[page 57↓]

6  TICT Formation and Antiquinoid Distortion in para- and meta-Derivatives of N-Phenyl Pyrrole

Abstract

The photophysical properties of meta- and para-cyano N-phenyl pyrrole (m- and p-PBN) are compared. Both compounds show highly red-shifted and strongly forbidden emission in polar solvents, assigned to a charge transfer state. The forbidden nature is indicative of very weak coupling between the two π-systems, and a twisted emissive structure is suggested (TICT state). Comparison to quantum chemical calculations indicates that the twisted structure possesses an antiquinoid distortion of the benzonitrile group, i.e. the bonds in the ring are lengthened instead of shortened, as in the quinoid state, which is reached in nonpolar solvents. m-PBN is the first meta compound which shows TICT emission. It differs from p-PBN by a less exergonic formation of the TICT state from the LE/ICT quinoid state. It therefore shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence in this solvent (LE/ICT and TICT).

6.1 Introduction

Since the ‘‘Twisted intramolecular charge transfer’’ (TICT) concept has been developed some two decades ago, the investigation of charge transfer (CT) in donor-acceptor systems, especially substituted benzenes, unraveled many processes occurring in their excited states. Particularly, this has led to the understanding of the dual fluorescence of 4-cyano-N,N-dimethylaniline (DMABN) which was discovered by Lippert in 1959 [52]. In many compounds, TICT states are formed via an adiabatic photoreaction: If a TICT forming molecule possessing a planar ground state is electronically excited, it relaxes spontaneously in the excited state towards a twisted conformation from where it decays by emission or nonradiatively. The rate of reaching the TICT state with its twisted conformation is determined, at least for compounds similar to DMABN and apart from other factors, by the initial (Franck-Condon) twist angle which is reached directly after light absorption from the ground state. The closer this initial twist angle is to the final one (about 90o), the faster is the TICT population kinetics. In that respect, CT can be explored by variation of the donor and acceptor groups on either side of the benzene moiety stabilizing the CT transfer state. Surprisingly, the dual fluorescence has not been observed in the case of meta derivatives of [page 58↓]DMABN [65]. This isin part due to the larger energy gap between the first two excited states (1La, 1Lb)for the meta as compared to the para derivatives, and therefore the driving force forthe adiabatic photoreaction is strongly reduced for m-DMABN as compared to p-DMABN [9].

Figure 6.1: Structures of the molecules

Pyrrolobenzonitriles constitute a group of donor-acceptor systems in which the dimethlyamino group is replaced by a better donor moiety, namely pyrrole. The absorption and fluorescence properties of N-pyrrolylbenzonitrile (p-PBN), its ester derivative PBAEE, and a more twisted model compound DPBN with stronger donor-acceptor propeties have been compared by Gude et al. [66].Recently, Yoshihara et al [67] reported on the determination of excited state dipole moments of N-phenylpyrroles and DMABN from solvatochromic and thermochromic measurements. Both the para and meta PBN derivatives were shown to populate a CT excited state but the conformational nature remained unclear as transition moments were not determined. This is to be contrasted with the recent investigation of a planar model compound (FPP) of phenyl pyrrole [68] as compared to phenyl pyrrole (PP) itself. In both cases, a red-shifted CT fluorescence band could be shown to be present. This leaves two possibilities: (i) either both FPP and PP possess a planar structure for the relaxed CT state corresponding to a “ Planar Intramolecular Charge Transfer ” PICT state [69, 70, 71, 72] or (ii) FPP has a planar structure and PP a twisted one. In this case, different emissive properties are expected. Unfortunately, the experimental investigation of transition moments in PP and FPP has not been reported [68].

Zilberg et al . [73]performed theoretical calculations on N-phenyl pyrroles. The calculations support the possible existence of two distinct structures for the CT state. One of [page 59↓]them has a quinoid structure with characteristically shortened bonds in the benzene ring, and the two rings are coplanar at the energy minimum. The other one is of anti-quinoid structure (where the quinoid bonds in between are lengthened instead of shortened), has a larger dipole moment than the quinoid one, and has an energy minimum with the pyrrolo group twisted by 90° with respect to the benzene ring.

Only the antiquinoid state is expected to possess forbidden emissive properties. Our results reveal that the CT emission is stronglyforbidden in both compounds investigated. The combination of experiment and theory therefore leads to the conclusion of antiquinoid properties and twisted structure of the emitting CT state in both compounds.

