Reply to Comment on StructureProperty Relationships for

FOCAS Institute and School of Physics, Dublin Institute of. Technology, KeVin Street, Dublin 8, Ireland. ReceiVed: August 16, 2005; In Final Form: Oct...
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J. Phys. Chem. B 2005, 109, 22082-22083

Reply to “Comment on ‘Structure-Property Relationships for Electron-Vibrational Coupling in Conjugated Organic Oligomeric Systems’” Luke O’Neill* and Hugh J. Byrne FOCAS Institute and School of Physics, Dublin Institute of Technology, KeVin Street, Dublin 8, Ireland ReceiVed: August 16, 2005; In Final Form: October 3, 2005 In response to the comment authored by Gierschner,1 time is taken to address the points outlined (1-5) and discuss their relevance and importance as we see them to the work published. While we accept point 1, we do not agree that points 2-5 require any further comment or correction. Point 1. The authors accept that this point is valid. The spectra and accompanying text (i.e., “the increased planarity of the backbone with increasing conjugation”) incorporated in the paper were added in a late revision, and inadvertently the spectra and text used were not appropriate or representative of the experimental data as a whole. Figure 1 shows the actual spectra that should have been included. The data employed in the subsequent plots of transition energies and Stokes shifts were not derived from the published spectra and so are not affected. As seen in Figure 1, there is no vibrational substructure present in the phenyl series, as correctly pointed out by Dr. Gierschner. Furthermore, there is no long wavelength tail on the spectra that can be indicative of scattering from aggregates. Previous concentration dependence studies3 have indicated that both terphenyl and anthracene suffered no loss of fluorescence attributable to aggregation up to 10-4 M in toluene (see also ref 11). Thus we are confident that the spectra in the revised figure represent those of molecular solutions. Originally, no spectra were included, as the 0-0 absorption peak is well-known for both the acene and phenyl series.2 The absorption and fluorescence were to be used mainly in an introductory manner, not as the principle point of the paper, and when all were included the paper became unnecessarily lengthy. The spectra of one representative series was included in response to reviewer/s suggestion. For reference, Figure 2 shows the acene spectral progression. These correlate well with literature. Point 2. The interpretation of much of the data is based on the nearly free electron model as stated in the paper. This model was used due to its simplicity and the accuracy to which it was shown in the application by Kuhn4 and furthermore by Rustagi.5 The limitations of the use of this model in terms of the its dependence on monomer structure (p2 line 22) and the effects of electron-vibrational coupling leading to quasi-particles such as solitons, polarons, and bipolarons, and thus limiting the effective conjugation in the longer chain limit, are clearly discussed and referenced in the text of the paper. That the molecules are well behaved in terms of this model does imply “that excited states are distributed over the extent of the oligomer and are thus molecular in nature”. This in turn indicates that these are ideal model systems to derive relationships between spectroscopically observable parameters and that the use of a simpler model does not negate the relationships shown. * Corresponding author. [email protected]

Figure 1.

Figure 2.

Point 3. It has already been recognized and accepted in the paper that use of both the Franck-Condon analysis and HuangRhys (page 12688 line 16, page 12687 line 11, respectively) can be used for a more detailed examination of the vibrational progression in these systems. As such, the comments are insightful but were known and accepted by both the authors of the paper and the reviewers. The aim was to establish if the Stokes shift did exhibit a well-definable relationship with both chain length progression and backbone variation, which is shown. For more novel systems, the Franck-Condon (FC), Huang Rhys (HR), and Stokes shift analyses will be carried out, but since FC and HR analyses have already been published on the systems studied here, the need to rehash the results seemed unnecessary. Point 4. In the treatment of the number of vibrational modes, we have taken the simplistic guidelines of 3n - 5 for linear molecules versus 3n - 6, normally quoted for nonlinear molecules.6 It is accepted that all organic polymers are polyatomic and nonlinear in nature; however, this slight oversight in no way affects the results. Any difference in the expression used will have negligible effect on the relationships elucidated. Point 5. The authors are confident that results and discussion are not “in contradiction to the known photophysics in these systems”. The Stickler-Berg equation has origin in the Einstein coefficient approach to photophysics of idealized systems, which

