With the perspective of future vibronic studies, ab initio calculations of the energies of vibrational relaxation that follows the processes of singlet or triplet excitation, and positive or negative ionization, are reported for four series of compounds of potential interest in the context of photovoltaic applications, notably those rooted in singlet exciton fission. The commonly used method of evaluating the energy of vibrational relaxation following ionization of an excited molecule is examined and found to be dubious, especially when used within approaches based on the concept of an effective progression-forming mode. In this regard, methodological consistency in computing the relaxation energies and Franck–Condon parameters for different excitation processes is found to be of paramount importance. The presented results respond to the existing demand for vibronic coupling constants relevant to singlet fission.

Vibronic relaxation energies of acene-related molecules upon excitation or ionization.

We report the first-principles $GW$-Bethe--Salpeter equation and quantum Monte Carlo calculations of the optical and electronic properties of molecular and crystalline rubrene $({\mathrm{C}}_{42}{\mathrm{H}}_{28})$. Many-body effects dominate the optical spectrum and quasiparticle gap of molecular crystals. We interpret the observed yellow-green photoluminescence in rubrene microcrystals as a result of the formation of intermolecular, charge-transfer, spin-singlet excitons. In contrast, spin-triplet excitons are localized and intramolecular with a predicted phosphorescence at the red end of the optical spectrum. We find that the exchange energy plays a fundamental role in raising the energy of intramolecular spin-singlet excitons above the intermolecular ones. Exciton binding energies are predicted to be around $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (spin singlet) to $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (spin triplet). The calculated electronic gap is $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The theoretical absorption spectrum agrees very well with recent ellipsometry data.

Optical spectra and exchange-correlation effects in molecular crystals

In this work, we introduce a method for modeling the evolving absorbance spectrum of an organic molecule, pseudoisocyanine (PIC), measured during the process of molecular aggregation. Despite being historically considered a J-aggregate, we find that the absorbance spectrum of PIC cannot be adequately modeled using solely J-aggregates either during molecular aggregation or in the final dry film. The collection of absorbance spectra during solution-casting is particularly difficult since a distribution of aggregates with various sizes and structures can coexist. Here, spectra measured during film formation are fit to a weighted sum of simulated spectra of two aggregate species, revealing the combinations of Coulombic coupling values, Huang–Rhys parameters, and aggregate sizes that provide good fits to measured spectra. The peak intensity ratios and relative peak positions are highly sensitive to the aggregate structure, and fitting only these features enables the rapid comparison of aggregate combinations. We find that the spectra of PIC aggregates cannot be modeled using the Huang–Rhys factor of the PIC monomer, as is typically assumed, leading us to consider models that utilize independent Huang–Rhys factors for each aggregate species. This method of fitting only the key spectral features allows an experimental spectrum to be modeled within 1 h–2 h when using a single Huang–Rhys factor, making the simulation of a series of in situ measurements during aggregation computationally feasible.

Revealing the evolving mixture of molecular aggregates during organic film formation using simulations of in situ absorbance.

Extremely solvent-enhanced absorption and fluorescence spectra of carbazole were investigated by performing a generalized multi-set damped Franck-Condon spectral simulation. Experimental absorption and fluorescence spectra of carbazole in the gas phase were first well reproduced by performing an un-damped Franck-Condon simulation, but a one-set scaling damped Franck-Condon simulation severely underestimated the intensities of the peaks of experimental absorption and fluorescence spectra of carbazole in n-hexane. Then, a multi-set scaling damped Franck-Condon simulation was proposed and carried out for simulating the extremely solvent-enhanced absorbance and fluorescence, and here, the simulated spectra agreed well with the experimental ones. Five (four) representative solvent-enhanced normal modes corresponding to the combination of ring stretching and ring breathing vibrational motions were determined to be responsible for enhanced absorbance (fluorescence) in n-hexane solution. Furthermore, different scalings were applied to the ground and first-excited states, resulting in different enhancement of absorbance and fluorescence, and this analysis revealed atoms in the carbazole interacting with n-hexane solvent molecules and, hence, leading to different normal-mode vibrational vector patterns in the ground and first-excited states, respectively. Basically, the same conclusion was drawn from a simulation with HF-CIS and the three functionals (TD)B3LYP, (TD)B3LYP-35, and (TD)BHandHLYP. The present multi-set scaling damped Franck-Condon simulation scheme was demonstrated to successfully interpret extremely solvent-enhanced absorbance and fluorescence of carbazole in n-hexane-solvent.

