Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman California State University Fresno, CA 93740
Luminescent Molecular Thermometers
Seiichi Uchiyama* and A. Prasanna de Silva School of Chemistry, Queen’s University, Belfast BT9 5AG, Northern Ireland: *
[email protected] Kaoru Iwai Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-Nishimachi, Nara 630-8506, Japan
Temperature is one of the most important factors that affects our daily lives. Everyone can recognize the arrival of spring by feeling the warmth in the air. Every chef pays attention to the temperature of a frying pan and an oven. In science, all phenomena, for example, chemical reactions or color of a star, are influenced by temperature. Therefore methods for measuring temperature remain in vogue. Until now, various types of thermometers have been developed, such as liquid-filled bulbs and stems (alcohol and mercury thermometers), bimetal strips, metallic resistors, thermistors, thermoelectric devices (thermocouples), and infrared radiation detectors (thermography) (1). A molecule that responds to heat and sends the information about temperature in the form of a light signal (e.g., fluorescence) is called a luminescent molecular thermometer (LMT). Such a “smart” molecule is able to function as a molecular-level thermometer (2, 3) in the scientific world. LMTs have a great advantage over other thermometers in measuring temperatures in a small space (with dimensions of < 10᎑3 m), since the fluorescence detection is highly sensitive and enables
us to observe even one molecule (with a size of ∼10᎑9 m) (4). To measure two- or three-dimensional temperature distribution in space and to obtain the time sequence of temperature in a dewdrop, a living cell, a microfluidic channel, or a microreactor are a few of the goals in the application of LMTs. All of these places are too small for macroscopic thermometers but large enough to accommodate the smart molecular reporter. Thermally-driven Boltzmann-type distribution between two competing or equilibrating states is a useful model to rationalize most observations with luminescent molecular thermometers. If the two states show a difference in fluorescence quantum yield (Φf) (or fluorescence intensity), fluorescence lifetime (τf), or maximum emission wavelength (λem), the molecule can be used as a LMT. In this article, LMTs are classified into nine categories based on the spatial relationship between two competing or equilibrating states as shown in Table 1. The materials outlined in this article are listed in Table 2. A related review has recently appeared (5).
Table 1. Classification of Luminescent Molecular Thermometers Category
Competing or Equilibrating States
Key Difference between the Two States
Intersystem crossing
S1 and Tn
spin of lumophore
within lumophore, complete overlap
TICT
ICT and TICT
bond twist within lumophore
within lumophore, complete overlap but with twist
Organometallic complexes: ruthenium
MLCT and MC (LF)
electronic delocalization over ligand
within lumophore, partial overlap
Organometallic complexes: lanthanide
MC and ligand T1
atomic orbital parentage
lumophore and ligand
Spatial Relationship
Photodissociation
ππ* and πσ*(or σσ*)
molecular orbital orthogonality
within lumophore, shared atom
Excimers and exciplexes
monomer and excimer/exciplex
number of excited π-systems
lumophore expansion
Spin crossover
high-spin and low-spin
spin of metal ion
within metal ion in macrocycle, spaced from lumophore
Guest–host interactions
extended and folded conformations
local environment near lumophore
within lumophore, complete overlap but with different CD position
Macromolecular systems
extended and folded conformations
local environment near lumophore
within lumophore, complete overlap but with different environment
Viscosity-sensitive probes and others
(bulk environment)
(viscosity or polarity of environment)
within lumophore, complete overlap but with different bulk environment
NOTE: Technical terms and abbreviations are described in the text.
720
Journal of Chemical Education
•
Vol. 83 No. 5 May 2006
•
www.JCE.DivCHED.org
Chemistry for Everyone
Intersystem Crossing The simplest LMTs are based on the competition of fluorescence from the singlet S1 state with a nonradiative relaxation pathway to a triplet Tn state by intersystem crossing (Figure 1). 9-Methylanthracene, 1,
Figure 1. Energy levels of a LMT with the competition of fluorescence with S1 → Tn intersystem crossing.
