J O U R N A L O F C H E M I S T R Y Materials Optochemical HCl gas sensor using substituted tetraphenylporphine– ethylcellulose composite films Katsuhiko Nakagawa,a Kazunari Tanaka,b Takahiro Kitagawa and Yoshihiko Sadaoka*b aDepartment of Industrial Chemistry, Niihama National College of T echnology, Niihama 792, Japan bCenter for Advanced T echnology, Ebara Research Co., L td., Fujisawa 251, Japan Department of Materials Science and Engineering, Faculty of Engineering, Ehime University, Matsuyama 790–77, Japan Composite films of various tetraphenylporphines embedded in ethylcellulose were prepared and their optical response to gaseous HCl was investigated.The absorbance of the Soret and Q-bands for free-base tetraphenylporphines is reversibly sensitive to ppm levels of HCl.The replacement of the para hydrogen in one phenyl group with a hydroxy group is eVective in enhancing the sensitivity of the Q-band region while prolonging response time. In addition, the temperature coeYcient of the sensitivity of TP(OH) (R)3PH2 is lower than that of TPPH2. Under filtered light (>600 nm) a significant deterioration of the sensitivity was not observed for more than 50 d.Recently, various optochemical sensors operating at room (451). The first band oV the column was the 5,10,15,20- tetrakis( p-alkylphenyl)porphine by-product. The third band, temperature have been reported to measure the emission of gaseous pollutants at ppm or ppb concentration levels.1–8 It is which moved very slowly, was eluted with DCM containing ethanol (1%, gradually increasing to 3%) and collected and well known that in nonaqueous media the tetrapyrrolic porphine macrocycle is oxidized in successive monoelectronic concentrated. For further purification, each component was subjected to column chromatography on silica gel, and eluted steps giving monocationic radicals and dications. Furthermore, the amphoteric nature of the porphine molecule permits the with DCM containing ethanol (0.5%, gradually increasing to 5%).Compounds 1–3 were isolated, then recrystallized from formation of acid salts involving the addition of protons to the porphine center. DCM–methanol–n-hexane and identified as follows: Recently, there has been a need for the detection of subppm levels of HCl gas concentrations from adsorbing towers 5-(p-Hydroxyphenyl )-10,15,20-triphenylporphine 1.Yield in semiconductor factories. Given that the environmental 3.5%, purple crystals, mp>300 °C. Rf, 0.43 (DCM). standard for HCl gas is 5 ppm, in the present work, the optical UV–VIS(CHCl3) lmax/nm (e/dm3 mol-1 cm-1) 417 (490 000), properties of immobilized substituted tetraphenylporphines 514 (19 000), 549 (9300), 591 (5900), 645 (5400).dH (CDCl3) were examined for their potential use in the detection of such 8.80 (s, 8H, pyrrole b-H), 8.17 (m, 6H, aromatic H), 7.70 (m, levels of HCl gas. Furthermore, we also discuss the eVects of 7H, aromatic H), 7.23 (m, 4H, aromatic H). MS(FAB) m/z: substituents on sensitivity, response behavior and long term 631.2 (M+H+). stability. 5-(p-Hydroxyphenyl )-10,15,20-tris(p-tertbutylphenyl )porphine 2.Yield 2.9%, purple crystals, Experimental mp>300 °C. Rf, 0.47 (DCM). UV–VIS(CHCl3) lmax/nm Chemicals (e/dm3 mol-1 cm-1) 420 (570 000), 517 (20 000), 554 (12 000), 592 (5800), 649 (7100). dH (CDCl3) 8.85 (s, 8H, pyrrole b-H), Synthesis of 5-(p-hydroxyphenyl)-10,15,20-tris( p-alkylphe- 8.10 (m, 10H, aromatic H), 7.76 (m, 4H, aromatic H), 7.20 (m, nyl)porphine [TP(OH) (R)3PH2] 1–3. 2H, aromatic H), 1.60 (s, 27H, But). MS(FAB) m/z: 799.4 The general method was modified by Adler et al.9 as follows: (M+H+). Anal. calc. for C56H54N4O (798.4): C, 84.17; H, 6.82; p-hydroxybenzaldehyde (12 mmol), p-alkylbenzaldehyde or p- N, 7.02. Found: C, 84.21; H, 6.80; N, 7.01%. alkoxybenzaldehyde (36 mmol) in propionic acid (150 cm3) was stirred and slowly heated to 80 °C until the p-hydroxybenz- 5-(p-Hydroxyphenyl )-10,15,20-tris(paldehyde dissolved.Pyrrole (50 mmol) was slowly added to octyloxyphenyl )porphine 3. Yield 2.8%, purple crystals, the above solution and heated at 150 °C, then the reaction mp>300 °C. Rf, 0.52 (DCM). UV–VIS(CHCl3) lmax/nm mixture was refluxed for 70 min and allowed to cool overnight.