6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Molecular rotors are a group of fluorescent molecules that form twisted intramolecular charge transfer (TICT) states upon photoexcitation. When intramolecular twisting occurs, the molecular rotor returns to the ground state either by emission of a red-shifted emission band or by nonradiative relaxation. The emission properties are strongly solvent-dependent, and the solvent viscosity is the primary determinant of the fluorescent quantum yield from the planar (non-twisted) conformation. This viscosity-sensitive behavior gives rise to applications in, for example, fluid mechanics, polymer chemistry, cell physiology, and the food sciences. However, the relationship between bulk viscosity and the molecular-scale interaction of a molecular rotor with its environment are not fully understood. This review presents the pertinent theories of the rotor-solvent interaction on the molecular level and how this interaction leads to the viscosity-sensitive behavior. Furthermore, current applications of molecular rotors as microviscosity sensors are reviewed, and engineering aspects are presented on how measurement accuracy and precision can be improved.

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Environment-sensitive behavior of fluorescent molecular rotors

Time-resolved infrared (IR) and Raman spectroscopy elucidates molecular structure evolution during ultrafast chemical reactions. Following vibrational marker modes in real time provides direct insight into the structural dynamics, as is evidenced in studies on intramolecular hydrogen transfer, bimolecular proton transfer, electron transfer, hydrogen bonding during solvation dynamics, bond fission in organometallic compounds and heme proteins, cis-trans isomerization in retinal proteins, and transformations in photochromic switch pairs. Femtosecond IR spectroscopy monitors the site-specific interactions in hydrogen bonds. Conversion between excited electronic states can be followed for intramolecular electron transfer by inspection of the fingerprint IR- or Raman-active vibrations in conjunction with quantum chemical calculations. Excess internal vibrational energy, generated either by optical excitation or by internal conversion from the electronic excited state to the ground state, is observable through transient frequency shifts of IR-active vibrations and through nonequilibrium populations as deduced by Raman resonances.

ULTRAFAST CHEMISTRY: Using Time-Resolved Vibrational Spectroscopy for Interrogation of Structural Dynamics

Recent advances in time-dependent density functional theory (TDDFT) have led to computational methods that can predict properties of photoexcited molecules with satisfactory accuracy at comparably moderate cost. We apply these methods to study the photophysics and photochemistry of 4-(dimethyl)aminobenzonitrile (DMABN). DMABN is considered the paradigm of photoinduced intramolecular charge transfer (ICT), leading to dual fluorescence in polar solvents. By comparison of calculated emission energies, dipole moments, and vibrational frequencies with recent results from transient spectroscopy measurements, a definitive assignment of the electronic and geometric structure of the two lowest singlet excited states of DMABN is possible for the first time. We investigate the mechanism of the ICT reaction by means of minimum energy path calculations. The results confirm existing state-crossing models of dual fluorescence. Our study suggests that analytical TDDFT derivative methods will be useful to predict and classify emissive properties of other donor-acceptor systems as well.

Photoinduced intramolecular charge transfer in 4-(dimethyl)aminobenzonitrile - a theoretical perspective

This photophysical study addresses the general question of how electron transfer in bichromophoric molecules influences the conformational relaxation, which can be toward either more or less π-conjugation. The effects of photoinduced intramolecular charge transfer on the electronic and molecular properties of a series of differently twisted 4-N,N-dimethylamino-4‘-cyanobiphenyls are investigated by steady-state and time-resolved fluorescence. The dipole moments, radiative rates, and torsional relaxations in the excited state are analyzed by comparison with the absorption spectra and interannular twist angle (φ)-dependent CNDO/S calculations. Independent of the twist angle φ and solvent polarity, the first excited singlet state of these donor−acceptor (D−A) biphenyls (I−III) is an emissive intramolecular 1CT state of the 1La-type transferring charge from the dimethylaminobenzene (D) to the cyanobenzene (A) subunit. Similar to the planar restricted D−A fluorene I, the flexible D−A biphenyl II shows only a we...