is a good example of this type of LMT. Compound 1 emits strongly in fluid ethanol at lower temperatures (Figure 2; ref 6 ). On the other hand, the measurement of triplet–triplet absorption of 1 shows that the triplet quantum yield (Φt) by intersystem crossing from the S1 state decreases with decreasing temperature (Figure 2; ref 7 ). These opposite trends of Φf and Φt data indicate clearly that the fluorescence of 1 competes with intersystem crossing. Since heat provides the S1 state of 1 with enough energy to go over an activation barrier to the Tn state, the fluorescence intensity of 1 is influenced by temperature. Delayed fluorescence is a phenomenon that is related to the energetic proximity of S1 and T1 states (8). For fluorophores with such a proximity, the reverse intersystem crossing from the T1 state to the S1 state can occur as well as the normal S1 → T1 intersystem crossing. The emission following the reverse T1 → S1 intersystem crossing is known as “delayed fluorescence”. Since the reverse intersystem crossing is
Figure 2. Temperature-dependence of Φf in ethanol and Φt in Lucite of 1. The original data are from refs 6 and 7.
Table 2. Luminescent Molecular Thermometers Commercial Availability
Solvent
9-Methylanthracene, 1
yes
ethanol
Parameter To Be Measured Φf
Eosin, 2
yes
ethanol
Compound
Temperature Range/⬚C
Parameter Variation
Special Requirementa
᎑42–20
0.69᎑0.42
liquid N2 or low temp. bath
delayed τf
᎑70–70
3.9–0.7/ms
liquid N2 bath and lifetime measuring apparatus none
Benzoxadiazole, 3
no
benzene
Φf
20–50
0.65–0.49
Ruthenium(II) complex, 4
yes
water
Φf
5–90
0.049–0.0075
Europium(III) complex, 5
yes
EPAb
Φf
᎑200–50
0.40–0.06
liquid N2 bath
Pyrazoline, 6
no
hexane
Φf
22–52
0.52–0.31
none
Bipyrenyl derivative, 7
no
dodecane
IE/IM
20–100
0.80–0.25
none
Polyamine, 8
no
NaCl soln, pH = 1
IE/IM
15–50
1.25–0.35
pH adjustment with hydrochloric acid
Anthracene derivative, 9
no
MCPc
IE/IM
᎑124–᎑50
0.34–5.3
liquid N2 bath
Quadridentate macrocycle, 10
no
acetonitrile
FId
27–65
100–250e
complexing by Ni(ClO4)2
β-CD derivative, 11
no
buffer, pH = 7f
FId
10–80
225–45e
d
Copolymer, 12
no
water
Pyridylaryloxazole, 13
no
ethanol
1,3-Bis(1pyrenyl)propane, 14
yes
[C4mpy][Tf2N]g
none
none
FI Φf
32–38
10–130e
none
᎑160–20
1.0–0.4
liquid N2 bath
IE/IM
25–140
0.1–1.1
heater or oil bath
a
b Standard fluorimeter and water bath are generally needed. UV lamp is useful for visualization. Dethyl ether–isopentane–ethanol (5:5:2). c Methylcyclohexane–isopentane (3:1). dFluorescence intensity. eArbitrary unit. fHEPES [N-(2-hydroxyethyl)piperazine-N’-ethanesulfonic acid] buffer, commercially available. g1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, commercially available.