(e/dm3 mol-1 cm-1) 423 (550 000), 515 (21 000), 555 (15 000), Ethanol (150 cm3) was added to the dark propionic acid 593 (6400), 651 (9700). dH (CDCl3) 8.90 (s, 8H, pyrrole b-H), residues under vigorous stirring at room temperature for 8.05 (d, 6H, aromatic H), 7.97 (m, 2H, aromatic H), 7.87 (m, 30 min, then the residue was filtered through a sintered funnel 6H, aromatic H), 7.50 (d, 6H, aromatic H) 7.08 (m, 8H, and washed with ethanol until the filtrate became clear.The aromatic H), 4.10 (t, 6H, MOMCH2M), 2.0 (m, 6H, MCH2M), violet solid obtained was dissolved in chloroform (150 cm3), 1.60–1.30 (br m, 30H, MCH2M), 0.90 (t, 9H, MCH3). washed with saturated aqueous sodium carbonate (3×50 cm3), MS(FAB) m/z: 1015.6 (M+H+). and dried over anhydrous sodium sulfate.After solvent evapor- Tetraphenylporphine (chlorin free) (TPPH2), and 5,10,15,20- ation, the purple needles obtained were chromatographed on tetrakis( p-hydroxyphenyl)porphine [TP(OH)PH2] were obalumina, and eluted with dichloromethane (DCM)–n-hexane tained from Aldrich Chemicals and Tokyo Kasei. The molecular structures of the porphines are shown. Ethylcellulose (EC) was obtained from Aldrich Chemicals.Porphines and *E-mail: sadaoka@en2.ehime-u.ac.jp J. Mater. Chem., 1998, 8(5), 1199–1204 1199HO 5-( p-hydroxyphenyl)-10,15,20-triphenylporphine [TP(OH)(H)3PH2] tetraphenylporphine (TPPH2) N NH N HN N NH N HN 5,10,15,20-tetrakis( p- tert-butylphenyl)porphine [TP(But)PH2] 5-( p-hydroxyphenyl)-10,15,20-tris( p- tert-butylphenyl)porphine [TP(OH)(But)3PH2] But N NH N HN But But But OH N NH N HN But But But 5,10,15,20-tetrakis( p-octyloxyphenyl)porphine [TP(OC)PH2] 5-( p-hydroxyphenyl)-10,15,20-tris( p-octyloxyphenyl)porphine [TP(OH)(OC)3PH2] C8H17O N NH N HN OC8H17 OC8H17 C8H17O HO N NH N HN OC8H17 OC8H17 C8H17O EC were dissolved in a mixture of toluene, ethanol and bis(2- channel spectrophotodetector (MCPD-1000, Otsuka electronics).The spectrum (I0) of the composite film was first ethylhexyl) phthalate (DOP) as a plasticizer. In the previous work,10 the absorption spectra of the EC-composite were measured in nitrogen and used as a reference for measuring the spectrum (I/I0) of the film in other environments. The examined for TPPH2 and TP(OH)PH2. The half width of the Soret band increases with porphine content indicating inter- reflectance (%) is defined as 100 I/I0.Standard dry gases (HCl, Cl2, NO2 and NO) diluted with nitrogen were obtained from molecular interactions, e.g. concentration dependent aggregations. A large proportion of the porphine results in formation Sumitomo Seika. The concentration was controlled by mixing the standard gas with nitrogen. of inhomogeneous films containing some porphine crystals.For the films containing 5×10-5 mol g-1 of EC or less, the half width of the band remains a constant, and smooth, homogeneous films are obtainable. To obtain reproducible Results and Discussion characteristics, the films on alumina and/or quartz substrates The absorption spectra in the UV–VIS region of free base from solutions of concentration 5×10-5 mol (g EC)-1 or less tetraphenylporphines have been extensively documented for were prepared with a spinner. The films were heated at 60 °C thin solid films and solutions.These spectra all consist of a in vacuo to remove the solvent. very strong band around 420 nm (the Soret band) and four moderately strong bands (Q-bands) in the 500–650 nm region. Optical measurements It is well known that in nonaqueous media the porphine macrocycle is oxidized in successive monoelectronic steps The spectra of the thin films (~5 mm) deposited on alumina plates were measured in reflection mode.Filtered light