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The excitation wavelength dependent fluorescence of porphyrins
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Chemical Physics Letters 454 (2008) 223–228 www.elsevier.com/locate/cplett
The excitation wavelength dependent fluorescence of porphyrins Mahesh Uttamlal *, A. Sheila Holmes-Smith * Department of Biological and Biomedical Sciences, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, Scotland, UK Received 16 August 2007; in final form 5 February 2008 Available online 13 February 2008
Abstract Spectroscopic studies of meso-tetraphenylporphyrin and other meso-substituted porphyrin derivatives have revealed a dual fluorescence emission process. The first emission is the usual 1(p p*) transition. The second alternative emission process is proposed to arise following tautomerism of the inner macrocyclic hydrogen atoms via an atom tunnelling process. This process occurs when exciting at virtually all wavelengths in the Q band region but, is at its most intense when exciting at the Qx(0,0) band and is not observed when exciting at the Soret band maximum, i.e., the emission spectrum of TPP is dependent on the excitation wavelength. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The absorbance and fluorescence emission spectra of meso-tetraphenylporphyrin (TPP) [1–4] are shown in Fig. 1. The accepted interpretation is as follows: there is strong absorbance in the near-UV called the Soret or B band and denotes the S2 S0 transition. There are three other absorption bands (not shown) in the deep UV known as the N, L and M bands. In the visible region of the spectrum there are four weak bands called the Q bands indicating S1 S0 transitions. The fluorescence emission spectrum consists of two strong emission bands at k > Qx(0,0). On closer inspection there are also other smaller fluorescence bands at wavelengths shorter and longer than the main bands. The spectra are typical of many free-base porphyrins. A theoretical description for the absorbance spectrum of free-base porphyrins has been reported by Gouterman based on p* p electronic transitions between the two highest occupied (HOMO) and the two lowest unoccupied (LUMO) molecular orbitals within an 18 membered macrocyclic ring [5–9]. The model explains the existence of the number of B and Q bands. Further interpretation of the model, in partic*
Corresponding authors. Fax: +44 141 331 3653. E-mail addresses: [email protected] (M. Uttamlal), a.s.smith@ gcal.ac.uk (A. Sheila Holmes-Smith). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.02.012
ular to the type of transitions has been discussed by Corwin et al. [10]. In the analysis they argue the absorption bands potentially arise from p* n and p* l transitions of lone pair electrons on the N groups of the macrocyclic ring. Computational methods have also been developed to predict the spectrum of porphin and substituted porphyrins to a varying degree of success [11]. To date no one model has been able to accurately predict the ground-state absorption spectrum of free-base porphyrins. Tautomerism in porphyrins has also been investigated experimentally and theoretically. Tautomers of porphyrins involving the exchange of the inner two hydrogen atoms can occur in the ground state via thermal activation or by a photo-induced process. This process has been studied both experimentally and theoretically for porphine [12–15] and porphycene [16,17] and is not yet fully understood, although it is now generally accepted that double proton transfer in porphine is via a two-step process involving a metastable cis intermediate. In this work, we have carried out a series of excitation and emission spectra of air saturated 5 lM TPP in toluene at room temperature (approx. 20 °C) and show how it can reveal new information about electronic transitions at (predominantly) the Qx(0,0) band not seen in the absorbance spectrum of the porphyrin. Our new observations are interpreted to potentially arise following tautomerism in the excited state. Following our observations for TPP we have
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3. Results and discussion
Fig. 1. Absorbance and fluorescence (kex = 418 nm) spectra of TPP in toluene. The TPP concentrations were 5 lM and 0.5 lM for absorbance and fluorescence, respectively.
made some preliminary measurements on other substituted porphyrins. 2. Experimental All chemicals and solutions were purchased from commercial suppliers and used without further purification unless otherwise stated. Porphyrin purity was checked by thin layer chromatography (TLC) before use and where necessary purified using preparative TLC. Absorbance spectra were obtained using a Lambda 2 (Perkin Elmer) dual-beam uv/vis spectrometer. Fluorescence spectra were collected on the Fluoromax 3 (HORIBA Jobin Yvon Ltd) spectrometer. The bandpass was 1.5 nm or 2 nm for both the excitation and emission light. The data shown is the S/R value where S and R are the signals from sample and lamp, respectively. Background fluorescence arising from the solvent has been subtracted. Signal smoothing has been applied using a means-movement procedure (convolution width = ±2 nm). Extreme care has been taken to ensure artefact free spectra. All instrumental and solvent effects have been removed by careful sample preparation and correction of the spectra for solvent Raman peaks and the quantum efficiency of the detector at the longer emission wavelengths. Luminescence lifetime measurements were performed on a 5000 U system (HORIBA Jobin Yvon IBH Ltd) in the single photon counting (TCSPC) mode with a time calibration of 0.056 ns per channel. A NanoLED light source (HORIBA Jobin Yvon IBH Ltd) of kmax 495 nm (FWHM = 1.3 ns) or a NanoLED laser source of kmax = 650 nm (FWHM = 288 ps) was used for 515 nm and 650 nm excitation, respectively. The repetition rate for both sources was 1 MHz. Data analysis was performed using the HORIBA Jobin Yvon IBH Ltd Data Analysis software, DAS6. All measurements were carried out in air saturated solutions at room temperature unless otherwise stated.
