[page 10↓]

2  Theoretical Background

2.1 Mechanism of Dual Fluorescence

In general, fluorescent compounds possess a single fluorescence band, there are however, where two fluorescence bands are observed in compounds even in the case of a simple donor-acceptor benzene called 4,N,N-dimethylaminobenzonitrile (DMABN). This phenomenon was first discovered by Lippert et al. [8] The first band around 350 nm corresponds to the “normal” band for closely related benzene derivatives, the other one, at considerably longer wavelengths was assigned to an anomalous band. Lippert proposed a solvent-induced reversal of excited states. The anomalous band was assigned to fluorescence from the more polar 1La-type state, which is preferentially stabilized by solvation. This has led to the nomenclature in photophysics: ‘A’ band for the “anomalous” emission from the 1La-type state or charge transfer (CT) state and ‘B’ band for the normal short wavelength emission from the 1Lb-type state or locally excited (LE) state. These emitting states are also called B* and A* states, and can be in thermal equilibrium. Lippert et al. [8] observed that the dual fluorescence strongly depends on the solvent polarity and on the temperature. In polar solvents, the long wavelength fluorescence band grows in relative intensity, while the intensity of the first band decreases with increasing polarity of the medium. The kinetic scheme for this process is shown below in Fig. 2.1

Figure 2.1: Kinetic scheme for the dual fluorescence of DMABN. Straight arrows represent radiative channels and dotted arrows represent non-radiative channels from the respective states.

The above scheme in Fig. 2.1 contains the reaction rate constants k BA (forward reaction) and k BA (backward reaction) as well as the radiative decay constants (k Bf and k Af ) and the non-radiative decay constants and to the ground state. The origin of the dual fluorescence can be well-described in terms of photoinduced charge separation via [page 11↓]twisted intramolecular charge transfer (TICT) [9]. It occurs by an adiabatic photoreaction [10] taking place on the excited state potential energy hypersurface. The following sections will give a short description on the principles of photoinduced electron transfer.

2.2 Photoinduced Charge Transfer

In order to describe an electron transfer process, it is useful to draw potential energy surfaces, a graphical representation that allows one to visualize the details of the complex mechanism. A potential energy surface is a topological representation of the approximated coordinate dependence of the total energy parabolic curve, which can give an overview of a chemical reaction. Intersections of parabolic curves were used to represent the course of electron transfer from reactant to product. According to Marcus electron transfer theory [11, 12, 13, 14], there are two types of electron transfer reactions [15] taking place in donor-acceptor systems:

Figure 2.2: Potential energy diagrams of adiabatic and non adiabatic intersecting curves during electron transfer.

  1. Adiabatic: Electron transfer reactions that take place on a single potential energy surface with out intersecting one another.
  2. Non adiabatic or diabatic: Process in which the potential energy surfaces that do not interact but intersect since the interaction between diabatic curves is weak as compared with the strong mixing in the adiabatic case.

There are different classes of adiabatic and non adiabatic photoreactions taking place in donor-acceptor systems. Here only the intramolecular and intermolecular electron transfer processes are considered in the following sections.

2.2.1 Intramolecular Electron Transfer

Intramolecular electron transfer is one of the main types of an adiabatic photoreaction, which forms the basis for dual fluorescence. The direct contact of the donor and acceptor molecules seems to be necessary for efficient electron transfer, particularly in the photoinduced electron transfer reactions. To be precise, after electronic excitation, electron transfer takes place from initial molecular orbital (MO) of the donor (D) state, to a MO of the final of the acceptor (A) state. When the donor and acceptor molecules are linked together by covalent bonds so that they are part of a single molecule, and the resulting electron transfer is called intramolecular. If an electron transfer from D to A is energetically feasible in the excited state, the product of such an intramolecular ET reaction is a charge-separated species, D+…. A-. Its electronic structure corresponds to the ground state of the free radical ion pair of opposite charges, consisting of a radical cation D+ and radical anion A-. This results in changes in the dipole moment values between ground and excited state that led to the charge transfer (CT) state. Relaxation processes e.g pyramidalization or planarization, linearization, bending or twisting etc. accompanied by during or after electron transfer reactions will cause various modifications in their electronic structure of the excited molecules. It is interesting to know which of the reaction coordinate determines the feasibility of electron transfer in this kind of donor-acceptor systems. There have been various mechanisms proposed to explain the phenomena of dual fluorescence, and these are as follows:


[page 13↓]

(i) TICT- mechanism (Twisted Intramolecular Charge Transfer)