from a giving monocationic radicals and dications. Carnieri and Harriman11 have reported that radical p-cations are very D2/I2 lamp (400–800 nm, 15W) was guided into a fiber and the reflected light was collected and analyzed using a multi- unstable in solution, being oxidized to the p-dications by 1200 J.Mater. Chem., 1998, 8(5), 1199–1204Fig. 2 Spectral changes of TPPH2–EC composite without DOP upon Fig. 1 Absorption spectra of TPPH2-benzene solution with and withexposure to HCl gas at 30 °C. TPPH2: 5×10-5 mol per g EC. HCl out HCl concentrations in ppm are shown.electron loss. Furthermore, as reported in 1951 by Dorough et al.,12 the amphoteric nature of the porphine molecule permits the formation of acid salts involving the addition of protons to the center of the porphine and the optical absorption spectra of these salts are influenced by the nature of the acid. The porphine nucleus can be regarded structurally as a polyvalent amphotyte or ampholite because all four nitrogen atoms are potential basic centers and the two pyrrolic type (NNH) nitrogen atoms are possible acidic centers.Absorption spectra of TPPH2–benzene solutions (5×10-5 mol cm-3, 5cm 3 ) with and without conc. HCl, 2H+ N NH N HN N NH N HN H+ H+ HNO3 and HF solution (5×10-4 cm3) were measured. For Scheme 1 Protonation process the benzene solution, the Soret band is at 446 nm for HCl, 439 nm for HNO3 and 435 nm for HF.The result for HCl is shown in Fig. 1. From these spectra, one can note that the spectral pattern changed upon addition of HCl; the four Qband spectrum, indicating D2h symmetry for free-base porphine, changed to a two Q-band spectrum, indicating D4h symmetry. The change in the spectra upon addition of HCl can be attributed in general to the attachment of protons (diprotonation) to two imino nitrogen atoms of the pyrroline-like ring in the free-base.A similar spectral change is expected for the EC composite film. When dry HCl gas was introduced into the chamber, the spectral response to HCl concentration changes is as shown in Fig. 2 for the DOP free film containing 5×10-5 mol g-1 of EC of TPPH2.The reflectance at lmax=446 and 662 nm decreased and some isosbestic points were detected. These changes were reversible. These results are very similar to the changes observed upon diprotonation of TPPH2 (Scheme 1). Fig. 3 shows the percent reflectance changes of TPPH2–EC composite at lmax=446 nm during exposure to nitrogen and 12 ppm HCl. The response and recovery times became shorter with increase in working temperature, while the degree of the Fig. 3 Response behavior of TPPH2–EC composite film without DOP at 446 nm. HCl concentration is changed from 0 ppm (nitrogen) to changes/sensitivity became smaller. The doping with DOP was 12.7 ppm and the reflectance measured again. ($) 30°C, (&) 45°C eVective for improving the response behavior without aVecting and (+) 60°C.the lmax values. The response behavior was examined for a TPPH2–EC composite containing DOP. The doping resulted in an enhancement of the sensitivity at the Soret and Q(0–0)- band under near-UV–VIS (400–800 nm) continuous irradiation for TPPH2–EC composite containing DOP were band; both the response and recovery time became shorter, as shown in Fig. 4. For free-base tetraphenylporphine, the examined, confirming that the sensitivity was halved after 30 d as shown in Fig. 5. To improve long-term stability, the use of absorbance of the Soret and Q-bands was reversibly sensitive to ppm levels of HCl at room temperature. Some fading was near-UV-cut light was considered. Under irradiation with filtered light (>600 nm), a marked deterioration of the detected for specimens exposed to light from a 50 W xenon lamp for 3 h.Irradiation with light from a D2/I2 lamp (15W) sensitivity was not observed for more than 50 d. To maintain long-term stability, operation in a longer wave- resulted in some deterioration of the sensitivity, indicating a lack of long-term stability. The sensitivity changes of the Soret length region (>600 nm) is preferable while the molar absorp- J.Mater. Chem., 1998, 8(5), 1199–1204 1201Fig. 4 Reflectance changes at 448 nm ($) and 662 nm (#) of Fig. 6 Spectral changes of TP(OH) (H)3)PH2–EC composite upon TPPH2–EC composite film at 40 °C. TPPH2: 2.5×10-5 mol per g EC exposure to HCl gas at 30 °C. TP(OH) (H)3PH2: 5×10-5 mol g per with DOP HCl concentration is changed from 0 ppm (nitrogen) to EC.HCl concentrations in ppm are shown. 