Fig. 2a shows a series of normalised corrected excitation spectra in the Q-band region obtained by monitoring the fluorescence emission spectra at the indicated wavelength. The absorbance spectrum is also included for comparison purposes. The figure shows that the excitation spectrum collected when monitoring the emission at Q(0,1) and the absorption spectrum have the same peak wavelengths and relative intensities for each Q band. The figure also shows that the relative intensity of the Qx(0,0) band is dependent upon the emission monitoring wavelength. There are also variations at the other Q bands but to a smaller extent. The maximum intensity for the Qx(0,0) excitation band occurs when monitoring at kem = 657 nm at which point the Qx(0,0) is larger than the Qy(1,0) band. Fig. 2b shows a series of normalised corrected emission spectra using a series of kex values along the Qx(0,0) band. For comparison purposes we have included the emission spectrum when exciting at kex = 580 nm. The figure shows that the ratio of the Q(0,0)/Q(1,0) intensities is the smallest when exciting at kex = 580 nm and largest when kex = 650 nm. The relative fluorescence quantum yield (QY) for excitation at 648 nm (Qx(0,0)) is 1.2 times that at 592 nm (Qx(1,0)). Therefore there is an increased radiative probability for the excited electron when the excitation wavelength falls in the Qx(0,0) band. QY variations for fluorescence emission at different excitation wavelengths have also been reported for ZnTPP when exciting in the Soret band region of the absorbance spectrum [18]. Now, if we consider the ‘normal’ excitation and emission spectra being those obtained using kem = 720 nm and kex = 580 nm, respectively, then subtraction from the normalized spectra obtained using kem = 660 nm and kex = 650 nm would reveal the spectra of the second excitation/emission process. Fig. 2c shows the extracted (absorbance corrected) excitation1 and emission spectra2 of the second photonic process together with the absorbance 1
The absorbance corrected ‘calculated’ excitation spectrum was determined from the data of two excitation spectra and the absorbance spectrum. The excitation spectra required were those collected when monitoring the emission at a point where the second emission process does not occur and the other at a point where it does. For TPP the spectra used were those obtained when monitoring the emission at 720 nm (Q(0,1)) and 660 nm (Q(0,0)), respectively. The excitation spectra are then normalised at an appropriate place. This is a trial an error operation, with the main criteria being that the subtraction should not produce any negative values for the fluorescence intensity. For TPP the normalisation was carried out at 580 nm. The normalised spectra are then subtracted to reveal the excitation spectrum of the second emission process. However, this spectrum has to be corrected for the absorbance. To do this we divide the uncorrected excitation spectrum for the second emission by the absorbance to give the absorbance corrected excitation spectrum for the second emission process. 2 The emission spectrum for the second emission process was extracted by subtracting the emission spectrum collected using kex = 580 nm from the spectrum obtaining using kex = 650 nm (Qx(0,0). The spectra are first normalised at 720 nm, the point at which there is no second emission process.
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Fig. 2. (a) Excitation spectra for 5 lM TPP in toluene as a function of emission wavelength in the Q band region. Spectra have been normalized at 580 nm. (b) Corrected emission spectra for 5 lM TPP in toluene as a function of excitation wavelength in the Qx(0,0) band region. Spectra have been normalized at 720 nm. (c) Normalized excitation (calculated and experimental) and emission spectra of the second emission process on excitation at Qx(0,0). Absorbance spectrum is also shown. The Stokes shift is 2 nm for the second emission process. (d) Corrected emission spectra for a series of excitation wavelengths.
spectrum. The emission spectrum consists of a single transition centred at 655 nm with some weak vibrational bands at longer wavelengths. The green circles in Fig. 2c represent the experimentally obtained values for the excitation spectrum3. There is very good agreement between the calculated and experimental values. The comparison between the excitation and the absorbance spectra indicates that; (i) the second emission process is most intense when exciting at the Qx(0,0) band, (ii) is less likely to occur at the peak position of the Q bands (except Qx(0,0)) and (iii) is more likely at the points of greatest overlap between two adjacent Q or other bands. Interestingly, the second emission process does not occur when exciting at the Soret band maximum. Fig. 2d shows a series of emission spectra for 3
The ‘experimental’ values for the excitation spectrum are determined as follows: A series of emission spectra are collected using selected excitation wavelengths including a base signal at which no second emission occurs. In the case of TPP we used the emission spectrum for excitation at 580 nm. The spectra were normalised at 720 nm (Q(0,1)). The spectrum from the base signal was subtracted from all other spectra to reveal the spectrum of the second emission process. The fluorescence peak intensity at each excitation wavelength is the experimental value for the second emission process.
