Triplet Formation Research Articles

Interchain Charge-Transfer States Mediate Triplet Formation in Purified Conjugated Polymer Aggregates

The pathways and dynamics of converting spin-allowed (S = 0) singlet excitons to spin-forbidden (S = 1) triplets have significant implications in determining performance metrics of conjugated polymers in optoelectronic devices. We study the effect of structural ordering factors on triplet formation in self-assembled aggregate π-stacked chains of poly(3-hexylthiophene) (P3HT) using single-molecule time-resolved intensity modulation and electric-field-dependent photoluminescence (PL) spectroscopy. Triplet generation is only efficient in P3HT aggregates of high purity, and formation yields are found to increase with nanofiber size. We propose that the high intrachain order in purified aggregates that extends exciton coherence lengths, leading to J-aggregate spectral signatures, is also important for populating interchain charge transfer (CT) states that, at longer times, recombine preferentially to triplets according to spin statistics. Electric-field-dependent PL decays of isolated P3HT aggregates show larg...

Synthesis, photophysical properties and spectroelectrochemical characterization of 10-(4-methyl-bipyridyl)-5,15-(pentafluorophenyl)corrole

Abstract The new bipyridyl-corrole dye 10-(4-methyl-bipyridyl)-5,15-(pentafluorophenyl)corrole, encompassing a coordenative methyl-bipyridine moiety (corrole 2 ), was prepared starting from 5-(pentafluorophenyl)dipyrromethane and reacting it with methyl-bipyridyl-carboxaldehyde by Grykos methodology. It was further characterized by spectroscopic and electrochemical methods. In addition, we investigated experimental photophysical properties, photostability, reactive oxygen species generation (ROS) and aggregation phenomena, which are relevant when selecting photosensitizers used in photodynamic therapy and many other applications. Photophysical properties have demonstrated that the corrole 2 dissolved in dichloromethane has a triplet quantum yield formation of about 51%. Fluorescence and internal conversion quantum yields of 4% and 45%, respectively, have also been determined in order to evaluate which could be was the main pathway of the excited state relaxation. Triplet state formation was also confirmed by measuring indirectly 1 O 2 generation. Additionally, quantum chemistry calculations indicated that the most probable intersystem-crossing pathway takes place through S 1 -T 1 , corroborating our rate equation model.

Aminopyridinyl-Pseudodeoxycytidine Derivatives Selectively Stabilize Antiparallel Triplex DNA with Multiple CG Inversion Sites.

The sequence-specific formation of triplex DNA offers a potential basis for genome-targeting technologies. In an antiparallel triplex DNA, the sequence-specificity is established by the formation of specific base triplets (G-GC, A-AT, and T-AT) between a triplex-forming oligonucleotide (TFO) and a duplex DNA. However, there are no natural nucleosides that can selectively recognize the inverted CG and TA base pairs. Therefore, the recognition of the CG and TA inversion sites to form a stable triplex DNA has been a long-standing goal for the triplex-forming technology. We now describe the design and synthesis of pseudo-deoxycytidine (ΨdC) derivatives for selective recognition of the CG base pair to expand the triplex-forming sequence. The aminopyridine-bearing ΨdC derivatives showed high selectivity and affinity toward the CG base pair in all neighboring base contexts. Remarkably, 3-methyl-2-aminopyridinyl-ΨdC ((Me) AP-ΨdC) formed a stable triplex with the promoter sequence of the hTERT gene containing four CG inversion sites, and effectively inhibited its transcription in human cancer cells. Thus, (Me) AP-ΨdC is expected to serve as a new starting point of triplex-forming oligonucleotides for a wide variety of genome-targeting applications.

Aminopyridinyl–Pseudodeoxycytidine Derivatives Selectively Stabilize Antiparallel Triplex DNA with Multiple CG Inversion Sites

AbstractThe sequence‐specific formation of triplex DNA offers a potential basis for genome‐targeting technologies. In an antiparallel triplex DNA, the sequence‐specificity is established by the formation of specific base triplets (G−GC, A−AT, and T−AT) between a triplex‐forming oligonucleotide (TFO) and a duplex DNA. However, there are no natural nucleosides that can selectively recognize the inverted CG and TA base pairs. Therefore, the recognition of the CG and TA inversion sites to form a stable triplex DNA has been a long‐standing goal for the triplex‐forming technology. We now describe the design and synthesis of pseudo‐deoxycytidine (ΨdC) derivatives for selective recognition of the CG base pair to expand the triplex‐forming sequence. The aminopyridine‐bearing ΨdC derivatives showed high selectivity and affinity toward the CG base pair in all neighboring base contexts. Remarkably, 3‐methyl‐2‐aminopyridinyl−ΨdC (MeAP−ΨdC) formed a stable triplex with the promoter sequence of the hTERT gene containing four CG inversion sites, and effectively inhibited its transcription in human cancer cells. Thus, MeAP−ΨdC is expected to serve as a new starting point of triplex‐forming oligonucleotides for a wide variety of genome‐targeting applications.

