The excited state dipole moments mue(ICT) and mue(LE) of the dual fluorescent molecules N-phenylpyrrole (PP), N-(4-cyanophenyl)pyrrole (PP4C) and N-(3-cyanophenyl)pyrrole (PP3C) are determined from solvatochromic and thermochromic measurements. It is shown that the best results are obtained when the solvatochromic as well as the thermochromic analysis of the spectral shifts is made relative to 4-(dimethylamino)benzonitrile (DMABN) as the model compound. Direct thermochromic experiments with PP4C, PP3C and DMABN in diethyl ether lead to reasonable results, but unrealistically large dipole moments mue(ICT) are found for PP, PP4C, PP3C and DMABN in acetonitrile, ethyl cyanide and n-propyl cyanide. The mue(ICT) values obtained for the N-phenylpyrroles from the thermochromic analysis in these solvents relative to DMABN (17 D) do not depend on solvent polarity: 13 D for PP, 15 D for PP4C and PP3C. The spectral shifts for the LE emission of the N-phenylpyrroles and aminobenzonitriles are much smaller than those for the ICT fluorescence, resulting in relatively small values for mue(LE). With PP and N-(4-methylphenyl)pyrrole (PP4M) the problem arises that one of the two values calculated by solving the quadratic equation for mue(LE) in the solvatochromic and thermochromic analysis cannot be discarded on photophysical or molecular grounds, as is the case for the other molecules. The experimental data for mue(ICT) of PP and PP4C are compared with theoretical values calculated for coplanar (PICT) and perpendicular (TICT) conformations of the pyrrole and phenyl or cyanophenyl groups. The experimental ICT dipole moment of PP4C has a value in between the theoretical results for mue(PICT) and mue(TICT), whereas the data for PP tend to favour the TICT configuration. It appears that in the LE state of PP and PP4M a negative charge remains on the pyrrole moiety, whereas a charge reversal takes place for the LE state of PP3C and the ICT state of PP, PP4C and PP3C.

The photophysical properties of the lowest excited singlet states, S1(π,π*), of two porphyrin diacids have been investigated. The diacids are H4TPP2+ and H4OEP2+, the diprotonated forms of free base tetraphenylporphyrin (H2TPP) and octaethylporphyrin (H2OEP), respectively. Both diacids exhibit perturbed static and dynamic characteristics relative to the parent neutral complexes in solution at room temperature. These properties include enhanced yields of S1 → S0 radiationless deactivation (internal conversion), which increase from ∼0.1 for H2TPP and H2OEP to 0.4 for H4OEP2+ and 0.6 for H4TPP2+. The fluorescence lifetimes of both diacids are strongly temperature dependent, with an activation enthalpy of ∼1400 cm-1 for S1-state deactivation. The enhanced nonradiative decays and many other photophysical consequences of diacid formation are attributed primarily to nonplanar macrocycle distortions. Both H4TPP2+ and H4OEP2+ have been shown previously by X-ray crystallography to adopt saddle-shaped conformations, and the magnitudes of the perturbed properties for the two diacids in solution correlate with the extent of the deviations from planarity in the crystals. A model is proposed to explain the nonradiative decay behavior of the porphyrin diacids that is relevant to nonplanar porphyrins in general. The model includes the existence of decay funnels on the S1(π,π*)-state energy surface that are separated from the equilibrium conformation and other minima by activation barriers. It is suggested that these funnels involve configurations at which the potential-energy surfaces of the ground and excited states approach more closely than at the equilibrium excited-state structure(s) from which steady-state fluorescence occurs. Possible contributions to the relevant nuclear coordinates are discussed.

