Second-derivative method: Application to vibrational spectroscopy in excited electronic states

The second-derivative method, which has been known to be useful for finding peaks overlapped in a broad band, is applied to u.v. and visible absorption bands to derive vibrational frequencies in excited electronic states of molecules in solution. By using this method, much better estimates of vibrational frequencies are possible for Franck—Condon active modes than can be made from the absorption spectrum itself. Computational procedure and the results obtained for trans-1,3,5-hexatriene, azulene, pyrazine and anthracene are described with remarks about the effectiveness and limitations of the second-derivative method.     

Deactivation pathways for metal-to-ligand charge-transfer excited states of ruthenium polypyridyl complexes with triphenylphosphine as a ligand

Emission, excitation spectra, quantum yields, and emission lifetimes are reported for the mixed-ligand, bis(2.2'-bipyridine)ruthenium(II) complexes, cis-[Ru(bpy)(2)(PPh(3))X](n+) with X = Cl(-), Br(-), CN(-), and NO(2)(-) (n = 1) and pyridine (py), 4-aminopyridine (NH(2)py), 4,4'- bipyridine (4,4'-bpy), NH(3), and MeCN (n = 2) in EtOH-MeOH, 4:1 (v:v), at 77 K. Radiative, k(r), and nonradiative, k(nr), decay rate constants were determined for the series of complexes, and a linear dependence of ln k(nr) on E(00), with E(00) being the 0-0 energy gap determined by emission spectral fitting, was obtained with a slope of -(0.6 ± 0.1) × 10(-3). On the basis of emission quantum yields and apparent k(r) values, possible metal-to-ligand charge-transfer (MLCT) deactivation by direct population of excited (1)dd states from initially excited (1)MLCT states is discussed.     

Self-assembly of heterobimetallic d-f hybrid complexes: sensitization of lanthanide luminescence by d-block metal-to-ligand charge-transfer excited states

Carboxylate-bearing d-transition metal bipyridyls form ternary complexes with seven-coordinate lanthanide centers. The neodymium,- ytterbium-, and erbium-containing complexes exhibit lanthanide-centered emissions sensitized by the MLCT states of the d-block components.     

Pump-probe spectroscopy of molecules driven by infrared field in both ground and excited electronic states

Pump-probe spectra of molecules driven by strong infrared (IR) field in both ground and excited states are studied theoretically. The role of the final state interaction becomes important when pump and probe pulses overlap, and the Rabi frequency is comparable with the lifetime broadening of the excited state and the duration of the pump pulse. Our theoretical approach is applied to x-ray absorption of nitrogen molecule and valence photoionization of carbon monoxide. It is shown that IR-x-ray pump-probe spectroscopy can directly evidence the delocalization of core hole.     

Contrasts in the 77 K emission spectra, structures, and dynamics of metal-to-metal and metal-to-ligand charge-transfer excited states