Dye photodegradation employing mesoporous organosilicas functionalized with 1,8-naphthalimides as heterogeneous catalysts

Abstract Mesoporous organosilicas containing covalently bound 1,8-naphthalimides (NI) were shown to be efficient heterogeneous catalysts for the photodegradation of methylene blue (MB), a typical textile pollutant. The NI-containing mesoporous silicas (SBANI materials) were prepared by a co-condensation reaction, in conditions similar to those used for inorganic SBA-15. For the photocatalytic tests, the SBANI materials were suspended in aqueous MB solutions, and the suspensions were irradiated with a 100 W Hg lamp (bandpass filter: 320–480 nm). The activity of the SBANI catalysts was proportional to the NI content in the sample. When MB was irradiated in the presence of a sample with high NI content, total bleaching of MB was observed after 300 min irradiation. In addition, blue shifts in the visible MB band were observed during irradiation. The results can be explained by concomitant mineralization and N -demethylation of MB. N -demethylation led to the formation of azure dyes, which were detected in the reaction mixtures after irradiation. The photocatalytic activity can be attributed to the formation of NI triplets, which can react with MB by both Type (II) (via oxygen reactive species) and Type I (via electron transfer) mechanisms.

Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells.

A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.

Bacteriopheophytin triplet state in Rhodobacter sphaeroides reaction centers.

It is well established that photoexcitation of Rhodobacter sphaeroides reaction centers (RC) with reduced quinone acceptors results in the formation of a triplet state localized on the primary electron donor P with a significant yield. The energy of this long-lived and therefore potentially damaging excited state is then efficiently quenched by energy transfer to the RC spheroidenone carotenoid, with its subsequent decay to the ground state by intersystem crossing. In this contribution, we present a detailed transient absorption study of triplet states in a set of mutated RCs characterized by different efficiencies of triplet formation that correlate with lifetimes of the initial charge-separated state P+HA−. On a microsecond time scale, two types of triplet state were detected: in addition to the well-known spheroidenone triplet state with a lifetime of ~4 μs, in some RCs we discovered a bacteriopheophytin triplet state with a lifetime of ~40 μs. As expected, the yield of the carotenoid triplet increased approximately linearly with the lifetime of P+HA−, reaching the value of 42 % for one of the mutants. However, surprisingly, the yield of the bacteriopheophytin triplet was the highest in RCs with the shortest P+HA− lifetime and the smallest yield of carotenoid triplet. For these the estimated yield of bacteriopheophytin triplet was comparable with the yield of the carotenoid triplet, reaching a value of ~7 %. Possible mechanisms of formation of the bacteriopheophytin triplet state are discussed.

Differential Roles of Carotenes and Xanthophylls in Photosystem I Photoprotection.

Carotenes and their oxygenated derivatives, xanthophylls, are structural elements of the photosynthetic apparatus and contribute to increasing both the light-harvesting and photoprotective capacity of the photosystems. β-Carotene is present in both the core complexes and light-harvesting system (LHCI) of Photosystem (PS) I, while xanthophylls lutein and violaxanthin bind exclusively to its antenna moiety; another xanthophyll, zeaxanthin, which protects chloroplasts against photooxidative damage, binds to the LHCI complexes under conditions of excess light. We functionally dissected various components of the xanthophyll- and carotene-dependent photoprotection mechanism of PSI by analyzing two Arabidopsis mutants: szl1 plants, with a carotene content lower than that of the wild type, and npq1, with suppressed zeaxanthin formation. When exposed to excess light, the szl1 genotype displayed PSI photoinhibition stronger than that of wild-type plants, while removing zeaxanthin had no such effect. The PSI-LHCI complex purified from szl1 was more photosensitive than the corresponding wild-type and npq1 complexes, as is evident from its faster photobleaching and increased rate of singlet oxygen release, suggesting that β-carotene is crucial in controlling chlorophyll triplet formation. Accordingly, fluorescence-detected magnetic resonance analysis showed an increase in the amplitude of signals assigned to chlorophyll triplets in β-carotene-depleted complexes. When PSI was fractioned into its functional moieties, it was revealed that the boost in the rate of singlet oxygen release caused by β-carotene depletion was greater in LHCI than in the core complex. We conclude that PSI-LHCI complex-bound β-carotene elicits a protective response, consisting of a reduction in the yield of harmful triplet excited states, while accumulation of zeaxanthin plays a minor role in restoring phototolerance.