12.7 ppm and the reflectance measured again. work,10 the spectral changes with time of 5,10,15,20-tetrakis( phydroxyphenyl) porphine–EC composite upon exposure to 0.09 ppm of dry HCl gas were examined. Exposure to 0.09 ppm HCl resulted in a gradual decrease in the percent reflectance at about 455 and 710 nm with blue-shifts. The response time was extremely long compared to that for TPPH2–EC composites. The recovery time was also long, e.g.the specimen must be held in pure nitrogen for 3 d or more at room temperature to recover the initial state. The sensitivity of the Q(0–0) band was very high and reversible when the blueshifts were detected. The response behavior and the sensitivity may be influenced by the number of p-hydroxyphenyl groups, so we synthesized and examined substituted tetraphenylporphines.In dimethylformamide (DMF) solution, lmax of the Soretand Q(0–0)-bands of the neutral form are 416 and 647 nm for TPPH2, 418 and 649 nm for TP(OH) (H)3PH2, 422 and 653 nm for TP(OH) (H)3PH2, 419 and 651 nm for TP(But)PH2, 420 and 651 nm for TP(OH) (But)3PH2, 421 and 651 nm for Fig. 5 Change of the sensitivity to 10 ppm HCl of TPPH2–EC com- TP(OC)PH2, 422 and 651 nm for TP(OH)(OC)3PH2, respectposite with DOP at 30 °C.During the measurement, the composite ively. For the DMF solution with conc. HCl, lmax values of was exposed continuously to light. the Soret- and Q(0–0)-band of the dication form are 446 and 664 nm for TPPH2, 451 and 674 nm for TP(OH) (H)3PH2, 456 and 703 nm for TP(OH)PH2, 450 and 674 nm for tion coeYcient of the Q-band is about tenfold lower than that of the Soret band for the acid dication form of TPPH2. For TP(But)PH2, 453 and 683 nm for TP(OH) (But)3PH2, 456 and 694 nm for TP(OC)PH2, 456 and 697 nm for TPPH2–EC composites, introducing 0.1 ppm HCl resulted in a decrease of only about 2% in reflectance at 662 nm [Q(0–0) TP(OH)(OC)3PH2, respectively .The ratio of the absorbance of the Q(0–0)/Soret band for the dication form is 0.13 for band]. To improve sensitivity, the use of TPPH2 derivatives having a higher molar absorption coeYcient at the Q-band is TPPH2, 0.20 for TP(OH) (H)3PH2, 0.22 for TP(OH)PH2, 0.15 for TP(But)PH2, 0.21 for TP(OH) (But)3PH2, 0.20 for desirable.Acid dications showed an enhanced bathochromic shift relative to the meso-tetraphenylporphine itself. Thus the TP(OC)PH2, 0.21 for TP(OH)(OC)3PH2. The absorbance ratio for the mono p-OH substituted tetraphenylporphines for molar absorption coeYcient of the Q(0–0) band appeared in a longer wavelength region (>650 nm); the larger the shift, the the dication form is higher than that for the porphines without a p-OH group.10 larger the electron donating power of the substituents.Meot- Ner and Adler13 reported that an increase in the electron- Fig. 6 shows the spectral changes of the TP(OH) (H)3PH2– EC composite films without DOP. The introduction of one donating power of the para substituents results in a red shift of all the observed peaks for both free-base and dication forms p-OH group resulted in red shifts of both the Soret- and Q-bands.Moreover, lmax of Q(0–0) band of the dication with an increase in the oscillator strength. It seems that the oscillator strength increases monotonically with the lmax of form was shorter than that of the dication form of tetrakis( phydroxyphenyl) porphine. In ambient HCl, lmax values of peak I [Q(0–0) band] for both free-base and dicationic