Internal Charge Transfer State Research Articles

Temperature and cholesterol composition-dependent behavior of 1-myristoyl-2-[12-[(5-dimethylamino-1-naphthalenesulfonyl)amino]dodecanoyl]- sn-glycero-3-phosphocholine in 1,2-dimyristoyl- sn-glycero-3-phosphocholine membranes

We present a steady-state and time-resolved fluorescence emission spectra analysis of the membrane probe 1-myristoyl-2-[12-[(5-dimethylamino-1-naphthalenesulfonyl)amino]dodecanoyl]- sn-glycero-3-phosphocholine (DANSYL) in 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and cholesterol multi-lamellar vesicles (MLV) prepared by modified rapid solvent exchange. We report that the dose-dependent cholesterol-induced blue shifts in the steady-state fluorescence emission spectra observed in DMPC MLV are due to complex solvent effects that include time-dependent dipolar relaxation and the formation of internal charge transfer (ICT) states. A key finding of this investigation is identification of two distinguishable DANSYL populations existing at both shallow and deep locations in the membrane; these two DANSYL populations are evidence of laterally phase-separated domains at cholesterol compositions between X chol = 0.30 and 0.60 at 30 °C in DMPC MLV.

The B–N bond controls the balance between locally excited (LE) and twisted internal charge transfer (TICT) states observed for aniline based fluorescent saccharide sensors

Three boronic acid based saccharide sensors with an aniline fluorophore have been prepared. One of the systems ( 1a ) contains an intramolecular boron–nitrogen (B–N) bond and displays fluorescence due to both LE and TICT states. The other two systems ( 1b and c ) have no B–N bond and only show fluorescence due to the LE state.

Ab initio study on the photochemical behavior of 4-dimethylamino,4 ′-cyanostilbene

Ab initio complete active space self-consistent field (CASSCF) and the second order perturbation (MRMP2) calculations have been performed to examine the photochemical behavior of 4-dimethylamino,4 ′-cyanostilbene (DCS) in the low-lying excited states. The potential energy curves with respect to four torsional angles at the CASSCF level is possible to give a reasonable interpretation on the experimental findings and the MRMP2 calculation well reproduces the experimental observable such as the excitation energy. The S 1 state in the Franck–Condon region is the internal charge transfer (ICT) state where the electron is partially transferred from the 4-dimethylanilino into the 4-cyanostyryl groups. Only the potential energy curve in S 1 with respect to the torsional motion of the ethylenic double bond has a small barrier to the perpendicularly twisted conformation leading to the cis–trans photoisomerization, though the others becomes energetically much unstable. DCS in S 1 in polar solvent is, on the other hand, stabilized by the torsional motion of the 4-dimethylanilino group.

Observation du processus de transfert de charge intramoléculaire dans le 4-dimethylamino-4'-nitrostilbene (DNS) par spectroscopie CARS résolue en temps

Charge Transfert (CT) process of 4-dimethylamino-4'-nitrostilbene in solution has been studied by pump-probe and transient CARS experiments with a 1 ps time resolution. While pump-probe experiments give good evidence for formation of CT state in polar solvent, CARS experiments reveal structural changes associated to CT process. In acetonitrile, a 37 cm -1 downshift of the NO 2 stretching mode is assigned to conformational relaxation of the Internal Charge Transfer state. In toluene such behavior is not observed correlated to the absence of CT stabilization in non-polar solvent.

Multiple fluorescences anomaly of 9,9′-bianthryl in solutions

UV–VIS fluorescence and excitation spectra of 9,9′-bianthryl (BA) in methylcyclohexane (MCH), n-butylchloride (NBC) and acetonitrile (CH 3CN) were measured at room and liquid nitrogen temperatures. Three fluorescence bands have been observed depending on the solvent polarity and temperature. In addition, approximate excited state dipole moments have been calculated, and found such as: μ e=0 D (debye) (in NBC at 77 K ), 11≤ μ e≤14.43 D (in all solvents at room temperature and in CH 3CN at 77 K), and μ e≈19.70 D (in CH 3CN at room temperature). On the basis of these results, a ‘multiple states’ model has been suggested. So, the fluorescences observed can originate either from: (i) a locally excited state (LE) with no charge transfer and with a twist angle of 90°, around the C 9–C 9′ bond; (ii) a partial intramolecular charge transfer (PICT) state with an intermediate twist angle; (iii) a twisted internal charge transfer state (TICT) with a full charge transfer and with a twist angle of 90°.

