Tuning of the excited state

In mixed ligand complexes containing bpy and an electron-rich ligand LL, [M(bpy)n(LL)3 n], n =1, 2, the lowest energy MLCT excited states are localized on bpy. The ligand LL is a spectator rather than a participant, as shown below for [Ru(bpy)2(Me2bpy)]2+  

The spectator however influences the energy gap by stabilization of the (dTC ) core by electronic donation. As LL becomes a better donor, the MLCT transition energy is decreased, as illustrated in figure 1. Using the above principle, extensive tuning in the energy of the Ru-dcbpy CT excited state has been obtained in complexes of the type [Ru(dcbpy)2X2] and [Ru(dcbpy)2(Y-bpy)] where X is a non-chromophoric ligand (e.g Cl, CN,..) and Y-bpy is a 4,4 -disubstituted-2,2 -bipyridine [22]. 

Similarly replacement of a bpy in Ru(bpy 32+ by electron-withdrawing groups can lead to increase of the energy of the MLCT transition. Scandola et al. demonstrated extensive tuning of the Ru-bpy CT transition using cyanide and methylisocyanide (CNMe) ligands in complexes of the type Ru(bpy)3 nXY, n= 1,2, X, Y= CN, CNMe [23]. The emission maximum of the Ru-bpy CT transition occurs at 2.32, 2.59, 2.78 and 3.25 eV in the complexes Ru(bpy)2(CN)2, Ru(bpy)2(CN)(CNMe), Ru(bpy)2(CNMe)2 and Ru(bpy)(CNMe)42-l- respectively. 

Shifting of the energy of the MLCT excited state has important consequences on the emission properties. In general, lowering of the energy is accompanied by decreased emission quantum yield and shorter lifetimes. Meyer et al. have demonstrated this behaviour for the CT excited state of Ru(II), Os(II) and Re(I) complexes [24], Table 4 illustrates this effect with some data on the Re(I)-carbonyl bipyridine complexes. In all these Re(I) complexes, the first reduction is bpy-based and occurs at a constant potential of -1.25 0.05 V vs. SCE. The changes in the radiative properties are due to increased occurrence of competitive non-radiative pathway. Data of this kind have been quantitatively interpreted in terms of the energy gap law . 

Both have their MLCT excited state energies nearly the same, though the first (ligand-based reduction) occurs more readily by - 0.5V. Due to better 7C-donation of the bpz ligand, the first oxidation potential is also raised nearly the same amount, causing the MLCT state energies to be approximately the same in both complexes. 

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Benincori, T., et al. 1998. Tuning of the excited-state lifetime by control of the structural relaxation in oligothiophenes. Phys Rev B 58 9082. 

Fig. 14.1 Two atoms, A and B have energy levels as shown. Initially atom A is in its excited state, and atom B is in its ground state. If the energy of the excited state of atom B could be tuned by some means, we would expect the cross section for resonant energy transfer from atom A to atom B to increase at resonance as shown on the right (from ref. 5).

The final physical method to be considered here, which allows further probing of an absorption band, is resonance Raman spectroscopy. The excitation laser wavelength is tuned into an absorption band and the vibrations enhanced in the Raman spectrum are detected. Only those vibrational modes associated with distortion of the excited electronic state relative to the ground-state geometry will be resonance enhanced. This method, therefore, not only allows observation of vibrations directly associated with the active site but also provides valuable information on the nature of the excited state. Usually, charge transfer transitions are probed due to the high intensity (e > 500 M"1 cm-1) required for resonance enhancement. These points are well illustrated by reso-... 

The largest families stem from substituted py, bipy, phen, and DAB but virtually any kind of O, N, S or P donor can be used. Both the nature of the hgands and their substituents as well as the nature of X (halide, alkoxide, alkyl, alkynyl) have been used as handles for tuning the photophysical properties of the excited state and emission of these complexes. 

The sample S is irradiated with a UV or visible light pulse (pump P) the wavelength of which is tuned to an electronic transition of the solute. The temporal behavior of the excited-state population and of the species produced is then probed with a second light pulse (probe p). The fluence of the probe pulse is much smaller than that of the pump pulse so that it does not perturb the existing populations but... 

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

The Franck-Condon factors, shown here for the process of absorption (blue line) followed by emission (green line). In a resonance Raman experiment, the LASER beam is tuned to be in resonance with an allowed electronic transition (indicated here by the blue arrow), which takes the electron from some initial state I to some intermediate state V. Unlike fluorescence, there is no vibrational relaxation of the excited state. The electron is scattered from the intermediate V state down to the i/ = 1 vibrational level of the ground electronic state, which we will call the final state F. The Franck-Condon overlap integrals involve the amount of overlap between the wave functions, which are shown on this diagram in brown. [ Mark M Samoza/CC-BY-SA 2.5/GFDL /Wimimedia Commons reproduced from http //en.wikipedia.org/wiki /Franck%E2%80%93Condon principle (accessed December 21,2013).]... 

It has been shown that in the limit of ultrashort laser pulses the stimulated-emission pump-probe signal is proportional to the population probability of the initially excited diabatic state [Tf)) Eq. (59) and Refs. 7, 99 and 141. As has been emphasized in Chapter 9, the electronic population probability P2 t) represents a key quantity in the discussion of internal-conversion processes, as it directly reflects the non-Born-Oppenheimer dynamics (in the absence of vibronic coupling, P2 t) = const ). It is therefore interesting to investigate to what extent this intramolecular quantity can be measured in a realistic pump-probe experiment with finite laser pulses. It is clear from Eq. (33) that the detection of P2(t) is facilitated if a probe pulse is employed that stimulates a major part of the excited-state vibrational levels into the electronic ground state, that is, the probe laser should be tuned to the maximum of the emission band. Figure 4(a) compares the diabatic population probability P2(t) with a cut of the stimulated-emission spectrum for uj2 3.4 eV, i.e. at the center of the red-shifted emission band. Apart from the first 20 fs, where the probe laser is not resonant with the emission [cf Fig. 2(b)], the pump-probe signal is seen to capture the overall time evolution of electronic population probability. Pump-probe experiments thus have the potential to directly monitor electronic populations and thus non-Born-Oppenheimer dynamics in real time. ... 

Figure 21 Potential energy diagram of the ground and the first excited electronic states of [Ag(CN)32 (eclipsed configuration) as plotted from extended Huckel calculations. The excimer [Ag(CN)32 corresponds to the potential minimum of the excited state. The optical transitions shown are (a) excimer emission, (b) solid state excitation and (c) dilute solution absorption. (Reproduced with permission from Omary MA and Patterson HH (1998) Luminescent homoatomic exciplexes in dicyanoargentate 0) ions doped in alkali halide crystals 1. Exciplex tuning by site-selective excitation. Journal of the American Chemical Society 120 7606-7706.

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