[page 80↓]

7  Meta- positioning effect in DPBN: a photophysical study

Abstract

Thephotophysical properties of the dimethyl derivative of N-Pyrrolo-4-benzonitrile (DPBN), with a change in the position of the acceptor moiety (m-DPBN), have been investigated and compared with the parent compound (p-DPBN). The values of the Stokes shift and of the excited-state dipole moment indicate that both meta- and para-DPBN possess similar excited-state properties regardless of the meta-positioning of the cyano group. The low values for the radiative rate constant suggest the presence of a strongly forbidden transition supporting the model of twisted intramolecular charge transfer (TICT) states.

7.1 Introduction

The charge transfer (CT) states occurring in donor-acceptor systems essentially are driven by two forces: (1) mesomeric forces which involve the resonance interaction between the donor-acceptor moieties. The corresponding states are usually called mesomeric intramolecular charge transfer (MICT) states [40]. (2) A second factor is dipolar solvation, which preferentially stabilizes the largest dipole, hence twisted conformations in CT systems. The CT is therefore often connected with twisting between donor and acceptor leading to so-called twisted intramolecular charge transfer (TICT) states [1, 9, 16, 85]. In this latter process, donor and acceptor moieties are completely decoupled in the CT excited state due to their perpendicular arrangement. The mesomerically stabilized CT state (MICT) always tends to be planar due to this mesomeric interaction, and it is also associated with a smaller dipole moment as compared to the TICT state [40]. This gives the possibility for observing two stable CT minima on the excited state hypersurface, and this can lead to dual fluorescence. The most well known compounds showing dual fluorescence are 4-N,N-dimethylaminobenzonitrile (DMABN) and its derivatives. It is also observed in other donor-acceptor molecules such as phenyl pyrroles with different substituents on the donor-acceptor part [29, 66, 82, 83]. According to the TICT model, the short wavelength fluorescence originates from the primarily excited “Locally Excited” (LE) state, from where the charge transfer (CT) state is accessible by an adiabatic photoreaction including a rotational motion around the bond linking the donor and acceptor moieties and leading to the long wavelength fluorescence band.


[page 81↓]

In previous work [67, 86] the excited state properties of N-Pyrrolo-4-benzonitrile (p-PBN) have been compared with those of its meta derivative, namely m-PBN. It was found that both compounds possess similar and very large excited state dipole moments. This emission from the excited CT state is of forbidden nature, consistent with the complete decoupling of the donor and acceptor orbitals.

Figure 7.1: Structures of the molecules investigated

Theoretical calculations by Zilberg et al [73] on N-Phenyl pyrroles also helped to interpret the experimental findings. These calculations support two distinct geometrical structures for two different states of CT nature. One has a quinoid (Q) structure and a planar geometry for the energy minimum consistent with the PICT model [69]. The other CT state is of anti-quinoid (AQ) nature, i.e., has lengthened central bonds and its dipole moment is larger than that of the quinoid state. The AQ state has an energy minimum with the pyrrolo group twisted by 90° with respect to the benzene ring.

In the present work, we can ask the question, how the two different CT states will be affected by a) increasing the donor strength b) introducing sterical hindrance to planarity in the ground state. This is achieved by comparing the dimethyl analogues of m- and p-PBN, namely m- and p-DPBN (see Fig. 7.1). The results show that the emissive ICT states of the PBN and DPBN are very similar (forbidden, with large dipole moment) indicating a decoupled ICT nature. Moreover, these properties are similar for meta and para substitution.

7.2 Experimental Section

Technical details about the measurement of absorption and fluorescence spectra and quantum yields are reported in chapter 3.


[page 82↓]

7.3  Results and Discussion

7.3.1 Absorption and Emission Spectra

Figure 7.2: Normalised Absorption and fluorescence spectra of p- DPBN and m-DPBN at room temperature in various solvents of different polarity (HEX = n-hexane;BCl = n-butyl chloride; THF = tetrahydrofuran).