The 77 K emission spectrum of trans-[(ms-Me6[14]aneN4)Cr(CNRu(NH3)5)2]5+ has components characteristic of ligand field (LF) and metal-to-metal charge transfer (MMCT) excited states (ms-Me6[14]aneN4=5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). The LF component of the emission is best resolved for irradiations at appreciably higher energies than the MMCT absorption band, while only the MMCT emission is observed for irradiations on the low-energy side of the MMCT absorption band. The LF emission component from this complex has vibronic structure that is very similar to that of the trans-[(ms-Me6[14]aneN4)Cr(CN)2]+ parent, but it is red-shifted by 560 cm-1 and the bandwidths are much larger. The red shift and the larger bandwidths of the ruthenated complex are attributed to configurational mixing between the LF and MMCT excited states, and the inferred mixing parameters are shown to be consistent with the known electron-transfer properties of the Ru(NH3)5 moieties. The MMCT excited-state lifetime is about 1 micros at 77 K and am(m)ine perdeuteration of this complex leads to an isotope effect of kNH/kND approximately 15-20. However, the contribution of the N-H stretching vibration to the emission sideband is too weak for a single vibrational mode model to be consistent with the observed lifetimes or the isotope effect. These features are very similar to those reported previously (J. Phys. Chem. A 2004, 108, 5041) for the MMCT emission of trans-[([14]aneN4)Cr{CNRu(NH3)5}2]5+ ([14]aneN4=1,4,8,11-tetraazacyclotetradecane), with the exception that the higher energy LF emission was not well resolved in the earlier work. The energies of the charge transfer absorption and emission maxima of both of these Cr(CN)Ru complexes are very similar to those of [Ru(NH3)4bpy]2+, but the latter has a 50-fold shorter 77 K excited-state lifetime, a 10-fold smaller NH/ND isotope effect, and a very different structure of its vibronic sidebands. Thus, the vibronic sidebands imply that the dominant excited-state distortions are in the metal-ligand vibrational modes for the Cr(CN)Ru complexes and in the bipyridine vibrational modes for the [Ru(NH3)4bpy]2+ complex. While an "equivalent" single vibrational mode model based on the frequencies and amplitudes of the dominant distortion modes is not consistent the observed lifetimes, such models do appear to be a good basis for qualitatively distinguishing different classes of excited-state dynamic behavior. A multimode, multichannel model may be necessary to adequately describe the excited-state dynamics of these simple electron-transfer systems.     

Vibronic coupling in excited charge-transfer states of solid-state charge-transfer complexes

Symmetry properties of CT excited states of some weak donor-acceptor complexes are discussed in the context of vibronic coupling with intermolecular vibrations. The results are applied to the analysis of electroabsorption spectra of anthracene-PMDA.     

Switching between ligand-to-ligand charge-transfer, intraligand charge-transfer, and metal-to-ligand charge-transfer excited states in platinum(II) terpyridyl acetylide complexes induced by pH change and metal ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2'-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.     

Rigid medium stabilization of metal-to-ligand charge transfer excited states

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.     

Monitoring the effect of ultrafast deactivation of the electronic excited states of DNA bases and polynucleotides following 267 nm laser excitation using picosecond time-resolved infrared spectroscopy

In this paper we demonstrate the use of picosecond time-resolved infrared spectroscopy (ps-TRIR) to monitor the early structural dynamics of DNA bases and polydeoxynucleotides following UV excitation in solution.     

Contribution of charge-transfer complexes to the structure of electronic states of dibenzotetraazaannulene in the crystalline phase

Based on analysis of the electronic absorption spectra and study of thin films, we have established the characteristic features of electronic processes involving participation of intermolecular charge transfer processes in crystalline dibenzotetraazaannulene. We have shown that the specific spectral characteristics of this compound are connected with the presence of pre-dimer pairs of molecules in the solid state.L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Nauki Prospekt, Kiev 252039, Ukraine. Translated from Teoreticheskaya i Éksperimertal'naya Khimiya, Vol. 32, No. 3, pp. 163–167, May–June, 1996. Original article submitted July 4, 1995.     

Localized excited triplet states in mixed charge-transfer single crystals: formation of excited multicomplexes

Single crystals of charge-transfer (CT) complexes between tetracyanobenzene as acceptor and different aromatic donors were doped with guest donors. The molecular arrangements of the guest CT complexes forming triple energy traps in the host crystal were determined from the triplet ESR spectra of the traps. A method for the determination of relative charge-transfer triplet energies is proposed. Extended electron delocalization over more than one donor-acceptor pair has been found.     

2-Aminopurine excited state electronic structure measured by stark spectroscopy

2-Aminopurine (2AP) is an adenine analogue that has a high fluorescence quantum yield. Its fluorescence yield decreases significantly when the base is incorporated into DNA, making it a very useful real-time probe of DNA structure. However, the basic mechanism underlying 2AP fluorescence quenching by base stacking is not well understood. A critical element in approaching this problem is obtaining an understanding of the electronic structure of the excited state. We have explored the excited state properties of 2AP and 2-amino,9-methylpurine (2A9MP) in frozen solutions using Stark spectroscopy. The experimental data were correlated with high level ab initio (MRCI) calculations of the dipole moments, mu0 and mu1, of the ground and excited states. The magnitude and direction of the dipole moment change, Deltamu01 = mu1 - mu0, of the lowest energy optically allowed transition was determined. While other studies have reported on the magnitude of the dipole moment change, we believe that this is the first report of the direction of Deltamu, a quantity that will be of great value in interpreting absorption spectral changes of the 2AP chromophore. Polarizability changes due to the transition were also obtained.     