different excitation wavelengths including the Soret band. From these spectra the calculated second emission process contribution is less than 5% of the total emission when exciting at both Qy and Qx(1,0) peak positions. To determine the effect of solvent on the second emission process, the excitation and emission spectra for TPP have been analysed in hexane, acetone, DMF and DMSO. All experiments were performed in air saturated solutions at room temperature and pressure. The same dual emission observations were made with the intensity of the second emission process decreasing with solvent polarity. Also, the removal of oxygen from the TPP/toluene solution increased the overall fluorescence quantum yield to both emission processes proportionally. On protonation of the TPP molecule by the addition of tetrafluoroacetic acid (TFA) to form H2TPP2+ [19] the second emission process is no longer observed and the excitation/emission spectra obtained are independent of the exciting/monitoring wavelength, respectively. This suggests that the second emission process in TPP is connected to the n-electrons at the N atoms in the macrocyclic ring. One possibility is that hydrogen atom tunnelling is responsible for the second emission process in TPP, as protonation
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Table 1 Fluorescence lifetime measurements using TCSPC of 5 lM TPP in air saturated toluene at various temperatures Temp [K]
kex/kema [nm]
s1 [ns]
285
515/660 650/660 515/660 515/720 650/660 650/720 515/660 515/720 650/660b 650/720
11.60 ± 0.03 10.01 ± 0.03 9.37 ± 0.03 9.63 ± 0.03 8.81 ± 0.03 9.20 ± 0.02 8.78 ± 0.02 8.99 ± 0.02 8.31 ± 0.02 8.66 ± 0.02
298
313
s2 [ns]
0.13 ± 0.03
0.10 ± 0.03
0.14 ± 0.01
%1 [%] 100 98.7 100 100 98.7 100 100 100 99.5 100
%2 [%] 1.3
1.3
0.5
v2 1.03 1.19 1.12 1.14 1.09 1.16 1.06 1.08 1.03 1.04
a Emission bandpass at kex = 515 nm and 650 nm was 32 nm and 8 nm, respectively. b Time calibration = 0.014 ns/channel.
of TPP will stop the migration of the inner-hydrogen atoms. Fluorescence lifetime measurements using the TCSPC method were carried out on the same 5 lM TPP solution in toluene at various temperatures and two excitation wavelengths, 515 nm (Qy(1,0)) and 650 nm (Qx(0,0)). The results are presented in Table 1. The fluorescence lifetime shows there is one component when exciting at the Qy(1,0) band and two components when exciting at the Qx(0,0) band where the second component is always less than 1.5% of the total decay. This short fluorescence lifetime component of 100–140 ps may be attributed to; (i) excitation light scattering due to the small wavelength difference between the excitation and emission light and (ii) potentially a fast fluorescence decay component as described by Baskin et al. [4]. With regards the
Fig. 3. Corrected excitation and emission spectra of a series of free-base and metallated porphyrins in the Q band region. Spectra have been normalized at an appropriate wavelength: (a) TpyP (5 lM in CHCl3); (b) TMepyP (5 lM in methanol); (c) THPP (5 lM in acetone); (d) TAPP (5 lM in toluene); (e) MAPTPP (5 lM in toluene); (f) ZnTPP (5 lM in toluene); (g) OEP (1 lM in toluene); (h) PP-ixDME (1.9 lM in toluene); (i) ZnPP-ixDME (0.2 lM in toluene).
M. Uttamlal, A. Sheila Holmes-Smith / Chemical Physics Letters 454 (2008) 223–228
major component the trends are; (i) the fluorescence lifetime decreases (as expected) with increasing temperature; (ii) the lifetime measured using Qy(1,0) excitation is always greater than that from Qx(0,0); and (iii) the lifetime measured at Q(0,1) is longer than Q(0,0) for each kex. For a molecule which has one major radiative transition the fluorescence lifetime is independent of excitation/emission wavelength. The variation in fluorescence lifetime noted for TPP would suggest that multiple radiative transitions are occurring. The fluorescence lifetime of each radiative transition being similar, with the percentage of emission from each transition changing at the different excitation/emission wavelengths. This would account for the trend observed for the average fluorescence lifetime of TPP. To confirm that this is the case the fluorescence lifetime of meso-(p-aminophenyl) triphenyl porphyrin (MAPTPP) which does not exhibit the second emission process (Fig. 3e) was measured. The fluorescence lifetime value obtained was 9.26 ± 0.03 ns, and was independent of the excitation/emission wavelength. For comparison purposes, a series of excitation and emission spectra for other meso, aryl and metallated porphyrins have been carried out. Fig. 3 shows sets of excitation and emission spectra collected around in the Q band region and reveals that multi-emission processes are observed for other porphyrins. Meso-tetra-(4-pyridyl) porphyrin (TpyP, Fig. 3a), meso-tetra-(N-methyl-4-pyridyl) porphyrin tetra tosylate salt (TMepyP, Fig. 3b), mesotetra-(p-hydroxyphenyl) porphyrin (THPP, Fig. 3c) and meso-tetra-(p-aminophenyl) porphyrin (TAPP, Fig. 3d) exhibit similar behaviour to TPP to a greater or lesser extent. Changing the meso group does not change the position of the Qx(0,0) band only its relative intensity, however, the excitation wavelength of the second emission process changes. THPP is blue shifted whereas TMepyP is red shifted relative to the Qx(0,0) band. The relative intensity of the second emission process varies and appears to be related to the absorbance value at the excitation maximum. The massively reduced second emission process for TAPP relative to TPP can be explained by the fact that charge transfer from the amine groups is known to quench the fluorescence of the molecule [20]. Interestingly, meso-(paminophenyl) triphenyl porphyrin (MAPTPP, Fig. 3e) exhibits no second emission process indicating electronic symmetry is a factor for a multi-emission process. ZnTPP (Fig. 3f) does not exhibit a second emission process in the Q band region but a second emission process has been reported recently for excitation in the Soret region of the spectrum [18]. The aryl substituted porphyrins, octaethylporphyrin (OEP, Fig. 3g) and protoporphyrin IX dimethyl ester (PP-ixDME, Fig. 3h) exhibit a very weak dual emission process in the Q band region, however, ZnPP-ixDME (Fig. 3i) does and most dramatically at kex = 430 nm. Examination of these spectra and comparison to the TPP spectra indicates that symmetry, molecular structure and planarity of the porphyrin in the excited state are all contributing factors to the observations noted for the dual
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Fig. 4. Proposed Jablonski diagram for absorbance and fluorescence emission processes in TPP in toluene.