The five 2,3,5,6-tetrafluoro-4-aminobenzonitriles XABN4F with a dimethyl-amino (DMABN4F), diethyl-amino (DEABN4F), azetidinyl (AZABN4F), methyl-amino (MABN4F) or amino (ABN4F) group undergo ultrafast intramolecular charge transfer (ICT) at room temperature, in the polar solvent acetonitrile (MeCN) as well as in the nonpolar n-hexane. ICT also takes place with the corresponding non-fluorinated aminobenzonitriles DMABN, DEABN and AZABN in MeCN, whereas for these molecules in n-hexane only minor (DMABN, DEABN) or no (AZABN) ICT fluorescence is detected. For the secondary (MABN) and primary (ABN) amines, an ICT reaction does not occur, which makes ABN4F the first electron donor/acceptor molecule with an NH(2) group for which ICT is observed. The ICT state of the XABN4Fs has a dipole moment of around 14 D, clearly smaller than that of DMABN (17 D). This difference is attributed to the electron withdrawing from the CN group to the phenyl ring, exerted by the four F-substituents. The reaction from the initially prepared locally excited (LE) to the ICT state in n-hexane proceeds in the sub-picosecond time range: 0.35 ps (DMABN4F), 0.29 ps (DEABN4F) and 0.13 ps (AZABN4F), as determined from femtosecond transient absorption measurements. In the highly polar solvent MeCN, an ICT reaction time of around 90 fs is observed for all five XABN4Fs, irrespective of the nature of their amino group. This shows that with these molecules in MeCN the ICT reaction rate is limited by the solvent dielectric relaxation time of MeCN, for which a value of around 90 fs has been reported. It is therefore concluded that, during this ultrashort ICT reaction, a large-amplitude motion such as a full 90 degrees twist of the amino group is unlikely to occur in the XABN4Fs. The ICT state of the XABN4Fs is strongly quenched via internal conversion (IC), with a lifetime tau'(0) (ICT) down to 3 ps, possibly by a reaction passing through a conical intersection made accessible due to a deformation of the phenyl group by out-of-plane motions induced by vibronic coupling between low-lying pisigma* and pipi* states in the XABN4Fs.

With 4-(dimethylamino)benzonitrile (DMABN), 4-(methylamino)benzonitrile (MABN), and 4-aminobenzonitrile (ABN) in an alkane solvent such as n-hexadecane, the fluorescence decay time τ and quantum yield Φf strongly decrease with increasing temperature. For DMABN in n-hexadecane, τ decreases from 3.43 ns at 25 °C to 0.163 ns at 284 °C, with a simultaneous drop in Φf from 0.14 to 0.006. Similar results are obtained for MABN and ABN. By measuring τ, Φf, and the intersystem crossing (ISC) yield ΦISC of the three aminobenzonitriles as a function of temperature in 2-methylpentane and n-hexadecane, covering a range from −151 to 284 °C, the rate constants for internal conversion (IC), ISC, and fluorescence are determined, together with their activation energies and preexponential factors. It is so established that DMABN, MABN, and ABN undergo efficient thermally activated IC. Upon increasing the temperature for DMABN in n-hexadecane from 18 to 287 °C, the IC yield ΦIC increases from 0.04 to 0.95. This goes at the expense of ISC and fluorescence, with a decrease from 0.81 to 0.04 for the yield ΦISC and from 0.15 to 0.005 for the fluorescence quantum yield Φf between these two temperatures. With MABN and ABN, likewise with ISC as the main decay channel at room temperature, IC becomes the dominating deactivation pathway of the first excited singlet state S 1 at temperatures above 125 °C. The IC activation energies E IC have similar values for the three aminobenzonitriles in the alkanes:  31.3 kJ/mol (DMABN), 34.3 kJ/mol (MABN), and 34.8 kJ/mol (ABN), with preexponential factors of around 5 × 1012 s-1. The ISC activation energies E ISC are considerably smaller, 3.9 kJ/mol (DMABN) and 5.6 kJ/mol (MABN and ABN), with relatively small preexponential factors of around 3 × 108 s-1, values in accord with the spin forbidden character of ISC. The different height of the barriers E IC and E ISC makes their separate determination at the high and low parts of the available temperature range possible. Contrary to what has previously been postulated for 1-aminonaphthalenes, the similarity of the barrier heights E IC of DMABN, MABN, and ABN shows that the IC reaction of these molecules in alkane solvents is not governed by the energy gap ΔE(S 1,S 2) between the two lowest excited singlet states, which gap substantially increases in the series DMABN, MABN, ABN. Because intramolecular charge transfer (ICT) does not take place with any of these three aminobenzonitriles in alkane solvents, the thermally activated IC process reported here is mechanistically not related to ICT. The IC decay channel obviously should be taken into account in discussions of excited-state processes of DMABN and its derivatives at higher excitation energies.