Exciton Correlations in Intramolecular Singlet Fission.

We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases.

Ultrafast electronic and vibrational dynamics in brominated aluminum corroles: Energy relaxation and triplet formation.

We combined femtosecond (fs) VIS pump–IR probe spectroscopy with fs VIS pump–supercontinuum probe spectroscopy to characterize the photoreaction of the hexacoordinated Al(tpfc-Br8)(py)2 in a comprehensive way. Upon fs excitation at ∼400 nm in the Soret band, the excitation energy relaxes with a time constant of (250 ± 80) fs to the S2 and S1 electronic excited states. This is evident from the rise time of the stimulated emission signal in the visible spectral range. On the same time scale, narrowing of broad infrared signals in the C=C stretching region around 1500 cm−1 is observed. Energy redistribution processes are visible in the vibrational and electronic dynamics with time constants between ∼2 ps and ∼20 ps. Triplet formation is detected with a time constant of (95 ± 3) ps. This is tracked by the complete loss of stimulated emission. Electronic transition of the emerging triplet absorption band overlaps considerably with the singlet excited state absorption. In contrast, two well separated vibrational marker bands for triplet formation were identified at 1477 cm−1 and at 1508 cm−1. These marker bands allow a precise identification of triplet dynamics in corrole systems.

A novel biopolymeric photoinitiator based on chitosan and thioxanthone derivative: Synthesis, characterization and efficiency in photopolymerization

Abstract A novel biopolymeric photoinitiator was synthetized from N -phthaloyl chitosan. The 10-oxo-10H-dibenzene thiopyran-3-4-dicarboximide/chitosan (TXICh) structure was characterized by 1D ( 1 H and 13 C) and 2D ( 1 H 1 H COSY, 1 H 13 C HSQC and 1 H 13 C HMBC) NMR experiments. TXICh showed absorption in a wide region of the visible, with bands at 385, 450 and 540 nm. Fluorescence measurements showed three emitting states with lifetimes of 3, 0.612 and 0.021 ns. Transient lifetimes of the same magnitude were found for TXICh by femtosecond time-resolved pump-probe transient absorption spectroscopy. TXICh triplet formation was observed by flash photolysis. This excited state is responsible for producing radicals, which initiate photopolymerization. The efficiency of TXICh as a biopolymeric photoinitiator was checked by photopolymerization kinetics of TEGDMA using photocalorimetry (photo-DSC). The ability and efficiency of TXICh to act as photoinitiator radical in different wavelengths was confirmed.

Origin of the Red-Shifted Optical Spectra Recorded for Aza-BODIPY Dyes.

The optical properties are compared for two boron dipyrromethene (BODIPY) dyes that differ by virtue of the substituent at the meso-site, namely, aza-N versus C-methine atoms. Both compounds are equipped with aryl rings at the 3- and 5-positions of the dipyrrin backbone, which help to extend the degree of π-delocalization. The aza-BODIPY dye absorbs and fluoresces at much lower energy than does the conventional BODIPY dye, with red shifts of about 100 nm being observed in fluid solution, but with comparable fluorescence yield and lifetime. Hydrogen bonding donors, such as alcohols, attach to the aza-N atom and promote nonradiative decay without affecting the properties of the conventional dye. Triplet formation is ineffective in the absence of a spin-orbit coupler. Quantum chemical calculations indicate that the electronegative aza-N atom lowers the energy of the LUMO while having little effect on the corresponding HOMO energy. The resultant decrease in the HOMO-LUMO energy gap is primarily responsible for the red shift. The HOMO-LUMO energy gap is also affected by the dihedral angle subtended by the aryl rings, but this is insensitive to the geometry around the central 6-membered ring. The aza-N atom, by virtue of restricting spatial overlap between the HOMO and LUMO, decreases the energy gap between excited-singlet and -triplet states.

Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer.