emission process described. In this work, the experimental evidence shows that, at certain excitation energies, particularly the Qx(0,0) band we observe tautomerism in the excited state, i.e., internal conversion of trans to cis to trans. This provides a second alternative/competitive route to the normal non-radiative relaxation process in the S1 state. The second emission will still arise from the energetically favourable trans form as in the normal fluorescence emission, however, the excited state of the ‘new’ trans form may be slightly different in energy to the normal S1 state. We therefore have to ascribe the energy levels in the ‘new’ trans form as S10 and S20 . The processes are summarised in the form of the Jablonski diagram (Fig. 4). The spectrum observed for the second emission process may not necessarily match that observed for the normal emission. TPP is known to exist in various forms, i.e., planar [21], wave [22] and ruffled [23], and their absorbance/ emission spectra will vary. For tautomerism to occur the structure, as well as energy barriers, is important. Only structural isomers meeting all the requirements will give rise to tautomerism and hence to emission. The structure of TPP following tautomerism may differ from the original structure, consequently its spectral profile and emission wavelength will change. Further experimentation is required to confirm this hypothesis and therefore, emission arising from the cis form or another intermediate cannot be ruled out at this stage. 4. Conclusion The photophysics and photochemistry of TPP (and hundreds of other porphyrins) has been extensively studied for over 70 years. The mechanisms of the relaxation processes of the molecule in the excited state have been investigated [1,2]. In this study, we have given evidence for an additional photonic process not previously reported experimentally. Preliminary studies show the additional alternative deactivation process, which may be associated with tautomerism of the macrocyclic ring, is present in other meso, aryl and metallated porphyrins. Further work is ongoing to establish the mechanism of the second emission process, which this work indicates to be dependent upon chemical composition and structure of the molecule in the excited
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state. These findings may have implications in the theoretical interpretation of porphyrin behaviour in natural systems and application of porphyrins for photodynamic therapy (PDT), energy harvesting systems and optical switches. Acknowledgements This work was funded by Glasgow Caledonian University. The authors wish to thank; HORIBA Jobin Yvon IBH Ltd for the loan of the SpectraLED 650 nm laser light source and Dr. Peter Wallace of Glasgow Caledonian University for helpful discussion. References [1] G.D. Dorough, J.R. Miller, F.M. Huennekens, J. Am. Chem. Soc. 73 (9) (1951) 4315. [2] R. Bonnett, D.J. McGarvey, A. Harriman, E.J. Land, T.G. Truscott, U.-J. Winfield, Photochem. Photobiol. 48 (3) (1988) 271. [3] H.N. Fonda, J.V. Gilbert, R.A. Cormier, J.R. Sprague, K. Kamioka, J.S. Connolly, J. Phys. Chem. 97 (27) (1993) 7024. [4] J.S. Baskin, H.-Z. Yu, A.H. Zewail, J. Phys. Chem. A. 106 (42) (2002) 9837. [5] M.J. Gouterman, J. Chem. Phys. 30 (5) (1959) 1139. [6] M.J. Gouterman, J. Mol. Spectrosc. 6 (1961) 138. [7] M. Gouterman, J. Mol. Spectrosc. 11 (1–6) (1963) 108.
