CROWN-CYCLE EFFECTS ON PHOTOPHYSICS OF 7-AMINOCOUMARINS

 

B.M. Uzhinov, S.I. Druzhinin, M.V. Rusalov, V.L. Lapteva, V.V. Samoshin.

Dept. of Chemistry, Moscow State University, Vorob'evy gory,

Moscow, 119899, Russia

e-mail: druzh@light.chem.msu.su

 

Abstr. XVIIIth Int. Conf. on Photochemistry. August 3-8, 1997, Warsaw, Poland.

Warsaw, In-t. Phys.Chem. Polish Acad. Sci. P. O5.4.1-O5.4.2.

 

In the present work photophysics of crowned aminocoumarins (Fig. 1), their protonated forms and complexes with alkali and alkaline earth metal cations has been studied. The regularities of solvent effects, protonation and complex formation on term energies and radiationless deactivation are discussed.

The wavelengths of absorption (labs) and fluorescence (lfl) maxima of free crown-ethers, their complexes and protonated forms are given in the Table. The low-frequency shift of absorption (up to 1800 cm-1) and fluorescence (up to 3000 cm-1) spectra is observed if nonpolar solvents are used instead of polar ones. The similar shifts are observed upon protonation of 3-substituent nitrogen atom (up to 2500 cm-1) and upon complex formation (up to 1000 cm-1). The influence of 3-substituent on photoinduced charge transfer from the 7-diethylamino to the carbonyl group is considered.

The introduction of macrocyclic moiety in the coumarin 1 molecule results in decreasing of fluorescence quantum yield (j ) (up to 15 times). Besides, the fluorescence decay ceases to be monoexponential (the fractional intensities c1 corresponding to the decay times t1 are less than a unity). Contrary to this, the fluorescence quantum yield increases and the fluorescence decay becomes monoexponential upon complexation with metal cations and protonation.

In order to elucidate the mechanism of intramolecular fluorescence quenching, the effects of solvent viscosity and polarity, and temperature on fluorescence quantum yield have been studied. The temperature quenching of C1A15C5 fluorescence has been investigated in nonpolar solvents of different viscosity (pentane, toluene, decalin).

The mechanism of intramolecular fluorescence quenching, consisting in the electron transfer from the nitrogen atom of macrocycle to the fluorophor part of molecule, is suggested. The data on fluorescence quenching of coumarin 1 in acetonitrile by the electron donor, triethylamine, corroborate this supposition. The Stern-Volmer plot is a straight line. The quenching constant is equal to 0.474± 0.009 M-1.

The efficiency of C1A15C5 fluorescence increases as temperature decreases. It has been shown that j does not depend on hydrocarbon viscosity (at room temperature the viscosities of pentane, toluene and decalin are equal to 0.24, 0.60 and 2.4 cP, while j are equal to 0.021, 0.019 and 0.017, respectively). A straight line is a satisfactory fit with a linearized expression 1/j =1/j 0+kd/kfЧ exp(-Ea/(RT)) for these solvents assuming that j 0=0.82± 0.08, kd/kf=(3.0± 0.3)Ч 103 , Ea=(2.32± 0.05) kcal/mol (Fig. 2). The fact that j is independent of solvent viscosity indicates that temperature-activated radiationless deactivation of crown-substituted 7-aminocoumarins is not related to the mutual motion of molecule fragments.

The nonexponentiality of fluorescence decay kinetics of free crown-ethers can be caused by different rates of intramolecular electron transfer for each crown-cycle conformation. This deactivation way disappears upon binding of a lone electron pair of the nitrogen atom, and fluorescence kinetics becomes exponential.

Equalization of j (» 0.8 in hexane, » 0.9 in THF, » 0.4 in acetonitrile, » 0.2 in methanol) upon protonation of free crown-ethers also testifies to the disappearance of this deactivation way.

This work was supported by Russian Fund of Basic Researches, grants ? ? 95-03-09482 and 96-03-34108.

 

Table

 compound

Solvent

labs, nm

lfl, nm

j

c1

t1, ns

t2, ns

C1A12C4

Hexane

356

404

0.069

 

 

 

 

THF

366

429

0.054

 

 

 

 

Acetonitrile

371

443

0.060

 

 

 

 

Methanol

380

459

0.070

0.75

0.28

1.1

C1A12C4H+

Hexane

389

422

0.82

 

 

 

 

THF

384

440

0.86

 

 

 

 

Acetonitrile

392

458

0.44

 

 

 

 

Methanol

392

465

0.22

1

0.94

 

C1A12C4Na+

Methanol

381

462

0.67

 

 

 

C1A12C4Ca2+

Methanol

398

473

0.49

 

 

 

C1A15C5

pentane

355

403

0.021

 

 

 

 

hexane

356

403

0.019

 

 

 

 

decalin

359

408

0.017

 

 

 

 

toluene

365

419

0.019

 

 

 

 

THF

365

430

0.028

 

 

 

 

acetonitrile

374

446

0.028

 

 

 

 

methanol

379

461

0.25

0.78

0.36

3.0

C1A15C5H+

hexane

389

427

0.86

 

 

 

 

THF

387

441

0.90

 

 

 

 

acetonitrile

393

461

0.34

 

 

 

 

methanol

394

468

0.20

1

0.84

 

C1A15C5Na+

methanol

381

463

0.64

1

2.9

 

C1A15C5Ca2+

methanol

401

474

1.0

 

 

 

C1A18C6

hexane

356

406

0.079

 

 

 

 

THF

366

431

0.079

 

 

 

 

acetonitrile

372

447

0.10

 

 

 

 

methanol

380

459

0.55

0.78

0.36

2.9

C1A18C6H+

hexane

388

425

0.76

 

 

 

 

THF

385

441

0.85

 

 

 

 

acetonitrile

390

458

0.38

 

 

 

 

methanol

393

467

0.23

1

0.87