The fluorescence spectra of 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), 2,4,6-tricyano-N-methylaniline (TCMA), and 2,4,6-tricyanoaniline (TCA) consist of a single emission band, even in the polar solvent acetonitrile (MeCN). This indicates that an intramolecular charge transfer (ICT) reaction from the initially prepared locally excited (LE) state does not take place with these molecules, in contrast to 4-(dimethylamino)benzonitrile (DMABN), although the electron accepting capability of the tricyanobenzene moiety in TCDMA, TCMA, and TCA is substantially larger than that of the benzonitrile group in DMABN. In support of this conclusion, the picosecond fluorescence decays of the tricyanoanilines are single-exponential. Only with TCDMA in MeCN at the highest time resolution, double-exponential decays are observed. On the basis of a similar temporal evolution of around 2 ps in the femtosecond excited-state absorption (ESA) spectra of TCDMA in this solvent, the time development is attributed to the presence of two rapidly interconverting S(1) conformers. The same conclusion is reached from CASPT2/CASSCF computations on TCDMA, in which two S(1) minima are identified. The ESA spectra of TCDMA, TCMA, and TCA resemble that of the LE state of DMABN, but are different from its ICT ESA spectrum, likewise showing that an ICT reaction does not occur with the tricyanoanilines. From the luminescence spectrum of TCDMA in n-propyl cyanide at -160 degrees C, it follows that intersystem crossing and not internal conversion is the main S(1) deactivation channel. The radiative rate constant of TCDMA in MeCN is smaller than that of TCMA and TCA, which indicates that the S(1) state of TCDMA has a larger ICT contribution than in the case of TCMA and TCA, in accordance with the results of the calculations, which show that the S(1) state displays ICT valence bond character. Extrapolated gas-phase data for TCDMA and TCA are compared with the results of the computations, revealing a good agreement. The calculations on TCDMA and TCA also lead to the conclusion that the lowest excited singlet state S(1) determines its photophysical behavior, without the occurrence of an LE --> ICT reaction, in the sense that the initially excited LE state has already a strong ICT character and there is no equilibrium between two electronic states with strongly different electronic structures (i.e., LE and ICT with very different dipole moments) leading to dual (LE + ICT) fluorescence.

T he development of highly selective and sensitive analytical techniques has been a driving force for unprecedented advances in biotechnology, gene engineering, and drug discovery. Capillary electrophoresis (CE) is becoming a wider accepted analytical method in biology and medicine. CE offers short analysis time, high resolution, and minute consumption of samples and reagents, making it an attractive technique for mass bioassays and drug screening. Since the last Analytical Chemistry review in this field, 1 there have been published over 10 000 articles with CE as a topic. Within a variety of studies concerning CE, we have identified the intensively developing area of reversible biomolecular interactions which are defined as highly selective noncovalent binding of ligands with biomolecules. These affinity interactions control cell recognition, signal transduction, immune response, DNA replication, gene expression, and other cellular processes. The knowledge of quantitative parameters of binding reactions (equilibrium and/ or rate constants) is essential for understanding the mechanisms of biological processes, which these reactions regulate. The present review covers a 3-year period between January 2012 and November 2014. We have attempted to select studies that demonstrate the newest and most impactive developments in the field of biomolecular affinity interactions.

The dynamics and mechanism of the photoinduced reversible process of formation and decay of an exciplex species created between the water-soluble cationic metalloporphyrin copper 5,10,15,20-tetrakis[4-(Nmethylpyridy1)lporphyrin ) and the DNA model compound poly(dA-dT) have been studied in detail. Such a photoinduced process had been previously observed in transient resonance Raman (RR) spectra under high-power laser irradiation of complexes of Cu(TMpy-P4) with calf thymus DNA and some oligoand polynucleotides containing thymine (T) or uracile (U) residues. It was found that the interaction of excited Cu(TMpy-P4) with carbonyl groups of T or U involved in polymers having an appropriate secondary structure was responsible for the new transient species detected in high-power Raman spectra. In the present work, direct kinetic measurements of the exciplex formation between Cu(TMpy-P4) and poly(dA-dT) were carried out by using both picosecond transient absorption pump-probe technique (10-ps time resolution) and two-color time-resolved RR technique (100-ps time resolution). A comparative nanosecond Raman study of this exciplex and of the excited (d,d) state of copper meso-tetraphenylporphyrin (CuTPP) model compound dissolved in a number of oxygen-containing solvents has also been performed, to clarify the excited electronic state which is at the origin of this process. It has been found that the binding of one of the CO-groups of T or U to Cu(TMpy-P4) in its lowest excited triplet state results in a shortening of the triplet-state lifetime to 35 f 7 ps. In addition, a population of an excited 2[d,2,d+y2] state, Le., the most low-lying and long-lived excited state for the five-coordinated Cu(TMpy-P4) (exciplex state), occurs in the process of excitation relaxation. Large wavenumber shifts of structure-sensitive vibrational marker lines from the porphyrin skeleton reveal the promotion of one of the copper d electrons into the half-filled d$-?2 orbital and the expansion of the porphyrin core to accommodate the occupation of this d orbital. The exciplex deactivation process (excited (d,d) state decay) has a time constant of 3.2 f 0.5 ns and is accompanied by the CO-group deattachment with a disruption of the exciplex into initial components.

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