[8] C. Weiss, H. Kobayashi, M.J. Gouterman, J. Mol. Spectrosc. (1965) 415. [9] M. Gouterman, in: D. Dolphin (Ed.), The Porphyrins, III, Academic Press, New York, 1978 (pp. 1). [10] A.H. Corwin, A.B. Chivvis, R.W. Poor, D.G. Whitten, E.W. Baker, J. Am. Chem. Soc. 90 (24) (1968) 6577. [11] Z.-L. Cai, M.J. Crossley, J.R. Reimers, R. Kobayashi, R.D. Amos, J. Phys. Chem. B. 110 (31) (2006) 15624 (and references therein). [12] J. Braun, M. Schlabach, B. Wehrle, M. Ko¨cher, E. Vogel, H.-H. Limbach, J. Am. Chem. Soc. 116 (1994) 6593. [13] J. Baker, P.M. Kozlowski, A.A. Jarzecki, P. Pulay, Theor. Chem. Acc. 97 (1997) 59. [14] D.K. Maity, R.L. Bell, T.N. Truong, J. Am. Chem. Soc. 122 (2000) 897. [15] Y. Zhang, P. Yao, X. Cai, H. Xu, X. Zhang, J. Jiang, J. Mol. Graph. Model. 26 (1) (2007) 319. [16] A. Vdovin, J. Sepiol, N. Urbanska, M. Pietraszkiewicz, A. Mordzinski, J. Waluk, J. Am. Chem. Soc. 128 (2006) 2577. [17] M. Gil, J. Waluk, J. Am. Chem. Soc. 129 (2007) 1335. [18] A. Lukaszewicz, J. Karolczak, D. Kowalska, A. Maciejewski, M. Ziolek, R.P. Steer, Chem. Phys. 331 (2007) 359. [19] V.N. Knyukshto, K.N. Solovyov, G.D. Egorova, Biospectroscopy 4 (1998) 121. [20] M. Vitasovic, M. Gouterman, H. Linschitz, J. Porphyrins Phthalocyanines 5 (3) (2001) 191. [21] M.P. Byrn et al., J. Am. Chem. Soc. 113 (1991) 6549. [22] S.J. Silvers, A. Tulinsky, J. Am. Chem. Soc. 89 (1967) 3331. [23] M.J. Hamor, T.A. Hamor, J.L. Hoard, J. Am. Chem. Soc. 86 (1964) 1938.
Chemical Physics Letters 454 (2008) 223–228 www.elsevier.com/locate/cplett
The excitation wavelength dependent fluorescence of porphyrins Mahesh Uttamlal *, A. Sheila Holmes-Smith * Department of Biological and Biomedical Sciences, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, Scotland, UK Received 16 August 2007; in final form 5 February 2008 Available online 13 February 2008
Abstract Spectroscopic studies of meso-tetraphenylporphyrin and other meso-substituted porphyrin derivatives have revealed a dual fluorescence emission process. The first emission is the usual 1(p p*) transition. The second alternative emission process is proposed to arise following tautomerism of the inner macrocyclic hydrogen atoms via an atom tunnelling process. This process occurs when exciting at virtually all wavelengths in the Q band region but, is at its most intense when exciting at the Qx(0,0) band and is not observed when exciting at the Soret band maximum, i.e., the emission spectrum of TPP is dependent on the excitation wavelength. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The absorbance and fluorescence emission spectra of meso-tetraphenylporphyrin (TPP) [1–4] are shown in Fig. 1. The accepted interpretation is as follows: there is strong absorbance in the near-UV called the Soret or B band and denotes the S2 S0 transition. There are three other absorption bands (not shown) in the deep UV known as the N, L and M bands. In the visible region of the spectrum there are four weak bands called the Q bands indicating S1 S0 transitions. The fluorescence emission spectrum consists of two strong emission bands at k > Qx(0,0). On closer inspection there are also other smaller fluorescence bands at wavelengths shorter and longer than the main bands. The spectra are typical of many free-base porphyrins. A theoretical description for the absorbance spectrum of free-base porphyrins has been reported by Gouterman based on p* p electronic transitions between the two highest occupied (HOMO) and the two lowest unoccupied (LUMO) molecular orbitals within an 18 membered macrocyclic ring [5–9]. The model explains the existence of the number of B and Q bands. Further interpretation of the model, in partic*
Corresponding authors. Fax: +44 141 331 3653. E-mail addresses: [email protected] (M. Uttamlal), a.s.smith@ gcal.ac.uk (A. Sheila Holmes-Smith). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.02.012
ular to the type of transitions has been discussed by Corwin et al. [10]. In the analysis they argue the absorption bands potentially arise from p* n and p* l transitions of lone pair electrons on the N groups of the macrocyclic ring. Computational methods have also been developed to predict the spectrum of porphin and substituted porphyrins to a varying degree of success [11]. To date no one model has been able to accurately predict the ground-state absorption spectrum of free-base porphyrins. Tautomerism in porphyrins has also been investigated experimentally and theoretically. Tautomers of porphyrins involving the exchange of the inner two hydrogen atoms can occur in the ground state via thermal activation or by a photo-induced process. This process has been studied both experimentally and theoretically for porphine [12–15] and porphycene [16,17] and is not yet fully understood, although it is now generally accepted that double proton transfer in porphine is via a two-step process involving a metastable cis intermediate. In this work, we have carried out a series of excitation and emission spectra of air saturated 5 lM TPP in toluene at room temperature (approx. 20 °C) and show how it can reveal new information about electronic transitions at (predominantly) the Qx(0,0) band not seen in the absorbance spectrum of the porphyrin. Our new observations are interpreted to potentially arise following tautomerism in the excited state. Following our observations for TPP we have
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3. Results and discussion
Fig. 1. Absorbance and fluorescence (kex = 418 nm) spectra of TPP in toluene. The TPP concentrations were 5 lM and 0.5 lM for absorbance and fluorescence, respectively.