forms.These results suggested to us that the substitution of the Soret- and Q(0–0)-bands were 454 and 677 nm for TP(OH)- (But)3PH2, and 456 and 693 nm for TP(OH)(OC)3 hydrogen para to the phenyl group with an electron donating substituent, such as hydroxy, is suitable to improve and PH2, respectively.For the mono p-OH substituted tetraphenylporphines, the sensitivity to HCl gas of the Soret- and enhance sensitivity in the Q(0–0) band region. Furthermore, it is expected that the HCl sorption ability may be enhanced Q(0–0)-band was considerably higher than that of the unsubstituted tetraphenylporphine. It is confirmed that for the EC by the presence of the phenolic hydroxy group.In previous 1202 J. Mater. Chem., 1998, 8(5), 1199–1204composite films replacing the para-hydrogen in the phenyl The response behavior of TP(OH)(OC)3PH2 is shown in Fig. 9. While both response and recovery times of mono p-OH group with a hydroxy group would also be eVective in enhancing the absorbance of the Q(0–0) band of the dication form substituted tetraphenylporphines are considerably longer than that of tetraphenylporphine, the response behavior was revers- with red shifts, in which the degree of the red shifts increases with the number of p-OH groups. ible and became faster with increasing temperature.The response and recovery times of 5-( p-hydroxyphenyl)-10,15,20- The calibration curves at the Soret- and Q(0–0)-bands are shown in Fig. 7 and 8, respectively, for the composite films triphenylporphine composite were faster than those of the 5,10,15,20-tetrakis( p-hydroxyphenyl)porphyrin composite. without DOP. The sensitivity at both bands was in the following order: TPPH2<TP(OH) (But)3PH2<TP(OH)- When the working temperature was increased from 45 to 60 °C, the percent reflectance of the Soret-band at 12 ppm HCl (OC)3PH2TP(OH) (H)3PH2.This trend can be ascribed to the electron donating power of the para substituents. The ratio changed from 86 to 94 for TPPH2 and remained the same (78) for TP(OH)(OC)3PH2. The introduction of a single p-OH of log(I0/I) at the Q(0–0) band and log(I0/I) at the Soret band upon exposure to 12.7 ppm HCl was evaluated to be group resulted in a decrease in the temperature coeYcient of the sensitivity for both the Soret- and Q(0–0)-bands, as shown 0.88, 0.85, 0.69, 0.68, 0.40 and 0.35 for TP(OH)(OC)3PH2, TP(OH) (H)3PH2, TP(OH) (But)3PH2, TP(OC)PH2, TPPH2 in Fig. 9. Significant deterioration of the sensitivity under irradiation with filtered light (>600 nm) was not observed for and TP(But)PH2, respectively. The introduction of one p-OH resulted in the enhancement of the HCl sensitivity at the more than 50 d for the 5-( p-hydroxyphenyl)-10,15,20-tris( p-Rsubstituted- phenyl)porphine composite films without DOP.Q(0–0)-band. It is interesting to note that at the Q(0–0) band, the sensitivity of the composite films with tetraphenylporphine Furthermore, tests in other gaseous environments showed that the Soret- and Q-bands of the films were also sensitive to substituted with a single p-OH group was higher than that of TPPH2-composite films doped with DOP (as mentioned the vapors from aqueous solution of HNO3, HF or HCOOH, while for a benzene solution, the Soret and Q-bands were HCl sensitivity of TPPH2 composite film was enhanced by the addition of DOP).insensitive to HCOOH which may be related to the water content.Furthermore, they were mostly insensitive to vapors from CH3COOH or C2H5COOH. To clarify the reasons for these observed diVerences in the sensitivity (selectivity), we needed to measure the sorption characteristics of the composite films, since the sensitivity is influenced by the sorption ability and solubility of the composite film for these organic acids and water.In dry conditions, the sensitivity to HCl gas was not influenced by the coexistence of 2000 ppm of H2 and CO2. The percent reflectance at the Soret band (lmax=440 nm) decreased by only 