Meta-positioning effect in sterically hindered N-phenyl-pyrroles: A photophysical study

Transcription

1 Chemical Physics Letters 412 (2005) Meta-positioning effect in sterically hindered -phenyl-pyrroles: A photophysical study Sukumaran Murali, Wolfgang Rettig * Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-str. 2, D Berlin, Germany Received 13 May 2005; in final form 16 June 2005 Available online 19 July 2005 Abstract The photophysical properties of the dimethyl derivative of -pyrrolo-4-benzonitrile (DPB), with a change in the position of the acceptor moiety (m-dpb), have been investigated and compared with the parent compound (p-dpb). The values of the Stokes shift and of the excited-state dipole moment indicate that both meta- and para-dpb 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 states. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction The charge transfer (CT) states occurring in donor acceptor systems are essentially driven by two forces: (1) mesomeric forces which involve the resonance interaction between the donor acceptor moieties. The corresponding states have been called mesomeric intramolecular charge transfer (MICT) states [1]. (2) A further important factor is dipolar solvation, which preferentially stabilizes the largest dipole. This occurs for twisted conformations, hence CT is often connected with twisting between donor and acceptor leading to so-called twisted intramolecular charge transfer (TICT) states [2 5]. 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 (MICT) state, on the other hand, 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 [1]. This gives the possibility for observing two stable CT minima on the excited-state hypersurface, and this can lead to dual fluorescence. The * Corresponding author. Fax: address: rettig@chemie.hu-berlin.de (W. Rettig). most well-known compounds showing dual fluorescence are 4-,-dimethylaminobenzonitrile (DMAB) and its derivatives. It is also observed in other donor acceptor molecules such as phenyl pyrroles with different substituents on the donor acceptor part [6 9]. According to the TICT model, the short wavelength fluorescence originates from the primarily excited Ôlocally excitedõ (LE) state (which is of MICT nature in many cases), 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. In the previous work [10,11], the excited-state properties of -pyrrolo-4-benzonitrile (p-pb) have been compared with those of its meta derivative, namely m-pb. It was found that both compounds possess similar and very large excited-state dipole moments. The emission from this excited CT state is of forbidden nature, consistent with the complete decoupling of the donor and acceptor orbitals. Theoretical calculations by Zilberg and Haas [12] on -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 /$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi: /j.cplett

2 136 S. Murali, W. Rettig / Chemical Physics Letters 412 (2005) geometry for the energy minimum consistent with the PICT model [13]. 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 Letter, we ask the question, how the two different CT states will be affected (a) by increasing the donor strength, (b) by introducing a sterical hindrance to planarity in the ground state. This is achieved by comparing the dimethyl analogues of m- and p-pb, namely m- and p-dpb (see Scheme 1). The results show that the emissive ICT states of PB and DPB are very similar (forbidden, with large dipole moment) indicating a decoupled ICT nature. Moreover, these properties are similar for meta and para substitution. 2. Experimental 2.1. Materials The compounds (for structures and abbreviations see Scheme 1) were synthesized by using the procedures described in [14,15] and were confirmed by elemental analysis and MR. The absence of possible traces of impurities was verified by thin layer chromatography and fluorescence spectra excited at different wavelengths. All solvents used were of spectrophotometric grade (Merck Uvasol) Steady-state spectra Absorption spectra were measured on an ATI UI- CAM UV Series Spectrometer UV All steadystate fluorescence spectra were measured by using an SLM Aminco-Bowman series 2 spectrofluorimeter fitted with a 150 W Xenon lamp, 2 nm excitation and emission band pass. All fluorescence spectra were corrected for detector response Fluorescence quantum yields The quantum yields of fluorescence were measured relative to quinine bisulfate in 0.1 H 2 SO 4 and calculated on the basis of Eq. (1) [16] R / f ¼ / 0 n 2 A 0 I f ðk f Þ dk R f f n 2 0 A I 0 f ðk ; ð1þ fþ dk f where n 0 and n are the refractive indices of the solvent, A 0 and A (60.1) are the absorbances, / 0 f ð¼ 52%Þ [17] and / f are the quantum yields, and the integrals denote the (computed) area of the corrected fluorescence bands, each parameter being for the standard (index 0) and the sample solution, respectively. The determination of lowtemperature fluorescence quantum yields took into account the temperature dependence of the refractive index as well as the increasing solvent contraction (density increase) with decreasing temperature. The relative experimental error of the quantum yields is around ±10% Fluorescence lifetime The fluorescence decay measurements were performed using time correlated single photon counting (TCSPC) [18] with synchrotron radiation from the Berlin Synchrotron facility BESSY II as excitation light source in conjunction with a monochromator (Jobin Yvon, II, 10 UV). BESSY II delivers a 1.25 MHz pulse train with characteristic pulse widths of ps. The fluorescence and decays were detected by a microchannel plate photomultiplier (MCP, Hamamatsu R U-01) cooled to 30 C, coupled to an emission monochromator (Jobin Yvon II, 10 VIR) by means of quartz fibre optics. The signal from a constant fraction discriminator (CFD, Tennelec 454) was used as the start pulse for the time-to-amplitude converter (TAC, Tennelec TC864) operating in the reverse mode. The BESSY II synchronisation pulse was used as the stop pulse. The MCP pulses were amplified by an amplifier (IA 10386) and coupled into the CFD. A multichannel analyser (Fast Comtec MCDLAP) was used for data accumulation. The instrument response function was obtained by the detection of Rayleigh scattered light in a scattering solution and had a width of 120 ps. The decays were analysed by the Ôleast squaresõ and iterative reconvolution method on the basis of the Marquardt Levenberg algorithm, which is implemented in the commercial global analysis program. The quality of the exponential fits was evaluated on the basis of the reduced v 2 values. 3. Results and discussion 3.1. Absorption and emission spectra C C C C PB p-pb m-pb m-dpb p-dpb Scheme 1. The absorption and emission spectra of p-dpb and m-dpb are almost identical regarding the Stokes shift but the absorption maximum is slightly blue shifted in the case of m-dpb. The spectra of the two compounds in solvents of varying polarity are shown in Fig. 1. The fluorescence spectra of both compounds show a signifi-

3 S. Murali, W. Rettig / Chemical Physics Letters 412 (2005) ormalised abs./flu. Intensity Abs. Emission p-dpb Abs. Emission Hex THF m-dpb Hex THF Wavelength (nm) Fig. 1. ormalized absorption and fluorescence spectra of p-dpb and m-dpb at room temperature in various solvents of different polarity (HEX, n-hexane;, n-butyl chloride; THF, tetrahydrofurane). cant 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- DPB and m-dpb in n-hexane as eðk max 275 Þ¼ M 1 cm 1 and eðk max 259 Þ¼6369 M 1 cm 1, respectively. Donor acceptor substituted benzenes in para position possess two close lying excited states. One is the long axis polarized 1 L a -type state, and the other one is the perpendicularly polarized 1 L b -type state with much weaker intensity which can be completely hidden under the main 1 L a band although the 1 L b -type state is often the energetically lower one. For p-pb, the main absorption band at 287 nm can be assigned to the 1 L a - type excited state (S 2 ) [6]. In the case of m-pb [10,11], the main absorption band is observed at 258 nm, i.e., blue shifted with respect to the same band in p-pb. The main absorption bands for p-dpb and m-dpb are at 275 and 259 nm, respectively, i.e., a blue shift is observed again for the meta derivative. In going from the para-substituted to the meta-substituted compound, m-dpb, also the weak shoulder at around 310 nm is blue shifted to around 285 nm. Also in comparing the two para derivatives, the main band of p-dbp (275 nm) is blue shifted with respect to that of p-pb (287 nm). With respect to the blue shift of the absorption maximum of p-dpb as compared to p-pb, two opposing effects have to be taken into account: (a) as the excited state possesses partial CT character connected with a quinoidal resonance structure, it should be lowered in energy for the compound with the better donor (p-dpb); (b) an opposing effect is the twisted ground-state structure in p-dbp which disfavours the quinoidal contribution and therefore leads to a blue-shift. The experiment shows a red shifted CT band for p-pb, which signifies that that the quinoidal contribution is higher in p-pb than in p-dpb, i.e., that the ground state twisting is the more important factor. In contrast, for both m-dpb and m-pb, the quinoidal contribution is less predominant or completely absent. Therefore, the main absorption band is observed at nearly the same energy. The Stokes shifts Dm S for both p-dpb and m-dpb are very large in polar solvents as compared to nonpolar solvents indicating a considerable energetic stabilization of the excited state in polar solvents. Moreover, the similar Stokes shift values for