have been tested as donor components in bulk heterojunction (BHJ) solar cells. Their efficiencies are compared with RuPc analogues previously reported by us