made some preliminary measurements on other substituted porphyrins. 2. Experimental All chemicals and solutions were purchased from commercial suppliers and used without further purification unless otherwise stated. Porphyrin purity was checked by thin layer chromatography (TLC) before use and where necessary purified using preparative TLC. Absorbance spectra were obtained using a Lambda 2 (Perkin Elmer) dual-beam uv/vis spectrometer. Fluorescence spectra were collected on the Fluoromax 3 (HORIBA Jobin Yvon Ltd) spectrometer. The bandpass was 1.5 nm or 2 nm for both the excitation and emission light. The data shown is the S/R value where S and R are the signals from sample and lamp, respectively. Background fluorescence arising from the solvent has been subtracted. Signal smoothing has been applied using a means-movement procedure (convolution width = ±2 nm). Extreme care has been taken to ensure artefact free spectra. All instrumental and solvent effects have been removed by careful sample preparation and correction of the spectra for solvent Raman peaks and the quantum efficiency of the detector at the longer emission wavelengths. Luminescence lifetime measurements were performed on a 5000 U system (HORIBA Jobin Yvon IBH Ltd) in the single photon counting (TCSPC) mode with a time calibration of 0.056 ns per channel. A NanoLED light source (HORIBA Jobin Yvon IBH Ltd) of kmax 495 nm (FWHM = 1.3 ns) or a NanoLED laser source of kmax = 650 nm (FWHM = 288 ps) was used for 515 nm and 650 nm excitation, respectively. The repetition rate for both sources was 1 MHz. Data analysis was performed using the HORIBA Jobin Yvon IBH Ltd Data Analysis software, DAS6. All measurements were carried out in air saturated solutions at room temperature unless otherwise stated.
Fig. 2a shows a series of normalised corrected excitation spectra in the Q-band region obtained by monitoring the fluorescence emission spectra at the indicated wavelength. The absorbance spectrum is also included for comparison purposes. The figure shows that the excitation spectrum collected when monitoring the emission at Q(0,1) and the absorption spectrum have the same peak wavelengths and relative intensities for each Q band. The figure also shows that the relative intensity of the Qx(0,0) band is dependent upon the emission monitoring wavelength. There are also variations at the other Q bands but to a smaller extent. The maximum intensity for the Qx(0,0) excitation band occurs when monitoring at kem = 657 nm at which point the Qx(0,0) is larger than the Qy(1,0) band. Fig. 2b shows a series of normalised corrected emission spectra using a series of kex values along the Qx(0,0) band. For comparison purposes we have included the emission spectrum when exciting at kex = 580 nm. The figure shows that the ratio of the Q(0,0)/Q(1,0) intensities is the smallest when exciting at kex = 580 nm and largest when kex = 650 nm. The relative fluorescence quantum yield (QY) for excitation at 648 nm (Qx(0,0)) is 1.2 times that at 592 nm (Qx(1,0)). Therefore there is an increased radiative probability for the excited electron when the excitation wavelength falls in the Qx(0,0) band. QY variations for fluorescence emission at different excitation wavelengths have also been reported for ZnTPP when exciting in the Soret band region of the absorbance spectrum [18]. Now, if we consider the ‘normal’ excitation and emission spectra being those obtained using kem = 720 nm and kex = 580 nm, respectively, then subtraction from the normalized spectra obtained using kem = 660 nm and kex = 650 nm would reveal the spectra of the second excitation/emission process. Fig. 2c shows the extracted (absorbance corrected) excitation1 and emission spectra2 of the second photonic process together with the absorbance 1
The absorbance corrected ‘calculated’ excitation spectrum was determined from the data of two excitation spectra and the absorbance spectrum. The excitation spectra required were those collected when monitoring the emission at a point where the second emission process does not occur and the other at a point where it does. For TPP the spectra used were those obtained when monitoring the emission at 720 nm (Q(0,1)) and 660 nm (Q(0,0)), respectively. The excitation spectra are then normalised at an appropriate place. This is a trial an error operation, with the main criteria being that the subtraction should not produce any negative values for the fluorescence intensity. For TPP the normalisation was carried out at 580 nm. The normalised spectra are then subtracted to reveal the excitation spectrum of the second emission process. However, this spectrum has to be corrected for the absorbance. To do this we divide the uncorrected excitation spectrum for the second emission by the absorbance to give the absorbance corrected excitation spectrum for the second emission process. 2 The emission spectrum for the second emission process was extracted by subtracting the emission spectrum collected using kex = 580 nm from the spectrum obtaining using kex = 650 nm (Qx(0,0). The spectra are first normalised at 720 nm, the point at which there is no second emission process.
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Fig. 2. (a) Excitation spectra for 5 lM TPP in toluene as a function of emission wavelength in the Q band region. Spectra have been normalized at 580 nm. (b) Corrected emission spectra for 5 lM TPP in toluene as a function of excitation wavelength in the Qx(0,0) band region. Spectra have been normalized at 720 nm. (c) Normalized excitation (calculated and experimental) and emission spectra of the second emission process on excitation at Qx(0,0). Absorbance spectrum is also shown. The Stokes shift is 2 nm for the second emission process. (d) Corrected emission spectra for a series of excitation wavelengths.
