Nature of the Lowest Excited States

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

First, we note that the cr-cr nature of the lowest excited state of long-chain polysilanes predicted by the semiempirical calculations described in detail below implies a transition moment direction lying approximately along the chain direction rather than perpendicular to it. Evidence for such an orientation of the transition moment has been obtained in measurements on solid polysilane samples (19,36). This... 

The HOMO-LUMO gap calculated by an ah initio method using an STO-3G basis set26 seems to support these ideas. A more definitive test will be possible after the nature of the lowest excited state in carbon unsaturated systems is understood27). 

Attempts to rationalize these and other photochemical reactions have been based on the nature of the lowest excited state or the stereochemistry of substituents. A wider-ranging explanation is based on stabilization of the common intermediate zwitterion (367) by either electron-releasing substituents at C-3 and C-5 or polar solvents (79JA7521). 

In this article the results of a detailed optical spectroscopic investigation of a series of related bis-cyclometalated Rh3+ and Ir3 + complexes of the general formula [M(CnN),NnN]+, M = Rh3+, Ir3+ (HCnN = 2-phenylpyridine or 2-(2-thienyl) pyridine NnN = 2,2 -bipyridine or ethylenediamine) are summarized. The nature of the lowest excited states of the compounds is... 

Interestingly, the photoisomerizing styrylpyridine and l,2-di(4-pyridyl)-ethylene (dpe) rhenium complexes behave as luminescence switches [123-125], This phenomenon originates in the different nature of the lowest excited state of the trans and cis isomers, TI.(F) and MLCT, respectively. Near-UV irradiation converts the nonemissive trans isomer to the cis form, which shows strong phosphorescence from its 3MLCT state. Emission can be switched off by UV (254 rnn) light that induces a reverse cis—>trans isomerization. 

Importantly, the MLCT nature of the lowest excited state of [Cu(phen)2j+-type complexes has been unambiguously demonstrated by several techniques, including resonance Raman [55] and transient absorption [56] spectroscopy. 

In order to proceed it is now necessary to consider the nature of the lowest excited state of these polymers. One description which appears to be particularly appropriate to these materials is that given by the molecular exciton theory (37,38). This of course is suggested by the nature of the fluorescence spectrum itself and in addition this approach has proven to be quite successful in the Interpretation of the electronic states of the alkanes, the structural analogs of the poly(organosllylenes) ( 3, 6). The basic assumption... 

The lowest excited state of simple ketones and aldehydes corresponds to excitation of an electron from the np lone pair to the 7t -MO. The transition is forbidden in compounds of C2v symmetry. The Tocal symmetry is the same in compounds such as acetaldehyde, so that n,7t transitions of ketones and aldehydes are generally weak, 20 50 m 1 cm and they are easily overlooked in absorption spectra or hidden by the red edge of stronger 71,71 absorption. The nature of the lowest excited state is, however, decisive for the photophysical properties and the photochemical reactivity of carbonyl compounds the reactivity of n,7t excited ketones is comparable to that of alkoxy radicals (see below). 

Solvent shifts are useful as a criterion to identify n,7t transitions in absorption spectra because hydrogen bonding of protic solvents with the carbonyl oxygen stabilizes the np lone pair and gives rise to a hypsochromic shift of the n,7t absorption bands see the positions of the n,7t absorption band of acetone in heptane and water (Figure 6.5, top). This contrasts with 7t,7t transitions that tend to be shifted bathochromically in polar solvents. Also, the photophysical and photochemical properties often serve to identify the nature of the lowest excited state. Lone-pair interaction in biacetyl splits the two np-orbitals giving rise to two n,7t transitions at v = 2.23 and 3.54 im In the spectrum of 1,4-naphthoquinone in methanol, the n,7t band is barely detectable as a shoulder on the red edge of the 71,71 absorption. 

Thiochromones and Thiocoumarins. - Thiochromone is converted into a mixture of stereoisomers on irradiation in benzenoid solvents, and the difference between its behaviour and that of the sulphone has been ascribed mainly to the differing natures of the lowest excited state of each compound. Thiochromones are not reduced by sodium borohydride alone, but the AH-thiochromene is the main product in the presence of cerium(lll) chloride. 

