2-Amino-7-nitro-fluorenes in Neat and Mixed SolventsOptical Band Shapes and Solvatochromism

Explicit Modelling of Spectral Bandshapes by a Mixed Quantum-Classical Approach: Solvent Order and Temperature Effects in the Optical Spectra of Distyrylbenzene

The absorption and emission spectral shapes of a flexible organic probe, the distyrylbenzene (DSB) dye, are simulated accounting for the effect of different environments of increasing complexity, ranging from a homogeneous, low-molecular- weight solvent, to a long-chain alkane, and, eventually, a channel-forming organic matrix. Each embedding is treated explicitly, adopting a mixed quantum-classical approach, the Adiabatic Molecular Dynamics - generalized vertical Hessian (Ad-MD|gVH) model, which allows a direct simulation of the environment-induced constraining effects on the vibronic spectral shapes. In such a theoretical framework, the stiff modes of the dye are described at a quantum level within the harmonic approximation, including Duschinsky mixing effects, while flexible degrees of freedom of the solute (e. g. torsions) and those of the solvent are treated classically by means of molecular dynamics sampling. Such a setup is shown to reproduce the distinct effects exerted by the different environments in varied thermodynamic conditions. Besides allowing for a first-principles rationale on the supramolecular mechanism leading to the experimental spectral features, this result represents the first successful application of the Ad-MD|gVH method to complex embeddings and supports its potential application to other heterogeneous environments, such as for instance, pigment-protein complexes or organic dyes adsorbed into metal-organic frameworks.

Molecular torsion controls the excited state relaxation pathways of multibranched tetraphenylpyrazines: effect of substitution of morpholine vs. phenoxazine

Multibranched donor-acceptor derivatives exhibiting desirable photophysical properties are efficiently used in optoelectronic devices, in which the excited state relaxation dynamics of the derivatives control the efficiency of the devices. Here, the effect of intramolecular torsion on the excited state relaxation dynamics of tetraphenylpyrazine (TPP) derivatives in non-polar (toluene) and polar (THF) solvents is investigated by substituting the electron donor of morpholine (TPP-4MOP) and phenoxazine (TPP-4PHO) leading to the planar and twisted configurations, respectively, using femtosecond and nanosecond transient absorption spectroscopy. In the steady state, TPP-4MOP showed feeble emission (ΦF ∼0.03) due to the weak donor by the delocalization of electron density supported by theoretical optimization. The TPP-4PHO exhibited strong emission (ΦF ∼0.18) in toluene compared to that in THF, in which it showed a large Stokes shift (∼9691 cm-1) with low fluorescence quantum yield (ΦF ∼0.01). The observation of large Stokes shifts, inherent nature and theoretical calculations of TPP-4PHO suggest the twisting of the dihedral angle between tetraphenylpyrazine and phenoxazine in the excited state leading to the twisted intramolecular charge transfer state (TICT). The femtosecond and nanosecond transient absorption and picosecond time-resolved emission spectra of TPP-4PHO revealed the signature of the existence of both the partial TICT and TICT states in THF leading to the triplet state. Whereas in the case of TPP-4MOP, the transient absorption spectra showed the formation of the triplet state from the local excited state without the involvement of the TICT state. Aggregation studies of TPP-4PHO in a THF and water mixture reflect the elimination of the TICT state by the restriction of intramolecular torsion in the aggregates leading to an increase of 12-fold of the fluorescence intensity along with shifting of the maximum towards the blue region. These studies revealed that the excited state relaxation pathways of the derivatives are controlled by polarity-dependent torsional motion.

Non-Phenomenological Description of the Time-Resolved Emission in Solution with Quantum-Classical Vibronic Approaches-Application to Coumarin C153 in Methanol

We report a joint experimental and theoretical work on the steady-state spectroscopy and time-resolved emission of the coumarin C153 dye in methanol. The lowest energy excited state of this molecule is characterized by an intramolecular charge transfer thus leading to remarkable shifts of the time-resolved emission spectra, dictated by the methanol reorganization dynamics. We selected this system as a prototypical test case for the first application of a novel computational protocol aimed at the prediction of transient emission spectral shapes, including both vibronic and solvent effects, without applying any phenomenological broadening. It combines a recently developed quantum-classical approach, the adiabatic molecular dynamics generalized vertical Hessian method (Ad-MD|gVH), with nonequilibrium molecular dynamics simulations. For the steady-state spectra we show that the Ad-MD|gVH approach is able to reproduce quite accurately the spectral shapes and the Stokes shift, while a ∼0.15 eV error is found on the prediction of the solvent shift going from gas phase to methanol. The spectral shape of the time-resolved emission signals is, overall, well reproduced, although the simulated spectra are slightly too broad and asymmetric at low energies with respect to experiments. As far as the spectral shift is concerned, the calculated spectra from 4 ps to 100 ps are in excellent agreement with experiments, correctly predicting the end of the solvent reorganization after about 20 ps. On the other hand, before 4 ps solvent dynamics is predicted to be too fast in the simulations and, in the sub-ps timescale, the uncertainty due to the experimental time resolution (300 fs) makes the comparison less straightforward. Finally, analysis of the reorganization of the first solvation shell surrounding the excited solute, based on atomic radial distribution functions and orientational correlations, indicates a fast solvent response (≈100 fs) characterized by the strengthening of the carbonyl-methanol hydrogen bond interactions, followed by the solvent reorientation, occurring on the ps timescale, to maximize local dipolar interactions.