Table 7.1: Spectral and photophysical data of p-DPBN and m-DPBN in various solvents at room temperature.

aλ = 275nm, solvent independent (used for excitation); bλ = 259nm, solvent independent (280nm is used for excitation); Percentage of error in the measurement: 10% in and 5% in τ f . c for p-PBN; d for m-PBN


[page 83↓]

The absorption and emission spectra of p-DPBN and m-DPBN are almost identical regarding the Stokes shift but the absorption maximum is slightly blue shifted in the case of m-DPBN. The spectra of the two compounds in solvents of varying polarity are shown in Fig.7.2. The corresponding photophysical parameters are collected in Table 7.1. The fluorescence spectra of both compounds show a significant solvatochromic effect, indicating a high charge transfer (CT) character of the emitting state. However, the absorption bands do not show any shift in their maximum in solvents of increasing polarity. The molar absorption coefficients were determined for both p-DPBN and m-DPBN in n-hexane as ε(λ) ) = 10350 M-1cm-1 and ε(λ)) = 6369 M-1cm-1 respectively. Donor-acceptor substituted benzenes in para position possess two close lying excited states. One is the long axis polarized, 1La- type state, and the other one is the perpendicularly polarized, 1Lb- type state with much weaker intensity which may be completely hidden under the main 1La band although the 1Lb- type state is often the lower one. The main absorption bands for p-DPBN and m-DPBN are 275 nm and 259 nm respectively. For p-PBN, the main absorption band at 287 nm can be assigned to the 1La-type excited state (S2) [66]. In the case of m-PBN [86], the main absorption band was observed at 258 nm, which was the similar case in m-DPBN. In going from the para-substituted to the meta-substituted compound, m-DPBN, the weak shoulder around 310 nm is blue shifted to around 285 nm.

The blue shift of the absorption maximum of p-DPBN as compared to p-PBN can be rationalized in terms of the partial CT character of this band connected with a quinoidal resonance structure, which is lowered in energy for the compound with the better donor (p-DPBN). In contrast, the red shifted CT band for p-PBN signifies that that the quinoidal contribution is higher in p-PBN than in p-DPBN. This is probably due to the twisted ground state structure in p-DBPN which disfavours the quinoid contribution. In contrast, for both m-DPBN and m-PBN, the quinoid contribution is less predominant or completely absent. Therefore, the main absorption band is observed at nearly the same energy.

The Stokes shift is also very large in nonpolar solvents indicating that it is not emission from the S1 state, which is visible in absorption but necessitates a photochemical reaction.

7.3.2 Fluorescence Quantum Yields and Rate Constants

The measurements of fluorescence quantum yields and lifetimes for the investigated compounds were done in various solvents of different polarity. The fluorescence [page 84↓]decay curves are monoexponential, which allows the evaluation of radiative and nonradiative rate constants according to equations 7.1 and 7.2. In eqn. 3, k corresponds to the sum of all nonradiative processes including triplet formation. The measured data and calculated photophysical values of p-DPBN and m-DPBN are collected in Table 7.1.

As one can see from this Table 7.1, the fluorescence quantum yield values show only a small variation in the range of solvents from low to high polarity for both compounds investigated. However, for p-DPBN, the quantum yield values in alkane solvents are significantly larger. For m-DPBN, the value in most other solvents does not differ from n-hexane. For p-PBN [66] and p-DPBN in n-hexane, the quantum yield values are comparable in contrast to m-DPBN as compared to m-PBN with a much larger quantum yield of 0.155 [86]. This latter observation suggests that all compounds except for m-PBN possess a similar electronic structure in this non-polar solvent, namely CT character, whereas the emission of m-PBN is of LE type n-hexane [86]. In highly polar solvents like acetonitrile, m-DPBN becomes non-fluorescent.

The k f values for both compounds decrease from alkanes to polar solvents by a factor of around 3, indicating a less allowed emission in solvent of high polarity. For both p-DPBN and m-DPBN, the significant decrease of k f values as compared to p-PBN and m-PBN(see Table 7.1) in going from low- to high-polarity solvents points to a change of the average emitting conformation of the CT state from a less twisted to or more twisted one in the excited state. For TICT systems this can occur with broad (hexane) and narrower angular distribution around the perpendicular minimum of the CT state [9].

7.3.3 Low Temperature Studies

Fluorescence measurements of m-DPBN at lower temperatures were done in the alkane mixture methylcyclohexane/isopentane (1:4), and in the medium polar solvent diethylether in order to study the temperature dependence of the emission maxima and the possibility of dual fluorescence. For the medium polar solvent EOE, with a lowering of temperature, a red shift of the emission maxima is observed (Fig. 7.3a). This thermochromic redshift can be explained by the enhancement of the dielectric constant with a lowering of [page 85↓]temperature and can be ascribed to the solvent stabilization of the CT state. The emission maxima and the associated quantum yields are collected in Table 7.2.

Table 7.2: Temperature dependence of the photophysical data of m-DPBN in the solvent mixture Methylcyclohexane:Isopentane (1:4) and in diethylether.