The meta isomer of p-PBN, m-PBN is the first compound where a m-substituted cyano derivative shows TICT formation. Like m-DMABN, m-PBN also shows a red-shift in the absorption maximum of S1. This reduces the exothermicity of CT formation for meta-compounds in general. On the other hand, when comparing p-PBN with DMABN, there is a large exothermicity from LE to TICT formation due to the better donor. As a matter of fact, already nonpolar solvents like n-hexane induce the appearance of dual fluorescence for p-PBN. The combined effect of both factors can achieve that both para isomers DMABN and p-PBN show CT formation whereas only m-PBN populates a CT state, but m-DMABN emits from an LE state.

6.2 Experimental

6.2.1 Materials

The compounds were synthesized according to the procedures described in refs. [29, 30] and sublimed. The experimental details about the absorption, fluorescence, lifetime and quantum yield measurements are described in chapter 3.

6.2.2 Quantum Chemical Calculations

The ab initio calculations were performed with GAMESS [74] using the cc-pVDZ basis set [75]in collaboration with Dr. Shmuel Zilberg at Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University of Jerusalem, Jerusalem, Israel. Full geometry optimization was performed for the doublet ground state of benzonitrile radical anion by using CASSCF calculations (CAS(11/10)/cc-pVDZ (11electrons on 10 orbitals)).


[page 60↓]

6.3  Results

6.3.1 Room Temperature Spectroscopy

The absorption and emission spectra of p-PBN and m-PBN in various solvents of different polarity are compared in Fig. 6.2. The main absorption band of m-PBN is shifted to the blue region compared to p-PBN in n-hexane. There is also a weak shoulder in the red edge of the absorption spectrum of m-PBN, which can be interpreted as a weak transition, which is red-shifted when compared to the corresponding one in p-PBN. All the emission spectra exhibit a strong red-shift of their maxima increasing from weakly polar to strongly polar solvents. The emission of p-PBN in n-hexane is broad as compared to the narrower band in m-PBN. Yoshihara et al.[67]reported dual fluorescence for m-PBN dissolved in the medium polar solvent diethyl ether (EOE). This is has been verified here by comparing an aerated and a deaerated solution, and the shoulder in the fluorescence spectrum is not present for both freshly prepared solutions but rises for the aerated solution only for a prolonged stay of the sample in the dark. The shoulder reported in ref. [67] is therefore assigned to the appearance of a thermal oxidation product. No shoulder appeared on photolysis of the deaerated solution. Emission maxima observed in this work differ somewhat from the previous work of Gude et al. [66]and Yoshihara et al. [67] due to the use of different fluorescence spectrometers and correction curves. The quality of the emission correction curve is especially important for broad and structureless CT spectra as observed for m-PBN and p-PBN in polar solvents. We have verified the quality of the correction curve used here by comparing to spectra measured on a freshly calibrated fluorimeter.


[page 61↓]

Figure 6.2: Normalised Absorption and fluorescence spectra of p-PBN and m-PBN in various solvents of different polarity (HEX- n-hexane, EOE – diethylether, ACN - Acetonitrile).

Comparison of the spectra of m-PBN with p-PBN exhibits rather similar features except in nonpolar solvents such as n-hexane where the emission spectrum is narrower for m-PBN than for p-PBN in n-hexane. The solvatochromic red-shift in going from low to high polarsolvents is a substantial indication of the charge transfer (CT) nature of the emitting state in both compounds. (see section 6.3.2)

The molar extinction coefficient values determined for p-PBN and m-PBN in n-hexane are ε (λ) = 25747 and ε (λ) = 25400 respectively. In the case of p-PBN, the bands due to the S1 and S2 states are strongly overlapping (see below), whereas the redshift of the S1 absorption in m-PBN leads to the appearance of the shoulder with small extinction coefficient in the absorption spectrum (Fig. 6.2). The blue shift of the main absorption band (see Fig. 6.2) and the small absorption shoulder at ca. 300nm for m-PBN is a consequence of [page 62↓]the meta-effect [76]. As the calculations described below show, the single main absorption band of p-PBN hides a weak band which can be assigned to the 1Lb state in Platt’s nomenclature. Due to the meta-effect, the S1 state of m-PBN is therefore at lower energy than for p-PBN.

Table 6.1: Photophysical Characteristics of m-PBN and p-PBN in Solvents of Different Polarity at Room Temperature

a λ = 287nm used as the excitation wavelength for all solvents (absorption maximum is independent of polarity). bThe Stokes shift is measured from the weak absorption shoulder at 300nm. c from ref. 4. . d λ = 258nm used as the excitation wavelength for all solvents (absorption maximum is independent of polarity). e S1 corresponds to the shoulder at 300nm in the absorption spectrum .

