Many emerging technologies depend on human's ability to control and manipulate the excited-state properties of molecular systems. These technologies include fluorescent labeling in biomedical imaging, light harvesting in photovoltaics, and electroluminescence in light-emitting devices. All of these systems suffer from non-radiative loss pathways that dissipate electronic energy as heat, which causes the overall system efficiency to be directly linked to quantum yield (Φ) of the molecular excited state. Unfortunately, Φ is very difficult to predict from first principles because the description of a slow non-radiative decay mechanism requires an accurate description of long-timescale excited-state quantum dynamics. In the present study, we introduce an efficient semiempirical method of calculating the fluorescence quantum yield (Φ fl) for molecular chromophores, which, based on machine learning, converts simple electronic energies computed using time-dependent density functional theory (TDDFT) into an estimate of Φ fl. As with all machine learning strategies, the algorithm needs to be trained on fluorescent dyes for which Φ fl 's are known, so as to provide a black-box method which can later predict Φ's for chemically similar chromophores that have not been studied experimentally. As a first illustration of how our proposed algorithm can be trained, we examine a family of 25 naphthalene derivatives. The simplest application of the energy gap law is found to be inadequate to explain the rates of internal conversion (IC) or intersystem crossing (ISC)-the electronic properties of at least one higher-lying electronic state (S n or T n) or one far-from-equilibrium geometry are typically needed to obtain accurate results. Indeed, the key descriptors turn out to be the transition state between the Franck-Condon minimum a distorted local minimum near an S 0 /S 1 conical intersection (which governs IC) and the magnitude of the spin-orbit coupling (which governs ISC). The resulting Φ fl 's are predicted with reasonable accuracy (± 22%), making our approach a promising ingredient for high-throughput screening and rational design of the molecular excited states with desired Φ's. We thus conclude that our model, while semi-empirical in nature, does in fact extract sound physical insight into the challenge of describing non-radiative relaxations.

Materials with magneto-optic (MO) properties have enabled critical fiber-optic applications and highly sensitive magnetic field sensors. While traditional MO materials are inorganic in nature, new generations of MO materials based on organic semi-conducting polymers could allow increased versatility for device architectures, manufacturing options, and flexible mechanics. However, the origin of MO activity in semiconducting polymers is far from understood. In this paper, we report high MO activity observed in a chiral helical poly-3-(alkylsulfone)thiophene (P3AST), which confirms a new design for the creation of giant Faraday effect with Verdet constants up to (7.63±0.78)×104 deg T−1 m−1 at 532 nm. We have determined that the sign of the Verdet constant and its magnitude are related to the helicity of the polymer at the measured wavelength. The Faraday rotation and the helical conformation of P3AST are modulated by thermal annealing, which is further supported by DFT and MD simulations. Our results demonstrate that helical polymers exhibit enhanced Verdet constants, and expand the previous design space for polythiophene MO materials that was thought to be limited to highly regular lamellar structures. The structure property studies herein provide insights for the design of next generation MO materials based upon semiconducting organic polymers.

The results of diffusion Monte Carlo (DMC) calculations of the ground and selected excited states of H5(+) and its deuterated analogues are presented. Comparisons are made between the results obtained from two recently reported potential surfaces. Both of these surfaces are based on CCSD(T) electronic energies, but the fits display substantial differences in the energies of low-lying stationary points. Little sensitivity to these features is found in the DMC results, which yield zero-point energies based on the two surfaces that differ by between 20 and 30 cm(–1) for all twelve isotopologues of H5(+). Likewise, projections of the ground state probability amplitudes, evaluated for the two surfaces, are virtually identical. By using the ground state probability amplitudes, vibrationally averaged rotational constants and dipole moments were calculated. On the basis of these calculations, all isotopologues are shown to be near-prolate symmetric tops. Further, in cases where the ion had a nonzero dipole moment, the magnitude of the vibrationally averaged dipole moment was found to range from 0.33 to 1.15 D, which is comparable to the dipole moments of H2D+ and HD2(+). Excited states with up to three quanta in the shared proton stretch and one quantum in the in-phase stretch of the outer H2 groups were also investigated. Trends in the energies and the properties of these states are discussed.