Photoinduced Intramolecular Charge Transfer in a Series of Differently Twisted Donor−Acceptor Biphenyls As Revealed by Fluorescence

The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles is discussed. The magnitude of the energy gap between the two lowest excited states is shown to determine the occurrence or absence of ICT. The excited state behaviour of a series of six 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, P3C to P8C, is studied by using photostationary and time-resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. ICT does not occur with P3C and P4C in diethyl ether. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The configurational change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate in the ICT process. The photophysics of the 4-aminobenzonitriles is different from that of other systems such as donor/acceptor-substituted stilbenes and 9,9′-bianthryl, which are governed by the charge distribution and macroscopic Coulombic interaction in their CT states.

Intramolecular charge transfer in the excited state Kinetics and configurational changes

A discussion is presented of the fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles. The occurrence or absence of this ICT reaction is shown to be determined by the magnitude of the energy gap ΔE (S 1 ,S 2 ) between the two lowest excited states. The strong decrease of the ICT efficiency in the series P8C to P3C of 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, is attributed to the increase with decreasing ring size of the barrier for the configurational change of the amino nitrogen from pyramidal to planar, which is considered to be an important reaction coordinate in the ICT process. Dual fluorescence does not occur with P3C nor with NC in diethyl ether. Application of a pressure of 6000 bar leads to a threefold increase of the CT/LE fluorescence quantum yield ratio for the 4(dialkylamino)benzonitrile DMABN (methyl) in diethyl ether, whereas this ratio only slightly increases for DDABN ( n -decyl) in n -hexane. This difference is attributed to the increase of the solvent dielectric constant with pressure, which effect is much larger for diethyl ether than for n-hexane. These results mean that a large amplitude motion does not occur during the ICT reaction of DMABN and DDABN, in accordance with the model of planar intramolecular charge transfer for dual fluorescence. Dual fluorescence is observed with the rigidized aminobenzonitrile NMC7, showing that a full perpendicular twist of the amino group is not essential for the CT state. I -(Dimethylamino) naphthalene ( I DMAN) undergoes a fast thermally activated internal conversion (IC), for which the activation energy increases with solvent polarity. This IC reaction depends on the magnitude of the ΔE (S 1 ,S 2 ) energy gap and is similar in this respect to the ICT reaction of DMABN.

Photo-induced intramolecular charge transfer and internal conversion in molecules with a small energy gap between S1 and S2. Dynamics and structure

Erratum to “Intramolecular charge transfer in the excited state. Kinetics and configurational changes” [J. Photochem. Photobiol. A: Chem., 102 (1996) 59–70]

Eleven conjoined coumarins possessing a chromeno[3,4-c]chromene-6,7-dione skeleton have been synthesized via the reaction of electron-rich phenols with esters of coumarin-3-carboxylic acids, catalyzed by either Lewis acids or 4-dimethylaminopyridine. Furthermore, Michael-type addition to angular benzo[f]coumarins is possible, leading to conjugated helical systems. Arrangement of the electron-donating amino groups at diverse positions on this heterocyclic skeleton makes it possible to obtain π-expanded coumarins with emission either sensitive to, or entirely independent of, solvent polarity with large Stokes shifts. Computational studies have provided a rationale for moderate solvatochromic effects unveiling the lack of collinearity of the dipole moments in the ground and excited states. Depending on the functional groups present, the obtained dyes are highly polarized with dipole moments of ∼14 D in the ground state and ∼20–25 D in the excited state. Strong emission in nonpolar solvents, in spite of the inclusion of a NO2 group, is rationalized by the fact that the intramolecular charge transfer introduced into these molecules is strong enough to suppress intersystem crossing yet weak enough to prevent the formation of dark twisted intramolecular charge transfer states. Photochemical transformation of the dye possessing a chromeno[3,4-c]pyridine-4,5-dione scaffold led to the formation of a spirocyclic benzo[g]coumarin.

O. Morawski

Papers

Intramolecular charge transfer in the excited state Kinetics and configurational changes

Photo-induced intramolecular charge transfer and internal conversion in molecules with a small energy gap between S1 and S2. Dynamics and structure

Erratum to “Intramolecular charge transfer in the excited state. Kinetics and configurational changes” [J. Photochem. Photobiol. A: Chem., 102 (1996) 59–70]

Polarized, V-Shaped, and Conjoined Biscoumarins: From Lack of Dipole Moment Alignment to High Brightness

Fluorene analogues of xanthenes - low molecular weight near-infrared dyes.