Extremely solvent-enhanced absorbance and fluorescence of carbazole interpreted using a damped Franck-Condon simulation.

Over the last decade, molecules capable of emitting circularly polarized light have attracted growing attention for potential technological and biological applications. The efficiency of such devices depend on multiple parameters, in particular the magnitude and wavelength of the peak of emitted light, and also on the dissymmetry factor for chiral applications. In light of these considerations, molecular systems with tunable optical properties, preferably in the visible spectral region, are particularly appealing. This is the case of boron dipyrromethene (BODIPY) dyes, which exhibit large molecular absorption coefficients, have high fluorescence yields, are very stable, both thermally and photochemically, and can be easily functionalized. The latter property has been extensively exploited in the literature to produce chromophores with a wide range of optical properties. Nevertheless, only a few chiral BODIPYs have been synthetized and investigated so far. Using a recently reported axially chiral BODIPY derivative where an axially chiral BINOL unit has been attached to the chromophore unit, we present a comprehensive computational protocol to predict and interpret the one-photon absorption and emission spectra, together with their chiroptical counterparts. From the physico-chemical properties of this molecule, it will be possible to understand the origin of the circularly polarized luminescence better, thus helping to fine-tune the properties of interest. The sensitivity of such processes require accurate results, which can be achieved through a proper account of the vibrational structure in optical spectra. Methodologies to compute vibrationally-resolved electronic spectra can now be applied on relatively large chromophores, such as BODIPYs, but require more extensive computational protocols. For this reason, particular attention is paid in the description of the different steps of the protocol, and the potential pitfalls. Finally, we show how, by means of appropriate tools and approaches, data from intermediate steps of the simulation of the final spectra can be used to obtain further insights into the properties of the molecular system under investigation and the origin of the visible bands.

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Theoretical Investigation of the Circularly Polarized Luminescence of a Chiral Boron Dipyrromethene (BODIPY) Dye.

Quantum chemistry calculations at the level of (TD)-DFT plus PCM solvent models are employed for analyzing potential energy surfaces and as a result two local minima with D2, two local minima with C2H, and one second-order transition state with D2H group symmetry are found in both ground S0 and excited-state S1 potential energy surfaces. Simulated vibronic coupling distributions indicate that only second-order transition states with D2H group symmetry are responsible for observed absorption and fluorescence spectra of rubrene and vibrational normal-motions related with atoms on the aromatic backbone are active for vibronic spectra. The Stokes shift 1120 cm−1 (820 cm−1) and vibronic-band peak positions in both absorption and fluorescence spectra in non-polar benzene (polar cyclohexane) solvent are well reproduced within the conventional Franck–Condon simulation. By adding damped oscillator correction to Franck–Condon simulation, solvent enhanced vibronic-band intensities and shapes are well reproduced. Four (three) normal modes with vibration frequency around 1550 cm−1 (1350 cm−1) related to ring wagging plus CC stretching and CH bend motions on the backbone are actually interpreted for solvent enhanced absorption (fluorescence) spectra of rubrene in benzene and cyclohexane solutions.

/pdf/franck-condon-simulation-for-unraveling-vibronic-origin-in-a0an0acq38.pdf

Fenglong Gu

Papers

Franck–Condon simulation for unraveling vibronic origin in solvent enhanced absorption and fluorescence spectra of rubrene