Intramolecular triplet energy transfer in benzophenone-{crown ether}-naphthalene system

Bichromophoric compound benzophenone-{crown ether}-naphthalene (Bp-C-Np) was synthesized and its photophysics was examined. Steady state and time-resolved spectroscopic measurements indicate that intramolecular energy transfer from the triplet state of the benzophenone chromophore to the naphthyl group in Bp-C-Np occurs with efficiency of ca. 33% and rate constant of ca. 3.3 × 10 5 s -1 . This long-distance triplet energy transfer is proposed to proceed via a through-bond exchange mechanism.


Phosphorescence Quenching
The absorption spectrum of Bp-C-Np in benzene is essentially identical with the sum of the spectrum of the models for the donor, Bp-C, and for the acceptor, Np-C as shown in Figure 1, indicating the absence of measurable interaction between the Bp and Np chromophores of Bp-C-Np in the ground state.Significantly, the absorption of the Bp group extends to longer wavelength than does that of the Np group, suggesting that singlet-singlet energy transfer from the excited Bp chromophore to the Np group is endothermic.Furthermore, this fact permits the selective excitation of the Bp moiety in the bichromophoric compound Bp-C-Np at longer wavelength.Figure 2 shows the phosphorescence spectra of Bp-C-Np and Bp-C in glassy methylcyclohexane at 77 K.The excitation light wavelength used was 350 nm where only the Bp chromophore absorbed.Both compounds exhibit a phosphorescence characteristic of the benzophenone chromophore with maxima at 435, 467, 504 nm and a shoulder at 545 nm.These two spectra were obtained using the solutions with identical optical density at the excitation wavelength and under identical instrument parameter setup.The general features of the phosphorescence spectra of the Bp chromophores from Bp-C-Np and Bp-C are essentially identical.However, the phosphorescence efficiency of the Bp group in Bp-C-Np is ca.30% less than that in the model compound Bp-C.6][27] Thus, the above observations suggest that triplet energy transfer from the Bp to the Np group in Bp-C-Np operates.Measurements at different concentrations reveal that this triplet energy transfer is intramolecular.Originally we expected to be able to observe the phosphorescence from the Np group, because the triplet-triplet energy transfer will result in the triplet state of Np.However, the quantum yield of the phosphorescence for Np is much smaller compared with that for Bp and the two phosphorescence spectra are extensively overlapped.Thus, we could not detect the phosphorescence of Np distinctly.

Flash Photolysis
The evidence for long-distance intramolecular triplet energy transfer in Bp-C-Np based on the phosphorescence efficiency is further strengthened by flash photolysis study.Pulse-laser photolysis of Bp-C-Np solution in benzene gives rise to a strong transient absorption spectrum with maximum at 560 nm immediately after the laser pulse as shown in Figure 3.The bleaching in the region of 400~500 nm is absolutely due to the phosphorescence of the Bp chromophore.The absorption spectrum with the maximum at 560 nm is assigned to the lowest triplet state of the Bp chromophore on the basis of the following observations.First, this absorption is essentially identical with that of the alkyl benzophenone-4-carboxylate triplet state independently generated. 15Secondly, this species is readily quenchable by O 2 .Significantly, the triplet state absorption of the Bp chromophore is progressively replaced by an absorption in the region of 400~500 nm (Figure 3).The latter absorption is assigned to the lowest triplet state of the Np chromophore by reference to the transient absorption of the triplet state of l-methylnaphthalene. 28 The decay of the Bp triplet state absorption (at 560 nm) occurs concomitantly with a growth in absorption of the Np triplet state (at 440 nm) as shown in Figure 4.This observation convincingly demonstrates that triplet energy transfer from Bp to Np group indeed occurs.Analysis of the transient spectrum of Bp group at 560 nm as a function of time yields a lifetime of the triplet state (τ 1 ) of ca.l.03 µs.Similarly, the lifetime of the triplet state of the Np group analyzed at 440 nm is ca.l3.18 µs.The feature of the transient absorption of the Bp group in the model compound Bp-C is identical with that in Bp-C-NP, and the lifetime of the triplet state (τ 2 ) is ca.l.55 µs.The shorter lifetime of the Bp triplet state in Bp-C-Np in comparison with that in Bp-C is consistent with the proposal that a long-distance intramolecular triplet energy transfer in Bp-C-Np operates.The rate constant (K Et ) and efficiency (φ Et ) for this energy transfer can be calculated from τ 1 and τ 2 according to eq l and 2 respectively.K Et was obtained to be 3.3 × l0 5 s -1 , and φ Et 33%, which is comparable to that obtained by phosphorescence efficiency measurements.
Originally, we also planed to investigate the intramolecular triplet energy transfer in Bp-C-Np with the spacer crown uncomplexing a metal ion.However, attempt to prepare ion-free Bp-C-Np was not successful, because sodium ion was already complexed into the crown in the course of the synthesis, and it was difficult to remove the ion from the crown (see Experimental Section).
It has been well established that macrocyclic polyethers complexing with metal ions are rigid and shaped like crowns. 29We used the Alchemy II program to calculate the energies of the various conformations of Bp-C-Np and found that the extended conformation has lowest energy.In this conformation the edge-to-edge distance between the Bp and Np groups, defined as the distance between the carboxyl oxygen atom of the Bp group and the methylene group neighbor to Np, is ca.l6.9 Å.At such separation between the chromophores triplet energy transfer via a through-space exchange process would be very inefficient.Thus, we infer that a through-bond exchange mechanism might operates in the intramolecular triplet energy transfer in Bp-C-Np.3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]