Stokes shift values with reference to the first absorption band have been calculated and tabulated for both m-PBN and p-PBN in Table 6.1. It is found that they are slightly smaller for m-PBN due to the red-shift of S1. But, in the case of highly polar solvents, the Stokes shift values are large in number for both compounds indicating emission from a CT state. The fluorescence decay curves are monoexponential, allowing the evaluation of radiative and nonradiative rate constants according to equations 6.2 and 6.3. In eq. 6.3, k corresponds to the sum of all nonradiative processes including triplet formation. The emission transition dipole moments were calculated using (eq. 6.4) [77] and are the characteristics of the excited states. The measured data and calculated photophysical values are collected in Table 6.1.


[page 63↓]

The k f values decrease when going from hexane to polar solvents in parallel to the quantum yield values for both p-PBN and m-PBN. In the case of m-PBN, the k f values in the more polar solvents are even smaller than in p-PBN, indicating the enhancement of forbidden character present in the emitting state.

6.3.2 Solvatochromic Measurements

The solvatochromic slopes were analysed to get quantitative information on the dipole moments for both compounds, by applying the Mataga equation (eq. 6.5) [52, 53, 54] and the resulting values are collected in Table 6.2. The plot of the emission maximum versus the polarity parameter, is shown in Fig. 6.3. Similarly, a new type of Mataga plot has also been made by using low temperature fluorescence spectra: In this case, the emission maxima at different temperatures in diethylether were plotted versus the temperature dependent values (fig. 6.4) In this case, was calculated by taking dielectric constant, ε and refractive index, n as a function of temperature [36]. The plots show a linear relationship in both cases. It is found that the solvatochromic slope values (Table 6.2), which are determined from the second type of Mataga plot at different temperatures, are somewhat larger than from the room-temperature Mataga plot, but in both types of Mataga plots, the slope values are slightly higher for m-PBN than for p-PBN. The Onsager radius, a=4.1 Å for p-PBN [66] and m-PBN was calculated by the mass-density formula (eq. 6.5) [55] by taking an average density ρ=0.95g/cm [66]. This approach neglects the different molecular shapes of p-PBN and m-PBN.

Table 6.2: Results of the Solvatochromic Measurements at Room Temperature and Low Temperature (Solvatochromic slopes, assumed Onsager factor a, μg(D) and derived μe(D) for the 2 methods)

a Error less than 10%. bBased on crystal density; ref. 5 cCalculated by AM1. d from the Mataga plot at room temperature. e from the Mataga plot in diethyl ether at low temperatures.


[page 64↓]

Figure 6.3: Solvatochromic fluorescence plot of a) m-PBN and b) p-PBN derived from differently polar solvents at room temperature. (HEX- n-hexane, BOB- dibutyl ether, EOE – diethyl ether, THF- tetrahydrofuran, DCM- dichloromethane, ACN - acetonitrile).

Figure 6.4: Solvatochromic fluorescence plot of a) m-PBN and b) p-PBN derived from measurements in diethyl ether at variable temperature

In the above equations, μ e and μ g are the excited and ground state dipole moments respectively, h is Planck’s constant, c is the velocity of light, and n and ε are the refractive index and dielectric constant, respectively.The resulting excited state dipole moments are determined using the calculated ground state dipole moments are very close for both [page 65↓]solvatochromic methods and are even slightly larger for m-PBN as compared to p-PBN (Table 6.2). These large μ e values suggest that the emissive state is of CT nature and is similar in both compounds regardless of the position of the cyano substituent.

6.3.3 Spectroscopic Measurements at Low Temperatures

Fluorescence measurements at low temperatures were done in the nonpolar solvent mixture methylcyclohexane – isopentane (1:4), and in the medium polar solvent diethyl ether. For m-PBN in EOE, with the lowering of temperature, a red-shift of the emission maxima is observed (Fig. 6.5) and analyzed using the Mataga equation. A red-shift of the emission has also been observed for p-PBN in EOE. This thermochromic red-shift is mainly due to the enhancement of the dielectric constant and the refractive index with the lowering of temperature and can be ascribed to the stabilization of the CT state.

Figure 6.5: Low temperature fluorescence spectra of a) m-PBN and b) p-PBN in diethyl ether (EOE). For Fig. 4b, the second order Rayleigh scattering in the spectral region 560-585 nm has been omitted.