A fluorescent temperature probe based on the association between the excited states of 4-(N,N-dimethylamino)benzonitrile and beta-cyclodextrin.

A temperature probe based on the fluorescence properties of the two excited states of 4-(N,N-dimethylamino)benzonitrile (DMABN) in equilibrium with beta-cyclodextrin (CD) in aqueous solution is presented. The fluorescence intensity of the Franck-Condon excited state (FB) as a function of temperature shows a straight line with a correlation better than 0.99 in the 283-308 K temperature interval. On the other hand, the fluorescence intensity of the twisted internal charge-transfer state (FA) remains constant in the same temperature interval because the binding of DMABN in the A* state to CD is isoenthalpic and entropy driven. It is found that the FA/FB ratio is independent of the excitation intensity at a specified temperature, shows a linear relationship with temperature, and allows temperature measurements with a resolution of +/- 2.5 K.

PM3 study of the electronic spectra of some substituted aminocoumarins

The photophysical properties of some 7-aminocoumarins have been investigated by PM3/CI method. Solvent effects have been evaluated by the continuum reaction field approach. The results show that all the molecules emit from a planar internal charge transfer (ICT) state. Radiationless decay occurs via the twisted ICT state, no evidence of a deactivation path via pyramidalization of the amino group has been found. Computed potential energy barriers for the torsional motion of the amino group are in line with the observed rate constants for nonradiative decay.

Analysis of electron correlation and vibronic coupling in the reaction center of bacterial photosynthetis

Approximate normal modes for the special pair dimer are calculated on the PM3 level. Their couplings to the lowest electronically excited states of the dimer and its cation are deduced from results based on INDO-CI calculations including electron correlation effects. Several modes are found which couple strongly to the internal charge-transfer states and are important for the internal relaxation after photoexcitation and the consecutive formation of the dimer cation.

Dual excited states in 4- dimethylamino 4′-cyanostilbene (DCS) revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-dimethylamino 4′-cyanostilbene (DCS) and p-dimethylamino, p′-cyano, 1-1′-bi-indanylidene (substituted “stiff stilbene” DCS-1324) are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentration, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in non-polar solvent (cyclohexane), only the presence of a single excited state is observed for the two studied compounds. In polar solvents, DCS-1324 derivative where twisting of the anilino moiety is prevented but double-bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the delocalized excited (DE) state, the internal charge transfer (ICT) state (highly polar, nearly planar configuration, formed quasi instantaneously from the DE state by electronic reorganization), the conformational relaxed ICT (CRICT) state (highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P ∗ on the trans ↔ cis isomerization pathway (twisted double bond, lowly polar, non-fluorescent).

Investigation of excited-state charge transfer with structural change in compounds containing anilino subunits by subpicosecond spectroscopy

Subpicosecond absorption spectroscopy and picosecond streak camera fluorometry are applied to the study of the photoinduced processes in three classes of compounds for which molecular torsion with charge localization leading to an emissive or non-emissive twisted internal charge transfer state was proposed. The compounds are classified according to their steady-state fluorescence properties: compounds having a single band without solvatochromism or a single band with solvatochromism and compounds exhibiting dual fluorescence in polar solvents. The selected compounds are di- and tri-phenylmethane and amino-rhodamine dyes for the first class, the laser dye DCM for the second class and a substituted triphenylphosphine for the third class. These compounds contain at least one electron-donating anilino-group able to rotate. In all cases, the transient spectroscopy gives evidence for an excited-state relaxation involving the formation of an intermediate state. The changes observed in the kinetics by changing the solvent viscosity and polarity and upon donor protonation bring support to a mechanism involving photoinduced charge transfer with structural change. The kinetics are tentatively discussed within the context of theories of electron transfer reactions in the condensed phase.

Photoinduced Electron Transfer in Pentaammineruthenium(II) Complexes of 1-(4-Cyanophenyl)imidazole.