both meta and para compounds indicate that a similar stabilization occurs in both compounds 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 decay curves are monoexponential, which allows the evaluation of radiative and non-radiative rate constants according to Eqs. (2) and (3). In Eq. (3), k tot nr corresponds to the sum of all non-radiative processes including triplet formation. The measured data and calculated photophysical values of p-dpb and m-dpb are collected in Table 1 k f ¼ / f =s f ; ð2þ k tot nr ¼ k f ð/ 1 f 1Þ. ð3þ As one can see from this table, the fluorescence quantum yield values show only a small variation in the range of medium polar to highly polar solvents for both compounds investigated. However, for p-dpb, the quantum yield values in non-polar alkane solvents are significantly larger. This increase was also found for p-pb [6]. On the other hand, for m-dpb the value in most other solvents does not differ from that in n-hexane. For p-pb [6] and p-dpb in n-hexane, the quantum yield values are comparable in contrast to m-dpb (/ f = 0.004) as compared with the much larger value (0.155) for m-pb [10]. This latter observation suggests that all compounds except for m-pb possess a similar electronic structure in this non-polar solvent, namely CT character, whereas the emission of m-pb is of LE type in n-hexane [10]. In highly polar solvents like acetonitrile, p-dpb and m-dpb are non-fluorescent. The above interpretation is supported by the k f values in Table 1. A general observation is that the k f values for the dimethylated compounds are significantly smaller than those of the non-methylated ones. Moreover, regarding the effect of alkane solvents, k f values decrease only moderately (factor around 3) for both compounds in polar solvents, indicating a somewhat less allowed

4 138 S. Murali, W. Rettig / Chemical Physics Letters 412 (2005) Table 1 Spectral and photophysical data of p-dpb and m-dpb in various solvents at room temperature Solvent k em max ðnmþ Dm st (10 3 cm 1 ) / f s f (ns) k f (10 6 s 1 ) (dimethylated compounds), (p-,m-dpb) k f (10 6 s 1 ) (non-methylated compounds), (p-,m-pb) p-dpb a Hex c 3.33 EOE c THF DCM c 1.01 AC <10 4 m-dpb b Hex d 2.80 BOB EOE d THF d 1.05 AC <10 4 a k abs b max k abs ¼ 275 nm, solvent independent (used for excitation). max ¼ 259 nm, solvent independent (280 nm is used for excitation); percentage of error in the measurement: 10% in / f and 5% in s f. c For p-pb (see [10]). d For m-pb (see [10]). k nr (10 8 s 1 ) emission in solvents of high polarity. This effect is somewhat higher in p-pb (factor around 5). Only for m- PB, a very large effect is observed (around 10). The medium-sized decrease of k f values in polar solvents for both dimethylated compounds p-dpb and m-dpb is consistent with the assumption of a forbidden CT state connected with a change of the average emitting conformation from a less twisted to a more twisted one in the excited state. For TICT systems this can occur with broad (hexane) and narrower angular distribution around a perpendicular minimum of the CT state for both solvents [2]. The smaller size of the k f values for the dimethylated compounds in all solvents can likewise be explained by a narrower angular distribution function induced by the sterically hindering methyl groups. This latter fact directly points to the perpendicular minimum of the emissive state. The larger hexane effect in p-pb can be interpreted by a partial contribution of a further state of more allowed character. In effect, the hexane spectra of p- PB clearly show a small short wavelength shoulder, i.e., dual fluorescence indicative of two emissive states [6,10,11]. This shoulder and second emissive state is absent for p- and m-dpb (see Figs. 1 and 2). For m- PB, the very large increase of k f in n-hexane is consistent with the spectral properties of an LE state [10]. The lower fluorescence quantum yield values of m-dpb as compared to p-dpb are linked to significantly lower k f values (Table 1) whereas the nonradiative rates are similar. This strong difference between meta and para substitution is not present for the corresponding PB pair of compounds [10]. At present, the reason for this different behaviour is unclear. Intensity (a.u.) a b Low temperature studies Wavelength (nm) 298K 273K 253K 233K 213K 193K 173K 298K 273K 253K 233K 213K 193K 173K Fig. 2. Temperature effects on the fluorescence spectra of m-dpb in (a) diethyl ether and in (b) methylcyclohexane/isopentane mixture (1:4); the second order Rayleigh scattering in the region nm has been omitted for (b). Fluorescence measurements of m-dpb 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. 2a). This thermochromic redshift can be explained by the enhancement of the dielectric constant with a lowering of 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 2.