Figure 6.6: Low temperature fluorescence spectra of a) m-PBN and b) p-PBN in the non-polar solvent mixture methylcyclohexane:isopentane, MCH/IP (1:4)


[page 66↓]

Neither for p-PBN nor for m-PBN, the low temperature spectra in EOE (Fig. 6.5) show an indication of dual fluorescence. But a more complex spectral behavior is exhibited by the development of the vibrational structure for p-PBN and m-PBN in the non polar solvent mixture (MCH-IP), upon cooling (Fig. 6.6).

Upon cooling, the behavior of m-PBN and p-PBN in the alkane solvent is opposite: Whereas the highest energy vibronic band is enhanced in m-PBN upon cooling, this spectral feature diminishes for p-PBN at low temperature. Moreover, the spectra are significantly broader for p-PBN. The behavior of m-PBN can be understood as the normal behavior at low temperature, where the structuring of vibronic bands is enhanced upon cooling. The diminishing of the vibronic feature at around 305 nm for p-PBN at low temperature can then be understood on dual fluorescence, with an LE/CT excited state equilibrium and a thermodynamically more stable CT state [66].

Upon closer inspection, the 0-0 vibronic band is seen to be situated at lower energy in m-PBN (~316 nm) than in p-PBN (~305 nm) reflecting the decreased energy of the emitting LE state for m-PBN (meta-effect), as is already visible in the absorption spectrum (see section 6.3.1 above).

6.4 Computational Results

6.4.1 AM1 Calculations

The ground state optimized geometries of the benzonitrile radical anion, p-PBN and m-PBN as calculated by using the AM1 method are shown in Fig. 6.7. Table 6.3 shows the equilibrium twist angles, rotational barriers to planarity and perpendicularity as well asdipole moments, which were calculated for the ground state. These values are compared with the parent compound, N-phenyl pyrrole (PP). The twist angle and rotational barrier for m-PBN are slightly higher than for p-PBN, but comparable with PP. The rotational barrier for m-PBN and PP to reach the perpendicular geometry is less than for p-PBN due to their more twisted nature in the equilibrium geometry and smaller intermoiety mesomeric contribution.

Table 6.3: Equilibrium Twist angles, Rotational barriers to each the Planar and the Perpendicular Geometry and Dipole moments in the S0 state as Calculated by AM1.


[page 67↓]

Figure 6.7: Equilibrium structures of benzonitrile (BN), BN radical anion, p-PBN and m-PBN in the S0 state.

The three ground state optimized conformations (equilibrium twist angle, as well as planar and perpendicular one) were also analysed regarding excited-state energies and properties. Configuration interaction (CI) for the excited states was used. The calculated values of transition energy, relative energy, dipole moments and oscillator strengths of the low lying excited states are collected in Table 6.4 and are discussed below.

6.4.2 CASSCF Calculations

The results on para-PBN have been given in detail previously [73]. The most important observation was that the charge transfer excited state of A symmetry has two minima on the hypersurface which involve both changes in the twist angle α between the pyrrole and the benzene ring as well as characteristic bond length changes in the benzene ring. The so-called quinoid minimum is situated at α=0° (planarity), and the central bonds in the benzene ring are shortened, characteristic for a quinoid distortion. On the other hand, the so-called antiquinoid minimum AQ is characterized by a perpendicular structure (α=90°) and by [page 68↓]long central benzene bonds, which are longer than the adjacent ones. It is called an antiquinoid structure [73].

Figure 6.8: The highest four occupied and lowest two unoccupied molecular orbitals for p-PBN and m-PBN in the equilibrium S0 geometry as calculated by AM1. The corresponding molecular orbitals for benzonitrile are also shown and arranged such that the coupling pattern with the orbitals of the pyrrole group becomes visible. The lower indices a and b denote subgroup orbitals transforming as symmetry species a and b in the symmetry point group C2. As can be seen, only the subgroup orbitals of b-symmetry can couple leading to the a + (bonding) and a (antibonding) combination. The position of the cyano group is not important here: Even for m-PBN, the orbitals correspond approximately to the a and b symmetry species.


