Polycyclic Aromatic Nitrogen Heterocycles. Part V: Fluorescence Emission Behavior of Select Tetraaza- and Diazaarenes in Nonelectrolyte Solvents SHERYL A. TUCKER, WILLIAM E. ACREE, JR.,* and CHRISTOPHER UPTON Department o[ Chemistry, University o[ North Texas, Denton, Texas 76203-5068, U.S.A. (S.A.T., W.E.A.), and School o[ Pharmacy and Pharmacology, University o[ Bath, Claverton Down, Bath, BA2 7A Y, England, U.K. (C.U.) Fluorescence emission spectra are reported for tricycloquinazoline, di- benzolc,fll2,Tlnaphthyridine, dibenzo[a,c]phenazine, dibenzlb,hl- indenoll,2,3de][l,6lnaphthyridine, and dibenz[c,f]indeno[1,2,3ij]- [2,7]naphthyridine dissolved in organic nonelectrolyte solvents of varying polarity and acidity. Results of these experiments were used to screen PANHs for potential probe character. The effect of nitromethane as a selective quenching agent on both the unprotonated and protonated PANHs was also examined. Nitromethane was found to quench fluo- rescence emission of dibenzo[c,fl[2,71naphthyridine. Emission intensities of the remaining four PANHs, and of the three protonated PANHs for which emission spectra could be obtained, remained essentially constant and were not affected by nitromethane. Index Headings: Fluorescence; Spectroscopic techniques; UV-visible spectroscopy. INTRODUCTION This paper continues a systematic, experimental investigation 1-5 of the effect that solvent polarity and acidity has on the photophysical properties of polycyclic aromatic nitrogen heterocycles (PANHs). Pertinent pho- tophysical processes of PANHs dissolved in fluid solution are summarized below: Unprotonated: PANH + by0 -~ PANH* (absorption/fluorescence excitation) PANH* -~ PANH + h~l (fluorescence emission) Protonated: PANH-H + + h,0 -~ PANH-H +* (absorption/fluorescence excitation) PANH-H +* -~ PANH-H + + h~2 (fluorescence emission) The emission spectrum of the neutral PANH monomer generally consists of several major bands labeled I, II, etc., in progressive order. Previous measurements revealed that 12-azabenzo[a]pyrene, phenanthro[2,3h]iso- quinoline, phenanthro[3,2h]isoquinoline, 1-azabenz[a]- anthracene, 2-azabenz[a]anthracene, and 9-azabenz- [a]anthracene exhibited modest selective emission Received 25 September 1992. * Author to whom correspondence should be sent. intensity reduction of band I relative to band II (or band III) in polar solvents. The ratio of emission intensities serves as a quantitative measure of solvent polarity and structure. Interestingly, not all PANHs behave in this fashion. Various emission intensity ratios of 1-azapyrene, 2-azapyrene, 4-azapyrene, 4-azachrysene, benzo[lmn]- [3,8]phenanthroline, and benzo[4,10]anthra[1,9,Scdef]- cinnoline remained constant, irrespective of solvent polarity. Fluorescence emission spectra of benzo[a]- phenazine, acenaphtho[1,2]pyridine, indeno[1,2,3ij]- isoquinoline, and indeno[1,2,3ij][2,7]naphthyridine showed very little structured emission. Depending upon the solute and solvent combination being studied, either one unsymmetrical band or two poorly resolved peaks were observed. Protonation of the nitrogen lone electron pair by a hydrogen ion results in loss of emission fine structure accompanied by a sizeable red shift in emission wave- length(s) (i.e., ~1 > v2). Degree of protonation is reflected by both the solvent acidity and PANH basicity. For example, 2,2,2-trifluoroethanol completely protonated 12- azabenzo[a]pyrene, 4-azapyrene, phenanthro[2,3h]- isoquinoline, phenanthro[3,2h]isoquinoline, and phen- anthro[9,10g]isoquinoline. The original three-band emission spectra of 12-azabenzo[a]pyrene at ~395-450 nm disappeared and was replaced by a fairly broad, single band red-shifted by over 60 nm into the 470-560 nm spectral region. The spectra remained unchanged when- ever a small perchloric acid drop was added to the sol- vent. As expected, the protonation was completely re- versible. Addition of sodium hydroxide completely restored the original PANH spectrum, though a slight loss in emission intensity was observed. 1-Azapyrene and 2-azapyrene, on the other hand, were only partially pro- tonated by trifluoroethanol, as indicated by the presence of both the neutral and protonated species in the emis- sion spectra. Protonation was complete upon addition of perchloric acid, as documented by the disappearance of the first emission band. 3 Concurrent with the polarity and protonation studies, we also investigated the ability of nitromethane and 1,2,4- trimethoxybenzene to act as selective quenching agents to distinguish between alternant and nonalternant PANH molecules. 1,~ This particular classification scheme is anal- ogous to the one used for polycyclic aromatic hydrocar- bons (PAHs). In alternant PAHs, every alternant carbon atom in the aromatic ring system(s) can be "starred." Volume 47, Number 2, 1993 ooo3-7o28/93/47o2-O2Ol,2.oo/o APPLIED SPECTROSCOPY 201 © 1993 Societyfor Applied Spectroscopy