1% for 8 ppm NO2 and 0.5% or less for 10 ppm NO. The changes for NOx may be attributed to the formation of HNO3 by the reaction with water as a contaminant. It is expected that the sorption of NO2 results in the formation of a radical p-cation or p-dication; the p-cations are very unstable in solution, being oxidized to p-dications as reported by Carnieri and Harriman.11 The p-dications are also reactive and react to give the acid dication by protonation.The lack of observation of spectral changes indicating the formation of radical p-cation or p-dication may be related to the diVerence in permeability of the EC composite for HNO3 and NOx and/or the diVerence in stability of acid dications Fig. 7 HCl concentration dependence of log(I0/I) at the Soret band at and radical cations in the EC matrix. The percent reflectance 30 °C. Porphine concentration: 5×10-5 mol per g EC; (#) TPPH2, ($) at the Soret band decreased by only 0.5% for 1 ppm Cl2 while TP(OH) (H)3PH2, (&) TP(OH) (But)3PH2, (+) TP(OH)(OC)3PH2.Fig. 9 Reflectance changes of TP(OH)(OC)3PH2–EC composite at Fig. 8 HCl concentration dependence of log(I0/I) at the Q(I)-band at (-- --) 45 and (—) 60 °C; and (#) 693 nm, ($) 456 nm. HCl concentration is changed from 0 ppm (nitrogen) to 12.7 ppm and the reflec- 30 °C; (#) TPPH2, ($) TP(OH) (H)3PH2, (%) TP(Bu t )PH2, (&) TP(OH) (But)3PH2, (6) TP(OC)PH2, (+) TP(OH)(OC)3PH2.tance measured again. J. Mater. Chem., 1998, 8(5), 1199–1204 1203the presence of a large excess of Cl2 (5 ppm) resulted in a of HCl gas, 5 ppm being the environmental standard for HCl gas. decrease of 10% and only partial recovery was observed when the introduction of Cl2 gas was stopped. It is well known that Cl2 interacts with water and forms HCl and HClO.HClO is References an unstable compound and decomposes to HCl and O2, especially under light irradiation. If Cl2 is changed completely 1 H. E. Posch and O. S. Wolfbeis, Sensors Actuators, 1988, 15, 77. 2 Q. Zhou, D. Kritz, L. Bonnell and G. Siger, Jr., Appl. Optics, 1989, to HCl, a reversible response behavior is expected. A lack of 28, 2022. reversibility to Cl2 may be due to some other side reactions 3 R.Gvishi and R. Reisfeld, Chem. Phys. L ett., 1989, 156, 181. with the porphine, such as chlorination. In any case, a more 4 O. S. Wolfbeis, Fiber Optic Chemical Sensors and Biosensors II, detailed discussion about cross-sensitivity requires other CRC Press, Inc., Boca Raton, USA, 1991. results. 5 K. Wang, K. Seiler, J.P. Haug, B. Lehmann, S. West, K. Kartman and W. Simon, Anal. Chem., 1991, 63, 970. 6 Y. Sadaoka, Y. Sakai and Y. Murata, T alanta, 1992 39, 1675. Conclusions 7 Y. Sadaoka, Y. Sakai and M. Yamada, J. Mater. Chem., 1993, 3, 877. Spectral changes of tetraphenylporphine and its derivatives 8 Y. Sadaoka, Y. Sakai and M. Yamada, Denki Kagaku, 1994, 62, dispersed in ethylcellulose were examined for detection of sub- 1066. ppm levels of HCl gas. The sensitivity in the Q(0–0) band 9 A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. KorsakoV, J. Org. Chem., 1967, 32, 476. region is enhanced by replacing the hydrogen para to the 10 K. Tanaka, C. Igarashi, P. Tagliatesta, T. Boschi and Y. Sadaoka, phenyl group with electron donating substituents, while the J.Mater. Chem., 1996, 6, 953. response time is prolonged. Marked deterioration of the sensi- 11 N. Carnieri and A. Harriman, Inorg. Chim. Acta., 1982, 62, 103. tivity under irradiation with filtered light (>600 nm) was not 12 G. D. Dorough, J. R. Millaer and F. M. Huennekens, J. Am. Chem. observed for more than 50 days, for the composite films Soc., 1951, 73, 4315. examined. An HCl gas sensor based on mono-substituted 13 M. Meot-Ner and A. D. Adler, J. Am. Chem. Soc., 1975, 97, 5107. tetraphenylporphine with a p-OH in one phenyl group shows superior performance for detection of emissions of ppm levels Paper 7/08482J; Received 24th November, 1997 1204 J. Mater. Chem., 1998, 8(5), 1199–1204