[page 69↓]

In the AQ minimum, the orbitals involved in the charge transfer state of symmetry A (HOMO-1 Pb to LUMO Bb, see Table 6.4 and Fig. 6.8), are localized on the subunits (Fig. 6.8 shows the near-planar equilibrium conformation where HOMO-1 is somewhat delocalized), so that the dipole moment is very high (16 D from the AM1 calculations reported in Tab. 6.4; the dipole moment value from the CASSCF calculations is 16.2 whereas for the quinoid minimum Q, the HOMO-1 is strongly delocalized over both rings (see Fig. 6.8: PbBb), and the calculated dipole moment is smaller (4.9 D from AM1 and 11.0 D from CASSCF). Note that according to the AM1 calculations (without geometry optimization in the excited state, the energy of the AQ state is very high (S7), but the Q state is the lowest CT state (S2). The CASSCF calculations with geometry optimization lower especially the AQ state, so that its minimum becomes lower lying than the minimum of the quinoid state Q, and the energy difference between Q and AQ states has been found to be -0.55 eV in the gas phase.

Table 6.4: Results of Semiempirical AM1-CI Calculations for the BN radical anion, and for p-PBN and m-PBN in the Planar and Perpendicular Geometry.

The calculations in Tab. 6.4, and also the results of Parusel [78] indicate that for the (rigid) perpendicular geometry (as optimized in the ground state), AQ is not the lowest CT state but a state B(90°) of B symmetry (S3, involving HOMO and LUMO orbitals Pa and Bb. The dipole moments are very large in both perpendicular (18 D) and planar conformation (1B(0°) state with 11.7 D) because the orbitals are rather localized even for the planar geometry. Of course, geometry relaxation is again expected to lead to changes in the relative ordering of 1B(0°) and 1B(90°). The energetics of this B-CT state have not yet been calculated with CASSCF geometry optimization. But the arguments and results given below indicate, that the same Q and AQ isomers should be expected for this state.

Because of the decoupled orbitals in the perpendicular conformation, the CT state can be regarded as the combination of an anion radical of benzonitrile and a cation radical of pyrrole. We can therefore expect to get a deeper insight by having a closer look on the benzonitrile anion radical alone, which is of doublet electronic structure but can be calculated as open-shell ground state molecule with somewhat simpler geometry optimization than for the excited state of PBN, and it can readily be treated with the CASSCF calculations.

The electronic structure of the benzonitrile anion radical is determined by the occupancy of the doubly occupied and one singly occupied orbital:

Benzonitrile has three low-lying LUMO’s, which provide its acceptor ability:

Figure 6.9: The lowest unoccupied MOs of benzonitrile, labelled according to C2v symmetry. The corresponding symmetry species in the C2 point group is given in square brackets (see also the lower indices of the orbitals shown in Fig. 7).


[page 72↓]

The first two MOs in Fig. 6.9 are normal π orbitals perpendicular to the benzene plane, corresponding to Bb L and Ba L in Fig. 7. If the lone electron occupies the b1 MO, this leads to a

Figure 6.10: Optimized structure with bond lengths given, and Mulliken charges (italic numbers in square brackets) of three states of the benzonitrile radical anion with different orbital occupation patterns. The 12B1 state is the global minimum on the PES of the anion radical. The relative energies in kcal/mol are given in round brackets.

12B1 electronic state of the anion-radical, which has some delocalization of the additional electron on the benzene ring and on the cyano group leads to a 12A2 electronic state of the anion radical and shows the acceptor activity of the benzene ring, without participation of the substituent. Occupation of MO b2 with one electron leads to a 12B2 electronic state of the anion radical and shows the pure acceptor activity of the cyano substituent.

The choice of the active space for CASSCF calculations was dictated by the needs to take into account all three possible situations. The active space includes four occupied πx and four unoccupied πx* orbitals, and also the orthogonal πy-system of the CN group: one occupied πy and one unoccupied πy*. The results of the CAS(11/10)/cc-pVDZ calculations (11electrons on 10 orbitals) show the expected changes:

  1. the optimized 12B1 state has a quinodal structure (see Fig 6.10 for the bond lengths, with the atomic charges given in square brackets (Lowdin atomic populations) distributed between the benzene ring and the CN group,
  2. The 12A2 state has an anti-quinodal structure with the additional electron delocalized on the benzene ring,
  3. the 12B2 state has a lengthened CN bond with charge localized on the CN group.

The 12A2 electronic state crosses the 12B1 state along the relaxation coordinate connecting these two states, but it is situated 13.3 kcal/mol above the global minimum. The 12B2 state is an excited state of the anion radical and 67.4 kcal/mol above the global minimum.