spectrum. The emission spectrum consists of a single transition centred at 655 nm with some weak vibrational bands at longer wavelengths. The green circles in Fig. 2c represent the experimentally obtained values for the excitation spectrum3. There is very good agreement between the calculated and experimental values. The comparison between the excitation and the absorbance spectra indicates that; (i) the second emission process is most intense when exciting at the Qx(0,0) band, (ii) is less likely to occur at the peak position of the Q bands (except Qx(0,0)) and (iii) is more likely at the points of greatest overlap between two adjacent Q or other bands. Interestingly, the second emission process does not occur when exciting at the Soret band maximum. Fig. 2d shows a series of emission spectra for 3
The ‘experimental’ values for the excitation spectrum are determined as follows: A series of emission spectra are collected using selected excitation wavelengths including a base signal at which no second emission occurs. In the case of TPP we used the emission spectrum for excitation at 580 nm. The spectra were normalised at 720 nm (Q(0,1)). The spectrum from the base signal was subtracted from all other spectra to reveal the spectrum of the second emission process. The fluorescence peak intensity at each excitation wavelength is the experimental value for the second emission process.
different excitation wavelengths including the Soret band. From these spectra the calculated second emission process contribution is less than 5% of the total emission when exciting at both Qy and Qx(1,0) peak positions. To determine the effect of solvent on the second emission process, the excitation and emission spectra for TPP have been analysed in hexane, acetone, DMF and DMSO. All experiments were performed in air saturated solutions at room temperature and pressure. The same dual emission observations were made with the intensity of the second emission process decreasing with solvent polarity. Also, the removal of oxygen from the TPP/toluene solution increased the overall fluorescence quantum yield to both emission processes proportionally. On protonation of the TPP molecule by the addition of tetrafluoroacetic acid (TFA) to form H2TPP2+ [19] the second emission process is no longer observed and the excitation/emission spectra obtained are independent of the exciting/monitoring wavelength, respectively. This suggests that the second emission process in TPP is connected to the n-electrons at the N atoms in the macrocyclic ring. One possibility is that hydrogen atom tunnelling is responsible for the second emission process in TPP, as protonation
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Table 1 Fluorescence lifetime measurements using TCSPC of 5 lM TPP in air saturated toluene at various temperatures Temp [K]
kex/kema [nm]
s1 [ns]
285
515/660 650/660 515/660 515/720 650/660 650/720 515/660 515/720 650/660b 650/720
11.60 ± 0.03 10.01 ± 0.03 9.37 ± 0.03 9.63 ± 0.03 8.81 ± 0.03 9.20 ± 0.02 8.78 ± 0.02 8.99 ± 0.02 8.31 ± 0.02 8.66 ± 0.02
298
313
s2 [ns]
0.13 ± 0.03
0.10 ± 0.03
0.14 ± 0.01
%1 [%] 100 98.7 100 100 98.7 100 100 100 99.5 100
%2 [%] 1.3
1.3
0.5
v2 1.03 1.19 1.12 1.14 1.09 1.16 1.06 1.08 1.03 1.04
a Emission bandpass at kex = 515 nm and 650 nm was 32 nm and 8 nm, respectively. b Time calibration = 0.014 ns/channel.
of TPP will stop the migration of the inner-hydrogen atoms. Fluorescence lifetime measurements using the TCSPC method were carried out on the same 5 lM TPP solution in toluene at various temperatures and two excitation wavelengths, 515 nm (Qy(1,0)) and 650 nm (Qx(0,0)). The results are presented in Table 1. The fluorescence lifetime shows there is one component when exciting at the Qy(1,0) band and two components when exciting at the Qx(0,0) band where the second component is always less than 1.5% of the total decay. This short fluorescence lifetime component of 100–140 ps may be attributed to; (i) excitation light scattering due to the small wavelength difference between the excitation and emission light and (ii) potentially a fast fluorescence decay component as described by Baskin et al. [4]. With regards the
Fig. 3. Corrected excitation and emission spectra of a series of free-base and metallated porphyrins in the Q band region. Spectra have been normalized at an appropriate wavelength: (a) TpyP (5 lM in CHCl3); (b) TMepyP (5 lM in methanol); (c) THPP (5 lM in acetone); (d) TAPP (5 lM in toluene); (e) MAPTPP (5 lM in toluene); (f) ZnTPP (5 lM in toluene); (g) OEP (1 lM in toluene); (h) PP-ixDME (1.9 lM in toluene); (i) ZnPP-ixDME (0.2 lM in toluene).
