The kinetics of formation of twisted intramolecular charge transfer (TICT) states in p-substituted dialkylanilines: consequences of conical intersections along the reaction coordinate

Intramolecular charge transfer in aminobenzonitriles and tetrafluoro counterparts: fluorescence explained by competition between low-lying excited states and radiationless deactivation. Part I: A mechanistic overview of the parent system ABN

The simplicity of the fluorescence pattern of ABN is in fact the outcome of an intricate interplay between locally excited and charge transfer excited states.

Synthesis and investigation of intra-molecular charge transfer state properties of novel donor-acceptor-donor pyridine derivatives: the effects of temperature and environment on molecular configurations and the origin of delayed fluorescence

A novel series of donor-acceptor-donor (D-A-D) structured pyridine derivatives were synthesised and detailed photo-physical investigations were made using mainly steady-state and time-resolved spectroscopy techniques at varying temperatures. The investigations showed that the molecules have solvent polarity and temperature dependent excited-state configurations, confirmed in two different polarity solvents (295-90 K), i.e. methyl cyclohexane (MCH) and 2-methyltetrahdrofurane (2-MeTHF). In MCH, the investigations revealed dual fluorescence over the temperature range of 295-90 K. At 295 K, the ground-state configuration of the molecules has a partially twisted geometry as determined by DFT calculation, yet the emission originates totally from a locally excited (LE) state, however once the temperature is lowered to 90 K, the twisted molecular configuration is stabilised, and the emission originates from a fully-relaxed intramolecular charge transfer state (ICT), this is contrary to the systems where structural reorganisation stabilises ICT and this is frozen out at low temperatures. The DFT calculations revealed different ground state molecular configurations due to the presence of different electron-donating groups, e.g. the molecule including anthracene groups has a near 90° twisted geometry whereas the triphenylamine including molecule has a pyramidal geometrical folding, therefore, the decrease in temperature restricts the donor degree of rotational freedom. In 2-MeTHF solution, the fluorescence spectrum of both molecules is always of ICT character, but gradually red-shifts through the fluid to glass transition temperature (∼135 K), in this case, the fluorescence occurs after structural and solvent-shell relaxations, however, upon cooling below 135 K, the spectra dramatically shift back to blue giving rise to strong emission from an ICT excited-state (but not the LE state) where the molecules have unrelaxed geometries. This significant change in the nature of the emitting species was explained with specific solvent-solute dynamic interactions in the vicinity of the solvation shell and the effect of thermal excitation of molecular vibrational modes of the C-C bond linking donor and acceptor units. Finally, we confirmed that the molecules have ICT ground-state geometry in the solid-state phase (spin-coated films), and the time-resolved decay dynamics were investigated comparing the spin-coated films (at RT and 25 K) and MCH solutions (at 295 K and 90 K).

Extensive reduction in back electron transfer in twisted intramolecular charge-transfer (TICT) coumarin-dye-sensitized TiO(2) nanoparticles/film: a femtosecond transient absorption study

We report the synthesis, characterization, and optical and electrochemical properties of two structurally similar coumarin dyes (C1 and C2). These dyes have been deployed as sensitizers in TiO2 nanoparticles and thin films, and the effect of molecular structure on interfacial electron-transfer dynamics has been studied. Steady-state optical absorption, emission, and time-resolved emission studies on both C1 and C2, varying the polarity of the solvent and the solution pH, suggest that both photoexcited dyes exist in a locally excited (LE) state in solvents of low polarity. In highly polar solvents, however, C1 exists in an intramolecular charge-transfer (ICT) state, whereas C2 exists in both ICT and twisted intramolecular charge-transfer (TICT) states, their populations depending on the degree of polarity of the solvent and the pH of the solution. We have employed femtosecond transient absorption spectroscopy to monitor the charge-transfer dynamics in C1- and C2-sensitized TiO2 nanoparticles and thin films. Electron injection has been confirmed by direct detection of electrons in the conduction band of TiO2 nanoparticles and of radical cations of the dyes in the visible and near-IR regions of the transient absorption spectra. Electron injection in both the C1/TiO2 and C2/TiO2 systems has been found to be pulse-width limited (<100 fs); however, back-electron-transfer (BET) dynamics has been found to be slower in the C2/TiO2 system than in the C1/TiO2 system. The involvement of TICT states in C2 is solely responsible for the higher electron injection yield as well as the slower BET process compared to those in the C1/TiO2 system. Further pH-dependent experiments on C1- and C2-sensitized TiO2 thin films have corroborated the participation of the TICT state in the slower BET process in the C2/TiO2 system.