Simulation of the VUV Absorption Spectra of Oxygenates and Hydrocarbons: A Joint Theoretical-Experimental Study

Vacuum UV absorption spectroscopy is regularly used to provide unambiguous identification of a target species, insight into the electronic structure of molecules, and quantitative species concentrations. As molecules of interest have become more complex, theoretical spectra have been used in tandem with laboratory spectroscopic analysis or as a replacement when experimental data is unavailable. However, it is difficult to determine which theoretical methodologies can best simulate experiment. This study examined the performance of EOM-CCSD and 10 TD-DFT functionals (B3LYP, BH&HLYP, BMK, CAM-B3LYP, HSE, M06-2X, M11, PBE0, ωB97X-D, and X3LYP) to produce reliable vacuum UV absorption spectra for 19 small oxygenates and hydrocarbons using vertical excitation energies. The simulated spectra were analyzed against experiment using both a qualitative analysis and quantitative metrics, including cosine similarity, relative integral change, mean signed error, and mean absolute error. Based on our ranking system, it was determined that M06-2X was consistently the top performing TD-DFT method with BMK, CAM-B3LYP, and ωB97X-D also producing reliable spectra for these small combustion species.

Insights for vibronic effects on spectral shapes of electronic circular dichroism and circularly polarized luminescence of aza[7]helicene

We present a systematic study of vibrationally resolved absorption (ABS), electronic circular dichroism (ECD), emission (EMI), and circularly polarized luminescence (CPL) of aza[7]helicene. Because of the rare experience of theoretical CPL calculation, a variety of harmonic models have been employed to compute the vibronic structures. To fully understand the vibronic effects on the spectral shapes, Franck-Condon (FC) and Herzberg-Teller (HT) contributions, Duschinsky mixings and temperature effect have all been taken into consideration. The performance of different alternative approximate methods has been carefully compared and discussed in detail. The results show that Vertical Hessian (VH) model has a slight better performance on the spectral shapes than Adiabatic Hessian (AH), especially for CPL spectrum. The thermal excitation effect has led to a reduced resolution and a broader spectral width. The moderate HT effects on the different spectral shapes have been addressed. The dissymmetry factors have been correctly reproduced and the main vibronic features of the four different spectral shapes have been successfully captured. A good estimation of the overall spectral width, relative position and relative height of different spectral bands has been presented. The nice agreement with the experiment allows us to present a detailed interpretation of the spectral shapes.

Vibrationally resolved electronic circular dichroism and circularly polarized luminescence spectra of a boron-fused double helicene: A theoretical study

In this work, we present the theoretical study of the vibrationally resolved absorption (ABS), electronic circular dichroism (ECD), emission (EMI), and circularly polarized luminescence (CPL) spectra of a boron-fused double helicene, with a detailed and complete discussion of the alternative possible approximate methods. Given the fact that few examples of CPL calculations exist, the potential energy surfaces (PESs) have been constructed and compared with Adiabatic (AH) and Vertical Hessian (VH) models. All the vibronic calculations have accounted for Duschinsky mixings, Franck-Condon (FC) effect and Herzberg-Teller (HT) contribution. Moreover, different HT expansions have been checked and compared, by computing the derivatives of the electric and magnetic dipole transition moments around the equilibrium geometries of the initial and final states. Our results show that both AH and VH models have well reproduced the experimental vibronic structures and VH model shows a better performance in the simulation of spectral lineshapes. They also show that HT effects dominate the shapes of EMI and CPL, tuning the relative heights of the different vibronic peaks, improving the agreement with the experiment for EMI. Moreover, HT effects are the main reason for the mirror-symmetry breaking between ECD and CPL spectra. Furthermore, interesting interference effects between FC and HT contributions have also been addressed.