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major component the trends are; (i) the fluorescence lifetime decreases (as expected) with increasing temperature; (ii) the lifetime measured using Qy(1,0) excitation is always greater than that from Qx(0,0); and (iii) the lifetime measured at Q(0,1) is longer than Q(0,0) for each kex. For a molecule which has one major radiative transition the fluorescence lifetime is independent of excitation/emission wavelength. The variation in fluorescence lifetime noted for TPP would suggest that multiple radiative transitions are occurring. The fluorescence lifetime of each radiative transition being similar, with the percentage of emission from each transition changing at the different excitation/emission wavelengths. This would account for the trend observed for the average fluorescence lifetime of TPP. To confirm that this is the case the fluorescence lifetime of meso-(p-aminophenyl) triphenyl porphyrin (MAPTPP) which does not exhibit the second emission process (Fig. 3e) was measured. The fluorescence lifetime value obtained was 9.26 ± 0.03 ns, and was independent of the excitation/emission wavelength. For comparison purposes, a series of excitation and emission spectra for other meso, aryl and metallated porphyrins have been carried out. Fig. 3 shows sets of excitation and emission spectra collected around in the Q band region and reveals that multi-emission processes are observed for other porphyrins. Meso-tetra-(4-pyridyl) porphyrin (TpyP, Fig. 3a), meso-tetra-(N-methyl-4-pyridyl) porphyrin tetra tosylate salt (TMepyP, Fig. 3b), mesotetra-(p-hydroxyphenyl) porphyrin (THPP, Fig. 3c) and meso-tetra-(p-aminophenyl) porphyrin (TAPP, Fig. 3d) exhibit similar behaviour to TPP to a greater or lesser extent. Changing the meso group does not change the position of the Qx(0,0) band only its relative intensity, however, the excitation wavelength of the second emission process changes. THPP is blue shifted whereas TMepyP is red shifted relative to the Qx(0,0) band. The relative intensity of the second emission process varies and appears to be related to the absorbance value at the excitation maximum. The massively reduced second emission process for TAPP relative to TPP can be explained by the fact that charge transfer from the amine groups is known to quench the fluorescence of the molecule [20]. Interestingly, meso-(paminophenyl) triphenyl porphyrin (MAPTPP, Fig. 3e) exhibits no second emission process indicating electronic symmetry is a factor for a multi-emission process. ZnTPP (Fig. 3f) does not exhibit a second emission process in the Q band region but a second emission process has been reported recently for excitation in the Soret region of the spectrum [18]. The aryl substituted porphyrins, octaethylporphyrin (OEP, Fig. 3g) and protoporphyrin IX dimethyl ester (PP-ixDME, Fig. 3h) exhibit a very weak dual emission process in the Q band region, however, ZnPP-ixDME (Fig. 3i) does and most dramatically at kex = 430 nm. Examination of these spectra and comparison to the TPP spectra indicates that symmetry, molecular structure and planarity of the porphyrin in the excited state are all contributing factors to the observations noted for the dual
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Fig. 4. Proposed Jablonski diagram for absorbance and fluorescence emission processes in TPP in toluene.
emission process described. In this work, the experimental evidence shows that, at certain excitation energies, particularly the Qx(0,0) band we observe tautomerism in the excited state, i.e., internal conversion of trans to cis to trans. This provides a second alternative/competitive route to the normal non-radiative relaxation process in the S1 state. The second emission will still arise from the energetically favourable trans form as in the normal fluorescence emission, however, the excited state of the ‘new’ trans form may be slightly different in energy to the normal S1 state. We therefore have to ascribe the energy levels in the ‘new’ trans form as S10 and S20 . The processes are summarised in the form of the Jablonski diagram (Fig. 4). The spectrum observed for the second emission process may not necessarily match that observed for the normal emission. TPP is known to exist in various forms, i.e., planar [21], wave [22] and ruffled [23], and their absorbance/ emission spectra will vary. For tautomerism to occur the structure, as well as energy barriers, is important. Only structural isomers meeting all the requirements will give rise to tautomerism and hence to emission. The structure of TPP following tautomerism may differ from the original structure, consequently its spectral profile and emission wavelength will change. Further experimentation is required to confirm this hypothesis and therefore, emission arising from the cis form or another intermediate cannot be ruled out at this stage. 4. Conclusion The photophysics and photochemistry of TPP (and hundreds of other porphyrins) has been extensively studied for over 70 years. The mechanisms of the relaxation processes of the molecule in the excited state have been investigated [1,2]. In this study, we have given evidence for an additional photonic process not previously reported experimentally. Preliminary studies show the additional alternative deactivation process, which may be associated with tautomerism of the macrocyclic ring, is present in other meso, aryl and metallated porphyrins. Further work is ongoing to establish the mechanism of the second emission process, which this work indicates to be dependent upon chemical composition and structure of the molecule in the excited
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state. These findings may have implications in the theoretical interpretation of porphyrin behaviour in natural systems and application of porphyrins for photodynamic therapy (PDT), energy harvesting systems and optical switches. Acknowledgements This work was funded by Glasgow Caledonian University. The authors wish to thank; HORIBA Jobin Yvon IBH Ltd for the loan of the SpectraLED 650 nm laser light source and Dr. Peter Wallace of Glasgow Caledonian University for helpful discussion. References [1] G.D. Dorough, J.R. Miller, F.M. Huennekens, J. Am. Chem. Soc. 73 (9) (1951) 4315. [2] R. Bonnett, D.J. McGarvey, A. Harriman, E.J. Land, T.G. Truscott, U.-J. Winfield, Photochem. Photobiol. 48 (3) (1988) 271. [3] H.N. Fonda, J.V. Gilbert, R.A. Cormier, J.R. Sprague, K. Kamioka, J.S. Connolly, J. Phys. Chem. 97 (27) (1993) 7024. [4] J.S. Baskin, H.-Z. Yu, A.H. Zewail, J. Phys. Chem. A. 106 (42) (2002) 9837. [5] M.J. Gouterman, J. Chem. Phys. 30 (5) (1959) 1139. [6] M.J. Gouterman, J. Mol. Spectrosc. 6 (1961) 138. [7] M. Gouterman, J. Mol. Spectrosc. 11 (1–6) (1963) 108.
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