6.5 Discussion

6.5.1 Absorption

Excited states of para donor-acceptor substituted benzenes possess two close lying π,π*excited states: the long axis polarized 1La-type constituting the main long wavelength absorption band and a perpendicularly polarized 1Lb-type state with much weaker absorption intensity which can cause some structural features in the long wavelength tail of the absorption spectra or which may be completely hidden underneath the much stronger 1La-type band. Depending on the substituents, the role of 1Lb- and 1La- states as S1 and S2 can interchange [79, 80, 81]. For both m-PBN and p-PBN, the main absorption band can be assigned to the 1La-state. The latter is blue shifted in the meta compound because resonance contributions are disfavoured. The 1Lb band, on the other hand, is slightly red shifted for m-PBN due to the meta-effect [76]. The weak shoulder around 300 nm in the absorption spectrum for m-PBN can therefore be attributed to the well-separated 1Lb absorption band.

6.5.2 Dual Fluorescence at Room Temperature

The fluorescence maxima of p-PBN and m-PBN show a continuous red shift from non-polar solvents to highly polar solvents indicating that a highly polar CT state is emitting in both compounds. The fluorescence spectrum of m-PBN in n-hexane appears structured fluorescence, and is assigned to an LE emission band (Δν1/2 = 4064 cm-1). In the case of p-PBN, however there is a significant broadening (Δν1/2 = 6233 cm-1) of the emission spectrum, [page 74↓]which arises due to the overlapping of two emission bands. The corresponding emitting states can be assigned as LE and CT, the former one with a structured spectrum, the latter one with completely structureless emission. The low-temperature experiment (Fig. 6.6) shows that the CT band is enhanced upon cooling. This corresponds to a situation, where CT is situated energetically below LE [66]. In the more polar solvents, this energetic preference is enhanced such that no trace of structured LE emission remains visible.

Figure 6.11: Energy differences between the LE State (broken line) and the CT State (full line) of p-PBN and m-PBN in the Gas Phase as calculated by AM1 and Schematic Energy Ordering of these States in Alkane Solvents.

In m-PBN, the LE state is energetically lowered, but the CT state remains approximately at the same energy such that the energetic order is reversed in non-polar solvents, and CT emission only appears in the more polar solvents due to the preferential lowering of the CT state. Moreover, the energy difference between the LE and CT state of m-PBN is only 1.1 eV which is slightly higher than that of p-PBN (0.8 eV) in the gas phase. The latter observations support the evidence for the appearance of a single fluorescence band of m-PBN in n-hexane (fig. 6.11).


[page 75↓]

6.5.3  Radiative rates and Dipole Moments

The fluorescence rate constant, k f is significantly larger in alkane solvents as compared to polar solvents indicating a more allowed character of the emission for m-PBN and p-PBN in alkanes, and the values were found to be similar (Table 6.1). With increasing solvent polarity, there is a sudden drop in k f values corresponding tothe change to a forbidden transition. This forbidden character present in medium and highly polar solvents for both compounds is most readily explainable by the population of a CT state with a twisted geometry (TICT state) due to the complete decoupling of the subgroup molecular orbitals [82, 83]. This can also be concluded from sterically hindered model compounds like p-PBN with two ortho methyl groups [66] where the structured LE-features observed for p-PBN are absent. p-PBN and m-PBN behave similar, with k f values being around 3x106 s-1 (radiative lifetimes longer than 300ns), quite comparable to the TICT state of DMABN and derivatives [9].The excited state dipole moment values (Table 6.2) for m-PBN and p-PBN resulting from room temperature and low temperature emission spectra are found to be similar (see ref.4). This gives additional evidence that both m-PBN and p-PBN possess the same highly polar TICT state. Yoshihara et al.[67] calculated the excited state dipole moment values for p-PBN and m-PBN by taking the Onsager radii derived from the molecular crystal density. Their values are in good agreement with the values obtained in this work.

Recently, dual fluorescence has been reported also for a rigidized planar derivative of p-PBN [68], however without giving any indication of the associated k f values. Two models can be discussed for the CT fluorescence in this case:

Model A would associate the CT state with the so-called PICT state [69] with strong mesomeric coupling and preferred planar geometry corresponding to the 1La-type stateaccording to Platt’s nomenclature. In this case, the subgroup orbitals (both of symmetry species b in C2) are strongly coupled, and the resulting k f values should be larger and the excited state dipole moment should be smaller than for the perpendicular twisted TICT state. The transition moment should be approximately in the long molecular axis (state with symmetry species A in C2)