The inverted solvatochromism of protonated ferrocenylethenyl-pyrimidines: the first example of the solvatochromic reversal of a hybrid organic/inorganic dye

Five new solvatochromic 2,6-diaryl-4-ferrocenylethenylpyrimidines were synthesized and their spectral variations in solution investigated in twenty-seven solvents of variable polarity.

Direct evidence of solvent polarity governing the intramolecular charge and energy transfer: ultrafast relaxation dynamics of push–pull fluorene derivatives

The occurrence of intramolecular charge transfer along with energy transfer controlled by the polarity of solvent is revealed by femtosecond and nanosecond transient absorption and emission spectroscopy.

Solvent-dependent dual fluorescence of the push-pull system 2-diethylamino-7-nitrofluorene

The solvent-dependent excited state behavior of the molecular push-pull system 2-diethylamino-7-nitrofluorene has been explored using femtosecond transient absorption spectroscopy in combination with density functional theory calculations. Several excited state minima have been identified computationally, all possessing significant intramolecular charge transfer character. The experimentally observed dual fluorescence is suggested to arise from a planar excited state minimum and another minimum reached by twisting of the aryl-nitrogen bond of the amino group. The majority of the excited state population, however, undergo non-radiative transitions and potential excited state deactivation pathways are assessed in the computational investigation. A third excited state conformer, characterized by twisting around the aryl-nitrogen bond of the nitro group, is reasoned to be responsible for the majority of the non-radiative decays and a crossing between the excited state and ground state is localized. Additionally, ultrafast intersystem crossing is observed in the apolar solvent cyclohexane and rationalized to occur via an El-Sayed assisted transition from one of the identified excited state minima. The solvent thus determines more than just the fluorescence lifetime and shapes the potential energy landscape, thereby dictating the available excited state pathways.

Solvatochromism of conjugated 4-N,N-dimethylaminophenyl-pyridinium donor–acceptor pairs

A series of solvatochromic dyes consisting of an N,N-dimethylaminophenyl donor group conjugated with a 1-butylpyridinium acceptor ring through a 1,4-phenylene spacer were synthesized by a three-step procedure.

Insights for an Accurate Comparison of Computational Data to Experimental Absorption and Emission Spectra: Beyond the Vertical Transition Approximation

In this work we carefully investigate the relationship between computed data and experimental electronic spectra. To that end, we compare both vertical transition energies, EV, and characteristic frequencies of the spectrum like the maximum, ν(max), and the center of gravity, M(1), taking advantage of an analytical expression of M(1) in terms of the parameters of the initial- and final-state potential energy surfaces. After pointing out that, for an accurate comparison, experimental spectra should be preliminarily mapped from wavelength to frequency domain and transformed to normalized lineshapes, we simulate the absorption and emission spectra of several prototypical chromophores, obtaining lineshapes in very good agreement with experimental data. Our results indicate that the customary comparison of experimental ν(max) and computational EV, without taking into account vibrational effects, is not an adequate measure of the performance of an electronic method. In fact, it introduces systematic errors that, in the investigated systems, are on the order of 0.1-0.3 eV, i.e., values comparable to the expected accuracy of the most accurate computational methods. On the contrary, a comparison of experimental and computed M(1) and/or 0-0 transition frequencies provides more robust results. Some rules of thumbs are proposed to help rationalize which kind of correction one should expect when comparing EV, M(1), and ν(max).

Solvation oscillations and excited-state dynamics of 2-amino- and 2-hydroxy-7-nitrofluorene and its 2'-deoxyriboside

Push-pull substituted fluorenes are considered for use as dynamic solvation probes in polynucleotides. Their fluorescence band is predicted (by simulations) to show weak spectral oscillations on the subpicosecond time scale depending on the nucleotide sequence. The oscillations reflect the local far-infrared spectrum of the environment around the probe molecule. A connection is provided by the continuum theory of polar solvation which, however, neglects molecular aspects. We examine the latter using acetonitrile solution as a test case. A collective librational solvent mode at 100 cm(-1) is observed with 2-amino-7-nitrofluorene, 2-dimethylamino-7-nitrofluorene, 2-hydroxy-7-nitrofluorene, and its 2'-deoxyriboside. Different strengths of the oscillation indicate that rotational friction of nearby acetonitrile molecules depends on the solute structure or that H bonding is involved in launching the librational coherence. Polar solvation in methanol is used for comparison. With hydroxynitrofluorenes, the observation window is limited by intersystem crossing for which rates are reported. A prominent excited-state absorption band of nitrofluorenes at 430 nm can be used to monitor polar solvation. Structural and electronic relaxation pathways are discussed with the help of quantum chemical calculations.