www.JCE.DivCHED.org
•
Vol. 83 No. 5 May 2006
•
Journal of Chemical Education
721
Chemistry for Everyone
activated in higher temperature and the lifetime of S1 is much smaller than that of T1, the delayed fluorescence lifetime is rather long as well as being temperature-dependent. Eosin, 2,
in benzene). Furthermore, a derivative of 3 is useful in intracellular studies (11). Its fluorescence intensity and Φf value were clearly variable in the biorelevent temperature range (10– 50 ⬚C) in living cells. Organometallic Complexes
Ruthenium In tris(2,2´-bipyridyl)ruthenium(II) complex, 4,
shows delayed fluorescence in ethanol (8). The delayed fluorescence lifetime of 2 changes from 3.9 to 0.7 ms as the temperature changes from ᎑70 to 70 ⬚C. The lifetime can be advantageous as a temperature-dependent parameter as opposed to the fluorescence intensity, since the former is hardly affected by the change in the concentration of the fluorescent dye (due to, for example, photobleaching and nonuniform distribution). Moreover, the phosphorescence, which is the emission accompanying the transition from the T1 state to the ground state, can be detected in the compound 2. The ratio of intensities of fluorescence to phosphorescence of 2 is also temperature-dependent and is independent of the concentration of dye as well. Similar fluorescence characteristics have been reported in acridine yellow in saccharide glass (9). However, solid thermometers seem to be limited in applicability for the tiny spaces that are easily accessible to molecular thermometers. Twisted Intramolecular Charge Transfer (TICT) 4-n-Propylamino-7-nitro-2,1,3-benzoxadiazole, 3,
shows temperature-dependent fluorescence characteristics that originate from the rotation of the substituent amine in the excited state (10). Because of the strong electron-donating group (n-propylamino group) and the strong electron-accepting group (nitro group) in 3, its S1 state is an intramolecular charge transfer (ICT) state. At this stage, the fluorescence from the ICT state competes with the process to form a TICT state with the rotation of the n-propylamino group (Figure 3). Thermal energy promotes the transition from the ICT state to the TICT state, which is nonemissive. Therefore, the Φf value of 3 drops with increasing temperature (Φf = 0.65, 0.58, 0.52, and 0.49 at 20, 30, 40, and 50 ⬚C, respectively,
722
Journal of Chemical Education
•
the emission originates from a triplet metal-to-ligand chargetransfer (MLCT) state in which an electron has been transferred from Ru(II) to a 2,2´-bipyridyl moiety (12). Nonemissive metal-centered (MC) triplet states (also called ligand field, LF, states) exist at energies slightly above the MLCT state. Since the population of excited 4 in the MC state becomes feasible by thermal activation, the emission intensity is temperature-dependent. Its quantum yield in water changes from 0.049 to 0.0075 with increasing temperature from 5 to 90 ⬚C. A more recent report on Ru(II) complexes with temperature-dependent emission characteristics is found in ref 13.
Lanthanides Lanthanide ions such as Eu(III) and Sm(III) form stable complexes with various chelating ligands. Among them, a large number of Eu(III) complexes emit strongly and the ligandcentered triplet states are involved as antennas in this process (14). If the triplet T1 state in a ligand exists at an energy slightly above the emissive state of Eu(III) ion, the energy can transfer from Eu(III) ion to the T1 state of the ligand with thermal assistance. The temperature-dependence of the Φf value of
Figure 3. Structural change from the ICT state to the TICT state of 3.
Vol. 83 No. 5 May 2006
•
www.JCE.DivCHED.org
Chemistry for Everyone
tris(thenoyltrifluoroacetonato)europium(III) complex, 5,
is caused by the competition of the luminescence with the energy transfer from Eu(III) ion to the ligand. The Φf value of 5 is 0.40 at ᎑200 ⬚C but is 0.06 at 50 ⬚C in the mixture of diethyl ether, isopentane, and ethanol (5:5:2) (15). A more recent article is also available (16). As described in the introduction, one of the merits of LMTs is their applicability to spatially resolved thermal analysis. By using a polymer film of perdeutero-poly(methyl methacrylate) doped with 5, the temperature distribution of an integrated circuit could be continuously monitored (17). The spatial resolution in this achievement was 15 µm. In this way, LMTs allow us to perform thermal imaging in a tiny space. Photodissociation In 3-(1,1´-biphenyl-4-yl)-1-phenyl-5-[(E )-2phenylvinyl]-4,5-dihydro-1H-pyrazole, 6,
(19, 20). Excimer (excited-state dimer) means a complex of two identical fluorophores in the excited state (21). N-(1Pyrenylmethyl)-1-pyrenebutanamide, 7,
is a nice representative (20). When one pyrene fluorophore is excited, then the following equilibrium (eq 1) holds, ∗ P + P
∗
E
(1)