Model B wouldbeassociatedwith an electronic configuration with full orbital decoupling available also for the planar geometry. In the case of PP and PBN [29, 55, 79, 80, 81], the donor orbitals on pyrrole should possess a node (this is actually the HOMO in all pyrrole derivatives studied here, see the calculational results), with symmetry species a in C2, and the acceptor orbital on benzonitrile should also possess a node through the carbon atom [page 76↓]linking the benzene to the pyrrolo group, with symmetry species a, resulting again in an emissive state of A symmetry, but large dipole moment and very small k f value. According to CASSCF calculations [73] on p-PBN, this state prefers to be in the perpendicular conformation and has an antiquinoid distortion at 90° (and a quinoid one in the planar minimum, see below). Chemical bridging enforcing planarity could allow this state to be populated provided that the CT state of B symmetry or an LE state are not lower in energy. Clearly further studies reporting k f values and polarization results are necessary to clarify this question.

6.6 Theoretical Investigations

6.6.1 AM1 Calculations

The twist angles of m-PBN (26.4°) and PP (26.5°) are similar in values (Table 6.3), and both compounds have the same rotational barriers towards both planar and perpendicular geometries. In both compounds, the mesomeric interactions stabilizing the planar geometry are similarly weak. In contrast, p-PBN has a smaller twist angle (23.3°), leading to a smaller rotational barrier to planarity: 0.27 kcal/mol as compared to m-PBN (0.40kcal/mol) and PP (0.39 kcal/mol). On the other hand, the rotational barrier (2.59 kcal/mol) of p-PBN towards perpendicularity is considerably higher than for m-PBN (2.05 kcal/mol) and PP (2.01 kcal/mol). This can be directly correlated with the increased importance of the quinoid resonance structure for p-PBN stabilizing the planar geometry. The increased quinoid contribution can also be seen from the S0 equilibrium structures calculated for both compounds. The benzene bond lengths are very similar for m-PBN, whereas in p-PBN, the middle benzene bonds are clearly shortened with respect to the adjacent ones. The quinoid stabilization becomes even more important in the excited state. Based on valence bond theory, Zilberg et al. [73]have proposed two excited state structures on the potential energy surface for donor-acceptor substituted benzene derivatives such as p-PBN. One has the quinoid form, in which the central C-C bond of the benzene ring is shorter than the adjacent bondsand which possesses a planar geometry at the energy minimum. The other excited state conformer possesses central C-C bonds, which are longer than the adjacent ones. This conformer was called the antiquinoid form or AQ state, and it has an energy minimum with the pyrrolo group twisted at 90° with respect to the benzene ring. Attempts to reproduce these results for p-PBN with excited-state optimization in AM1 did not lead to stable AQ geometries and were abandoned. AM1-geometries also did not converge for the AQ-[page 77↓]minimum of ground-state benzonitrile radical anion (see however the next section on ab initio CASSCF calculations), whereas the minimum reached for neutral p-PBN showed a clear quinoid bond length distribution (Fig. 6.7)

Fig. 6.8 and Table 6.4 contain results for single-point calculations of the excited state of p-PBN and m-PBN for three selected optimized ground state geometries which performed in order to understand and assign the various excited states for differently twisted conformations. The calculations include configuration interaction between single and multiple excited configurations and are considered to contain a large part of dynamic correlation, approaching quite closely the experimental absorption energies consistent with the experiment, the main absorption band (S2) is shifted to the blue when going from p-PBN to m-PBN (testifying for the smaller quinoid stabilization in m-PBN), but the S1 state shifts to the red, as is typical for meta-substituted donor-acceptor benzenes. In m-PBN, S1 and S2 are clearly separated (0.4 eV) and can therefore be seen as separate bands, whereas in p-PBN, both states are calculated to be nearly degenerate,therefore appearing as one of the single absorption band in the experimental spectrum.

According to the calculations (p-PBN, 0°) [84], S1 is the forbidden 1Lb-type state (Platt nomenclature) with b-symmetry in C2, closely followed by S2 (a-symmetry, 1La-type state) which possess some CT character and is allowed. It is equivalent to the “PICT” state in literature [69]. It is followed by a further state S3 of b-symmetry with stronger CT (11 D) but forbidden character, which corresponds to the HOMOtoLUMO transition (orbital Pa H to Bb L in Fig. 6.8). The high dipole moment and forbidden character derives from the localized nature of Pa H (pure HOMO of pyrrole, Fig. 6.8), whereas the reduced dipole moment (5-6 D) and allowed character of S2 (PICT) can be traced back to the delocalized nature of the corresponding occupied orbital (Pa H Bb L) in Fig.7. At 90° twist, S1 and S2 develop into pure LE states with a small dipole moment (1Lb and 1La of benzonitrile) whereas S3 develops into the TICT state of b-symmetry (18 D dipole moment).