Solvent Dependence of the Spectra and Kinetics of Excited-State Charge Transfer in Three (Alkylamino)benzonitriles

Steady-state absorption and emission spectra and emission decay kinetics are reported for 4-aminobenzonitrile (ABN), 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C) in 24 room temperature solvents. In solvents of modest to high polarity, P4C, P5C, and P6C exhibit dual fluorescence and emission decays characteristic of the transformation from an initially prepared (LE) state to a more polar charge transfer (CT) state, whereas ABN does not undergo this reaction. The frequencies of the steady-state absorption and emission spectra of all of these solutes can be rationalized using a dielectric continuum description of the solvent and considering only the minima on the reactive surfaces, which are assumed to involve both an intramolecular (twisting) and a solvation coordinate. Characteristics of the gas-phase solutes deduced from this analysis are in good agreement with electronic structure calculations and indicate that differences in their spectra mainly reflect differences in the relative energies of the gas-phase LE and CT states. The relative yields of LE and CT emission are not described as satisfactorily by this model, and reasons for this failure are discussed. The kinetics of the LE --> CT reaction vary considerably with solute and solvent. In many solvents, the emission decays of P4C are reasonably described by a simple two-state kinetic scheme with time-independent rate constants. In P5C and P6C multiexponential decays are observed that reflect time-dependent shifts of the component spectra as well as time-dependent reaction rates. A simplified analysis of these complex dynamics provides estimates for both the free energy change Delta(r)G and (average) LE --> CT rate constant k(f) for a wide range of solute and solvent combinations. The driving force for reaction (-Delta(r)G) follows the order P6C > P5C > P4C and increases with increasing solvent polarity. The reaction rates are correlated to Delta(r)G and follow the opposite trend. The relationships observed between k(f) and Delta(r)G suggest that static solvent effects, i.e., barrier height changes, are the primary determinants of the solvent dependence in P4C. Correlations between barrier-corrected rates and solvation times suggest that dynamical solvent effects contribute substantially to the solvent dependence of the rates in P5C, and especially P6C.

New fluorescent probes for visual proteins. Part II. 5-(Oxo)penta-2,4-dienyl-p-(N,N-dimethylamino)benzoate

A new dual-fluorescent compound, 5-(oxo)penta-2,4-dienyl-p-(N,N-dimethylamino)benzoate (1), a derivative of dimethylaminobenzoic acid, has been synthesised and studied photophysically. This compound continues the series of potential fluorescent probes for visual and proton-pumping opsin proteins. The photophysical behaviour of this molecule, including charge-transfer interaction in the ground state and dual-fluorescence emission, is similar to that of the previously studied analogue cis-3-(oxo)propenyl-p-(N,N-dimethylamino)benzoate (cis-2). The presence of several theoretically calculated conformers of compound 2 was suggested to be responsible for the observed strongly red-shifted absorption and excitation wavelength dependence. These photophysical anomalies were also observed for molecule 1, though the models put forward to explain them in the cases of 1 and 2 are rather different. Based on theoretical calculations and experimental results, we propose that some of the stable conformers might be connected with either a charge-transfer complex or mesomeric interactions in the ground state. Upon changing the electronic nature of the oxo-pentadienyl acceptor moiety, e.g. protonation, chemical or biochemical reaction, the charge-transfer absorption disappears, which leads to a dramatic increase in the fluorescence quantum yield.