where P*, P, and E* represent the excited pyrene, another pyrene in the ground state, and the pyrene excimer, respectively. The emission band of E* is distinct from that of P*. In the case of 7, the reaction of eq 1 is exothermic and thus the relative fluorescence intensity from E* compared to that of P* decreases with increasing temperature. Figure 4 shows the fluorescence spectra and the excimer–monomer emission ratio (IE at 448 nm; IM at 396 nm) of 7 in dodecane at the different temperatures. We can see a clear relationship between the temperature and the signal ratio in Figure 4, inset. As opposed to this relationship, the excimer–monomer emission ratio increases with increasing temperature in other cases
the photodissociation of the C5⫺N1 bond plays the key role in its sensitivity to temperature (18). High temperature accelerates the photohomolysis of the C⫺N bond that competes with fluorescence. Therefore the Φf value of 6 decreases with increasing temperature (Φf = 0.52, 0.43, 0.37, and 0.31 at 22, 30, 39, and 52 ⬚C respectively, in n-hexane). The temperature sensitivity can be easily tuned by varying the group at the C3 position, since the rate of the photodissociation depends on the energy of the S1 state of the fluorophore. Excimers and Exciplexes The luminescent molecular thermometers presented in the former sections are based on the competition of fluorescence with another nonemissive process. Here we describe situations in which two different fluorescent states equilibrate. Several of these function by forming intramolecular excimers
www.JCE.DivCHED.org
•
Figure 4. Fluorescence spectra of 7 in dodecane. Spectra are normalized to the intensity of the monomer emission. Inset: relationship between IE/IM and temperature. Reprinted with permission from ref 20. Copyright (1997) American Chemical Society.
Vol. 83 No. 5 May 2006
•
Journal of Chemical Education
723
Chemistry for Everyone
(22, 23). Recently, a tripodal polyamine containing three naphthalene fluorophores, 8,
Spin Crossover A quadridentate macrocycle, 10, with a naphthalene fluorophore forms a complex with Ni(II) ion (27). Interestingly, the Ni(II) complex of 10
gives an equilibrium mixture of high- and low-spin states in acetonitrile,
[Ni(10)S2]2+ high-spin octahedral
was reported to have thermo-dependent fluorescence characteristics owing to an intramolecular excimer (24). Polyamine 8 operates in water (pH 1.0) in the range of 15– 50 ⬚C. Although the conditions are perhaps too acidic for normal use, water is the most desirable solvent for the operation of luminescent molecular thermometers as far as biological applications are considered. In contrast, a complex of a fluorophore and another moiety in the excited state is called an exciplex (excited-state complex) (21). The intramolecular exciplex also contributes to temperature-dependent fluorescence characteristics. For example, 1-(N-p-anisyl-N-methyl)-amino-3-(9-anthryl)-propane, 9,
shows temperature-dependent fluorescence spectra (25). The relative emission intensity from anthracene (395 nm) to that from the exciplex (the intramolecular complex of anthracene and anisidine moieties, 560 nm) varies with the change in temperature. Similarly, intermolecular excimers and exciplexes are adopted as a basis of luminescent thermometers. However, such a thermometer is not unimolecular. When compared with intramolecular types, an impractically high concentration of molecules is required for the formation of an intermolecular excimer or exciplex. Nevertheless, a polymer film in which two compounds are doped to allow formation of an intermolecular exciplex functions as a luminescent thermometer (26). 724
Journal of Chemical Education
•
[Ni(10)]2+ + 2S low-spin square-planer
(2)
where S represents acetonitrile (solvent). In this equilibrium, the conversion from the high-spin state to the low-spin state is endothermic, and hence the proportion of the high-spin state becomes lower at high temperature. The fluorescence of naphthalene is quenched by the Ni(II)–attached macrocyclic moiety in both states, but the quenching efficiencies are different (the quenching mechanism has been suggested to be an energy transfer). Therefore, the fluorescence intensity of the Ni(II) complex of 10 becomes temperature-dependent. Figure 5 shows the fluorescence spectra of the Ni(II) complex of 10 at various temperatures and the relationship between its fluorescence intensity and temperature. This thermometer indicates an interesting behavior in that the fluorescence intensity increases at higher temperatures, whereas most luminescent molecular thermometers display an opposite behavior.
Figure 5. Fluorescence spectra of [Ni(10)](ClO4)2 in acetonitrile. Inset: relationship between fluorescence intensity and temperature. Reproduced by permission of The Royal Society of Chemistry (27).