A new bridging ligand, 1-(4-cyanophenyl)imidazole (CPI) has been prepared, as well as its N-methylated derivative 1-methyl-3-(4-cyanophenyl)imidazolium iodide (CPI-Me(+)I(-)). The mononuclear and binuclear complexes [(NH(3))(5)Ru-CPI-Me](3+) and [(NH(3))(5)Ru-CPI-Ru(NH(3))(5)](4+) have been obtained. Free CPI is planar, according to theoretical calculations (MMX and MNDO), and its luminescence properties suggest the occurence of a twisted internal charge transfer (TICT) state. The comparison of the two ruthenium complexes reveals the spectral and electrochemical features of coordination by the cyanophenyl or by the imidazole groups. Controlled oxidation of the binuclear complex [(NH(3))(5)Ru-CPI-Ru(NH(3))(5)](4+) yields the mixed valence species [(NH(3))(5)Ru-CPI-Ru(NH(3))(5)](5+) in which the ruthenium coordinated to the cyanophenyl group is ruthenium(II) while the ruthenium linked to imidazole is ruthenium(III). An intervalence band is observed at 640 nm (epsilon = 188), from which the effective metal-metal coupling through the bridging ligand is determined as 0.032 eV. This value is satisfactorily reproduced by a theoretical calculation using the effective Hamiltonian theory. Finally the binuclear complex exhibits a weak luminescence when excited either on the ligand band near 260 nm or on the metal-to-ligand charge transfer band near 410 nm. The CPI ligand is the first example of a TICT-forming species with appreciable coupling between metallic sites and can be considered as a first step toward a molecular switch.

Ultrafast Investigation of Vibrational Relaxation in the S1Electronic State of HITC

The condensed phase relaxation dynamics of electronically excited 1,1‘,3,3,3‘,3‘-hexamethylindotricarbocyanine iodide HITC (a cyanine dye) has been examined by transient bleach/stimulated emission experiments. These measurements were performed using tunable pump and probe laser pulses with ∼200 fs time resolution. The dynamics observed was assigned to vibrational relaxation in the S1 state of HITC. Solvation effects make a negligible contribution to these experiments because the dipole moment of HITC only changes by a small amount when the S1 ← S0 transition is excited. Experiments performed with variable wavelength pump and probe pulses show that vibrational relaxation is faster at high energies in the S1 state. At low energies in the S1 state the vibrational relaxation times depend on the solvent. The measured relaxation times at low energies are1.7 ps in acetonitrile, 3.2 ps in dimethyl sulfoxide, 2.4 ps in methanol, 3.5 ps in ethanol, 7.1 ps in 1-butanol, and 6.4 ps in ethylene glycol. These results show that the vibrational relaxation rate decreases with solvent viscosity and increases with solvent dipole moment. To explain these observations, we propose that the torsional motion associated with isomerization in the S1 state of HITC is responsible for vibrational deactivation at low energies. Rotation about the central C−C bond in the HITC polyene chain produces an internal charge transfer state, creating a large dipole moment along the long axis of the molecule. Thus, the torsional vibration generates an oscillating dipole which can couple to the dipole moments of the solvent molecules, providing a mechanism for energy exchange between HITC and the solvent. The rate of energy exchange will increase with the solvent dipole moment and decrease with the solvent viscosity because highly viscous solvents hinder the torsional vibration and, so, reduce the magnitude of the induced dipole moment in HITC.

Proton and charge transfer in the intercalating antitumour drug pazelliptine

The photophysical properties of the synthetic drug pazelliptine (PZE) have been investigated in order to characterize the geometric and electronic structure of the molecule bound to DNA. Whatever the pH of the solution, a proton tranfer reaction occurs in the excited state: this leads to the excited 9-N monoprotonated form of PZE. Deexcitation of this excited species is mainly non-radiative. A study of the fluorescence properties of sterically hindered derivatives allows us to propose the existence of a twisted internal charge transfer state to explain this non-radiative deexcitation. The formation of this state occurs on the picosecond timescale (τ∼ 200 ps) when PZE is fully protonated ring nitrogens (i.e. in acidic aqueous solutions).The acido-basic properties of PZE and five related amino and amino-substituted derivatives have been previously studied by spectroscopic measurements. The absorption, fluorescence spectra and the fluorescence quantum yields as a function of pH (range 2–12) and buffer concentration (50 mmol dm–3 to 1 mol dm–3) have been measured. Three pKa values have been determined, at 5.5, 6.5 and 9.2. This is good evidence for PZE having an appropriate pKa when bound to a hydrophobic and/or a hydrophilic structure.