Singlet fission is a process in which a singlet exciton converts into two triplet excitons. To investigate this phenomenon, we synthesized two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of intertetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibits less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by absorption and emission spectroscopy in solution, in PMMA and neat thin films. The results show that singlet fission occurs within 1 ps in an amorphous thin film of BET-B with high efficiency (triplet yield: 154%). In solution and the PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair (1)(T1T1) on a time scale of 2 ps, which decays to the ground state without forming separated triplets, suggesting that triplet energy transfer from (1)(T1T1) to a nearby chromophore is essential for producing free triplets. In support of this hypothesis, selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) leads to formation of the (1)(T1T1) state of BET-B, followed by generation of both DPT and BET-B triplets. For the structurally cofacial BET-X, an intermediate forms in <180 fs and returns to the ground state more rapidly than BET-B. First-principles calculations predict a 2 orders of magnitude faster rate of singlet fission to the (1)(T1T1) state in BET-B relative to that of crystalline tetracene, attributing the rate increase to greater coupling between the S1 and (1)(T1T1) states and favorable energetics for formation of the separated triplets.

Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution.

The photochemical dynamics of the thione 2-mercaptobenzothiazole (MBT) initiated by absorption of 330 nm ultraviolet light are investigated by ultrafast transient absorption spectroscopy. The lowest energy triplet state (T1) has mixed 3ππ*/3nπ* character and is populated with a quantum yield of 0.58 ± 0.01 from the photo-excited 1ππ* S2 state in methanol solution via rapid internal conversion to the 1nπ* S1 state (with time constant τ1 < 150 fs). The spectroscopic evidence points to a mechanism involving intersystem crossing from S1 to the 3nπ*/3ππ* T2 state (τ2 = 400 ± 100 fs) and internal conversion to T1 (with time constant for growth τ3 = 6.1 ± 0.4 ps). The remainder of the photoexcited molecules return to the ground state by S1 → S0 internal conversion. In methanol solution, the T1 state is long-lived but when the solvent is changed to styrene, triplet quenching is observed with a time constant of 107 ± 8 ps and assigned to the adduct-mediated energy transfer process MBT (T1) + styrene (S0) → 3[MBT-styrene] → MBT (S0) + Styrene (T1). Transient vibrational absorption spectroscopy observes the 3[MBT-styrene] biradical intermediate and determines its lifetime to be 700 ± 80 ps. Computational studies identify the mechanistic pathway for triplet quenching, which involves a curve crossing between two triplet states of the MBT-styrene adduct. The quenching process occurs with high efficiency, and no long-lived isomers of the initial adduct are observed.

A comprehensive picture of the ultrafast excited-state dynamics of retinal.

All-trans retinal is the chromophore of microbial rhodopsins initiating energy conversion and cellular signalling by subpicosecond photoinduced switching. Here, we provide detailed UV-Vis transient absorption experiments to disentangle the complex photochemistry of this polyene, which is governed by its terminal aldehyde group. After photoexcitation to the S2((1)Bu(+)) state, the system exhibits polarity-dependent branching, populating separate S1((1)Ag(-)) and intramolecular charge transfer (ICT) species. In all solvents, population of a singlet nπ* state from S1 is observed which represents the precursor of the T1 triplet state. While triplet formation dominates in nonpolar solvents (67% quantum yield), it is dramatically reduced in polar solvents (4%). The channel closes completely upon replacing the aldehyde by a carboxyl group, due to an energetic up-shift of (1)nπ*. In that case, internal conversion via the ICT species becomes the main pathway, with preferential formation of the initially excited isomer.

Nitropyrene Photoprobes: Making Them, and What Are They Good for?

AbstractPyrene derivatives are among the most widely used organic fluorescent photoprobes. Many of them are photosensitizers for hole injection. Pyrenes, however, are mostly UV absorbers, limiting their utility for photonic applications. Nitration of pyrene shifts its absorption to the visible region. Conversely, nitration of pyrene that is already derivatized for covalent labeling, produces mixtures of isomers that are challenging to separate. We present a robust procedure for attaining isomerically pure nitropyrenes. NMR analysis provides unequivocal assignments of the regioisomers and of the structures of the disubstituted nitropyrenes. The added substituents negligibly affect the electronic properties of the nitropyrenes. Photoexcited nitropyrenes undergo efficient triplet formation, making them an attractive choice for triplet sensitizers and photooxidants. Hence, facile and reliable preparation of disubstituted nitropyrenes provides venues for exploring their electronic and photonic utility.