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Comments does indeed predict dependence of the absorption cross-section and therefore the radiative decay rate on the transition energy (see for example refs 2 and 8). This approach does not, however, take into account any degree of freedom of the atomic skeleton, and hence does not make any predictions about Stokes shift or subsequent nonradiative decay. The excited-state lifetime is dependent on a combination of the radiative decay rate and the nonradiative decay rate, and although a decrease of the mean transition energy via a Stokes shift can have some effect of decreasing the radiative rate, the effect of increasing the decay rate due to the increased number of available nonradiative decay channels also contributes. In this context the so-called energy gap law for nonradiative transitions (although not quoted in the manuscript) can be evoked, whereby the internal conversion rate increases as the energy gap decreases. Implicit in this equation is a dependence on the number of vibrational modes available for electron vibrational coupling. As outlined in the text (page 3 line 11), the Huang Rhys factor (and therefore Stokes shift) is also dependent on the number of atoms9 (and therefore vibrational modes), linking the efficiency of this form of electron-phonon coupling to the vibrational structure. It is clearly stated in the text that the carbon-carbon backbone vibrations (ring stretching and vibrating) are indeed most strongly coupled to the excitation and are of most interest in the exploration of nonradiative decay, but also that “Although all modes will not couple equally to the electronic excitation, the role of the C-H vibrational modes in the determination of the nonradiative decay rate has been demonstrated in deuterated systems whereby an increase of the radiationless transition rates without effect on the band-gap is affected”, linking not just the modes coupled to the electronic excitation with the nonradiative decay process. Indeed it has been shown that the efficiency of nonradiative decay depends also on the vibrational coupling to the solvent environment10 (see also ref 13). Far from contradicting the known photophysics of these systems, this work demonstrates the correlation between the Stokes shift as well as the Raman scattering with molecular structure, both of which have origin in electron vibrational coupling. Nonradiative decay is implicitly linked with this coupling of the electronic state to the vibrational structure of the molecule, and the correlations identified here indicate that “vibrational activity and thus nonradiative decay processes are controllable through molecular structure (abstract)”, that “a further understanding of the nonradiative decay processes in these materials, and their structural dependence (introduction)” can be developed and that ultimately “nonradiative processes could be shown to be monitored though relatively routine spectroscopic techniques (conclusion)”. Erratum Based on the acceptance of Point 1, we propose the following erratum. We apologize for the inconvenience, which arose from an oversight while going through the phases of refereeing.

J. Phys. Chem. B, Vol. 109, No. 46, 2005 22083 The correct wording of the first paragraph of the Results and Discussion section and Figure 2a (on page 12686 of ref 14) are given below. “In Figure 2a, the absorbance spectra of the phenyl series are shown for example. It is evident that with increased conjugation there is a considerable bathochromatic shift, as predicted by Kuhn et al.16 Biphenyl has an absorption shoulder at 270 nm, however, by the time the series reaches a six-ringed structure (sexiphenyl), the longest wavelength absorption peak has shifted to 380 nm. By addition of four phenyl monomer units, the band-gap has reduced by 110 nm (1.32 eV). There is a notable absence of vibronic substructure in the relatively flexible phenyl series compared to the more rigid planar acene series.17”

Figure 2a. Progression of absorption spectra of phenyl oligomers from 2 to 6 repeat units.

References and Notes (1) Gierschner, J. J. Phys. Chem. B 2005, 109, 22081. (2) Organic Molecular Photophysics; Birks, J. B.; Ed.; Wiley-Interscience: New York, 1973. (3) Hedderman, T. G.; Keogh, S. M.; Chambers, G.; Byrne, H. J. Phys. Chem. B 2004, 108, 49. (4) Kuhn, H. Fortsch. Chem. Org. Naturstoffe 1958, 16, 169. (5) Rustagi, C.; Ducuing, K. Opt. Commun. 1974 10(3), 258-261. (6) (7) Karabunarliev, S.; Baumgarten, M.; Bittner, E. R.; Mu¨llen, K. J. Chem. Phys. 2000, 113, 11372. (8) Henderson, B.; Imbusch, G. F. Optical Spectroscopy of Inorganic Solids; Clarendon: New York, 1989. (9) Lee, J. Y.; Lee, S. J.; Kim, K. S. J. Chem. Phys. 1997, 107(11). Yu, J. Synth. Met. 1997, 85 1115-1116. (10) Henderson, K.; Kretsch, K. P.; Drury, A.; Maier, S.; Davey, A. P.; Blau, W.; Byrne, H. J. Synth. Met. 2000, 111-112, 559-561. (11) Hedderman, T. G.; Keogh, S. M.; Chambers, G.; Byrne, H. J.; SPIE 2005, 12, 5826. (12) Frank, H. A.; Desamero, R. Z. B.; Chynwat, V.; Gebhard, R.; van der Hoef, I.; Jansen, F. J.; Lugtenburg, J.; Gosztola, D.; Wasielewski, M. R. J. Phys. Chem. A 1997, 101, 149-157. (13) Henderson, K., et al. Synth. Met. 2001, 119, 555-556. (14) O’Neill, L.; Byrne, H. J. J. Phys. Chem. B 2005, 109, 1268512690.