Figure 7.3: Temperature effects on the fluorescence spectra of m-DPBN in a) diethyl ether and in b) methylcyclohexane and isopentane mixture (1:4)

In the case of m-DPBN in the non-polar solvent mixture (MCH:IP), the emission maxima observed do not show a noticeable shift (Fig. 7.3b). A similar behaviour has been observed in the case of p-DPBN [66]. For p-PBN, a dual fluorescence was observed in the non-polar solvent mixture [66, 67, 86]. In this case, there is a continuous red shift of the [page 86↓]maximum in going down from higher to lower temperature with the disappearance of the short-wavelength shoulder [86]. Neither for m-DPBN in the alkane solvent mixture or in EOE nor for p-DPBN [66] in the alkane mixture and in the highly polar solvent ethanol in the fluid range, there is any dual fluorescence detected upon cooling. The latter observation can be rationalized by the strong donor nature of the ortho-dimethyl pyrrolo part as compared to pyrrole, which leads to a corresponding stabilization of the CT band even in alkanes such that the LE state is considerably higher in energy than the CT state. The temperature dependent quantum yield values do not show any significant trend in both measurements for different polarity. There does not seem to be a strong temperature dependent fluorescence quenching channel in m-DPBN.

7.3.4 Excited State Dipole Moments

The excited state dipole moments μe are calculated from a plot of the solvatochromic shift of the emission maxima versus solvent polarity (see Fig. 7.4), and are calculated using the Mataga equation (eq. 7.3), where and are the ground and excited state dipole moments, respectively.

Figure 7.4:Mataga plot of p-DPBN and m-DPBN in various solvents of different polarity.


[page 87↓]

Table 7.3: Dipole moments for the ground and excited states derived for p-DPBN and m-DPBN from the Mataga plot (see fig. 7.4).

a) from eq. (7.4) by assuming equal densities; b) calculated from AM1 calculation

where h is the Planck’s constant, is permittivity constant of vacuum and c is the velocity of light. is the solvent polarity parameter, consisting of dielectric constant ε and refractive index n. The Onsager radii ‘a’ for p-DPBN and m-DPBN were calculated from the mass-density formula eq. (7.4) by assuming equal densities for both compounds, and the ground state dipole moments, μ g, are calculated by using the AM1 semiempirical method embedded in the Ampac software package [44]. The resulting μ e values of p-DPBN and m-DPBN are shown in Table 7.3. By taking similar Onsager radii for both compounds, the calculated excited state dipole moments values are close to 14 D. The high dipole moment values in the excited state indicate that CT state exist in these compounds. This value is comparable to the parent compound PBN (14.8 D) [86]. These observations suggest that the o-dimethyl pyrrole derivatives of PBN possess similar CT excited state properties.

7.4 Discussion

The lower fluorescence quantum yield values of m-DPBN as compared to p-DPBN are linked to significantly lower k f values (Table 7.1) whereas the non-radiative rates are similar. This strong difference between meta and para substitution is not present for the [page 88↓]corresponding PBN pair of compounds [86]. At present, the reason for this different behaviour is unclear.

In principle, another type of CT state can also be discussed, namely the highly coupled and mesomerically stabilized CT state with a preference for the planar conformation (mesomeric intramolecular charge transfer (MICT) state [40]). In this case, high k f values would be expected, and the population of a MICT state can therefore be ruled out. A further possibility is the so-called planar intramolecular charge transfer (PICT) state [69]. This model has been formulated in conjunction with a crossing of both S1 (1Lb-type) and S2 (1La-type) states, and is thought to possess a planar quinoid structure with high coupling and allowed emissive character. These expectations are not supported by our observations on k f . Zilberg et al [73] proposed a model where the most stable CT state is twisted and is of antiquinoid nature and decoupled. This latter model is consistent with the findings in this work.

7.5 Conclusion

Both p-DPBN and m-DPBN have similar values in the amount of Stokes shift and polarity induced red shifts evidencing their large excited-state dipole moments. The lower k f values of m-DPBN and p-DPBN as compared to the PBN pair give evidence that the transition is more forbidden in the DPBN pair. Since the introduction of the two methyl groups in ortho position considerably increases the average twist angle of the donor moiety both in the ground and the excited state, this sterical influence will also narrow the rotational distribution function around the perpendicular TICT minimum and therefore lead to the reduced transition moment values [1, 9]. In summary, it is concluded that the DPBN compounds possess similar excited state properties irrespective of the position of the cyano group in the acceptor moiety and similarly as the PBN pair form a TICT state of forbidden emissive properties and very large dipole moment.


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