6.6.2 CASSCF Calculations

When comparing the structures of the benzonitrile anion from AM1 (Fig. 6.7) and from CASSCF ab initio (Fig. 6.10), it is seen, that AM1 only converges to the Q form for AM1, although effort was given to find the minimum of the AQ conformer also with AM1. In the neutral benzonitrile, the Q deformation is very small, but significantly larger, if benzene possesses both a donor and an acceptor substituent (p-PBN, Fig. 6.7).


[page 78↓]

The CASSCF calculations show that the charge distribution in the Q and AQ conformers is quite different. In the former, much charge is localized on the atoms linking the substituent, whereas in the AQ conformer, most negative charge is concentrated on the four central carbon atoms. The Q conformer (12B1) is more stable than the AQ (12A2) one. In terms of orbital involvement, the Q (12B1) state possesses a singly occupied orbital with an antinode through the linking atoms (Bb L in Fig. 6.8), the AQ (12A2) state a singly occupied orbital with a node through the linking atoms (Ba L in Fig. 6.8).

When the AQ radical anion conformer of benzonitrile is linked to a π-system at any twist angle such that a CT state is formed, (e.g. linkage to the pyrrolo radical cation in p-PBN), the overall symmetry point group is reduced to C2, and Q and AQ states of benzonitrile radical anion are of A and B symmetry, respectively, and the singly occupied orbitals similarly. The CT transition from pyrrole to benzonitrile can involve orbitals of the same or of different symmetry on the pyrrolo unit. In the Q state of A symmetry (as calculated in ref. 11), the singly occupied orbital on pyrrole must have an antinode through the linking atom (Pb H in Fig. 6.8) in order to yield an overall symmetry species A. In the AQ state of A symmetry, both singly occupied orbitals on the two units must have B symmetry in order to yield the overall symmetry A. This means that the planar Q-CT state of A symmetry in PBN involves the transition between orbitals Pb HBb H and Bb L in Fig. 6.8, but the AQ-CT state of A symmetry involves the orbitals Pa H and Ba L. This can be nicely followed by the CASSCF calculation, which shows that for intermediate twist angles, these two conformation mix through configuration interaction. The single occupancy of Ba L in the AQ state explains both the long central bonds in benzene (orbital node between the central atoms) and the accumulation of negative charge on the adjacent atoms (orbital coefficients only on these atoms in Ba L).

There are, of course, further CT states present which can most readily be identified at 90° twist. There is a further A state with CT between Pb H and Bb L which corresponds in nature to the TICT state in DMABN. And there are two further CT states of B symmetry, combining the orbital pairs Pb H with Ba L and Pa H with Bb L (see Fig. 6.8). All these states have been calculated for the rigid ground state geometry with AM1 and are contained in Table 6.4 (last column).

The main discrepancy between the CASSCF optimized excited state energies (ref [73]) and the semiempirical (this work) or DFT/MRC results for the rigid geometries [78] is that the A-CT of AQ character is the lowest CT state at 90° after geometrical relaxation [73] whereas for the Franck Condon situation, the CT state of B symmetry (and Q character [page 79↓]involving Bb L and Pa H) is the lowest one. We conclude that the excited-state geometrical relaxation must switch these two CT states.

We can also conclude on the two CT states present in DMABN for 90° twist: Because the amino group possesses a donor orbital of B symmetry, two of the CT states in PBN (involving Pa H) must be absent in DMABN. The lowest TICT state is of A symmetry involving the Bb L orbital on benzonitrile. We therefore expect that this state has the Q bond length pattern in benzonitrile, as found by all calculations until now [9]. But the second TICT state, somewhat higher lying in energy, will be of B symmetry and involve Ba L as accepting orbital and is therefore expected to possess the AQ bond length pattern.

Finally, due to the localized LUMO for meta-PBN (Fig. 6.8) we can likewise expect that the geometry-relaxed lowest CT state at 90° will be an AQ state of A symmetry, similarly as in para-PBN.

6.7 Conclusion

The photophysical properties of meta- and para-cyano-N-phenylpyrrole (m- and p-PBN) have shown that both compounds show highly red shifted and strongly forbidden emission in polar solvents, assigned to a TICT state. It is concluded that m-PBN differs from p-PBN by a less exergonic formation of the TICT state from the LE/ICT quinoid state, and it therefore shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence (LE/ICT and TICT).


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