Observation of triplet intraligand excited states through nanosecond step-scan Fourier transform infrared spectroscopy

Nanosecond step-scan Fourier transform infrared spectroscopy permits the observation of triplet intraligand ((3)IL) character in the excited states of [Ru(bpy)2(PNI-phen)]2+ and [Ru(PNI-phen)3]2+ where PNI is 4-piperidinyl-1,8-naphthalimide. After pulsed 355-nm laser excitation, the two ground-state imide C=O bands in each compound are bleached and two substantially lower energy vibrations are produced; the lower energy feature appears as two distinct bands split by an overlapping transient bleach. Model studies confirm that the time-resolved vibrational data are consistent with photoinduced sensitization of the 3IL excited state. Density functional theory calculations also support these assignments because localization of triplet electron density on the PNI moiety is expected to lead to red-shifted C=O vibrations of magnitude similar to those measured experimentally. The current results illustrate that triplet electron density can be directly tracked by time-resolved infrared measurements in metal-organic chromophores and that frequency shifts comparable to those observed in charge-transfer systems can be realized.     

Ultrafast vibrational dynamics observed in higher electronic excited states of iodine using pump-UV DFWM spectroscopy

By using a combination of an initial pump pulse and a degenerate four-wave mixing process, we show that an interrogation of the vibrational dynamics occurring in different electronic states of molecules is possible. The technique is applied to iodine. The initial pump pulse is used to populate the B((3)Pi) state of molecular iodine in the gas phase. Now, by using an internal time delay in the DFWM process, which is resonant with the transition between the B state and a higher lying ion-pair state, the vibrational dynamics of the B state and the ion-pair state could be observed. States of even symmetry are investigated, which are accessed by a one photon transition from the B state. By a proper choice of the wavelengths used for the pump and DFWM beams, the dynamics of ion-pair states belonging to two different tiers are monitored.     

Probing the excited states of d6 metal complexes containing the 2,2'-bipyrimidine ligand using time-resolved infrared spectroscopy. 1. Mononuclear and homodinuclear systems

This paper reports time-resolved infrared (TRIR) spectroscopic studies on a series of weakly luminescent or nonluminescent 2,2'-bipyrimidine-based complexes to probe their electronic structure and the dynamic behavior of their excited states on the picosecond and nanosecond time scales. The complexes are mononuclear [Re(CO)3Cl(bpm)] (1), [Ru(CN)4(bpm)]2- (2), and [Ru(bpyam)2(bpm)]2+ (3) [bpm=2,2'-bipyrimidine; bpyam=2,2'-bipyridine-4,4'-(CONEt2)2] and their homodinuclear analogues [{Re(CO)3Cl}2(mu-bpm)] (4), [{Ru(CN)4}2(mu-bpm)]2- (5), and [{Ru(bpyam)2}2(mu-bpm)]4+ (6). Complex 1 shows the characteristic shift of the three nu(CO) bands to higher energy in the Re-->bpm triplet metal-to-ligand charge-transfer (3MLCT) state, which has a lifetime of 1.2 ns. In contrast, the dinuclear complex 4 shows nu(CO) transient bands to both higher and lower energy than the ground state indicative of, on the IR time scale, an asymmetric excited state [(OC)3ClReI(bpm*-)ReII(CO)3Cl] whose lifetime is 46 ps. The cyanoruthenate complexes 2 and 5 show comparable behavior, with a shift of the nu(CN) bands to higher energy in the excited state for mononuclear 2 but two sets of transient bands-one to higher energy and one to lower energy-in dinuclear 5, consistent with an asymmetric charge distribution [(NC)4RuII(bpm*-)RuIII(CN)4]4- in the 3MLCT state. These cyanoruthenate complexes have much longer lifetimes in D2O compared with CH3CN, viz., 250 ps and 3.4 ns for 2 and 65 ps and 1.2 ns for 5 in CH3CN and D2O, respectively. In complex 3, both higher-energy Ru-->bpyam and lower-energy Ru-->bpm 3MLCT states are formed following 400 nm excitation; the former decays rapidly (tau=6-7 ps) to the latter, and the subsequent decay of the Ru-->bpm 3MLCT state occurs with a lifetime of 60 or 97 ns in D2O or CH3CN, respectively. Similar behavior is shown by dinuclear 6 in both D2O and CH3CN, with initial interconversion from the Ru-->bpyam to the Ru-->bpm 3MLCT state occurring with tau approximately 7 ps and the resultant Ru-->bpm 3MLCT state decaying on the nanosecond time scale.     