In the framework of density functional theory (DFT), the lowest triplet excited state, T 1 , can be evaluated using multiple formulations, the most straightforward of which are unrestricted DFT (UDFT) and time-dependent DFT (TDDFT). Assuming the exact exchangecorrelation (XC) functional is applied, UDFT and TDDFT provide identical energies for T 1 (E T ), which is also a constraint that we require our XC functionals to obey. However, this condition is not satisfied by most of the popular XC functionals, leading to inaccurate predictions of low-lying, spectroscopically and photochemically important excited states, such as T 1 and the lowest singlet excited state (S 1 ). Inspired by the optimal tuning strategy for frontier orbital energies [T. Stein, L. Kronik, and R. Baer, J. Am. Chem. Soc. 131, 2818 (2009)], we proposed a novel and non-empirical prescription of constructing an XC functional in which the agreement between UDFT and TDDFT in E T is strictly enforced. Referred to as "triplet tuning", our procedure allows us to formulate the XC functional on a case-by-case basis using the molecular structure as the exclusive input, without fitting to any experimental data. The first triplet tuned XC functional, TT-ωPBEh, is formulated as a long-range-corrected (LRC) hybrid of Perdew-Burke-Ernzerhof (PBE) and Hartree-Fock (HF) functionals [M. A. Rohrdanz, K. M. Martins, and J. M. Herbert, J. Chem. Phys. 130, 054112 (2009)] and tested on four sets of large organic molecules. Compared to existing functionals, TT-ωPBEh manages to provide 1 arXiv:1806.00317v3 [physics.chem-ph] 15 Jan 2019more accurate predictions for key spectroscopic and photochemical observables, including but not limited to E T , the optical band gap (E S ), the singlet-triplet gap (∆E ST ), and the vertical ionization potential (I ⊥ ), as it adjusts the effective electron-hole interactions to arrive at the correct excitation energies. This promising triplet tuning scheme can be applied to a broad range of systems that were notorious in DFT for being extremely challenging.

Boron-dipyrromethene (BODIPY) molecules are widely used as laser dyes and have therefore become a popular research topic within recent years. Numerous studies have been reported for the rational design of BODIPY derivatives based on their photophysical properties, including absorption and fluorescence wavelengths (λ abs and λ fl ), oscillator strength ( f), nonradiative pathways, and quantum yield (Φ). In the present work, we illustrate a theoretical, semiempirical model that accurately predicts Φ for various BODIPY compounds on the basis of inexpensive electronic structure calculations, following the data-driven algorithm proposed by us in a previous study [Kohn et al. J. Phys. Chem. C. 2019, 123, 15394]. The model allows us to identify the dominant nonradiative channel of any BODIPY molecule using its structure exclusively and to establish a correlation between the activation energy (E a ) and the fluorescence quantum yield (Φ fl ). On the basis of our calculations, either the S 1 → S 0 or L a → L b internal conversion (IC) mechanism dominates in the majority of BODIPY derivatives, depending on the structural and electronic properties of the substituents. In either case, the nonradiative rate (k nr ) exhibits a straightforward Arrhenius-like relation with the associated E a . More interestingly, the S 1 → S 0 mechanism proceeds via a highly distorted intermediate structure in which the core BODIPY plane and the substituent at the 1-position are twisted, while the internal rotation of the very same substituent induces the L a → L b transition. Our model reproduces k fl , k nr , and Φ fl to mean absolute errors (MAEs) of 0.16 decades, 0.87 decades, and 0.26, when all outliers are considered. These results allow us to validate the predictive power of the proposed data-driven algorithm in Φ fl . They also indicate that the model has a great potential to facilitate and accelerate the machine learning aided design of BODIPY dyes for imaging and sensing applications, given sufficient experimental data and appropriate molecular descriptors.