Ultrafast temporary charge transfer in pyrrolidinyl–benzonitrile and pyrrolyl–benzonitrile in the gas phase

4-(N-pyrrolidinyl)benzonitrile and 4-(N-pyrrolyl)benzonitrile were excited by an ultrashort pulse at 270 nm to their La (S2) states and then probed by ionization at long wavelengths. Parent and fragment ion signals show components with time constants < 100 fs which we attribute to ultrafast relaxation to the Lb (S1) state. From this short time we infer a conical intersection between the Lb and La surfaces. The wave packet can branch there, one part going temporarily to a strongly displaced state. Its shift was concluded from an anisotropy observed only there. The only excited state known to have a large displacement is the charge-transfer (CT) state. The positive anisotropy indicates that the CT state belongs to the 2A species for both molecules. For pyrrolylbenzonitrile, this is in contrast to previous assignments. The anisotropy, and a coherent oscillation observed in pyrrolidinylbenzonitrile, support the idea that the amino-group twist is an important component of the CT reaction coordinate.

Experimental evidence for the photoisomerization of higher fullerenes

The initial relaxation dynamics of the photoexcited fullerenes C60, C70, C76, C84, C86, and C90 were investigated by dispersion-free femtosecond pump-probe spectroscopy. Under identical excitation conditions, the formation of the lowest excited state slows down for the larger fullerenes. This trend in dynamics, monitored throughout the visible and NIR range, is found to correlate with the number of isomers in accordance with the isomerization mechanism suggested by Stone and Wales. The Stone-Wales isomerization was calculated as thermally inaccessible but photoinduced barrierless. The energy difference of the isomers is in the 1 meV range, and back-isomerization is observed on the picosecond time scale. The characteristic spectrally broad transient absorption of the investigated fullerenes is promising for fast optical gating applications.

Ultrafast charge transfer via a conical intersection in dimethylaminobenzonitrile

The L(a)-like S2 state (2A) of 4-(dimethylamino)benzonitrile was pumped at 267 nm in the gas phase at 130 degrees C. Nonresonant multiphoton ionization at 800 nm with mass-selective detection then probed the subsequent processes. Whereas ionization at the Franck-Condon geometry only gave rise to the parent ion, fragmentation increased on motion towards the charge-transfer (CT) state. This useful difference is ascribed to a geometry-dependent resonance in the ion. The time constants found are interpreted by ultrafast (approximately 68 fs) relaxation through a conical intersection to both the CT and the L(b)-type S1 state (1B). Then the population equilibrates between these two states within 1 ps. From there the molecule relaxes within 90 ps to a lower excited state which can only be a triplet state (T(n)) and then decomposes within 300 ps. Previous experiments either investigated only 1B --> CT relaxation-which does not take place in the gas phase or nonpolar solvents for energetic reasons--or, starting from S2 excitation, typically had insufficient time resolution (>1 ps) to detect the temporary charge transfer. Only recently temporary population of the CT state was found in a nonpolar solvent (Kwok et al., J. Phys. Chem. A. 2000, 104, 4188), a result fully consistent with our mechanism. We also show that S2 --> S1 relaxation does not occur vertically but involves an intermediate strong geometrical distortion, passing through a conical intersection.

Ultrafast [2 + 2]-cycloaddition in norbornadiene

Excitation of norbornadiene (bicyclo[2.2.1]hepta-2,5-diene) at 200 nm populates two states in parallel, the second pi(pi*) state and a Rydberg state. We monitored both populations by transient nonresonant ionization. From the pi(pi*) state the molecule relaxes in consecutive steps with time constants 5, 31 and 55 fs down to the ground-state surface, whereas the Rydberg population merges to the other path on the pi(pi*) surface within 420 fs. The relaxation steps are discussed in terms of conical intersections (CoIns) between different surfaces Information on them is inferred from known spectroscopy and, for the last CoIn, from published calculations on Dewar benzene-->prismane conversion and on ethylene photodimerization for which norbornadiene with its two nonconjugated double bonds is a model. The calculation predicts symmetry breaking for this CoIn, the two ethylenes forming a rhombus Although this distortion is hindered in norbornadiene by ring strain, this CoIn seems easily accessible as indicated by the short time (<55 fs) found for passing through it.