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Laser cooling of large, complex molecules is a long-standing goal, instrumental for enabling new quantum technology and precision measurements. A primary consideration for the feasibility of laser cooling, which determines the efficiency and technical requirements of the process, is the number of excited-state decay pathways leading to vibrational excitations. Therefore, the assessment of the laser-cooling potential of a molecule begins with estimate of the vibrational branching ratios of the first few electronic excited states theoretically to find the optimum cooling scheme. Such calculations, typically done within the Born-Oppenheimer and harmonic approximations, have suggested that one leading candidate for large, polyatomic molecule laser cooling, alkaline earth phenoxides, can most efficiently be laser cooled via the third electronically excited ( $\tilde{C}$) state. Here, we report the first detailed spectroscopic characterization of the $\tilde{C}$state in CaOPh and SrOPh. We find that nonadiabatic couplings between the $\tilde{A}, \tilde{B}$, and $\tilde{C}$states lead to substantial mixing, giving rise to vibronic states that enable additional decay pathways. Based on the intensity ratio of these extra decay channels, we estimate a nonadiabatic coupling strength of $\sim 0.1{\mathrm{cm}}^{-1}$. While this coupling strength is small, the large density of vibrational states available at photonic energy scales in a polyatomic molecule leads to significant mixing. Only the lowest excited state $\tilde{A}$is exempt from this coupling because it is highly separated from the ground state. Thus, this result is expected to be general for large molecules and implies that only the lowest electronic excited state should be considered when judging the suitability of a molecule for laser cooling. 

Dimension-engineered synthesis of atomically thin II–VI nanoplatelets (NPLs) remains an open challenge. While CdSe NPLs have been made with confinement ranging from 2 to 11 monolayers (ML), CdTe NPLs have been significantly more challenging to synthesize and separate. Here we provide detailed mechanistic insight into the layer-by-layer growth kinetics of the CdTe NPLs. Combining ensemble and single-particle spectroscopic and microscopic tools, our work suggests that beyond 2 ML CdTe NPLs, higher ML structures initially appear as heteroconfined materials with colocalized multilayer structures. In particular, we observe strongly colocalized 3 and 4 ML emissions, accompanied by a broad trap emission. Accompanying transient absorption, single-particle optical, and atomic force microscopy analyses suggest islands of different MLs on the same NPL. To explain the nonstandard nucleation and growth of these heteroconfined structures, we simulated the growth conditions of NPLs and quantified how the monomer binding energy modifies the kinetics and permits single NPLs with multi-ML structures. Our findings suggest that the lower bond energy associated with CdTe relative to CdSe limits higher ML syntheses and explains the observed differences between CdTe and CdSe growth. 

We introduce an individually fitted screened-exchange interaction for the time-dependent Hartree–Fock (TDHF) method and show that it resolves the missing binding energies in polymethine organic dye molecules compared to time-dependent density functional theory (TDDFT). The interaction kernel, which can be thought of as a dielectric function, is generated by stochastic fitting to the screened-Coulomb interaction of many-body perturbation theory (MBPT), specific to each system. We test our method on the flavylium and indocyanine green dye families with a modifiable length of the polymethine bridge, leading to excitations ranging from visible to short-wave infrared. Our approach validates earlier observations on the importance of inclusion of medium range exchange for the exciton binding energy. Our resulting method, TDHF@vW, also achieves a mean absolute error on a par with MBPT at a computational cost on a par with local-functional TDDFT. 

The total photon economy of a chromophore molecular species represents a study of how absorbed photons partition among various electronic states and ultimately dissipate their excited energy into the environment. 

Achieving ultranarrow absorption linewidths in the condensed phase enables optical state preparation of specific non-thermal states, a prerequisite for quantum-enabled technologies. 

Abstract Changes in the viscosity of intracellular microenvironments may indicate the onset of diseases like diabetes, blood‐based illnesses, hypertension, and Alzheimer's. To date, monitoring viscosity changes in the intracellular environment remains a challenge with prior work focusing primarily on visible light‐absorbing viscosity sensing fluorophores. Herein, a series of near‐infrared (NIR, 700–1000 nm) absorbing and emitting indolizine squaraine fluorophores (1PhSQ,2PhSQ,SO₃SQ,1DMASQ,7DMASQ, and1,7DMASQ) are synthesized and studied for NIR viscosity sensitivity.2PhSQexhibits a very high slope in its Forster‐Hoffmann plot at 0.75 which indicates this dye is a potent viscosity sensor. The properties of the squaraine fluorophores are studied computationallyviadensity functional theory (DFT) and time‐dependent (TD)‐DFT. Experimentally, both steady‐state and time‐resolved emission spectroscopy, absorption spectroscopy, and electrochemical characterization are conducted on the dyes. Precise photophysical tuning is observed within the series with emission maxima wavelengths as long as 881 nm for1,7DMASQand fluorescence quantum yields as high as 39.5 and 72.0 % for1PhSQin DCM and THF, respectively. The high tunability of this molecular scaffold renders indolizine squaraine fluorophores excellent prospects as viscosity‐sensitive biological imaging agents with2PhSQgiving a dramatically higher fluorescence quantum yield (from 0.3 to 37.1 %) as viscosity increases. 

Near infrared (NIR, 700–1000 nm) and short-wave infrared (SWIR, 1000–2000 nm) dye molecules exhibit significant nonradiative decay rates from the first singlet excited state to the ground state. While these trends can be empirically explained by a simple energy gap law, detailed mechanisms of nearly universal behavior have remained unsettled for many cases. Theoretical and experimental results for two representative NIR/SWIR dye molecules reported here clarify the key mechanism for the observed energy gap law behavior. It is shown that the first derivative nonadiabatic coupling terms serve as major coupling pathways for nonadiabatic decay processes from the first excited singlet state to the ground state for these NIR and SWIR dye molecules and that vibrational modes other than the highest frequency modes also make significant contributions to the rate. This assessment is corroborated by further theoretical comparison with possible alternative mechanisms of intersystem crossing to triplet states and also by comparison with experimental data for deuterated molecules. 

In vivo fluorescence imaging in the shortwave infrared (SWIR, 1,000–1,700 nm) and extended SWIR (ESWIR, 1,700–2,700 nm) regions has tremendous potential for diagnostic imaging. Although image contrast has been shown to improve as longer wavelengths are accessed, the design and synthesis of organic fluorophores that emit in these regions is challenging. Here we synthesize a series of silicon-RosIndolizine (SiRos) fluorophores that exhibit peak emission wavelengths from 1,300–1,700 nm and emission onsets of 1,800–2,200 nm. We characterize the fluorophores photophysically (both steady-state and time- resolved), electrochemically and computationally using time-dependent density functional theory. Using two of the fluorophores (SiRos1300 and SiRos1550), we formulate nanoemulsions and use them for general systemic circulatory SWIR fluorescence imaging of the cardiovascular system in mice. These studies resulted in high-resolution SWIR images with well-defined vasculature visible throughout the entire circulatory system. This SiRos scaffold establishes design principles for generating long-wavelength emitting SWIR and ESWIR fluorophores.