Vol. 83 No. 5 May 2006
•
www.JCE.DivCHED.org
Chemistry for Everyone
Guest–Host Interactions The fluorescence intensity of a β-cyclodextrin (CD) derivative bearing a naphthoamide group, 11,
is considered to relate to the change in the relative position of the naphthalene moiety with respect to the β-CD cavity. Experimental data suggested that the naphthalene moiety was always located outside of the β-CD cavity in the range of 10–80 ⬚C but the distance between the naphthalene moiety and the β-CD cavity increased with the increase in temperature. Naturally, the emission efficiency of the fluorophore depends on the local environment. Macromolecular systems The polymeric LMT developed recently by us gives the largest fluorescence enhancement known in a narrow temperature range (29, 30). The copolymer, 12,
decreases with increasing temperature (its intensity at 80 ⬚C is ca. 20% of that at 10 ⬚C) in neutral aqueous buffer (28). The mechanism of this temperature sensitivity is unclear but
Figure 6. Mechanism of polymeric LMT 12.
Figure 7. Relationship between fluorescence intensity of 12 at λem in water and temperature. Fluorescence enhancement is observed between ca. 32–38 ⬚C. Inset: fluorescence spectra. Reprinted with permission from ref 29. Copyright (2003) American Chemical Society.
www.JCE.DivCHED.org
•
of N-isopropylacrylamide (NIPAM) and 4-N-(2-acryloyloxyethyl)-N-methylamino-7-N,N-dimethylaminosulfonyl2,1,3-benzoxadiazole (DBD-AE) functions as a luminescent molecular thermometer with a unique mechanism: As shown in Figure 6, the copolymer 12 in aqueous solution undergoes a temperature-induced phase transition at ca. 32 ⬚C. Below 32 ⬚C, the main chain of 12 becomes extended with considerable hydration. In this situation, the DBD-AE unit is weakly fluorescent since this fluorophore emits strongly only in a nonpolar environment. On the other hand, the copolymer 12 folds into a contracted structure above 32 ⬚C and water molecules become remote from the main chain. Then the polarity-sensitive DBD-AE unit becomes fluorescent. Figure 7 shows the fluorescence spectra of 12 and the relationship between its fluorescence intensity and temperature. Over 1300% enhancement of the fluorescence intensity can be seen in the range of only 6 ⬚C. Moreover, the sensitive temperature range of 12 can be easily tuned by the substitution of the NIPAM units. The capability of operating in aqueous solution is also a nice feature of this type of LMT. Another polymeric LMT has been reported (22) where a pyrene-labeled poly(N-propargylamide) in chloroform changes its conformation from helix into random coil by increasing temperature, resulting in the temperature-dependent pyrene excimer fluorescence. DNA is perhaps the most celebrated single macromolecule and it has been exploited for an LMT as well (31). The rhodamine-labeled DNA oligomer (5´-RhG-
Vol. 83 No. 5 May 2006
•
Journal of Chemical Education
725
Chemistry for Everyone
perature, that is, Φf ≈ 1.0, 0.8, 0.5, 0.4 at ᎑160, ᎑120, ᎑60, 20 ⬚C, respectively (33). This variation is due to the change in viscosity of ethanol and the sensitivity of 13’s fluorescence to the latter property. Some viscoelastic ionic liquids can also lead to temperature-dependent fluorescence characteristics. For example, the efficiency of intramolecular excimer emission of 1,3-bis(1-pyrenyl)propane, 14,
Figure 8. Fluorescence spectra of 14 in 1-butyl-1-methylpyrrolidium bis(trifluoromethylsulfonyl)imide; Inset: relationship between IE/IM and temperature. Reproduced by permission of The Royal Society of Chemistry (23).