5 S. Murali, W. Rettig / Chemical Physics Letters 412 (2005) Table 2 Temperature dependence of the photophysical data of m-dpb in the solvent mixture methylcyclohexane:isopentane (1:4) and in diethylether Solvent 298 K 273 K 253 K 233 K 213 K 193 K 173 K MCH/IP EOE k em max ðnmþ / f k em max ðnmþ / f In the case of m-dpb in the non-polar solvent mixture (MCH:IP), the emission maxima observed do not show a noticeable shift (Fig. 2b). A similar behaviour has been observed in the case of p-dpb [6]. For p- PB, a dual fluorescence was observed in the non-polar solvent mixture [6,10,11]. In this case, there is a continuous red shift of the maximum in going from higher to lower temperature with the disappearance of the shortwavelength shoulder [10]. either for m-dpb in the alkane solvent mixture or in EOE nor for p-dpb [6] 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 orthodimethyl pyrrolo part as compared to pyrrole, which leads to a corresponding stabilization of the CT state even in alkanes such that the LE state is considerably higher in energy than the CT one. The temperature dependent quantum yield values for m-dpb 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-dpb Excited-state dipole moments The excited-state dipole moments l e are calculated from a plot of the solvatochromic shift of the emission maxima vs. solvent polarity (see Fig. 3), and are calculated using the Mataga equation (Eq. (4)), where l g and l e are the ground and excited-state dipole moments, respectively m f ¼ 2Df 0 4p2 0 hca l eðl 3 e l g Þþconst. ð4þ with Df 0 ¼ðe 1Þ=ð2eþ1Þ 1=2ðn 2 1Þ=ð2n 2 þ 1Þ p a ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 3M=4p A q; ð5þ where h is the PlanckÕs constant, 2 0 the permittivity constant of vacuum and c is the velocity of light. Df 0 is the solvent polarity parameter, consisting of the dielectric constant e and the refractive index n. The Onsager radii ÔaÕ for p-dpb and m-dpb were calculated from the mass density formula Eq. (5) by assuming equal densities q for both compounds, and the ground-state dipole moments, l g, are calculated by using the AM1 semiempirical method embedded in the AMPAC software ν max ( cm ) f HEX HEX BOB EOE f ' EOE package [19]. The resulting l e values of p-dpb and m-dpb are shown in Table 3. By taking similar Onsager radii for both compounds, the calculated excitedstate dipole moments values are close to 14 D. The high dipole moment values in the excited state indicate that the excited state is of CT nature in these compounds. This dipole moment value is comparable to that of the parent compound PB (14.8 D) [10]. These observations suggest that the dimethylated pyrrole derivatives of PB possess similar CT excited-state properties Conformation of the emitting CT state p-dpb Etac THF DCM m-dpb THF Fig. 3. Mataga plot of p-dpb and m-dpb in various solvents of different polarity (HEX, n-hexane;, n-butyl chloride; Etac, ethyl acetate; EOE, diethyl ether; BOB, n-dibutyl ether; DCM, dichloromethane; THF, tetrahydrofurane). Table 3 Dipole moments for the ground and excited states derived for p-dpb and m-dpb from the Mataga plot (see Fig. 3) a (Å) a Slope (10 3 cm 1 ) l g (D) b l e (D) p-dpb m-dpb a From Eq. (2) by assuming equal densities. b Calculated from AM1 calculation. 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 [1]). In this case, high k f values would be expected, and the population of a MICT state can there-