Effects of hydrogen bonding to a bacteriochlorophyll-bacteriopheophytin dimer in reaction centers from Rhodobacter sphaeroides.

The properties of the primary electron donor in reaction centers from Rhodobacter sphaeroides have been investigated in mutants containing a bacteriochlorophyll (BChl)--bacteriopheophytin (BPhe) dimer with and without hydrogen bonds to the conjugated carbonyl groups. The heterodimer mutation His M202 to Leu was combined with each of the following mutations: His L168 to Phe, which should remove an existing hydrogen bond to the BChl molecule; Leu L131 to His, which should add a hydrogen bond to the BChl molecule; and Leu M160 to His and Phe M197 to His, each of which should add a hydrogen bond to the BPhe molecule [Rautter, J., Lendzian, F., Schulz, C., Fetsch, A., Kuhn M., Lin, X., Williams, J. C., Allen J. P., & Lubitz, W. (1995) Biochemistry 34, 8130-8143]. Pigment extractions and Fourier transform Raman spectra confirm that all of the mutants contain a heterodimer. The bands in the resonance Raman spectra arising from the BPhe molecule, which is selectively enhanced, exhibit the shifts expected for the addition of a hydrogen bond to the 9-keto and 2-acetyl carbonyl groups. The oxidation--reduction midpoint potential of the donor is increased by approximately 85 mV by the addition of a hydrogen bond to the BChl molecule but is only increased by approximately 15 mV by the addition of a hydrogen bond to the BPhe molecule. An increase in the rate of charge recombination from the primary quinone is correlated with an increase in the midpoint potential. The yield of electron transfer to the primary quinone is 5-fold reduced for the mutants with a hydrogen bond to the BPhe molecule. Room- and low-temperature optical absorption spectra show small differences from the features that are typical for the heterodimer, except that a large increase in absorption is observed around 860-900 nm for the donor Qy band in the mutant that adds a hydrogen bond to the BChl molecule. The changes in the optical spectra and the yield of electron transfer are consistent with a model in which the addition of a hydrogen bond to the BChl molecule increases the energy of an internal charge transfer state while the addition to the BPhe molecule stabilizes this state. The results show that the properties of the heterodimer are different depending on which side is hydrogen-bonded and suggest that the hydrogen bonds alter the energy of the internal charge transfer state in a well-defined manner.

On the validity of the standard model for primary charge separation in the bacterial reaction center

In the standard model the primary electron donor state (P∗) is localized on the special pair, meaning that the coupling between the lowest excited state (P_) of the special pair and charge transfer states involving radical anions of accessory cofactors is weak. This model is scrutinized using hole burning data for bacterial and green plan reaction centers (RC), antenna protein complexes and ππ∗ states of probe molecules imbedded in glasses and polymers. Attention is focused on the linear electron-phonon coupling strength (optical reorganization energy) and the dynamics afforded by the zero-phonon hole of the P∗ ← P transition. The hole burning data, together with the structural differences between the special pair of bacterial and green plant RC as well as quantum chemical calculations pertaining to the contribution of internal charge transfer states (of P) to P_, indicate that the standard model is invalid. A model is proposed that has P_ strongly coupled to a quasi-degenerate P+BL− state, where BL is the necessary bacteriochlorophyll on the L branch of the RC. In the adiabatic limit this model appears to be capable of explaining the non-single exponential decay of P870∗ (Rhodobacter sphaeroides) while resolving a serious discrepancy between the hole burning and time domain results on the decay of P∗. In the strong coupling limit, the long-standing question of whether or not P+BL− serves as a real or virtual state in the primary charge separation process becomes irrelevant. The model, especially in the adiabatic limit, may well provide the basis for understanding the extraordinarily fast energy transfer dynamics from higher energy Qy-states to P∗ in the bacterial RC. Potential problems with the strong coupling model are considered and possible approaches to its testing discussed.