The photoisomerization of / -ionone (114) and / -ionylidene (115) derivatives in aqueous solution was inhibited in the presence of P-CD because of the restrictions imposed on the rotation of the double bond by the cavity. The only photoreaction observed was 1,5-H migration, which was probably promoted by a change in the nature of the lowest excited state from n,it to K,7t as induced by lowering the environment polarity [300]. 

The detailed symmetry-analysis by Trindle and the author [17] of the parent reaction is summarized concisely in Fig. 10.3. Two 0 atoms, two C atoms and two H atoms provide twenty-two valence electrons altogether. Ten of these six in the CC and CH bonds of ethylene and four oxygen lone-pair electrons - need not be considered explicitly, because the five orbitals housing them retain their symmetry properties across the diagram. Fig. 10.3 makes do with six MOs for the closed shell dioxetene intermediate these are supplemented by one more in the reactants and product, where open shell states have to be taken into account. The ordering of the orbitals in the three species is intuitive, but - apart from the upper two, that determine the nature of the lowest excited states - it is immaterial. 

Fig. 3.4 Tuning the nature of the lowest excited state in Pt(II) chromophores. The charge of the compounds with cyclometallating ligands (right hand side) depends on the ligand...

These examples demonstrate the wealth of excited states in metal chromoph-ores and how subtle changes in structure can alter the nature of the lowest excited state and consequently the overall light-induced properties. 

S Tuning the Nature of the Lowest Excited State by the Nature of the Ligand... 

These differences in the nature of the lowest excited state arise from the balance between the relative orbital energies of the M2(i5) or M2(i5 ) and the ligand n orbitals, as well as the magnitudes of the orbital overlap. Such differences in the nature and the degree of delocalisation between singlet and triplet excited states, fine-tuned by ligand design in a set of structurally similar M2 compounds, demonstrates the breadth of excited states and associated potential reactivity in qua-druply-bonded complexes. 

In the examples above, the reactivity of the -N=N- unit was exploited. A recent example discusses an N2-bridged Os dimer in which the -NN-bridge largely preserves its triple bond character. It demonstrates how homoleptic N2-sphtting can be achieved photochemically with the use of Os complexes, as well as how tuning of the nature of the lowest excited state can dramatically alter the photochemical properties and the nature of the reaction products [106]. 

Starcke JH, Wormit M, Dreuw A (2009) Nature of the lowest excited states of neutral polyenyl radicals and polyene radical cations. J Chem Phys 131 144311... 

This reaction has been extended further to other porphyrins, and also to other axial ligands such as DMSO, THF, and amines. A similar photodecarbonylation is observed when the osmium(II) carbonyl complex Os(OEP)(CO)py is photolyzed in the presence of pyridine. Room temperature transient absorption studies on ruthenium(II) porphyrin carbonyl complexes show that there are significant differences in the nature of the lowest excited state for series of complexes having different axial ligands. For the complexes Ru(OEP)L2 and Ru(TPP)L2, where L is a (T-donor ligand such as pyridine, the complexes exhibit charge transfer... 

The nature of the lowest excited state Ao is still subject of discussion. The position of this band is independent of the preparation conditions and the film thickness and therefore independent of the form of aggregates. The most probable explanation attributes Ao to the 0-0 vibronic band of the 1 Ag —> 1 Bu transition. For a more detailed discussion compare [250] and Chapter 6 of this book [28]. 

In addition to the absorption, PL spectra were also analysed to disentangle the nature of the lowest excited state. The general luminescence properties of thin T6 films were quite poor and a typical example is shown in Fig. 12 [149, 150]. By going from solution to thin films the PL quantum yields decreases by three orders of magnitude [84] and apparently broad emission lines dominate the spectra. Even at very low temperatures the resolution of the optical spectra is rather poor (several 100 cm ) and spectroscopic details are smeared out. In most cases a considerable red-shift between the absorption and PL onsets and multiple different PL-components could be found within the spectra (Fig.l2) [149, 151]. The main radiative decay channels were attributed to deep trap levels [149] or aggregates [8], which are strongly depending on the preparation conditions and film thicknesses (Fig. 12) [149]. By site selective PL-spectroscopy the positions of at least three trap levels could be located which are up to 2 000 cm" lower than the absorption onset... 

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