Photodynamic Therapy with Blended Conducting Polymer/Fullerene Nanoparticle Photosensitizers.

In this article a method for the fabrication and reproducible in-vitro evaluation of conducting polymer nanoparticles blended with fullerene as the next generation photosensitizers for Photodynamic Therapy (PDT) is reported. The nanoparticles are formed by hydrophobic interaction of the semiconducting polymer MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) with the fullerene PCBM (phenyl-C61-butyric acid methyl ester) in the presence of a non-compatible solvent. MEH-PPV has a high extinction coefficient that leads to high rates of triplet formation, and efficient charge and energy transfer to the fullerene PCBM. The latter processes enhance the efficiency of the PDT system through fullerene assisted triplet and radical formation, and ultrafast deactivation of MEH-PPV excited stated. The results reported here show that this nanoparticle PDT sensitizing system is highly effective and shows unexpected specificity to cancer cell lines.

Functional Characterization of a LOV-Histidine Kinase Photoreceptor from Xanthomonas citri subsp. citri.

The blue-light (BL) absorbing protein Xcc-LOV from Xanthomonas citri subsp. citri is composed of a LOV-domain, a histidine kinase (HK) and a response regulator. Spectroscopic characterization of Xcc-LOV identified intermediates and kinetics of the protein's photocycle. Measurements of steady state and time-resolved fluorescence allowed determination of quantum yields for triplet (ΦT =0.68±0.03) and photoproduct formation (Φ390 =0.46±0.05). The lifetime for triplet decay was determined as τT =2.4-2.8μs. Fluorescence of tryptophan and tyrosine residues was unchanged upon light-to-dark conversion, emphasizing the absence of significant conformational changes. Photochemistry was blocked upon cysteine C76 (C76S) mutation, causing a seven-fold longer lifetime of the triplet state (τT =16-18.5μs). Optoacoustic spectroscopy yielded the energy content of the triplet state. Interestingly, Xcc-LOV did not undergo the volume contraction reported for other LOV domains within the observation time window, although the back-conversion into the dark state was accompanied by a volume expansion. A radioactivity-based enzyme function assay revealed a larger HK activity in the lit than in the dark state. The C76S mutant showed a still lower enzyme function, indicating the dark state activity being corrupted by a remaining portion of the long-lived lit state.

Broad-Band N(∧)N Pt(II) Bisacetylide Visible Light Harvesting Complex with Heteroleptic Bodipy Acetylide Ligands.

Pt(II) dbbpy bisacetylide (dbbpy = 4,4'-di(tert-butyl)-2,2'-bipyridine) complex (Pt-1) with two different Bodipy ligands was prepared with the goal to attain broad-band visible light absorbing, efficient funneling of the photoexcitation energy (via resonance energy transfer, RET) to the energy acceptor and high triplet formation quantum yields. Construction of the above-mentioned molecular structural motif is challenging because two different arylacetylide ligands are incorporated in the complex; normally two homoleptic acetylide ligands were used for this kind of N(∧)N Pt(II) complexes. A reference complex with trans bis(tributylphosphine) Pt(II) bisacetylide protocol (Pt-4) was prepared for comparison of the photophysical properties. The two different Bodipy ligands in Pt-1 and Pt-4 constitute singlet/triplet energy donor/acceptor, as a result the harvested photoexcitation energy can be funneled to the triplet state confined on one of the two Bodipy ligands. The photophysical properties of the complexes were studied with steady state UV-vis absorption and luminescence spectroscopies, femto- and nanosecond transient absorption spectroscopies, cyclic voltammetry, as well as DFT/TDDFT calculations. Fluorescence/phosphorescence dual emission were observed for the complex. The ultrafast intramolecular singlet/triplet energy transfer in Pt-1 was confirmed by the transient absorption spectroscopy (kFRET = 2.6 × 10(11) s(-1), ΦFRET = 87.1%) followed by an intersystem crossing (kISC = 1.9 × 10(10) s(-1)), and the triplet state lifetime (τT) is 54.1 μs. The reference complex Pt-4 shows drastically different kinetics with kFRET = 6.9 × 10(10) s(-1), ΦFRET = 81.0%, kISC = 5.83 × 10(9) s(-1), and τT = 147.9 μs. Different singlet oxygen ((1)O2) quantum yields (ΦΔ = 75% and 70%) and triplet state quantum yields (ΦT = 91% and 69%, respectively) were observed for complexes Pt-1 and Pt-4.

Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers.

Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm(-1) of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.