The TICT model was first put forward by Grabowski and coworkers [16, 17, 18] to account for the observation that the dual fluorescence of DMABN with its “normal” band (B- band) and its “anomalous” one depends on the conformational freedom of the dimethylamino (DMA) group, coupled with an electron transfer in the orthogonal conformation. In the case of DMABN, there exists a reaction path in the excited state leading from the near planar conformation (emitter of the B-band) to an excited photochemical product with an energetic minimum at the perpendicular conformation (emitter of the A-band). These two emitting states possess a mother-daughter relationship, which has been revealed by direct kinetic measurements [19]. In many cases, the back reaction A* → B* also occurs leading to an excited state equilibrium. The ground state of DMABN is known to possess an energy barrier for the perpendicular conformation (the rotational barrier), therefore the emission from the perpendicular excited-state minimum occurs to a repulsive potential and is expected to lead to structureless spectra. The key point here is that the reaction coordinate is not only the intramolecular twisting motion but involves other coordinates, too, such as electron transfer, solvent dipolar relaxation and, most probably, some rehybridization at the amino nitrogen. For the perpendicular TICT conformation, donor (dialkylamino group) and acceptor (benzonitrile) π-orbitals are orthogonal (zero overlap) and thus decoupled leading to a maximum for the dipole moment in the excited state (and a minimum in the ground state). This maximum of the dipole moment (near full electron transfer from donor to acceptor) connected with the energetic minimum for the perpendicular conformation are essential ingredients of the so-called “minimum overlap rule” [16]. For the near planar conformation (B* state), mesomeric interaction between the donor and acceptor π-systems exists and diminishes the dipole moment of B* state, and as schematically shown below:

Figure 2.3: The TICT model involves a twisted product species with charge transfer or charge shift properties (A* state) formed through an adiabatic photoreaction from the precursor (B* state) with a nearly coplanar conformation.


[page 14↓]

The above equations 2.1 and 2.2 can be used to predict possible new TICT systems. Whether or not the energetic minimum of the A*/TICT state is lower than that of the precursor B* state (inequality Eq. 2.1 fulfilled) sensitively depends on the electron donor-acceptor properties of the sub systems which can be quantified by ionization (or oxidation) potential and electron affinity EA (or reduction potential) of donor D and acceptor A.

The B* state responds much less to changes in donor and acceptor properties than the TICT state, and Eq. 2.1 can often easily be fulfilled by increasing donor and /or acceptor strength. In addition to these two factors which deliver the decisive part of the reaction driving force, polar solvent stabilization Esol and the mutual Coulombic attraction C of the linked donor and acceptor radical anion/cation pair also help to preferentially stabilize the TICT state with respect to the precursor B* state.

(ii) Pseudo – Jahn-Teller Mechanism:

Zachariasse et al. found a new explanation for the occurrence of dual fluorescence in DMABN based on a Pseudo – Jahn-Teller (PJT) distortion of the molecular structure. It correlates between the efficiency of the CT state formation and the 1La1Lb energy gap in the absorption spectrum. They postulated that the proximity of these two electronic states favors the CT state. The PJT coupling of 1La and 1Lb states via the inversion mode (rehybridization) of the amino group is assumed to lead to a pyramidal geometry in the ICT state [20, 21, 22].

(iii) Rehybridization of the acceptor (RICT model):

Apart from the amino group (donor) involvement in the CT state, there can also be another site of structural changes in the cyano substituent (acceptor), that is, a bending of the cyano group (rehybridization) taking place in the excited state. It was suggested that the latter could be responsible for the anomalous emission from the A* state [23, 24].


[page 15↓]

2.3  Intermolecular Electron Transfer

Intermolecular electron transfer is defined as the transfer of an electron density from one molecule to another molecule in the excited state. As a result, it forms a complex. It can be either excimer (excited dimer) or exciplex (excited complex). Collision between an excited and an identical unexcited molecule forms excimer whereas exciplexes are formed by collision of an excited molecule (electron donor or acceptor) with an unlike unexcited molecule (electron acceptor or donor). Excimer and exciplex formation processes are diffusion-controlled. The photophysical effects can thus be detected at relatively high concentrations of the species so that a sufficient number of collisions can occur during the excited-state lifetime. Temperature and viscosity are major governing parameters.

2.4 TICT Model Compounds

The TICT phenomenon is observed not only in DMABN, but also in its numerous derivatives and analogous compounds, with modified donor or acceptor groups in the benzene ring. When methyl groups were introduced in ortho or meta position to the N-Me2 group of DMABN, different effects were observed. To analyse the possible role of the steric effect, a series of model compounds was synthesized [16, 17, 25, 26, 27], with the dialkylamino group structurally fixed nearly coplanar to the ring (MIN), or strongly sterically hindered (TMABN), or rigidly fixed in a position perpendicular to the aromatic ring (CBQ).


[page 16↓]

Figure 2.4:Scheme of model compounds

Compounds with a possible rotational degree of freedom around the benzene – amine bond such as DMABN, m-DMABN, MO-DMABN [17], PYRBN and PIPBN [28] exhibit a dual fluorescence, whereas for compounds MIN and CBQ with fixed rigid structure, only one band was observed: the ‘B’ band in MIN and the ‘A’ band in CBQ similar to DMABN. In the case of o-DMABN, only ‘A’ band has been observed. This was interpreted in terms of a steric effect: the methyl substituent in the position ortho to the –NMe2 group sterically hinders the coplanar (quinoid) structure. Similarly, for pretwisted compounds such as TMABN and CBQ, where the nitrogen lone pair is nearly in-plane with the benzonitrile skeleton and perpendicular to the π-orbital system, only the A-band was observed. Thus the emission spectra of the model compounds exemplify the effect of the substituents sterically hindering the coplanarity of the –NR2 group with the ring.


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