6 140 S. Murali, W. Rettig / Chemical Physics Letters 412 (2005) fore be ruled out. A further possibility is the so-called planar intramolecular charge transfer (PICT) state [13]. This model has been formulated initially in conjunction with a crossing of both S 1 ( 1 L b -type) and S 2 ( 1 L a -type) states, and is therefore connected with a planar quinoid structure with high coupling and allowed emissive character, identical to the MICT state, which is, however, more general because it also explains cases where the planar conformation cannot be reached in the excited state as in p- and m-dpb and further sterically hindered compounds [20,1]. These expectations are not supported by the small observed values of k f. Recently, Zilberg and Haas [12] proposed a model from CASSCF calculations where the most stable CT state is twisted and is of antiquinoid nature and decoupled. This latter model is consistent with the very small k f values and the decrease of k f for the dimethylated compounds as determined in this Letter. The observation of a CT emission band in a rigidly planar phenyl pyrrole [21] shows that dual emission can also occur for planar geometries. A possible explanation is that the CT emission is of MICT nature with the expectation of allowed emission. A further possible explanation is that also at planar geometries, a CT state with forbidden emission can be populated. Such a state is possible for pyrroles (in contrast to DMAB) even for the planar geometry because the pyrrole HOMO orbital and one of the benzonitrile unoccupied orbitals is completely localized on the corresponding moiety [22,7]. At present, it cannot be decided yet regarding these two possibilities, because the radiative rate constant has not been published in [21]. 4. Conclusion Both p-dpb and m-dpb have similar values of polarity-induced red shifts evidencing their large excited-state dipole moments. The lower k f values of m- DPB and p-dpb as compared to the PB pair give evidence that the transition is more forbidden in the DPB 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 narrow the rotational distribution function around the perpendicular TICT minimum and therefore explains the reduced transition moment values [2,4]. In summary, it can be concluded that both p- and m-dpb possess similar excited-state properties irrespective of the position of the cyano group in the acceptor moiety. Similarly to the p- and m-pb pair, they form a TICT state of forbidden emissive properties and very large dipole moment. Acknowledgements We kindly acknowledge Sascha Jautze for the synthesis of m-dpb and BESSY for beamtime. References [1] W. Weigel, W. Rettig, M. Dekhtyar, C. Modrakowski, M. Beinhoff, A.D. Schlueter, J. Phys. Chem. A 107 (2003) [2] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. USA 103 (2003) [3] Z.R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D.J. Cowley, W. Baumann, ouv. J. Chim. 3 (1979) 443. [4] W. Rettig, Angew. Chem., Int. Ed. 25 (1986) 971. [5] A. Sarkar, S. Chakravorti, Chem. Phys. Lett. 235 (1995) 195. [6] C. Cornelissen-Gude, W. Rettig, J. Phys. Chem. A 102 (1998) [7] W. Rettig, J. Mol. Struct. 84 (1982) 303. [8] W. Rettig, F. Marschner, ew J. Chem. 14 (1990) 819. [9] W. Rettig, F. Marschner, ouv. J. Chim. 7 (1983) 425. [10] S. Murali, W. Rettig (to be published). [11] T. Yoshihara, V.A. Galievsky, S.I. Druzhinin, S. Saha, K.A. Zachariasse, Photochem. Photobiol. Sci. 2 (2003) 342. [12] S. Zilberg, Y. Haas, J. Phys. Chem. A 106 (2002) 1. [13] K.A. Zachariasse, Chem. Phys. Lett. 320 (2000) 8. [14] H. Fischer, H. Orth., Die Chemie des Pyrrols, Bd. I, Akademische Verlagsgesellschaft, Leipzig, [15] G. Penieres, V. Soto, C. Alvarez, O. Garcia, J.G. Garcia, Heterocycl. Commun. 4 (1998) 31. [16] C.A. Parker, Photoluminescence of Solutions, Elsevier, Amsterdam, [17] S.R. Meech, D. Phillips, J. Photochem. 23 (1983) 193. [18] D.V. OÕConnor, D. Phillips, Time Correlated Single Photon Counting, Academic Press, London, [19] AMPAC 6.0 and AMPAC Semichem, Inc., Shawnee, USA, [20] M. Maus, W. Rettig, D. Bonafoux, R. Lapouyade, J. Phys. Chem. A 103 (1999) [21] T. Yoshihara, S.I. Druzhinin, K.A. Zachariasse, J. Am. Chem. Soc. 126 (2004) [22] A.B.J. Parusel, Phys. Chem. Chem. Phys. 2 (2000) 5545.