Twisted internal charge transfer (TICT) effect in bis(4-cyanophenyl)piperazine, a pseudo-dimer of dimethylaminobenzonitrile: a comparative study

Dimethylaminobenzonitrile (DMABN) and N,N′-bis(4-cyanophenyl)piperazine (BCPPZ) have been comparatively studied by X-ray diffraction, fluorescence and time-resolved emission and absorption. BCPPZ can be considered as a pseudo-dimer made up of two weakly interacting DMABN moieties. The crystal structure of DMABN is reported for the first time: it shows an almost perfect planarity. By contrast, BCPPZ is appreciably twisted around the C-N bonds connecting the piperazine part and the cyanophenyl group. The twist is assigned to intramolecular H·H repulsions and thus should also be present in solution. The photophysical properties of BCPPZ and DMABN are very similar and, in particular, the dual luminescence typical of the formation of a twisted internal charge transfer (TICT) state is also observed in BCPPZ. The only difference lies in the values of the kinetic and thermodynamic parameters describing the interconversion between the planar excited state and the TICT state: this latter state is thermodynamically more stable for BCPPZ than for DMABN. The general analogy in the photophysical behaviour implies that, while the ground state of BCPPZ is centrosymmetric, the excited states exhibit a broken symmetry which can be qualitatively attributed to the weak orbital interaction between the constituent parts.

Ultrafast internal charge transfer in a donor-modified rhodamine

A rhodamine dye ARh with a 4-dimethylamino (electron donor) instead of the usual 2-carboxylate (electron acceptor) substituent is described and its photophysical properties are rationalized within the twisted internal charge transfer (“TICT”) state model. ARh is nearly non-fluorescent in fluid solution but can be transformed into a highly fluorescent dye by protonation. This intramolecular fluorescence quenching in fluid neutral solutions is characterized by stationary and time-resolved fluorescence measurements and the product state is evidenced by subpicosecond transient absorption and gain spectroscopy.

The nature of the ground and excited singlet states of p-substituted ( N, N-dialkylamino)cinnamates and benzalmalonate

The basic spectroscopic properties of benzyl 4-( N,N-dimethylamino)cinnamate (BDC), ethyl 4-( N, N-diethylamino)cinnamate (EDC) and diethyl 4-( N, N-dimethylamino)benzalmalonate (DBM) are described and discussed. The dipole moments, differences between the low and ambient temperature fluorescence, quantum yields of fluorescence, and the solvatochromic properties observed for BDC, EDC and DBM are shown to reflect different pathways for singlet state relaxation. The behaviors of BCD and EDC are attributed to the existence of a relaxed internal charge transfer state. The dual fluorescence of DBM is explained by the theory of twisted intramolecular charge transfer states. Results show that DBM is another molecule which relaxes to the ground state from two different excited states.

Photophysics of 7-diethylamino-4-methylcoumarin: picosecond time-resolved absorption and amplified emission study

Picosecond time-resolved pump-probe excited state absorption measurements (with a 400–700 nm window) were used to investigate the conformationally sensitive excited charge transfer state of the 7-diethylamino-4-methylcoumarin ( C1) laser dye in moderately polar to polar solvents. Time-dependent spectral changes due to amplified (probe beam) emission (AE) were observed in the short wavelength region of our window, together with excited state absorption (ESA) at longer wavelengths. These time-dependent changes are consistent with a single excited state species. Since the transient ESA spectrum of C1 is similar to that of the “rigid” C102 aminocoumarin in aqueous solution and to that of C1 in less polar solvents, the transient is assigned to an excited internal charge transfer (ICT) state rather than to a twisted internal charge transfer (TICT) state of C1. These results do not appear to support previous evidence for the formation of a TICT—solvent exciplex in moderately polar to polar solvents.

Towards molecular switching: Photophysical properties of N,N′-bis(4-cyanophenyl)piperazine, a bridging TICT molecule

The properties of N,N′-bis(4-cyanophenyl)piperazine are described. This molecule has been devised to exhibit bridging properties, together with the possibility of giving twisted internal charge transfer (TICT) states following irradiation. The crystal structure shows that the molecule is almost planar in the ground state. Photochemical excitation on the charge transfer band at 300 nm gives rise to a luminescence which is very sensitive to the solvent polarity. In butanol, the dual luminescence characteristic of TICT is observed, which is attributed to the twisting motion of the acceptor(s) part of the molecule with respect to the donor. Transient absorption measurements have been also performed, which characterize the triplet state. A comparison with dimethylaminobenzonitrile is made.