ABSTRACT
Preparing electronically excited trans-stilbene molecules in deuterated chloroform using both one-photon excitation and excitation through an intermediate vibrational state explores the influence of vibrational energy on excited-state isomerization in solution. After infrared excitation of either two quanta of C-H stretch vibration |2ν(CH)> at 5990 cm(-1) or the C-H stretch-bend combination |ν(CH) + ν(bend)> at 4650 cm(-1) in the ground electronic state, an ultraviolet photon intercepts the vibrationally excited molecules during the course of vibrational energy flow and promotes them to the electronically excited state. The energy of the infrared and ultraviolet photons together is the same as that added in the one-photon excitation. Transient broadband-continuum absorption monitors the lifetime of electronically excited molecules. The lifetime of excited-state trans-stilbene after one-photon electronic excitation with 33,300 cm(-1) of energy is (51 +/- 6) ps. The excited-state lifetimes of (55 +/- 9) ps and (56 +/- 7) ps for the cases of excitation through |2ν(CH)> and |ν(CH) + ν(bend)>, respectively, are indistinguishable from that for the one-photon excitation. Vibrational relaxation in the electronically excited state prepared by the two-photon excitation scheme is most likely faster than the barrier crossing, making the isomerization insensitive to the method of initial state preparation.
ABSTRACT
The spectroscopic behavior of mono- and dinuclear Ru(II) complexes (P, T, PP and TT, Figure 1) that contain the extended planar ligand tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' '',3' ''-j]acridine (TPAC) and either 1,10-phenanthroline (phen) or 1,4,5,8-tetraazaphenanthrene (tap) as ancillary ligands is examined in water and as a function of the pH. These four complexes luminesce in aqueous solution. The analyses of the data in absorption lead to the pKa values in the ground state, and the data in emission show that the excited 3MLCT states are much more basic than the ground state. When the complex contains tap ligands (T and TT), a decrease in pH transforms the luminescent excited basic form into another luminescent excited protonated species, which emits more bathochromically. In contrast, with phen ancillary ligands (P and PP), the protonated excited state does not luminesce. The rate constant of first protonation of the 3MLCT state is diffusion controlled, except for the dinuclear PP complex, whose protonation takes place on the nitrogen of the acridine motif. For P, in which the protonation process is the fastest, it would take place on the nitrogen atoms of the nonchelated phen moiety of the TPAC ligand. These results allow also us to gain information on the localization of the excited electron in the 1MLCT state populated upon absorption as well as in the relaxed 3MLCT emissive state. Moreover as these complexes are interesting for their study with DNA, it can be concluded from these data that a portion of the excited species in interaction with DNA will be protonated.
Subject(s)
Acridines/chemistry , Organometallic Compounds/chemistry , Ruthenium/chemistry , Spectrophotometry/methods , Absorption , Chemistry, Physical/methods , DNA/chemistry , Hydrogen-Ion Concentration , Ligands , Models, Chemical , Protons , Temperature , Thermodynamics , Water/chemistryABSTRACT
Four novel dinuclear RuII compounds and, for comparison purposes, two corresponding mononuclear complexes containing the PHEHAT or TPAC ligand (PHEHAT=1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene and TPAC=tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' '',3' ''-j]acridine) have been synthesized and characterized. Conclusions on the effects of dinucleation of these two bridging ligands can be drawn only for the compounds for which the results demonstrate that the bridging ligand is involved in the first electrochemical reduction and lowest emission energy. The behavior of these complexes, which is not always predictable, is discussed, and the differences are highlighted in this work. Interestingly, all of the compounds are luminescent except one dinuclear species, [(phen)2Ru-mu-PHEHAT-Ru(TAP)2]4+, which does not luminesce in MeCN and BuCN at room temperature.
