The molecular exciton model has received its most extensive development and application in the field of molecular crystals1'2. More recently, numerous applications to non-crystalline molecular composite systems have been made, including van der Waals and hydrogen-bonded dimers, trimers, and higher order aggregates. Another type of composite system has also been investigated, namely the composite molecule consisting of covalently bonded molecular units, with intrinsic individual unsaturated electronic systems so isolated by single bonds that but little or insignificant electronic overlap between units may occur. It is now well established that in molecular aggregates and in composite molecules, exciton effects may be observed if sufficiently strong electronic transitions exist in the component sub-units. The result of exciton splitting of excited states in the composite molecule may be the appearance of strong spectral shifts or splittings (which may be of the order of 2000 cm—1) of the absorption bands for the component molecules. At the same time, as a consequence of the exciton splitting of the excited state manifold, an enhancement of triplet state excitation may result. The purpose of this paper is to present a summary of the various type cases for molecular dimers, trimers and double and triple molecules in the description of the molecular exciton strong-coupling model. Then it will be shown by new experimental examples that, even in those cases where no significant exciton effect is observable in the singlet—singlet absorption spectrum for the composite molecule (intermediate and weak coupling cases), the enhancement of lowest triplet state excitation may still be conspicuous and significant. The ideas which are summarized in this paper have a curious history. Long ago, Kautsky and Merkel3 demonstrated experimentally that aggregation of dyes facilitated their action as photophysical sensitizers in photochemical reactions, at the same time diminishing their fluorescence efficiency. Kautsky attributed these easily demonstrated effects to enhancement of metastable state excitation in the aggregate dye. There is no doubt today that the metastable state he described is the lowest triplet state of the molecules studied. However, he did not distinguish between intrinsic and enhanced metastable (triplet) state excitation, so his interpretations were largely overlooked. Forster in l946 used the quasi-classical vector model to

/pdf/the-exciton-model-in-molecular-spectroscopy-2vwkowivjh.pdf

The exciton model in molecular spectroscopy

6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

https://www.cell.com/cell/pdf/S0092-8674(14)00826-5.pdf

Insights into secondary metabolism from a global analysis of prokaryotic biosynthetic gene clusters.

In this Review, the extreme care that must be taken when predicting the optical properties of conjugated polymers via the oligomer approach, and when comparing theoretical and experimental data, is illustrated. In the first part, conceptual strategies for the correct determination of optical transitions from experimental spectra and relevant extrapolation procedures at the polymer limit are introduced. The impact of conformational, substitution, solvent, and solid-state effects on the optical properties is discussed in light of experimental data reported for molecular backbones based on phenylene, phenylenevinylene, and thiophene repeat units. A comparison is then made between experimental results and those provided by standard quantum-chemical methods, to assess their reliability.

Optical Bandgaps of π‐Conjugated Organic Materials at the Polymer Limit: Experiment and Theory

We describe a new method for the simulation of excited state dynamics, based on classical trajectories and surface hopping, with direct semiempirical calculation of the electronic wave functions and potential energy surfaces ~DTSH method!. Semiempirical self-consistent-field molecular orbitals ~SCF MO’s! are computed with geometry-dependent occupation numbers, in order to ensure correct homolytic dissociation, fragment orbital degeneracy, and partial optimization of the lowest virtuals. Electronic wave functions are of the MO active space configuration interaction ~CI! type, for which analytic energy gradients have been implemented. The time-dependent electronic wave function is propagated by means of a local diabatization algorithm which is inherently stable also in the case of surface crossings. The method is tested for the problem of excited ethylene nonadiabatic dynamics, and the results are compared with recent quantum mechanical calculations. © 2001 American Institute of Physics. @DOI: 10.1063/1.1376633#

Direct semiclassical simulation of photochemical processes with semiempirical wave functions

Theoretical study of trans-cis photoisomerism in polymethine cyanines

The photophysical and photochemical properties and the ground-state conformational equilibrium of trans-n-styrylphenanthrene (n-StPh, with n = 1, 2, 3, 4, 9) have been studied in inert solvents. The kinetic parameters of the competitive radiative and reactive decay processes have been obtained. A detailed analysis of the fluorimetric behavior as a function of the excitation wavelength and temperature has allowed the distinct decay parameters and the ground-state energy difference of the two rotamers of trans-3-StPh to be obtained. Parallel theoretical calculations of the potential energy curves for the internal rotation of the phenanthryl group in the ground state, of the energies and oscillator strengths of the lowest transitions, and of the activation energies for trans ..-->.. cis isomerization in the ground and lowest excited singlet states have been carried out with a modified (CS) INDO method. The results of the experimental and theoretical studies are in satisfactory agreement and provide a general description of the photophysical and photochemical behavior of this class of compounds.

Photophysical and theoretical studies of photoisomerism and rotamerism of trans-styrylphenanthrenes

Fluorescence and absorption spectra of polyphenyls. Theoretical study on the band shape

The solvaton–CS INDO model, previously successfully used to describe the solvatochromic properties of merocyanines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2 was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the ET(30) scale. Solvato- and fluorosolvatochromism were described by calculating the S0(eq.)→S1(Franck–Condon) and S1(eq.)→S0(Franck–Condon) transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and ranging from 0 to 1. The absorbing S0(eq.) and emitting S1(eq.) units (solute molecule + solvent cage) were approximated using the S0 and S1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value of k(e). The calculation results fully confirm that S0 and S1 states of merocyanines can be viewed as a mixture of a neutral and a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral data (absorption and emission maxima and corresponding Stokes shifts) vs k(e) are in fairly good agreement with those of the experimental data over almost the entire range of the normalized ETN values, thus showing that specific solvent interactions are at least partly simulated within the solvaton–CS INDO scheme. The methodological prerequisites for a correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations.

Solvent influence on absorption and fluorescence spectra of merocyanine dyes: a theoretical and experimental study

A new indo-type procedure for conjugated non-rigid molecules. I. Ground-state conformations and barriers to internal rotation

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