GATGATGAGAAGAAC-3´, RhG: rhodamine-G dye, G: guanine, A: adenine; T: thymine, C: cytosine) shows temperature-dependent τf values between 15 and 35 ⬚C. The reason for the temperature-dependence is because the conformation of the DNA oligomer is changed by heat and therefore the quenching efficiency of guanine and thymine toward the rhodamine varies. This thermometer is expected to be effective for monitoring the temperature of the PCR (polymerase chain reaction) in a microfluidic channel. The temperature-induced conformational change is also observed in naphthalene-labeled peptides that contain αaminoisobutyric acid residues (32). The peptide in acetonitrile at a given temperature displays fluorescence with two distinguishable lifetimes. These two lifetimes originate from two equilibrating conformations, and therefore the proportion of fluorescence with each lifetime is temperature-dependent. Viscosity-Sensitive Probes and Others There are some situations in which temperature-dependent characteristics of a bulk solvent influence a fluorescence behavior of a solute (33, 34). The Φf value of 2-(4-pyridyl)5-(4-N,N-dimethylaminophenyl)-1,3-oxazole, 13,
is temperature-dependent in the ionic liquid 1-butyl-1methylpyrrolidium bis(trifluoromethylsulfonyl)imide (23). Figure 8 clearly shows that the excimer (476 nm)–monomer (376 nm) ratio of 14 in the ionic liquid increases with increasing temperature. The utilization of the ionic liquid raises the upper limit of the sensitive temperature range of 14 to 140 ⬚C. In closing, we note that atomic species and small molecules can achieve the thermometer function as adequately as the designed molecules of intermediate or large sizes discussed above. For example, the precious stone ruby has fluorescence characteristics that are affected by temperature (35). This fluorescence behavior is caused by two competing relaxation pathways in its main optical component, Cr(III) ion. However, this is clearly a hard solid that can not function at the molecular or atomic level. Similarly, a gas phase example would be the fluorescence of nitric oxide from various rotational levels, which is temperature-dependent. This feature is useful in the measurement of the temperature of flames that are seeded with nitric oxide (36). Conclusion The luminescent molecular thermometers discussed in this article vary in their type as well as in sensitivity, color, effective temperature range, stability, operating medium, and durability. Therefore we can choose a suitable thermometer for a given purpose. For example, a thermometer operating in water around room temperature is probably the best for the biological application. Since LMTs are active in tiny spaces, they are ready for the nano-world that is important in 21st century chemistry. Acknowledgments
in ethanol is significantly changed by the increase in tem-
726
Journal of Chemical Education
•
We are grateful for the support of the Ministry of Education, Culture, Sports, Science, and Technology of Japan, Department of Employment and Learning of Northern Ireland, Invest NI (RTD COE 40), and European Union (HPRN-CT-2000-00029). S.U. also thanks the Japan Society for the Promotion of Science for a JSPS Research Fellowship for Young Scientists.
Vol. 83 No. 5 May 2006
•
www.JCE.DivCHED.org
Chemistry for Everyone
Literature Cited 1. Childs, P. R. N.; Greenwood, J. R.; Long, C. A. Rev. Sci. Instrum. 2000, 71, 2959–2978. 2. Stimulating Concepts in Chemistry; Vögtle, F., Stoddart, J. F., Shibasaki, M., Eds.; Wiley-VCH: Weinheim, Germany, 2000. 3. de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice T. E. Chem. Rev. 1997, 97, 1515–1566. 4. Valeur, B. Molecular Fluorescence; Wiley-VCH: Weinheim, Germany, 2002; pp 372–380. 5. Chandrasekharan, N.; Kelly, L. A. Progress towards Fluorescent Molecular Thermometers. In Reviews in Fluorescence 2004; Geddes, C. D., Lakowicz, J. R., Eds.; Kluwer Academic/Plenum: New York, 2004; Vol. 1, pp 21–40. 6. Lim, E. C.; Laposa, J. D.; Yu, J. M. H. J. Mol. Spectrosc. 1966, 19, 412–420. 7. Bennett, R. G.; McCartin, P. J. J. Chem. Phys. 1966, 44, 1969–1972. 8. Parker, C. A. Adv. Photochem. 1964, 2, 305–383. 9. Fister, J. C., III.; Rank, D.; Harris, J. M. Anal. Chem. 1995, 67, 4269–4275. 10. Fery-Forgues, S.; Fayet, J.-P.; Lopez, A. J. Photochem. Photobiol. A: Chem. 1993, 70, 229–243. 11. Chapman, C. F.; Liu, Y.; Sonek, G. J.; Tromberg, B. J. Photochem. Photobiol. 1995, 62, 416–425. 12. Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853–4858. 13. Fernando, S. R. L.; Maharoof, U. S. M.; Deshayes, K. D.; Kinstle, T. H.; Ogawa, M. Y. J. Am. Chem. Soc. 1996, 118, 5783–5790. 14. Crosby, G. A.; Whan, R. E.; Alire, R. M. J. Chem. Phys. 1961, 34, 743–748. 15. Bhaumik, M. L. J. Chem. Phys. 1964, 40, 3711–3715. 16. Sabbatini, N.; Guardigli, M.; Bolletta, F.; Manet, I.; Ziessel R. Angw. Chem., Int. Ed. Engl. 1994, 33, 1501–1503. 17. Kolodner, P.; Tyson, J. A. Appl. Phys. Lett. 1982, 40, 782– 784. 18. de Silva, A. P.; Gunaratne, H. Q. N.; Jayasekera, K. R.; O’Callaghan, S.; Sandanayake, K. R. A. S. Chem. Lett.