Study on excited states of hexamethoxybenzene-NO+ charge-transfer complex: incorporation of excited radical cation

A charge-transfer (CT) complex of NOBF₄ and hexamethoxybenzene (HMB), which gives out HMB^?+ as a “fluorescent radical cation probe,” upon one-electron oxidation, has been designed to explore the excited state dynamics of contact radical ion pairs by laser-induced fluorescence and femtosecond transient absorption spectroscopic techniques. The acetonitrile solution of the CT complex showed weak fluorescence with a similar spectrum to that observed for free excited HMB radical cation (HMB^?+*), suggesting the formation of HMB^?+* upon the one-photonic excitation of the CT complex. The laser-power dependence of the fluorescence intensity supported the one-photonic excitation event. We have also observed a short-lived transient species but no long-lived species by femtosecond laser flash photolysis of the CT complex. The lifetime (6.5 ps) was in good accordance with its fluorescence quantum yield (2.5 × 10^?5) and was able to assign the transient species to the fluorescent state, an excited radical ion pair [HMB ^?+*/NO^?]. All the events were completed within the inner sphere and the short lifetime of the transient species could be attributed to rapid back-electron transfer. It is concluded that the excited radical cation character in the excited state of the CT complex originates from the radical ion character in the CT complex in the ground state and that a relatively long lifetime of HMB^?+* facilitates its observation even in the contact ion pair.     

The low-lying electronic excited states of NiCO

Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X 1Sigma+, and for the 3Delta, 3Sigma+, 3Phi, 1 3Pi, 2 3Pi, 1Sigma+, 1Delta, and 1Pi excited states of NiCO. The X 1Sigma+ ground state's geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of pi and sigma bonding for the Ni-C bond is discussed based on the structures of excited states.     

Nature and location of excited charge-transfer states in the porphine-magnesium porphine dimer

The ground and excited states of a cofacial porphine-magnesium porphine dimer with a ring separation of 5.35 Å are investigated by ab initio configuration interaction calculations, using a floating gaussian basis. A pair of charge-transfer states are found ≈23000 cm^?1 above the ground state, but are lowered by ≈7400 cm^?1 upon coordination of the Mg atom with chloride ion.     

The electronic structure of low-lying excited states of two model systems of retinal

2,4-pentadienal and 2,4,6,8-nonatetraenal were studied as simple model systems of retinal. Four kinds of CI were performed on low-lying excited states of 2,4-pentadienal by using a split valence basis set. The results show that MR SD π CI is not adequate for the π–π* state and the single excitation σπ CI is a good compromise between cost and effectiveness as far as singly excited states are concerned. This CI was applied to the bigger model system. All-trans and 11-cis forms of aldehyde, Schiff base, and protonated Schiff base of the model system were studied. A fairly large energy lowering of about 1 eV of the first allowed excited state (π → π*) occurs when the Schiff base is protonated for both all-trans and 11-cis forms.