Experimental Section
Material.Unless otherwise noted, materials were purchased from Beijing Chemical Work and were used without further purification.Spectral-grade methylcyclohexane and benzene were used for absorption and emission spectra, and flash photolysis measurements.Instrumentation. 1 H NMR spectra were recorded at 300 MHz with a Brucker spectrometer.MS spectra were run on a VG ZAB or VG TR-200 spectrometer.UV spectra were measured with a Hitachi UV-340 spectrometer.Steady-state emission spectra were recorded either on a Hitach Em850 or a Hitachi MPF-4 spectrofluorimter.Phosphorescence Measurements.Phosphorescence studies were performed in methylcyclohexane at 77 K, and the sample solutions were degassed by at least three freeze-pump-thaw cycles at a pressure of 5 × 10 -5 Torr.The excitation wavelength was 355 nm.For comparison of the emission efficiency of Bp-C-Np with Bp-C, the spectra were run using solutions with identical optical density at the excitation wavelength and under identical instrument parameter setup.Laser Flash Photolysis.The pump light source for the laser flash photolysis was the third harmonic (355 nm) of an Nd:YAG laser (Spectra-Physics, GCR-11-1).The probe light source was a xenon arc lamp (Vshio, UXL-500-0).The probe light transmitting through the sample cell was fed to a detection system which consists of a monochromater (Ritsu, MC-10L), photomultiplier (Hamamatsu, R928), digital oscilloscope (Tektronix, 2440), and microcomputer.The decay curves were analyzed by using nonlinear least-squares fitting.Synthesis of Bp-C-Np.Bp-C-Np was prepared by four steps as shown in Scheme 1. Reaction of dibenzo-18-crown-6 (DBC) with hexamethylenetetramine in the presence of trifluoroacetic acid gave bis(4'-fomylbenzo)-18-crown-6 (BFBC) according to the literature method. 30After separation, BFBC was reduced by sodium borohydride to yield bis(4'-hydroxymethylbenzo)-18crown-6 (BHMBC).Reaction of BHMBC with 2-(bromomethyl)naphthalene in the presence of metal sodium in DMF produced (2-naphthyl-derivative-benzo)-(4'-hydroxymethylbenzo)-18crown-6 (Np-C-OH).Electrospray-ionization mass spectrometry (ESI-MS) analysis revealed that the crown-ether in Np-C-OH complexed with a sodium ion, and this sodium ion was difficult to be removed.Np-C-OH (complexing sodium ion) was used as a starting material to prepare Bp-C-Np by its reaction with p-benzoylbenzoyl chloride.It was found that the crown ether in Np-C-Bp also complexed a sodium ion.2,3,11,12-Bis(4′-formylbenzo)-18-crown-6 (BFBC).This compound was prepared according to the literature. 30A mixture of dibenzo-18-crown-6 (DBC) (10.8 g, 0.03 mol), trifluoroacetic acid (0.21 mol) and hexamethylenetetramine (8.8 g, 0.02 mol) was stirred at 90 o C under nitrogen for 12 h.After the mixture was cooled, 50 ml of concentrated KOH and 200 ml of water were successively added.The product was precipitated as a brown solid.The crude product was collected by suction filtration and washed with acetone several times, resulting in white powder, yield 8.2 g (65%). 1 H-NMR δ 9.85 (2H, s, CHO), 7.5-6.9(6H, m, ArH), 4.35-3.90(16H, m, OCH 2 CH 2 O).After neutralization with sulfuric acid the mixture was extracted with 5 × 30 ml of methylene chloride.The solvent was evaporated, and 2.1 g of white solid was obtained.Yield 50%. 1 H-NMR δ 7.78-6.98(6H, m, ArH), 4.61 (4H, s, CH 2 OH), 4.35-3.95(16H, m, OCH 2 CH 2 ).Np-C-OH.To the solution of 4.20 g (0.01 mol) of BHMBC in 25 ml of dry fresh distilled DMF was added slowly 0.1 g of sodium metal.The mixture was stirred for half an hour.Then 2.4 g (0.011 mol) of 2-(bromomethyl)naphthalene in 10 ml of DMF was added dropwise.The mixture was stirred for 4 h at room temperature, and then 100 ml of water was added.The mixture was acidified with 2 N HCl.The solid was isolated by filtration and purified by chromatography on silica eluting with chloroform /ethyl ether (1:2, V/V).Yield 0.6 g (11%). 1  Bp-C-Np.p-Benzoylbenzoyl chloride was prepared by refluxing the solution of 1.13 g (5 mmol) of p-benzoylbenzoic acid and 3 ml of thionyl chloride in 10 ml of chloroform for 3 h.After the excess thionyl chloride and chloroform were evaporated under reduced pressure, the resultant benzoylbenzoyl chloride was added to a solution of 4.2 g of Np-C-OH in 20 ml of chloroform, then 1 ml of pyridine was added.The reaction mixture was stirred and refluxed for 4 h.To the mixture 20 ml of water was added and then the solution was extracted with 3 × 15 ml of chloroform.Evaporation of the solvent afforded a yellow solid.The crude product was purified by column chromatography on silica eluting with chloroform /ethyl ether (1:2, V/V).Yield 10%.