1995, 123–124. 19. Gossage, H. E.; Melton, L. A. Appl. Opt. 1987, 26, 2256– 2259. 20. Lou, J.; Hatton, T. A.; Laibinis, P. E. Anal. Chem. 1997, 69, 1262–1264. 21. Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: London, 1970; p 301. 22. Nomura, R.; Yamada, K.; Masuda, T. Chem. Commun. 2002, 478–479. 23. Baker, G. A.; Baker S. N.; McCleskey T. M. Chem. Commun. 2003, 2932–2933. 24. Albelda, M. T.; García-España, E.; Gil, L.; Lima, J. C.; Lodeiro, C.; de Melo, J. S.; Melo, M. J.; Parola, A. J.; Pina, F.; Soriano, C. J. Phys. Chem. B 2003, 107, 6573–6578. 25. Pragst, F.; Hamann, H.-J.; Teuchner, K.; Naether, M.; Becker, W.; Daehne, S. Chem. Phys. Lett. 1977, 48, 36– 39. 26. Chandrasekharan, N.; Kelly, L. A. J. Am. Chem. Soc. 2001, 123, 9898–9899. 27. Engeser, M.; Fabbrizzi, L.; Licchelli, M.; Sacchi, D. Chem. Commun. 1999, 1191–1192. 28. Nakashima, H.; Takenaka, Y.; Higashi, M.; Yoshida, N. J. Chem. Soc., Perkin Trans. 2 2001, 2096–2103. 29. Uchiyama, S.; Matsumura, Y.; de Silva, A. P.; Iwai, K. Anal. Chem. 2003, 75, 5926–5935. 30. Uchiyama, S.; Kawai, N.; de Silva, A. P.; Iwai, K. J. Am. Chem. Soc. 2004, 126, 3032–3033. 31. Jeon, S.; Turner, J.; Granick, S. J. Am. Chem. Soc. 2003, 125, 9908–9909. 32. Hungerford, G.; Martinez-Insua, M.; Birch, D. J. S.; Moore, B. D. Angew. Chem., Int. Ed. Engl. 1996, 35, 326– 329. 33. Khimich, M. N.; Volchkov, V. V.; Uzhinov, B. M. J. Fluoresc. 2003, 13, 301–305. 34. Schrum, K. F.; Williams, A. M.; Haerther, S. A.; BenAmotz, D. Anal. Chem. 1994, 66, 2788–2790. 35. Anghel, F.; Iliescu, C.; Grattan, K. T. V.; Palmer, A. W.; Zhang, Z. Y. Rev. Sci. Instrum. 1995, 66, 2611–2614. 36. Tamura, M.; Luque, J.; Harrington, J. E.; Berg, P. A.; Smith, G. P.; Jeffries, J. B.; Crosley, D. R. Appl. Phys. B 1998, 66, 503–510.
The structures of a number of the molecules discussed in this article are available in fully manipulable Jmol and Chime format as JCE Featured Molecules in JCE Online (see page 740).
Featured Molecules
an interactive modeling feature, Only@JCE Online
http://www.JCE.DivCHED.org/JCEWWW/Features/MonthlyMolecules
www.JCE.DivCHED.org
•
Vol. 83 No. 5 May 2006
•
Journal of Chemical Education
727
